Many viruses are acquired by ingestion. They may either be swallowed or infect cells in the oropharynx and then be carried to the intestinal tract. The esophagus is rarely infected, probably because of its tough stratified squamous epithelium and the rapid passage of swallowed material over its sur-
It orrtincin cold)
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Fig. 6-2 (A) Pathways of .infection and mechanical protective mechanisms in the respiratory tract At right are listed clinical syndromes produced by infection at various levels of the respiratory tract (B) Enlarged section of turbinates showing (he narrow and complicated pathway of inspired air, and thus Ihe ease with which slight cellular swelling "blocks the nose " (From I A Mims and D. O White, "Viral Pathogenesis and Immunology " HJackwell, Oxford, 1984 )
Viruses That Initiate Infection of Humans via the Respiratory Tract With production of local respiratory symptoms
Piccruiwirrdiie Rhmoviruses, some enterovuuses Co (nmviruitif Pmnm/xmnndac
Otfhtmntxovuttlar Influenza virus Adenmimfkif Most types
Parainfluenza viruses, respiratory syncytial virus
Producing generalized disease, usually without initial respiratory symploms
ParamijxoPtndnc Mumps, measles viruses
Tognvtridae Rubella virus
Herjfcsvmdac Varicella virus
Prtvrrnwrr/drif Some enteroviruses
Papwiwvn t due Polyomaviruses
Biitit/nvindar Hantaan virus
Amtai'iridnc South American hemorrhagic fever viruses
Puxi'iridiic Variola virus (smallpox is now extinct)
face. The intestinal tract is partially protected by mucus, which may contain specific secretory antibodies (IgA), but (he constant movement of the contents provides opportunities for virions to attach to specific receptors Virions may also be taken up by specialized M cells that overlie Peyer's patches in the ileum, from which they are passed to adjacent mononuclear cells in which they may replicate
There are other protective mechanisms in the intestinal tract; from the stomach downward, acid, bile, and proteolytic enzymes may inactivate viruses. In general, viruses that cause intestinal infection, such as enteroviruses, rotaviruses, and caliciviruses (Table 6-2), are acid- and bile-resistant. Rotaviruses and caliciviruses are now recognized as the major causes of viral diarrhea, whereas the great majority of intestinal infections by enteroviruses and adenoviruses are asymptomatic. Some of the enteroviruses (e.g., polio-viruses), and hepatitis A and E viruses, are important causes of generalized infection but do not produce signs referable to the intestinal tract.
The emergence of AIDS has drawn attention to the importance of the rectum as a route of infection with HIV and other sexually transmitted agents
The skin is the largest organ in the body, and since its outer layer consists of keratinized cells it provides a tough and usually impermeable barrier to the entry of viruses. However, after entry through minor abrasions or by artificial puncture, some viruses replicate in the skin to produce local lesions, for example, papillomaviruses and poxviruses (Table 6-3).
The most efficient way by which viruses are introduced through the skin is via the bite of an arthropod vector, such as a mosquito, tick, or sandfly Such insects may be mechanical vectors (e.g , for tanapox), but most viruses
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