Anti-Infectives
One of the most feared complications of animal-bite wounds is infection with rabies. Although the dog is the major animal reservoir for rabies worldwide, the principal wildlife vectors in the United States are raccoons, skunks, foxes, and bats. An exposure to rabies is defined as an animal bite or contamination of an open wound or mucous membrane with saliva or infected tissue. Local health departments should be contacted regarding rabies prevalence among wild animals.
Rabies is caused by a highly neurotropic RNA virus. A bite from a rabid animal may inoculate the surrounding soft tissue with rabies virus. The incubation period varies from approximately 5 days to 1 year, although the usual period varies from 20 to 60 days. Usually, the virus is first amplified in adjacent skeletal muscle cells near the site of inoculation. Once the concentration of virus is sufficient, it can directly enter peripheral nerves (unmyelinated sensory and motor terminals) and spread by retrograde axoplasmic flow until it reaches the spinal cord, causing pain at the wound site. After the virus has reached the spinal cord, it disseminates to the central nervous system and causes rapidly progressive encephalitis. It then spreads throughout the body along peripheral nerves, including those of the salivary glands, where it is shed. Death occurs by respiratory collapse, and infection is thought to be 100% fatal.
When exposure to rabies is confirmed or a high risk identified (particularly in unprovoked animal attacks), rabies prophylaxis must be initiated promptly. Prophylactic therapy after exposure includes local wound care, passive immunization with human rabies immunoglobulin (HRIG), and vaccination. HRIG should be administered at the started of antirabies prophylaxis (day 0) as a single 20 IU/kg dose, half infiltrated around the site of exposure and the other half given intramuscularly. Three vaccines are available in the United States: human diploid cell vaccine (HDCV), rabies vaccine adsorbed (RVA), and purified chick embryo cell vaccine (PCEC). All three types of rabies vaccine are considered equally safe and efficacious. HDCV, RVA, and PCEC should be administered intramuscularly in a series of five 1.0 mL injections given on days 0, 3, 7, 21, and 28. The deltoid muscle of adults or the anterolateral thigh of small children is recommended. The gluteal region has been associated with treatment failures and is not advised. Vaccine induces an active immune response within 7 to 10 days, and immunity persists for approximately 2 years. Previously vaccinated patients do not need HRIC but should receive two doses of rabies vaccination on days 0 and 3.
Depending on the type of bite, animals should either be observed for erratic behavior for a 5- to 10-day period or euthanized at once to determine whether rabies virus is present. Examination of the animal’s brain tissue is the only reliable method of diagnosis. Healthy-appearing cats and dogs can be quarantined for 10 days and euthanized if signs of illness appear.
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