Infections caused by bacteria that live in the mouth of the biting animal are much more common and less frightening than rabies. Surprisingly, a single species, Pasteurella multocida, is responsible for bite infections from a number of kinds of animals, including dogs, cats, monkeys, and humans, among others.
There are no reliable symptoms or signs that distinguish among most bacterial bite wound infections. Spreading redness and tenderness and swelling of tissues adjacent to the wound, followed by the discharge of pus, are early indications of infection.
Pasteurella multocida is a Gram-negative, facultatively anaerobic, coccobacillus that is easily cultivated in the laboratory. Most isolates are encapsulated. There are a number of different antigenic types.
The details of pathogenesis are not yet known. There is evidence for one or more adhesins. Some strains produce a toxin that kills cells. The capsules of P. multocida are antiphagocytic. Abscesses form. When opsonized by specific antibody, the organisms are ingested and killed by phagocytes.
Pasteurella multocida is best known as the cause of a devastating disease of chickens called fowl cholera. This disease is historically significant because while studying it, Pasteur first discovered that the virulence of a pathogen could be attenuated by repeated laboratory culture, and the attenuated organism used as a vaccine. Pasteurella multocida also causes diseases in a number of other animal species. Epidemics of fatal pneumonia and bloodstream infection occur in rabbits, cattle, sheep, and mice.
Many healthy animals, however, carry the bacterium among their normal oral and upper respiratory flora. Both diseased animals and healthy carriers constitute a reservoir for human infections. Cats are more likely to carry P. multocida than dogs, and so cat bites are more likely than dog bites to cause the infection.
No vaccines are available for use in humans. Immediate cleansing of bite wounds and prompt medical attention usually prevent the development of serious infection and possible permanent impairment of function. Unlike many Gram-negative pathogens, P. multocida is susceptible to penicillin. Usually, before the cultural diagnosis is known, amoxicillin plus a ^-lactamase inhibitor are administered, available in a single tablet under the trade name Augmentin. This combination is used because amoxicillin is active against P. multocida, and with the addition of the ^-lactamase inhibitor, the antibiotic is also active against many strains of ^-lactamase-producing Staphylococcus aureus, another common cause of bite wound infections. This and other antibacterial medications are effective if given early in the infection.
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