Vector (bitinq arthropod) Sandfly fever, dengue o /O
Vertebrate reservoir Rabies, cowpox
Most arbovirus infections
Fig- 14-2 Modes of transmission of human viral diseases. (Modified from C A. Mims, "The Pathogenesis of Infectious Disease," 2nd Ed. Academic Press, London, 1982 )
hospital or clinic. The lethal Ebola virus outbreaks in Zaire and the Sudan in 1976 were classic examples of iatrogenic nosocomial infections; more common examples of nosocomial virus infections are the occurrence of chickenpox, influenza, and respiratory syncytial virus cross-infections in hospital settings. Hepatitis B and C viruses, and to a lesser extent HIV, can also be transmitted by doctors, dentists, acupuncturists, tattooists, etc., but are more of a risk to attending staff and laboratory personnel, via needle stick and similar injuries.
Because most viruses are host-specific, most human viral infections are maintained in nature within the human population. However, there are a number of viruses that spread naturally between several different species of animals, for example, rabies and the arboviral encephalitides. The term zoonosis is used to describe infections that are transmissible from animals to humans (see Fig. 14-2). The zoonoses, whether involving domestic or wild animal reservoirs, usually occur only under conditions where humans are engaged in activities involving close contact with animals (Table 14-2), or if the viruses are transmitted by arthropods (see Table 14-6)
Non-Arthropod-Borne Viral Zoonoses
Mode of transmission to humans
I Irrfwftnrntar f'ii.vi'fniffle
Fibpiridae Orlhomi/xiwirnlnc Bunt/aim idnc Arcntwmdne
Vesicular stomatitis Ebola, Marburg Influenza A< Hantaan
Lymphocytic choriomeningitis, Junin, Machupo, Lassa
Rodents, cats, cattle Squirrel, monkeys Cattle
Contact, through abrasions
Animal bite, scratch, respiratory Contact with secretions" Contact; iatrogenic (injection)'' Respiratory
Contact with rodent uiine Contact with rodent uiine
" May also be arthropod-borne '' Also human-to-human spread.
' Usually maintained by human-to-human spread; zoonotic infections occur only rarely, but reassortants between human and avian influenza viruses (perhaps arising during confection of pigs) may result in human pandemics due to antigenic shift.
The term vertical transmission is usually used to describe infection that is transferred from mother to embryo/fetus/newborn prior to, during, or shortly after parturition, although some authorities prefer to restrict the term to situations where infection occurs before birth. Certain retroviruses of birds and mammals are vertically transmitted via the integration of proviral DNA directly into the DNA of the germ line of the fertilized egg. Rubella virus and cytomegalovirus are transmitted to the fetus via the placenta, whereas others, such as herpes simplex type 2 virus, are transmitted during passage through the birth canal. Yet others, such as hepatitis B virus, may be transmitted perinatally or postnatally via saliva, milk, or other secretions. Vertical transmission of a virus may be lethal to the fetus, and the cause of abortion, or it may be associated with congenital disease or cause congenital abnormalities.
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