Chapter 9: Respiratory Viruses

Respiratory disease accounts for an estimated 75% to 80% of all acute morbidity in the United States, and most of these illnesses (approximately 80%) are viral infections. Although a majority of the episodes may not require medical attention, the overall average is three to four illnesses per year per person. Although the incidence varies inversely with age (ie, greater among younger children than healthy young adults), the morbidity is significantly higher in elderly population. Seasonality is also a feature; incidence is lowest in the summer months and highest in the winter.

The viruses that are major causes of acute respiratory disease (ARD) include influenza viruses, parainfluenza viruses, respiratory syncytial virus (RSV), adenoviruses, rhinoviruses, respiratory coronaviruses, human metapneumovirus (hMPV), and bocaviruses (a member of parvovirus group). Reoviruses can also affect respiratory tract and are included in this chapter. Other viruses, such as enterovirus, measles virus, Epstein-Barr virus (EBV), cytomegalovirus (CMV), varicella-zoster (VZV), herpes simplex virus (HSV), and hantavirus can also cause respiratory symptoms but are discussed in other chapters of their principal diseases.

In addition to the ability to cause a variety of ARD syndromes, this group of viruses discussed in this chapter shares a relatively short incubation period (1-4 days) and a person-to-person mode of spread. Transmission is direct, by infective droplet nuclei, or indirect, by hand transfer of contaminated secretions to nasal or conjunctival epithelium. These respiratory viral agents are associated with an increased risk of bacterial superinfection of the damaged tissue of the respiratory tract, and all have a worldwide distribution.

INFLUENZA VIRUS GROUP CHARACTERISTICS

Influenza viruses are members of the orthomyxovirus group or family, which are enveloped, pleomorphic, single-stranded negative-sense segmented RNA viruses. They are classified into three major types, A, B, and C, based on antigenic differences in their ribonucleoprotein (NP) and matrix (M) protein antigens. Influenza A viruses are the most extensively studied because of their predominance in epidemics, and much of the following discussion is based on knowledge of this type. They generally cause more severe disease and more extensive epidemics than the other types; naturally infect a wide variety of species, including mammals and birds; and have a great tendency to undergo significant antigenic changes (Table 9–1). Influenza B viruses are more antigenically stable, are known to infect humans and seals, and usually occur in more localized outbreaks. Influenza C viruses appear to be relatively minor causes of disease, affecting humans and pigs.

Influenza A and B viruses each consist of a nucleocapsid containing eight segments of negative-sense, single-stranded RNA, which is enveloped in a lipid bilayer membrane derived from the host cell plasma membrane. The inner side of the envelope contains a layer of virus-specified matrix protein (M1). Two virus-specified glycoproteins, hemagglutinin (HA or H) and neuraminidase (NA or N), are embedded in the outer surface of the lipid bilayer envelope and appear as “spikes” over the surface of the virion. The ratio of H to N is generally 4 or 5 to 1. There is another integral membrane protein in influenza A known as M2 ion channel protein. Figure 9–1 illustrates the makeup of influenza A virus. Influenza B is somewhat similar but has a unique integral membrane protein, NB instead of M2 that is also believed to function as an ion channel. Influenza C differs from the others in that it possesses only seven RNA segments and has no neuraminidase, although it does possess other receptor-destroying capability (see further). In addition, the hemagglutinin of influenza C binds to a cell receptor different from that for types A and B.

The virus-specific glycoproteins are antigenic and have special functional importance to the virus in pathogenesis and immunity. Hemagglutinin is so named because of its ability to agglutinate red blood cells from certain species (eg, chickens and guinea pigs) in vitro. Its major biologic function is to serve as a point of attachment to N-acetylneuraminic (sialic) acid-only containing glycoprotein or glycolipid receptor sites on human respiratory cell surfaces, which is a critical first step in initiating infection of the cell.

Neuraminidase is an antigenic hydrolytic enzyme that acts on the hemagglutinin receptors by splitting off their terminal neuraminic (sialic) acid. The result is destruction of receptor activity, which may help in preventing superinfection or aggregation of virus particle in the infected cell. Neuraminidase serves several functions. It may inactivate a free mucoprotein receptor substance in respiratory secretions that could otherwise bind to viral hemagglutinin and prevent access of the virus to the cell surface. Neuraminidase may promote fusion of the viral envelope with the host cell membrane for viral entry. More importantly, neuraminidase aids in the release of newly formed virus particles from infected cells. The newly formed virus particles aggregate on the cell surface by attaching to sialic acid through their hemagglutinins but neuraminidase removes the sialic acid from the cell surface receptor allowing the virus to be released and infect other cells. Type-specific antibodies to neuraminidase appear to inhibit the spread of virus in the infected host and to limit the amount of virus released from host cells.

Figure 9–2 illustrates the replication cycle of influenza virus. After viral entry in the cytoplasm of the host cells, nucleocapsids (viral RNA-protein complex) with viral RNA-dependent RNA polymerase complex (PB2, PB1, PA; see Table 9–2) move into the nucleus for transcription and replication (unique to RNA viruses). The priming of viral mRNA transcription is done by using host capped RNA primers, whereas viral RNA synthesis is performed by viral RNA-dependent RNA polymerase. Viral mRNAs are transported in the cytoplasm for protein synthesis followed by proteins translocation at various sites such as H and N on cell surface and nucleocapsid in the nucleus. Viral genomic (–) RNAs replication is carried out by viral RNA polymerase via positive-sense RNA intermediates followed by nucleocapsids assembly.

Nucleocapsids assembly take place in the cell nucleus, but final virus assembly takes place at the plasma membrane. The ribonucleoproteins are enveloped by the plasma membrane, which by then contains hemagglutinin and neuraminidase. Virus “buds” are formed, and intact virions are released from the cell surface (Figure 9–2).

Influenza A viruses were initially isolated in 1933 by intranasal inoculation of ferrets, which developed febrile respiratory illnesses. The viruses replicate in the amniotic sac of embryonated hen’s eggs, where their presence can be detected by the hemagglutination test. Most strains can also be readily isolated in cell culture systems, such as primary monkey kidney cells. Some cause cytopathic effects in culture.

The most efficient method of detection is demonstration of hemadsorption by adherence of erythrocytes to infected cells expressing hemagglutinin or by agglutination of erythrocytes by virus already released into the extracellular fluid. The virus can then be identified specifically by inhibition of these properties by addition of antibody directed specifically against hemagglutinin. This method is called hemadsorption inhibition or hemagglutination inhibition (HI), depending on whether the test is conducted on infected cells or on extracellular virus, respectively. Because the hemagglutinin is antigenic, HI tests can also be used to detect antibodies in infected subjects. Research has shown that antibody directed against specific hemagglutinin is highly effective in neutralizing the infectivity of the virus.

Influenza A

Influenza A is considered in detail because of its great clinical and epidemiologic importance.

The influenza A virion contains eight segments of negative-sense, single-stranded RNA with defined genetic responsibilities. These functions include coding for virus-specified proteins (Figure 9–1; Table 9–2). A unique aspect of influenza A viruses is their ability to develop a wide variety of subtypes through the processes of mutation and whole-gene “swapping” between strains, called reassortment. Recombination, which occurs when new genes are assembled from sections of other genes, is thought to occur rarely, if at all. These processes result in antigenic changes called drifts (mutation) and shifts (reassortment or recombination), which are discussed shortly.

The 18 recognized subtypes of hemagglutinin (H) and 11 neuraminidase (N) subtypes known to exist among influenza A viruses that circulate in birds and mammals represent a reservoir of viral genes that can undergo reassortment, or “mixing” with human strains. Although, 16 subtypes of hemagglutinins and 9 subtypes of neuraminidases have been identified in aquatic birds, pigs are infected with two major hemagglutinins (H1 and H3) and neuraminidases (N1 and N2) and horses with two H (H3 and H7) and two N (N7 and N8). Two new subtypes, H17N10 and H18N11 have been identified in bats. Three hemagglutinins (H1, H2, and H3) and two neuraminidases (N1 and N2) appear to be of greatest importance in human infections. These subtypes are designated according to the H and N antigens on their surface (eg, H1N1, H3N2). There may also be more subtle, but sometimes important, antigenic differences (drifts) within each subtype. These differences are designated according to the major representative virus to which they are most closely related antigenically, using the place of initial isolation, number of the isolate, and year of detection. For example, two H3N2 strains that differ antigenically only slightly are A/Texas/1/77(H3N2) and A/Bangkok/1/79(H3N2).

Antigenic drifts within major subtypes can involve either H or N antigens, as well as the genes encoding other structural and nonstructural proteins, and may result from as little as a single mutation in the viral RNA. These mutations are caused by viral RNA polymerase enzyme because it lacks proof reading ability. The mutant may come to predominate under selective immunologic pressures in the host population (Figure 9–3). Such drifts are common among influenza A viruses, occurring at least every few years and sometimes even during the course of a single epidemic. In addition, drifts can develop in influenza B viruses but considerably less frequently.

In contrast to the frequently occurring mutations that cause antigenic drift among influenza A strains, major changes (>50%) in the nucleotide sequences of the H or N genes can occur suddenly and unpredictably. These are referred to as antigenic shifts. (Figure 9–3 illustrates the difference between antigenic drifts and shifts). They almost certainly result from reassortment that can be readily reproduced in the laboratory. Simultaneously infecting a cell with two influenza A subtypes yields progeny that contain antigens derived from either of the original viruses. For example, a cell infected simultaneously with influenza A (H3N2) and influenza A (H1N1) may produce a mixture of influenza viruses of the subtypes H3N2, H1N1, H1N2, and H3N1. When “new” epidemic strains emerge, they most likely have circulated into animal or avian reservoirs, where they have undergone genetic reassortment (and sometimes also mutation) and then are readapted and spread to human hosts when a sufficient proportion of the population has little or no immunity to the “new” subtypes. An example was the appearance of avian influenza A (H5N1) virus in Hong Kong in 1997 that caused infection in humans. The global spread of avian influenza (H5N1 and others) continued through 1997 and onward with several more cases every year. Studies indicated that all RNA segments were derived from an avian influenza A virus, but a single insert coding for several additional amino acids in the hemagglutinin protein facilitated cleavage by human cellular enzymes. In addition, a single amino acid substitution in the PB2 polymerase protein occurred. These two mutations together made the virus more virulent for humans; fortunately, human-to-human transmission was poor as discussed further. A recent example is the emergence of swine influenza virus (H1N1) in Mexico and the southwestern United States in 2009 that contained segments from avian, human, and swine influenza A viruses, and was easily transmitted to humans and caused a severe disease, mainly in young immune-competent adults, including deaths. In 2012, a new influenza virus strain, H3N8 has been identified in the autopsies of seals, which is closely related to a strain circulating in North American birds since 2002. As this strain has the ability to target the sialic acid receptor found in the human respiratory tract, the new strain H3N8 poses risk to humans. In 2013, a new strain of avian flu (H7N9) infected humans in eastern China resulting in severe illness, including deaths. H7N9 has been found in chickens, ducks, and pigeons in live poultry markets in eastern China. H7N9 continues to cause infection and deaths in humans in China even in 2017. There is no solid evidence of human-to-human transmission because most of the infected people had contacts with sick poultry. Based on genetic analysis, H7N9 is responsive to neuraminidase inhibitors and that the virus has acquired some mutations that may allow it to infect mammals and humans.

Additional molecular barriers limit human-to-human transmission of avian influenza virus (H5N1). One of the most important barriers is that avian and human influenza viruses target different regions of the human respiratory tract. Although the receptor for influenza viruses is sialic acid (SA) glycoprotein, there is a major difference in the sialic acid sugar positions with SA α 2,6 galactose for human influenza virus and SA α 2,3 galactose for avian influenza virus (H5N1). Human influenza virus receptor, SA α 2,6 galactose, is dominant on epithelial cells of nasal mucosa, paranasal sinuses, pharynx, trachea, and bronchi, whereas the H5N1 receptor SA α 2,3 galactose is mainly found on nonciliated bronchiolar cells at the junction between respiratory bronchioles and alveolus. It is interesting that A/Hong Kong/213/03 (H5N1) isolated from a patient recognized both SA α 2,6 galactose and SA α 2,3 galactose and is bound extensively to both bronchial and alveolar cells. More importantly, H1N1 swine influenza of 2009 was transmitted from human-to-human easily because it binds to the receptor SA α 2,6 galactose found in the upper respiratory tract, and caused greater severity because it infected the lower portion of the lungs by interacting with the receptor SA α 2,3 galactose.

Major antigenic shifts, which occurred approximately every 8 to 10 years in the 20th century, often resulted in serious epidemics or pandemics among populations with little or no preexisting antibody to the new subtypes. Examples include the appearance of an H1N1 subtype in 1947, followed by an abrupt shift to an H2N2 strain in 1957, which caused the pandemic of Asian flu. A subsequent major shift in 1968 to an H3N2 subtype (the Hong Kong flu) led to another, but somewhat less severe epidemic. The Russian flu, which appeared in late 1977, was caused by an H1N1 subtype very similar to that which dominated between 1947 and 1957 (Table 9–3). The swine flu that appeared in April, 2009, in Mexico and southwestern United States was a previously unrecognized H1N1 strain, which caused a severe acute respiratory distress syndrome, including deaths, especially in young healthy immune-competent adults. Further analysis revealed that H1N1 swine influenza virus of 2009 was a reassortant that contained genetic components from four different flu viruses—North American swine influenza, North American avian influenza, human influenza, and swine influenza virus of Eurasian origin. Over the subsequent 3 months, this strain, designated swine-origin 2009 A (H1N1) rapidly spread globally. Fortunately, the pandemic tapered down in the following seasons. So, the key requirements for a pandemic influenza strain are: (1) generation of a new influenza A subtype, (2) causing a serious illness, and (3) easily transmitted from human-to-human. Although two of these three requirements were met in 2006 by H5N1, all these three prerequisites were fulfilled in 2009 by H1N1 swine. Each new human infection is an opportunity for the virus to change.

The concepts of antigenic shift and drift in human influenza A virus infections can be approximately summarized as follows. Periodic shifts in the major antigenic components appear, usually resulting in major epidemics in populations with little or no immunologic experience with the subtype. As the population of susceptible individuals is exhausted (ie, subtype-specific immunity is acquired by increasing numbers of people), the subtype continues to circulate for a time, undergoing mutations with subtle antigenic drifts from season to season. This allows some degree of virus transmission to continue. Infectivity persists because subtype-specific immunity is not entirely protective against drifting strains; for example, an individual may have antibodies reasonably protective against influenza A/Texas/77(H₃N₂), yet be susceptible in succeeding years to reinfection by influenza A/Bangkok/79(H₃N₂). Eventually, however, the overall immunity of the population becomes sufficient to minimize the epidemic potential of the major subtype and its drifting strains. Unfortunately, the battle is never entirely won; the scene is set for the sudden and usually unpredictable appearance of an entirely new subtype that may not have circulated among humans for 20 years or more. One example we saw in 2009 was when an H1N1 swine influenza virus appeared that had not been seen previously, and the existing population had no immunity to its components.

INFLUENZA

EPIDEMIOLOGY

Humans are the major hosts of the influenza viruses, and severe respiratory disease is the primary manifestation of infection.

However, influenza A viruses closely related to those prevalent in humans circulate among many mammalian and avian species. As noted previously, some of these may undergo antigenic mutation or genetic recombination (reassortment) and emerge as new human epidemic strains.

Characteristic influenza outbreaks have been described since the early 16th century, and outbreaks of varying severity have occurred nearly every year. Severe pandemics occurred in 1743, 1889-1890, 1918-1919 (the Spanish flu), 1957-1958 (the Asian flu), 1968-1969 (Hong Kong flu), 1977-1978 (Russian flu), and 2009-2010 (Swine flu). These episodes were associated with particularly high mortality rates; the Spanish flu was thought to have caused at least 30 to 50 million deaths, and some historians estimate the worldwide toll was closer to 100 million deaths. Usually, the elderly and persons of any age group with cardiac or pulmonary disease have the highest death rate. However, the severity in 2009 swine flu was mainly seen among the young healthy adult population.

Direct droplet spread is the most common mode of transmission. Influenza infections in temperate climates tend to occur most frequently during midwinter months. Major epidemics of influenza A usually occur at 2- to 3-year intervals, and influenza B epidemics occur irregularly, usually every 4 to 5 years. The typical epidemic develops over a period of 3 to 6 weeks, and can involve 10% of the population. Illness rates may exceed 30% among school-aged children, residents of closed institutions, and industrial groups. One major indicator of influenza virus activity is an abrupt rise in school or industrial absenteeism. In severe influenza A epidemics, the number of deaths reported in a given area of the country often exceeds the number expected for that period. This significant increase, referred to as excess mortality, is another indicator of severe, widespread illness. Influenza B rarely causes such severe epidemics. In general, human influenza viruses are not stable in the environment and are sensitive to heat, acid pH, and solvents. In contrast, avian influenza viruses (H5N1 and others) retain infectivity for several weeks outside the host. The avian virus is shed in respiratory secretions and feces, and the virus survives in the feces for a long time.

PATHOGENESIS

Influenza viruses have a predilection for the respiratory tract because of the presence of their receptors. They multiply in ciliated respiratory epithelial cells, leading to functional and structural ciliary abnormalities and viremia is rarely detected. This is accompanied by a switch-off of protein and nucleic acid synthesis in the affected cells, the release of lysosomal hydrolytic enzymes, and desquamation of both ciliated and mucus-producing epithelial cells. Thus, there is substantial interference with the mechanical clearance mechanism of the respiratory tract. The process of programmed cell death (apoptosis) results in the cleavage of complement components, leading to localized inflammation. Early in infection, the primary chemotactic stimulus is directed toward mononuclear leukocytes, which constitute the major cellular inflammatory component. The respiratory epithelium may not be restored to normal for 2 to 10 weeks after the initial insult.

The virus particles are also toxic to tissues. This toxicity can be demonstrated by inoculating high concentrations of inactivated virions into mice, which produces acute inflammatory changes in the absence of viral penetration or replication within cells. Other host cell functions are also severely impaired, particularly during the acute phase of infection. These functions include chemotactic, phagocytic, and intracellular killing functions of polymorphonuclear leukocytes and, perhaps, of alveolar macrophage activity.

The net result of these effects is that, on entry into the respiratory tract, the viruses cause cell damage, especially in the respiratory epithelium, which elicits an acute inflammatory response and impairs mechanical and cellular host responses. This damage renders the host highly susceptible to invasive bacterial superinfection. In vitro studies also suggest that bacterial pathogens such as staphylococci can more readily adhere to the surfaces of influenza virus-infected cells. Recovery from infection begins with interferon (α/β) production, which limits further virus replication, and with rapid generation of natural killer cells. Shortly thereafter, class I major histocompatibility complex (MHC)-restricted cytotoxic T cells appear in large numbers to participate in the lysis of virus-infected cells and, thus, in initial control of the infection. This is followed by the appearance of local and humoral antibody together with an evolving, more durable cellular immunity. Finally, there is repair of tissue damage.

IMMUNITY

Although cell-mediated immune responses are undoubtedly important in influenza virus infections, humoral immunity has been investigated more extensively. Typically, patients respond to infection within a few days by producing antibodies directed toward the group ribonucleoprotein antigen, the hemagglutinin, and the neuraminidase. Peak antibody titer levels are usually reached within 2 weeks of onset and then gradually wane over the following months to varying low levels. Antibody to the ribonucleoprotein appears to confer little or no protection against reinfection because it is an internal protein of the virus particle that cannot be recognized by circulating antibody. Antihemagglutinin antibody is considered the most protective; it has the ability to neutralize virus on reexposure because it is a surface protein of the virus easily recognized by the antibody. However, such immunity is relative, and quantitative differences in responsiveness exist among individuals. Furthermore, antigenic shifts and drifts often allow the virus to subvert the antibody response on subsequent exposures. Antibody to neuraminidase antigen is not as protective as antihemagglutinin antibody, but plays a role in limiting virus spread within the host.

CLINICAL ASPECTS

MANIFESTATIONS

Influenza A and B viruses tend to cause the most severe illnesses, whereas influenza C seems to occur infrequently and generally causes milder disease. The typical acute influenza syndrome is described here.

The incubation period is brief, lasting an average of 2 days. Onset is usually abrupt, with symptoms developing over a few hours. These include fever, myalgia, headache, and occasionally shaking chills. Within 6 to 12 hours, the illness reaches its maximum severity, and a dry, nonproductive cough develops. The acute findings persist, sometimes with worsening cough, for 3 to 5 days, followed by gradual improvement. By about 1 week after onset, patients feel significantly better. However, fatigue, nonspecific weakness, and cough can remain frustrating lingering problems for an additional 2 to 6 weeks.

Occasionally, patients develop a progressive infection that involves the tracheobronchial tree and lungs. In these situations, pneumonia, which can be lethal, is the result. Other unusual acute manifestations of influenza include central nervous system dysfunction, myositis, and myocarditis. In infants and children, a serious complication known as Reye syndrome may develop 2 to 12 days after onset of the infection. It is characterized by severe fatty infiltration of the liver and by cerebral edema. This syndrome is associated not only with influenza viruses but with a wide variety of systemic viral illnesses. The risk is greatly enhanced by exposure to salicylates, such as aspirin.

The most common and important complication of influenza virus infection is bacterial superinfection. Such infections usually involve the lung, but bacteremia with secondary seeding of distant sites can also occur. The superinfection, which can develop at any time in the acute or convalescent phase of the disease, is often heralded by an abrupt worsening of the patient’s condition after initial stabilization. The bacteria most commonly involved include Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus.

In summation, there are essentially three ways in which influenza may cause death:

• Underlying disease with decompensation. Individuals with limited cardiovascular or pulmonary reserves can be further compromised by any respiratory infection. Thus, the elderly and those of any age with underlying chronic cardiac or pulmonary disease are at particular risk.

• Superinfection. Superinfection can lead to bacterial pneumonia and, occasionally, disseminated bacterial infection.

• Direct rapid progression. Less commonly, progression of the viral infection can lead to overwhelming viral pneumonia with asphyxia. This phenomenon has been seen most commonly in severe pandemics; for example, the Spanish flu in 1918-1919 often produced fulminant death in healthy young soldiers and H1N1 swine flu epidemic in 2009 that caused a high fatality in young people.

Clinical manifestations of avian flu (H5N1) and swine flu (H1N1) varied with high fever, respiratory symptoms, neurologic symptoms, lymphopenia, and diarrhea. The virus replicated in the lower portion of the lung via interacting with the SA α 2,3 galactose receptor resulting in primary viral pneumonia in the absence of any secondary bacterial infection, including deaths, especially in healthy young adults. The cause of death was believed to be related to systemic dissemination, alveolar flooding, Na+ channel blockage, and cytokine storm (see Figure 7–5).

DIAGNOSIS

During the acute phase of illness, influenza viruses can be readily detected or isolated from respiratory tract specimens, such as nasopharyngeal, nasal, and throat swabs. However, nasopharyngeal specimens typically have higher yield of virus than nasal or throat swabs. Various diagnostic tests, including virus culture, serology, rapid antigen, and molecular (viral nucleic acid) assays, immunofluorescence and reverse transcription polymerase chain reaction (RT-PCR) are available. Rapid antigen and RNA assays take about 15 to 20 minutes, rapid immunofluorescence (antigen) assay 1 to 4 hours, real time RT-PCR1 takes 8 hours, and virus culture takes 3 to 10 days. These tests detect both influenza A and B viruses. Most strains grow in primary monkey kidney cell cultures, and they can be detected by hemadsorption or hemagglutination. Serologic diagnosis (antibody test) is of considerable help epidemiologically and is usually made by demonstrating a fourfold or greater increase in HI antibody titers in acute and convalescent specimens collected 10 to 14 days apart. For details about the HI assay, see Chapter 4.

TREATMENT

The two basic approaches to management of influenza disease are symptomatic care and anticipation of potential complications, particularly bacterial superinfection. After the diagnosis has been made, rest, adequate fluid intake, conservative use of analgesics for myalgia and headache, and antitussives for severe cough are commonly prescribed. It must be emphasized that nonprescription drugs must be used with caution. This applies particularly to drugs containing salicylates (aspirin) given to children, because the risk of Reye syndrome must be considered.

Bacterial superinfection is often suggested by a rapid worsening of clinical symptoms after patients have initially stabilized. Antibiotic prophylaxis has not been shown to enhance or diminish the likelihood of superinfection, but can increase the risk of acquisition of more resistant bacterial flora in the respiratory tract and make the superinfection more difficult to treat. Ideally, physicians should instruct patients regarding the natural history of the influenza virus infection and be prepared to respond quickly to bacterial complications, if they occur, with specific diagnosis and therapy.

Two classes of antiviral agents are available for use against influenza viruses—neuraminidase (N) inhibitors and viral protein M2 inhibitors (Table 9–4). Neuraminidase inhibitors include oseltamivir (Tamiflu) and zanamivir (Relenza) that were approved in 1999 and block the function of neuraminidase enzyme of both influenza A and B viruses, which is required for viral release, spread, and infectivity. The mechanism of action of these neuraminidase inhibitors is to competitively inhibit the function of the viral neuraminidase enzyme. As neuraminidase removes sialic acid from the glycoprotein receptors, the inhibitors do not cleave sialic acid residues on the surfaces of host cells and influenza viral envelopes. Therefore, viral hemagglutinin (H) binds to the uncleaved sialic acid residues, resulting in viral aggregation at the surface of the host cell and inhibition of virus release and reinfection of uninfected cells. These drugs are effective in reducing the severity of influenza virus if taken within 48 hours of onset of illness. Oseltamivir is recommended for treatment in subjects 2 weeks and older, and chemoprophylaxis in 1 year and older. Zanamivir is recommended for treatment in subjects 7 years and older, and chemoprophylaxis in 5 years and older. Zanamivir that is administered as oral inhalation is not recommended for people with underlying respiratory disease. Viral resistance has now been demonstrated for some strains of influenza A and is currently low, but this might change in the future.

Antivirals amantadine and rimantadine (the two symmetric amines) that were considered for influenza A treatment and prophylaxis but not for influenza B virus are not currently recommended because they have developed resistance against influenza A virus. Amantadine or rimantadine used to show a modest benefit to some patients against influenza A but not B when the drug was administered early in the illness (within 12-24 hours of onset). Unfortunately, the incidence of resistance to these amines by influenza A (H3N2), the dominant circulating strains, has risen dramatically from 0.8% before 1995 to higher than 95% by 2005. The mechanism of action of both amantadine and rimantadine was to block the ion channel of the viral M2 protein, resulting in interference with the key role for M2 protein in early virus uncoating. Later, virion assembly was also affected. Regrettably, virus resistance to both drugs can be readily developed in vitro or in vivo due to a single amino acid substitution in the transmembrane portion of the M2 protein.

PREVENTION

The best available method of controlling influenza infection is to annually vaccinate all people aged 6 months and older. Although everybody older than age 6 months should be vaccinated, it is important that vaccination be directed primarily toward the elderly, individuals of all ages who are at high risk (eg, those with chronic lung or heart disease), and their close contacts, including medical personnel and household members and pregnant women.

There are two types of influenza vaccines—killed or inactivated (flu shot) and live attenuated influenza vaccine (nasal spray or FluMist). These viral vaccines are newly formulated each year to most closely match the influenza A and B antigenic subtypes [two influenza A viruses and one B virus (trivalent) or two influenza A and two influenza B viruses (quadrivalent)] currently causing infections. There are three different technologies approved by to FDA to produce influenza vaccines in the United States: (1) egg-based flu vaccine, (2) cell-based flu vaccine, and (3) recombinant flu vaccine. Egg-based flu vaccine is the most common and oldest technology used to produce both inactivated and live attenuated vaccines. The candidate vaccine viruses are grown in eggs, harvested and either inactivated (flu shot) or weakened for live attenuated vaccine (nasal spray). In the cell-based vaccine, the candidate vaccine viruses are grown in mammalian cell cultures followed by harvesting and preparing the vaccines. This method requires less time than egg-based vaccine and prevents allergic reaction with eggs to some people. The recombinant flu vaccine utilizes expression of hemagglutinin (HA), the major antigen of influenza that produces protective immune response in people, in insect cells followed by purification of the antigen. This is the fastest technology to produce influenza vaccine, free of egg allergies. Some of these vaccines also include adjuvants.

There are three different types of flu shots made available recently: (1) a regular flu shot for ages 6 months and older; (2) a high-dose flu shot approved for people aged 65 years and older; and (3) an intradermal flu shot approved for people aged 18 years to 64 years. The flu shots are commonly used in two doses given 1 month apart to immunize children (aged 6 months to 8 years) who may not have been immunized previously. Among older children and adults, single annual doses are recommended just before influenza season. Vaccine efficacy is variable, and annual revaccination is necessary to ensure maximal protection. Live attenuated influenza vaccine (LAIV), which is made up of live, weakened influenza viruses (same strains used in killed vaccine) and given in the form of mist (FluMist) in the nostrils, is approved for use in healthy people 2 to 49 years of age. It is not given to pregnant women. Two weeks after vaccination, protective antibodies against influenza viruses are formed in the body that provide variable protection.

A problem unique to influenza vaccinology is the inherent, often unexpected variation in antigenic drift from year to year. This often requires annual reformulation of vaccines that are hoped to provide the best protection before the onset of the next influenza season. Prediction of which strains should be used for vaccine production is based on international surveillance—always a difficult task indeed. Thus, in some years, vaccine efficacy (prevention of serologically confirmed influenza infection) has been estimated to be as high as 70% to 90%. At other times, efficacy may be only at levels estimated at 40% to 60%. After the emergence of the swine-origin 2009 A (H1N1) virus, this virus was added to annual influenza vaccine strains. The dilemma to vaccine composition will continue as new strains abruptly develop.

A major factor contributing to this dilemma is related to difficulties in timely production of a vaccine. Up until very recently, all available vaccines had to be prepared in embryonated hen’s eggs—a cumbersome process that required at least 22 weeks of preparation. There are new methods whereby new strains can be identified quickly and mass produced in mammalian cell culture (as described earlier) instead of eggs, thus reducing the production time by as much as 50% with far higher vaccine quantities. In addition, the cell-based vaccine could be given to people with egg allergies.

VIROLOGY

Parainfluenza viruses belong to the paramyxovirus group. There are four serotypes of parainfluenza viruses: parainfluenza 1, 2, 3, and 4. These enveloped viruses contain linear (nonsegmented), negative-sense, single-stranded RNA genome. Similar to influenza viruses, parainfluenza viruses possess a hemagglutinin and neuraminidase, but on the same spike. The structure of paramyxovirus is shown in Figure 9–4. The single-stranded, negative-sense linear RNA genome is bound to a nucleoprotein (helical nucleocapsid), and the matrix protein surrounds the nucleoprotein complex, which is packaged into a lipid bilayer envelope containing attachment protein (H and N on the same spike) and the fusion protein (F). Their mode of spread and pathogenesis are similar to those of the influenza viruses. They differ from the influenza viruses in that RNA synthesis occurs in the cytoplasm rather than in the nucleus. All events related to parainfluenza virus replication occur in the cytoplasm, similar to any other negative-sense RNA viruses (see Chapter 6 for details about replication of negative-sense RNA viruses). The virus buds out through plasma membranes. In addition, the antigenic makeup of the four serotypes is relatively stable, and significant antigenic shift or drift does not occur. Each serotype is considered separately.

PARAINFLUENZA DISEASE

The parainfluenza viruses are important because of the serious diseases they can cause in infants and young children. Parainfluenza 1 and 3 are particularly common in this regard. Overall, the group is thought to be responsible for 15% to 20% of all nonbacterial respiratory diseases requiring hospitalization in infancy and childhood. Immunity to reinfection is transient. Although repeated infections can occur in older children and adults, they are usually milder than the illnesses of infancy and early childhood. Humoral immunity plays an important role in controlling parainfluenza virus infection. Antibodies against surface protein, HN, and F are detected in infected patients. Cell-mediated immunity may also be involved in protecting infected patients with severity of disease.

CLINICAL ASPECTS

MANIFESTATIONS

The onset of illness from parainfluenza virus may be abrupt, as in acute spasmodic croup, but usually begins as a mild upper respiratory infection (URI) with variable progression over 1 to 3 days to involvement of the middle or lower respiratory tract. Duration of acute illness can vary from 4 to 21 days but is usually 7 to 10 days.

Parainfluenza 1

Parainfluenza 1 is the major cause of acute croup (laryngotracheitis) in infants and young children, but also causes less severe diseases such as mild URI, pharyngitis, and tracheobronchitis in individuals of all ages. Outbreaks of infection tend to occur most frequently during the fall months.

Parainfluenza 2

Parainfluenza 2 is of slightly less significance than parainfluenza 1 or 3. It has been associated with croup, primarily in children, with mild URI, and occasionally with acute lower respiratory disease. As with parainfluenza 1, outbreaks usually occur during the fall months.

Parainfluenza 3

Parainfluenza 3 is a major cause of severe lower respiratory disease in infants and young children. It often causes bronchitis, pneumonia, and croup in children younger than 1 year of age. In older children and adults, it may cause URI or tracheobronchitis. Infections are common and can occur in any season; it is estimated that nearly 50% of all children have been exposed to this virus by 1 year of age.

Parainfluenza 4

Parainfluenza 4 is the least common of the group. It is generally associated with mild upper respiratory illness only.

DIAGNOSIS, TREATMENT, AND PREVENTION

Specific diagnosis is based on RT-PCR, rapid antigen assay (by enzyme-immunoassay or immunofluorescence), virus isolation, usually in monkey kidney cell cultures, or serology using HI, enzyme immunoassay (EIA), or neutralization assays on paired sera to detect a rising antibody titer. Currently, there is no method of control or specific therapy for these infections. However, symptoms could be relieved by using some over the counter medication to relieve pain and fever. Rest and drinking plenty of fluids are recommended.

VIROLOGY

Respiratory syncytial virus (RSV) is classified as a pneumovirus within the paramyxovirus family. Its name is derived from its ability to produce cell fusion in tissue culture (syncytium formation). Unlike influenza or parainfluenza viruses, RSV possesses no hemagglutinin or neuraminidase. The virion structure is similar to parainfluenza virus except that the envelope glycoproteins are an attachment (G) protein and a fusion (F) protein. The RNA genome is linear (nonsegmented), negative-sense, and single stranded and codes for at least 10 different proteins. Among these are a nucleoprotein bound to genomic RNA (helical nucleocapsid), a phosphoprotein and two matrix (M) proteins in the viral envelope. One forms the inner lining of the viral envelope; the function of the other is uncertain. The virion also contains the viral RNA polymerase enzyme (RNA-dependent RNA polymerase). RSV, similar to other paramyxoviruses, replicates in the cytoplasm and buds out from the plasma membrane.

The antigens on the surface spikes of the viral envelope include the G glycoprotein, which mediates virus attachment to host cell receptors, and the fusion (F) glycoprotein, which induces fusion of the viral envelope with the host cell surface to facilitate entry. F glycoprotein is also responsible for fusion of infected cells in cell cultures, leading to the appearance of multinucleated giant cells (syncytium formation). Antibodies directed at the F glycoprotein are more efficient than G glycoprotein antibodies in neutralizing the virus in vitro.

At least two antigenic subgroups (A and B) of RSV are known to exist. This dimorphism is due primarily to differences in the G glycoprotein. The epidemiologic and biologic significance of these variants is not yet certain; however, epidemiologic studies have suggested that group A infections tend to be more severe. RSV is the single most important etiologic agent in respiratory diseases of infancy, and it is the major cause of bronchiolitis and pneumonia among infants under 1 year of age.

RESPIRATORY SYNCYTIAL VIRUS DISEASE

EPIDEMIOLOGY

Community outbreaks of RSV infection occur annually, commencing at any time from late fall to early spring. The usual outbreak lasts 8 to 12 weeks, and can involve nearly 50% of all families with children. In the family setting, it appears that older siblings often introduce the virus into the home, and secondary infection rates can be almost 50%. The usual duration of virus shedding is 5 to 7 days; young infants, however, may shed virus for 9 to 20 days or longer.

Spread of RSV in the hospital setting is also a major problem. Control is difficult, but includes careful attention to handwashing between contacts with patients, isolation, and exclusion of personnel and visitors who have any form of respiratory illness. Masks are not effective in controlling nosocomial spread.

PATHOGENESIS

RSV is spread to the upper respiratory tract by contact with infective secretions. Infection appears to be confined primarily to the respiratory epithelium, with progressive involvement of the middle and lower airways. Viremia occurs rarely. Viral surface F protein plays an important role in pathogenesis by forming syncytia and multinucleated giant cells leading to cell death. The direct effect of virus on respiratory tract epithelial cells is similar to that previously described for influenza viruses, and cytotoxic T cells appear to play a similar role in early control of the acute infection.

The apparent enhanced severity of RSV, particularly in very young infants, is not yet clearly understood but may have an immunologic basis. Factors that have been proposed to play a role include: (1) qualitative or quantitative deficits in humoral or secretory antibody responses to critical virus-specified proteins; (2) formation of antigen–antibody complexes within the respiratory tract resulting in complement activation; and (3) excessive damage from inflammatory cytokines. Experimental evidence suggests that patients who respond to RSV infections with CD4+ T cells that are predominantly of the T_H type 2 have more severe disease than those with predominant T_H type 1 responses. This is thought to be due to the inflammatory cytokines produced by T_H type 2 cells, including interleukin (IL)-4, IL-5, IL-6, IL-10, and IL-13. Several of these cytokines are involved in promoting increased infiltrations of eosinophils and neutrophils into the lung tissues. In addition, this allergic like response diminishes the activation and effector functions of cytotoxic CD8+T cells followed by a delay in RSV clearance, induction of lung damage, and dissemination of the virus.

The major pathologic findings of RSV are in the bronchi, bronchioles, and alveoli. These include necrosis of epithelial cells; interstitial mononuclear cell inflammatory infiltrates, which sometimes also involve the alveoli and alveolar ducts; and plugging of smaller airways with material containing mucus, necrotic cells, and fibrin (Figure 9–5). Multinucleated syncytial cells with intracytoplasmic inclusions are occasionally seen in the affected tracheobronchial epithelium.

IMMUNITY

Infection with RSV results in IgG and IgA humoral and secretory antibody responses. However, immunity to reinfection is tenuous, as shown by patients who have recovered from a primary acute episode and have become reinfected with disease of similar severity in the same or succeeding year. Illness severity appears to diminish with increasing age and successive reinfection. Cell-mediated immunity is dampened due to activation of the Th2 response.

CLINICAL ASPECTS

MANIFESTATIONS

The usual incubation period for RSV is 2 to 4 days, followed by the onset of rhinitis; severity of illness progresses to a peak within 1 to 3 days. In infants, this peak usually takes the form of bronchiolitis and pneumonitis, with cough, wheezing, and respiratory distress. Clinical findings include hyperexpansion of the lungs, hypoxemia (low oxygenation of blood), and hypercapnia (CO₂ retention). Interstitial infiltrates, often with areas of pulmonary collapse, may be seen on chest radiography (Figure 9–6). Fever is variable. The duration of acute illness is often 10 to 14 days.

The fatality rate among hospitalized infected infants is estimated to be between 0.5% and 1%; however, this rises to 15% or more in children receiving cancer chemotherapy, infants with congenital heart disease, and those with severe immunodeficiency. Infants with underlying chronic lung disease are also at high risk. Causes of death include respiratory failure, right-sided heart failure (corpulmonale), and bacterial superinfection. Death has sometimes resulted from unnecessary procedures in patients in whom RSV infection was not considered. Bronchoscopy, lung biopsy, or overly aggressive therapy with corticosteroids and bronchodilators for presumed asthma all can pose a danger to such patients.

Older infants, children, and adults are also readily infected. The clinical illnesses in these groups are usually milder and include croup, tracheobronchitis, and URI; however, elderly persons can experience severe morbidity. In addition, RSV can cause acute flare-ups of chronic bronchitis and trigger acute wheezing episodes in asthmatic children.

DIAGNOSIS

Rapid diagnosis of RSV infection can be made by detection of viral genome by PCR, and/or viral antigen by immunofluorescence or immunoenzyme from respiratory tract specimens. The virus can also be isolated from the respiratory tract by prompt inoculation of specimens into cell cultures. Syncytial cytopathic effects develop over 2 to 7 days. Serodiagnosis may also be used but requires acute and convalescent sera and is less sensitive than antigen-detection methods, PCR, or culture.

TREATMENT AND PREVENTION

Treatment for RSV is directed primarily at the underlying pathophysiology and includes adequate oxygenation, ventilatory support when necessary, and close observation for complications such as bacterial superinfection and right-sided heart failure. Some studies suggest that ribavirin aerosol treatment may be effective in selected circumstances.

No vaccine is currently available for RSV. Attenuated live virus vaccines and immune globulin containing high antibody titers to RSV are also under active investigation. However, a high-titered monoclonal antibody against F protein called palivizumab has been used for prophylaxis in high-risk infants (those born prematurely or with chronic lung disease). This method requires monthly injections during the RSV season (usually 5 months). This monoclonal antibody can prevent development of severity of RSV disease, but cannot cure or treat infants already infected with RSV.

Human metapneumovirus (HMPV), a Pneumovirus of paramyxovirus group, was discovered in 2001 and can cause upper and lower respiratory tract infection in people of all age groups. HMPV has subsequently been found to be a significant cause of ARD in infants and young children. It may account for approximately 10% of the respiratory tract infections for which there are no previously identified causative agents. It is second only to RSV as a cause of bronchiolitis during the winter-spring seasons, and it produces illnesses that are comparable in their severity and symptoms to those of RSV. Infection with HMPV generally occurs in slightly older children compared with RSV that infects younger children. The incubation period is approximately 3 to 6 days. Symptoms include fever, nasal congestion, cough, and shortness of breath, which may progress to bronchiolitis or pneumonia. Both viruses, HMPV and RSV, can coinfect the same child, and this is generally associated with worse disease. Two genotypes are known to exist, but it is not known whether either produces more severe disease or protective immunity. The usual diagnostic methods of choice are viral genome amplification by RT-PCR or viral antigen detection by enzyme immunoassay or immunofluorescence. The virus culture is rarely done. No specific treatment is available. Preventive measures such as handwashing, avoiding sharing drinks and kissing, and covering coughs and sneezes may prevent the spread to others.

VIROLOGY

Adenoviruses are naked capsid, icosahedral, and double-stranded DNA viruses. There are 68 different serotypes of adenoviruses that infect humans, which are classified into one of seven subgroups (A-G) based on multiple biologic properties of the virus. The virion size is in the range of 90 to 100 nm and it contains a linear double-stranded DNA genome covered with an icosahedral capsid (Figure 9–7). The capsid is composed of 252 subunits (capsomeres), including 240 hexons and 12 pentons and fibers. The penton on the surface of the capsid contains a base and a projecting fiber that varies in length based on the serotypes. The fiber is modified by addition of glucosamine. The fiber is like a spike that interacts with the receptor on host cells. The variability in the fiber determines cellular tropism for the virus. The hexon and the fiber contain most of the neutralizing antibodies’ epitopes, although some epitopes on penton base have been recognized. Adenoviruses enter cells via viropexis and replication occurs in the nucleus by using host RNA polymerase for transcription and viral DNA–dependent DNA polymerase (viral DNA polymerase) for replication of DNA genomes. The assembly of the virus occurs in the nucleus, and virions are released by cell destruction (see Chapter 6 for DNA virus replication). All adenoviruses share a common group-specific, complement-fixing antigen associated with the hexon component of the viral capsid. Adenoviruses are characterized by their ubiquity and persistence in host tissues for periods ranging from a few days to several years. Their ability to produce infection without disease is illustrated by the frequent recovery of virus from tonsils or adenoids removed from healthy children (the group name is derived from its discovery in 1953 as a latent agent in many adenoid tissue specimens) and by prolonged intermittent shedding of virus from the pharynx and intestinal tract after initial infection. Adenoviruses generally cause respiratory infection but depending on the serotypes, they can also cause gastroenteritis, conjunctivitis, cystitis, hepatitis, myocarditis, and, unusually, neurologic diseases.

EPIDEMIOLOGY

Types 1, 2, and 3 adenoviruses are highly endemic; type 5 is the next most common. Most primary infections with these viruses occur early in life and are spread by the respiratory route, fecal–oral route, and by contact with contaminated fomites. Neonates can acquire infection from exposure to cervical secretions at birth. Adenoviruses can survive for a long period on surfaces and are relatively resistant to disinfectants but inactivated by heat, formaldehyde or bleach. Overall, only about 45% of adenovirus infections result in disease. Their most significant contribution to acute illness is in children, particularly those younger than 2 years of age (~10% of acute febrile illness). Adenoviruses are also major causes of ARD in military recruits, usually by types 4 (prevalence > 90%), 14, 7, 3, and 21.

Infections caused by serotypes 1, 2, and 5 are generally most common during the first few years of life. All serotypes can occur during any season of the year, but are encountered most frequently during late winter or early spring. Sharp outbreaks of disease caused by serotypes 3 and 7 have been traced to inadequately chlorinated swimming pools. Conjunctivitis is the illness most commonly associated with these episodes. Other outbreaks of conjunctivitis have been traced to physicians’ offices, and appear to have been spread by contaminated ophthalmic medications or diagnostic equipment.

PATHOGENESIS

The adenoviruses usually enter the host by inhalation of droplet nuclei or by the oral route. Direct inoculation onto nasal or conjunctival mucosa by hands, contaminated towels, or ophthalmic medications may also occur. The virus replicates in epithelial cells, producing cell necrosis and inflammation. Viremia sometimes occurs, and can result in spread to distant sites, such as the kidney, bladder, liver, lymphoid tissue (including mesenteric nodes), and, occasionally, the central nervous system. In the acute phase of infection, the distant sites may also show inflammation; for example, abdominal pain is occasionally seen with severe illnesses and is believed to result from mesenteric lymphadenitis caused by the viruses.

After the acute phase of illness, the viruses may remain in tissues, particularly lymphoid structures such as tonsils, adenoids, and intestinal Peyer patches, and may become reactivated and shed without producing illness for 6 to 18 months thereafter. This reactivation is enhanced by stressful events (stress reactivation), such as infection by other agents. Integration of adenoviral DNA into the host cell genome has been shown to occur; this latent state can persist for years in tonsillar tissue and peripheral blood lymphocytes.

Like the viruses described previously, adenoviruses have a primary pathology involving epithelial cell necrosis with a predominantly mononuclear inflammatory response. In some instances, smudgy intranuclear inclusions may be seen in infected cells (Figure 9–8). A potentially important pathogenic feature of the virion is the presence of pentons, which are located at each of the 12 corners of the icosahedron. These fiber-like projections with knob-like terminal structures are believed to bind to a cellular receptor that is similar or identical to the one for group B coxsackieviruses. Moreover, the pentons appear to be responsible for a toxic effect on cells, which manifests as clumping and detachment in vitro.

In addition, adenoviruses have developed other novel strategies to survive in the host, yet produce deleterious effects. These include encoding a protein in its early E3 genomic region that binds class I MHC antigens in the endoplasmic reticulum, thus restricting their expression on the surface of infected cells and interfering with recognition and attack by cytotoxic T cells. This ability to evade immunosurveillance may be vital to establishment of latency. Another early protein (E1A) has been associated with increased susceptibility of epithelial cells to destruction by tumor necrosis factor and other cytokines. Other adenoviral proteins have been described that have a variety of effects on cell function and susceptibility to cytolysis. One of these, called the adenovirus death protein, is considered important for efficient lysis of infected cells and release of newly formed virions.

IMMUNITY

Immunity to adenoviruses after infection is serotype-specific and usually long lasting. In addition to type-specific immunity, group-specific complement-fixing antibodies appear in response to infection. These antibodies are useful indicators of infection, but do not specify the infecting serotype.

CLINICAL ASPECTS

MANIFESTATIONS

The diversity of major syndromes and serotypes commonly associated with adenoviruses are summarized in Table 9–5. The acute respiratory syndromes vary in both clinical manifestations and severity. Symptoms include fever, rhinitis, pharyngitis, cough, and conjunctivitis. Adenoviruses are also common causes of nonstreptococcal exudative pharyngitis, particularly among children younger than 3 years of age. Acute and, occasionally, chronic conjunctivitis and keratoconjunctivitis have been associated with several serotypes. More severe disease, such as laryngitis, croup, bronchiolitis, and pneumonia, may also occur. A syndrome of pharyngitis and conjunctivitis (pharyngoconjunctival fever) is classically associated with adenovirus infection. Adenoviruses can also cause acute hemorrhagic cystitis, in which hematuria and dysuria are prominent findings. Some serotypes are significant causes of gastroenteritis (see Chapter 15).

DIAGNOSIS

Adenovirus infection can be diagnosed by genome amplification by PCR, antigen detection by enzyme immunoassay, virus isolation and serology. Many serotypes of adenoviruses, other than those associated with acute gastroenteritis, can be readily isolated in heteroploid cell cultures. There is little difficulty in relating the virus detected to the illness in question when the isolate has been obtained from a site other than the upper respiratory or gastrointestinal tract (eg, lung biopsy, conjunctival swabs, urine). However, because of the known tendency for intermittent asymptomatic shedding into the oropharynx and feces, isolates from these latter sites must be interpreted more cautiously. Serologic testing of acute and convalescent sera may be necessary to confirm the relation between the virus and the illness in question.

TREATMENT AND PREVENTION

There is no specific treatment for adenovirus infection. Most infections are treated or managed based on the symptoms. Some in vitro and in vivo data, combined with clinical observations in patients with severe disseminated infections suggest that cidofovir (nucleotide analog) might be effective for adenovirus infection. A live virus vaccine containing serotypes 4 and 7, enclosed in enteric-coated capsules and administered orally, has been used in military recruits. The viruses are released into the small intestine, where they produce an asymptomatic, nontransmissible infection. This vaccine has been found effective, but is neither available nor recommended for civilian groups.

The rhinovirus group comprises of more than 100 serotypes as well as more that are not yet classified, all of which are members of the picornavirus family. They are small (20-30 nm), naked capsid virus particles containing single-stranded, positive-sense RNA genomes. They are distinguished from other picornaviruses, namely enteroviruses by their acid lability and an optimum temperature of 33°C for in vitro replication. This temperature approximates that of the nasopharynx in the human host, and may be a factor in the localization of pathologic findings at that site. Rhinoviruses are most consistently isolated in cultures of human diploid fibroblasts. The receptor for most rhinoviruses (and some coxsackieviruses) is glycoprotein intercellular adhesion molecule 1 (ICAM-1), a member of the immunoglobulin supergene family. ICAM-1 is best known for its role in immunologic cell adhesion; its ligand is the lymphocyte function-associated antigen-1.

Rhinoviruses are known as the common cold viruses. They represent the major causes of mild URI syndromes in all age groups, especially older children and adults. Lower respiratory tract disease caused by rhinoviruses is uncommon. The usual incubation period is 2 to 3 days, and acute symptoms commonly last 3 to 7 days. It is interesting to note that mucosal cell damage is minimal during the illness. Data suggest that activation and an increase in kinins, particularly bradykinin, may have a major role in the pathogenesis of increased secretions, vasodilation, and sore throat. Rhinovirus infections may be seen at any time of the year. Epidemic peaks tend to occur in the early fall or spring months.

DIAGNOSIS, TREATMENT, AND PREVENTION

Rhinoviruses can be diagnosed by viral genome amplification by RT-PCR in nasopharyngeal specimens. At present, there is no specific therapy and no method of prevention with vaccines. Prospects for the development of an appropriate vaccine appear less promising. The multiplicity of serotypes and their tendency to be type-specific in the production of antibodies seem to demand the development of a multivalent vaccine, which would be extremely difficult to accomplish. However, recent studies have suggested that a monoclonal antibody directed at the virus receptor or the use of a recombinant soluble receptor (ICAM-1) might block attachment of rhinoviruses. It remains to be seen whether these observations can be translated into effective preventive or therapeutic applications. At present, the attitude toward these viruses is best summed up by Sir Christopher Andrewes, who suggested that we should accept these infections as “one of the stimulating risks of being mortal.”

Coronaviruses contain a positive-sense single-stranded linear RNA genome, which is surrounded by an envelope that includes a lipid bilayer derived from intracellular rough endoplasmic reticulum and Golgi membranes of infected cells. Petal- or club-shaped spikes (peplomers), measuring approximately 13 nm, project from the surface of the envelope, giving the appearance of a crown of thorns or a solar corona. The peplomers play an important role in binding to the host cell receptor and inducing neutralizing and cellular immune responses. The structure of coronavirus is shown in Figure 9–9. Coronaviruses replicate in the cytoplasm, generally like other positive-sense RNA viruses, but acquire an envelope from endoplasmic reticulum or Golgi apparatus. Like the rhinoviruses, coronaviruses are considered primary causes of the common cold. Based on serologic studies, it is estimated that they may cause up to 5% to 10% of common colds in adults, and a similar proportion of lower respiratory illnesses in children.

The number of serotypes is unknown. Two strains of human coronaviruses (hCoV)—hCoV-229E and hCoV-OC43—have been studied to some extent; they can cause outbreaks like those of the rhinoviruses, and that reinfection with the same serotype can occur. The cellular receptors for these strains are a cell-surface metalloprotease and a sialic acid receptor similar to that bound by influenza C virus. Moreover, three other strains have also been described. They include NL63 (NL coronavirus), a similar species called NH (New Haven coronavirus), and HKU1 (Hong Kong coronavirus). All produce similar syndromes ranging from upper to lower respiratory illness.

In late 2002, an illness called severe acute respiratory syndrome (SARS) appeared in China, spread throughout Asia, and is now found worldwide. The etiology has been identified as another previously undescribed coronavirus named as SARS-CoV, with unusually high virulence for humans. The genome of SARS causing coronavirus has been sequenced, and the virus has the ability to mutate like other RNA viruses. The route of transmission is similar to that of other common cold viruses such as direct contact, via the eyes, nose, and mouth with infectious droplet. The risk of transmitting the disease to a person is greatest around day 10 of the illness, when the maximum amount of virus is shed from the respiratory tract. The older population is at a higher risk than younger people and children.

In 2012, an illness called Middle East respiratory syndrome (MERS) appeared in Saudi Arabia that caused a fatal ARD leading to renal failure that was caused by new coronavirus known as MERS coronavirus (MERS-CoV). This virus is similar to SARS, but still different than SARS and other human coronaviruses. Most of the MERS cases have been linked with travel or living in the Arabian Peninsula. MERS affect people of all age groups. Although the exact mechanisms of transmission are not known, it most likely occurs through close contact with sick people or caring or living with an infected people. The incubation period is from 2 to 14 days (average 5-6 days). Symptoms of severe acute respiratory syndrome include fever, cough, and shortness of breath. While some patients experience nausea, vomiting, and diarrhea, many patients develop severe complications such as pneumonia and renal failure. The fatality with MERS is about 30% to 40%. People with other underlying conditions and weakened immune system develop severe disease. There is no specific treatment or vaccine. Supportive care is recommended. Preventive measures to protect respiratory infection are recommended.

Human bocavirus was first discovered in 2005 by using molecular screening methods. It is a novel parvovirus which is a small (20 nm) naked capsid, single-stranded DNA virus. Unlike another human parvovius, parvovirus B19 that causes erythema infectiosum (slapped face—see Chapter 10 detailed virology of parvovirus), it has been primarily implicated as a cause of wheezing and other respiratory illnesses in children. In addition, bocavirus has also been isolated from feces of infants with gastroenteritis symptoms. Three strains of human bocavirus (HBoV), HBoV-1, 2, and 3 have been identified. Symptoms include cough, fever, and wheezing. Diagnosis requires PCR methods. Further studies are ongoing to determine its epidemiologic behavior and relative contribution to respiratory morbidity.

Reoviruses have been associated with upper respiratory tract infection, fever, gastroenteritis, febrile illness, and childhood exanthem. The reoviruses (also known as respiratory enteric orphans for “reo”) are naked capsid virions that contain segmented, double-stranded RNA genomes and an outer and inner (double) protein shell. In addition, the virions contain RNA-dependent RNA polymerase. These double-stranded viruses transcribe and replicate in the cytoplasm by using its own RNA polymerase. The progeny viruses are assembled in the cytoplasm of infected cells and released by cell lysis. They are ubiquitous and have been found in humans, simians, rodents, cattle, and a variety of other hosts. They have been studied in detail as experimental models, revealing much basic knowledge about viral genetics and pathogenesis at the molecular level. Three serotypes are known to infect humans and associated with several conditions; however, their role and mechanisms in human disease remain understood. Reoviruses causing diarrheal disease are discussed in Chapter 15 and arboviral diseases are discussed in Chapter 16.