Diagnosis in animals and humans

Diagnosis in animals

A diagnosis of rabies can be made after detection of rabies virus from any part of the affected brain, but in order to rule out rabies, the test must include tissue from at least two locations in the brain, preferably the brain stem and cerebellum.

The test requires that the animal be euthanized. The test itself takes about 2 hours, but it takes time to remove the brain samples from an animal suspected of having rabies and to ship these samples to a state public health or veterinary diagnostic laboratory for diagnosis.

In the United States, the results of a rabies test are typically available within 24 to 72 hours after an animal is collected and euthanized. Because rabies exposure to suspect animals is a medical urgency, but not an emergency, testing within this period is more than adequate for determining if a person was exposed to a rabid animal, and requires rabies postexposure vaccinations.

Approximately 120,000 animals or more are tested for rabies each year in the United States, and approximately 6% are found to be rabid. The proportion of positive animals depends largely on the species of animal and ranges from <1% in domestic animals to >10% of wildlife species.

Based on routine public health surveillance and pathogenesis studies, we have learned that it is not necessary to euthanize and test all animals that bite or otherwise potentially expose a person to rabies. For animals with a low probability of rabies such as dogs, cats, and ferrets, observation periods (10 days) may be appropriate to rule out the risk of potential human rabies exposure.

Consultation with a local or state health official following a potential exposure can help determine the best course of action based on current public health recommendations.

Diagnosis in humans

Several tests are necessary to diagnose rabies ante-mortem (before death) in humans; no single test is sufficient. Tests are performed on samples of saliva, serum, spinal fluid, and skin biopsies of hair follicles at the nape of the neck. Saliva can be tested by virus isolation or reverse transcription followed by polymerase chain reaction (RT-PCR). Serum and spinal fluid are tested for antibodies to rabies virus. Skin biopsy specimens are examined for rabies antigen in the cutaneous nerves at the base of hair follicles.