O

Vector (bitinq arthropod) Sandfly fever, dengue o /O

Vertebrate reservoir Rabies, cowpox

Vector-vertebrate

Most arbovirus infections

Vector-vertebrate

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.

Zoonotic Transmission

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)

Table 14-2

Non-Arthropod-Borne Viral Zoonoses

Virus family

Vitus

Reservoir host

Mode of transmission to humans

I Irrfwftnrntar f'ii.vi'fniffle

Rliiibdovnidih'

Fibpiridae Orlhomi/xiwirnlnc Bunt/aim idnc Arcntwmdne

Herpes B

Cowpox

Monkeypox

Pseudoooivpnx

Rabies

Vesicular stomatitis Ebola, Marburg Influenza A< Hantaan

Lymphocytic choriomeningitis, Junin, Machupo, Lassa

Monkey

Rodents, cats, cattle Squirrel, monkeys Cattle

Sheep, goats

Various mammals

Cattle

'Monkeys

Birds, pigs

Rodents

Rodents

Animal bile

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.

Vertical Transmission

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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