Human disease from rabies virus infection is very uncommon in the United States. The number of human cases in the United States is usually less than five per year, and only nine were reported from 1980 to 1987. However, there are always questions about the disease, and several thousand cases of animal rabies are confirmed each year (6975 in 1991). Until 1990, wildlife rabies was most common in skunks; beginning in 1990, raccoon cases have been most frequent, followed by skunks, bats, and foxes in order of decreasing numbers. Actually most animal bites are from caged rodent pets such as rabbits, gerbils, or mice. This is not a problem since rodents (including squirrels) very rarely are infected by the rabies virus even when wild. Interestingly, of the nine human rabies cases mentioned above, six did not give a history of bat or other animal contact when they were hospitalized. Non-bite transmission of rabies from human to human (e.g., contact with saliva or CSF) has not been proven in the United States to date.

The standard procedure for suspected rabies in domestic dogs or cats is to quarantine the animal under observation of a veterinarian for 10 days. The incubation period in humans is 1-3 months (although inoculation through facial bites may have an incubation as short as 10 days), which provides enough time for diagnosis of the animal before beginning treatment in the person who was bitten. Animal rabies will produce symptoms in the animal within 10 days in nearly all cases.

For wild animals, if the animal was defending itself normally, it might be captured and quarantined. If it was thought to be exhibiting abnormal behavior, the animal is usually killed and the head is sent for rabies examination of the brain. The head (or brain) should be kept at refrigerator temperature (not frozen) and sent to a reference laboratory (usually a public health laboratory) as soon as possible. If the specimen will be received within 1 day, it should be sent refrigerated with ordinary ice; it if is to be stored longer than 1 day, dry ice should be used.

Diagnosis. Laboratory diagnosis consists of stained impression smears of the brain, mouse inoculation, and serologic tests. Impression smears from Ammon’s horn of the hippocampal area in the temporal lobe stained with Seller’s stain is the traditional method for diagnosis. The smears are examined microscopically for Negri body inclusions in neurons. Use of Seller’s stain has approximately 65% sensitivity, with sensitivity reported to be somewhat greater than this in dogs (75%-80%) and somewhat less in skunks and bats. Fluorescent antibody stains on the smears have more than 95% sensitivity and currently are the standard method for diagnosis. Mouse inoculation with fresh brain extracts also has more than 95% sensitivity but may eventually be replaced by tissue culture. Saliva from animals or humans can be used for mouse inoculation, but the sensitivity is not as great as brain testing. All specimens should be taken with sterile instruments, which should be immediately decontaminated by autoclaving after use. Serologic tests (ELISA method) for rabies antibody in serum or CSF can be done if the patient has not been immunized against rabies. For serum, this requires two specimens drawn at least 1 week apart. For CSF, a single specimen positive result is diagnostic.