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Various transmissible infections may be acquired from others by direct contact, by aerosol transmission of infectious secretions, or indirectly through contaminated inanimate objects or materials. 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 transmission—from mother to fetus. The major horizontal routes of transmission of infectious diseases are summarized in Table 5–1 and discussed in the following text.
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Horizontal transmission = direct or indirect person to person
Vertical transmission = mother to fetus
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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).
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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 by the unsuspecting victim.
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Droplet nuclei are usually less than 6 μm in size
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Some infections, such as herpes simplex and infectious mononucleosis, can be transferred directly by contact with infectious saliva through kissing. 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.
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Handwashing is especially important
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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 their personal hygienic practices are inadequate. 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.
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Reduced gastric hydrochloric acid can facilitate enteric infections
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Skin-to-Skin Transfer
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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 probably involved in infection. Other diseases may be spread 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.
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Syphilis, ringworm, and impetigo are examples of skin-to-skin transfer
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Bloodborne Transmission
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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. Such is the case with the mosquito and the malarial parasite. Direct transmission from human to human through blood has become increasingly important in modern medicine 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 blood screening tests.
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Parenteral drug abuse is a major risk factor
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Asymptomatic carriage and recurrence are common
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Disease transmission through the genital tract has emerged as one of the most common infectious problems, and reflects changing social and sexual mores. Spread can occur between sexual partners or from the mother to the infant at birth. A major factor in these infections has been the persistence, high rates of asymptomatic carriage, and frequency of recurrence of organisms such as Chlamydia trachomatis, cytomegalovirus (CMV), herpes simplex virus, and Neisseria gonorrhoeae.
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Eye-to-Eye Transmission
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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.
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Fomites, unsterile ophthalmologic instruments are associated with transmission
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Zoonotic Transmission
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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.
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Zoonotic = animals to humans
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Vertical Transmission
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Certain diseases can spread from mother to fetus through the placental barrier. This mode of transmission involves organisms such as rubella virus that can be present in the mother's bloodstream and may occur at different stages of pregnancy with different organisms. Another form of transmission from mother to infant occurs by contact during birth with organisms such as group B streptococci, C trachomatis, and N gonorrhoeae, which colonize the vagina. Herpes simplex virus and CMV can spread by both vertical methods, as it may be present in blood or may colonize the cervix. In addition, CMV may be transmitted by breast milk, a third mechanism of vertical transmission.
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Vertical transmission can occur transplacentally, during birth, or through breast milk