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B virus infection in humans usually occurs as a result of bites or scratches from macaques—a genus of Old World monkeys that serve as the natural host—or from direct or indirect contact of broken skin or mucous membranes with infected monkey tissues or fluids. The virus can be present in the saliva, feces, urine, or nervous tissue of infected monkeys and may be found in cell cultures derived from infected monkeys.

Possible routes of transmission to humans include

  • Bite or scratch from an infected animal
  • Needlestick from contaminated syringe
  • Scratch or cut from contaminated cage or other sharp-edged surface
  • Exposure to nervous tissue or skull of infected animal (especially brain)

B virus can survive for hours on the surface of objects, particularly on surfaces that are moist. The injury need not be severe for infection to occur, although less severe wounds (those that don't break the skin) are thought to carry a lower risk of transmission.

Risk of B virus transmission to humans should be considered in the context of how rare infection occurs, even among broadly infected populations of animals. Hundreds of macaque bites and scratches occur annually in primate facilities in the United States, but B virus infection in humans is rare. In a study of more than 300 animal care workers, among whom, 166 reported possible transmission exposures to macaques, none of the workers were found to be B virus positive.

Only one case of human-to-human transmission has been documented; the case, which was reported in a study of a B virus outbreak involving four persons in Florida, resulted from direct physical contact with lesions (see Epidemiologic Notes and Reports B-virus Infection in Humans -- Pensacola, Florida. Morbidity and Mortality Weekly Report 1987). Among the four persons, three were animal handlers (two suffered bite wounds and one had close contact with the sick macaque but was not injured or exposed to other bodily fluids and did not develop symptoms). The fourth person was the wife of one of the animal handlers. She used an ointment to treat her husband's lesions and subsequently used it on herself to treat contact dermatitis. She seroconverted to B virus but never developed symptoms. The study found no evidence of B virus infection among 130 close contacts of the four patients, healthcare workers, or primate workers. Moreover, even though B virus seroprevalence among adult macaques is >70%, only a few people in the study developed laboratory evidence of B virus exposure. Thus, transmission of this virus, both human-to-human and primate-to-human, is quite rare.