Various transmissible infections may be acquired from others by direct contact, indirectly through contaminated inanimate objects or materials, or by aerosol transmission of infectious secretions. Some infections, such as malaria, involve an animate insect vector. These routes of spread are often referred to as horizontal transmission, in contrast to vertical or perinatal transmission—from mother to fetus or infant.
Vertical or Perinatal Transmission
Some infections can spread from mother to fetus through the placenta, during childbirth, or during breastfeeding. For example, rubella virus may cause birth defects when transmitted from the mother’s bloodstream across the placenta during the first trimester of pregnancy. Neonatal infections with group B streptococci, Chlamydia trachomatis, and Neisseria gonorrhoeae can occur following passage through the birth canal. Cytomegalovirus (CMV) can be acquired prenatally (across the placenta) or perinatally (from passage through an infected cervix, contact with blood, or through breast milk).
✺ Vertical transmission can occur transplacentally, during birth, or through breast milk
✺ Vertical transmission = mother to fetus
Important effects of perinatal infection include prematurity, intrauterine growth retardation (IUGR) and low birth weight, developmental abnormalities, congenital disease, and persistent perinatal infection. Historically, common causes of IUGR and developmental abnormalities associated with congenital infection during the first trimester include toxoplasmosis, rubella, CMV, herpes simplex, and syphilis. Zika virus, transmitted by Aedes mosquitos, was recognized recently to cause similar abnormalities when acquired in the first trimester of gestation.
The major routes of horizontal transmission of infectious diseases are summarized in Table 5–1 and discussed in the following text.
✺ Horizontal transmission = direct or indirect person to person
TABLE 5–1Common Routes of Transmission of Infectiona ||Download (.pdf) TABLE 5–1 Common Routes of Transmission of Infectiona
|ROUTE OF EXIT ||ROUTE OF TRANSMISSION ||EXAMPLE |
|Respiratory ||Aerosol droplet inhalation ||Influenza virus; tuberculosis |
| ||Nose or mouth → hand or object → nose ||Common cold (rhinovirus) |
|Salivary ||Direct salivary transfer (eg, kissing) ||Oral-labial herpes; Epstein-Barr virus, cytomegalovirus |
| ||Animal bite ||Rabies |
|Eye ||Conjunctival ||Adenovirus |
|Skin ||Skin discharge → air → respiratory tract ||Varicella, smallpox, or monkeypox |
| ||Skin to skin ||Human papillomavirus (warts); syphilis |
|Genital secretions ||Urethral or cervical secretions ||Gonorrhea; herpes simplex; Chlamydia |
| ||Semen ||Cytomegalovirus |
|Gastrointestinal ||Fecal-oral (Stool → hand → mouth and/or stool → object, water or food → mouth) ||Enterovirus; hepatitis A |
| ||Stool → water or food → mouth ||Salmonellosis; shigellosis |
|Blood ||Transfusion or needle prick ||Hepatitis B; cytomegalovirus infection; malaria; HIV |
| ||Mosquito bite ||Malaria; arboviruses |
|Urine ||Urine → hand → catheter ||Hospital-acquired urinary tract infections |
|Zoonotic ||Animal bite ||Rabies |
| ||Contact with carcasses ||Tularemia |
| ||Tick bite ||Rickettsia; Lyme disease |
Respiratory Spread: Airborne or Contact with Respiratory Secretions
Many infections are transmitted by the respiratory route, often by aerosolization of respiratory secretions with subsequent inhalation by other persons. The efficiency of this process depends, in part, on the extent and method of propulsion of discharges from the mouth and nose, the size of the aerosol droplets, and the resistance of the infectious agent to desiccation and inactivation by ultraviolet light. In still air, a particle 100 μm in diameter requires only seconds to fall the height of a room; a 10 m particle remains airborne for about 20 minutes, smaller particles even longer. When inhaled, particles with a diameter of 6 μm or more are usually trapped by the mucosa of the nasal turbinates, whereas particles of 0.6 to 5.0 μm attach to mucous sites at various levels along the upper and lower respiratory tract and may initiate infection. These “droplet nuclei” are most important in transmitting many respiratory pathogens (eg, M tuberculosis).
Droplet nuclei are usually less than 6 μm in size (eg, tuberculosis)
Respiratory secretions are often transferred on hands or inanimate objects (fomites) and may reach the respiratory tract of others in this way. For example, spread of the common cold may involve transfer of infectious secretions from nose to hand by the infected individual, with transfer to others by hand-to-hand contact and then from hand to nose. Transmission of infectious secretions by direct contact with the nasal mucosa or conjunctiva often accounts for the rapid dissemination of agents, such as respiratory syncytial virus and adenovirus. The risk of spread in these instances can be reduced by simple hygienic measures such as handwashing.
Handwashing is especially important to decrease transmission of the common cold
Salivary Spread: Kissing or Bite
Some infections, such as herpes simplex and infectious mononucleosis, can be transferred directly by contact with infectious saliva by drooling small children or through kissing. Saliva containing rabies virus can transmit rabies when the rabid animal bites.
Infections of the conjunctiva may occur in epidemic or endemic form. Epidemics of adenovirus and Haemophilus conjunctivitis may occur, and are highly contagious. The major endemic disease is trachoma, caused by Chlamydia, which remains a common cause of blindness in developing countries. These diseases may be spread by direct contact via ophthalmologic equipment or by secretions passed manually or through fomites such as towels.
Fomites and unsterile ophthalmologic instruments are associated with transmission
Skin-to-skin transfer occurs with a variety of infections in which the skin is the portal of entry such as the spirochete of syphilis (Treponema pallidum), strains of group A streptococci that cause impetigo, and the dermatophyte fungi that cause ringworm and athlete’s foot. In most cases, an unapparent break in the epithelium is involved in infection. Other diseases may be spread indirectly from skin-to-skin through fomites such as shared towels and inadequately cleansed shower and bath floors. Skin-to-skin transfer usually occurs through abrasions of the epidermis, which may be unnoticed.
Syphilis, ringworm, and impetigo are examples of skin-to-skin transfer
Disease transmission through the genital tract has been and remains one of the most common infections worldwide. Spread can occur between sexual partners or from the mother to the infant at birth. Major factors related to the persistence of these infections are high rates of asymptomatic carriage and the frequency of recurrence of organisms such as C trachomatis, CMV, herpes simplex virus, and Neisseria gonorrhoeae.
Asymptomatic carriage and recurrence are common
Foodborne or Waterborne Transmission: Fecal–Oral Spread
Fecal–oral spread involves direct or finger-to-mouth spread, the use of human feces as a fertilizer, or fecal contamination of food or water. Food handlers who are infected with an organism transmissible by this route constitute a special hazard, especially when they fail to wash their hands. Some viruses disseminated by the fecal–oral route infect and multiply in cells of the oropharynx and then disseminate to other body sites to cause infection. However, organisms that are spread in this way commonly multiply in the intestinal tract and may cause intestinal infections. They must, therefore, be able to resist the acid in the stomach, the bile, and the gastric and small intestinal enzymes. Many bacteria and enveloped viruses are rapidly killed by these conditions, but members of the Enterobacteriaceae and unenveloped viral intestinal pathogens (eg, enteroviruses) are more likely to survive. Even with these organisms, the infecting dose in patients with reduced or absent gastric hydrochloric acid is often much smaller than in those with normal stomach acidity.
✺ Reduced gastric hydrochloric acid can facilitate the spread of enteric infections
Blood or Transfusion-Borne
Bloodborne transmission of infection through insect vectors requires a period of multiplication or alteration within an insect vector before the organism can infect another human host, as occurs with the female Anopheles mosquito and the malarial parasite. Direct transmission from human to human through blood has become increasingly important because of the use of blood transfusions and blood products and the increased self-administration of illicit drugs by intravenous or subcutaneous routes using shared nonsterile equipment. Hepatitis B and C viruses, as well as HIV, were frequently transmitted in this way before the institution of universal screening of blood.
✺ Parenteral drug abuse or transfusion is a major risk factor
Vector-borne and Zoonotic
Zoonotic infections are spread from animals, where they have their natural reservoir, to humans. Some zoonotic infections such as rabies are directly contracted from the bite of the infected animal, whereas others are transmitted by vectors, especially arthropods (eg, ticks, mosquitoes). Many infections contracted by humans from animals are dead-ended in humans, whereas others may be transferred between humans once the disease is established in a population. Plague, for example, has a natural reservoir in rodents. Human infections contracted from the bites of rodent fleas may produce pneumonia, which may then spread to other humans by the respiratory droplet route.
✺ Zoonotic = animals to humans
Classically the term vector was restricted to arthropods like ticks and mosquitoes; however, it is often used to refer to any animal that can transmit a pathogen to a human host. The probability of vector-borne transmission depends on the biology of the vector (mosquito, tick, snail, etc) and the infectivity of organism.
✺ Vectorborne = vectors (eg, mosquitos, ticks, snails) to humans