Rabies is a viral disease that attacks the central nervous system of mammals, leading to a fatal outcome once clinical signs appear. Testing for this infection is highly specialized, managed almost exclusively through state public health laboratories following strict protocols. Veterinarians manage exposure cases and prepare specimens for the specialized testing that determines the presence of the virus.
Why Testing Requires Post-Mortem Samples
The gold standard for rabies diagnosis requires the examination of brain tissue, meaning testing can only be performed after the animal has died or been humanely euthanized. Currently, no reliable ante-mortem (live animal) test exists that can definitively rule out rabies infection. The virus travels along nerve pathways to the brain, where it concentrates before spreading to the salivary glands.
A definitive diagnosis requires the laboratory to examine full cross-sections of two specific brain regions: the brain stem and the cerebellum. The brain stem is the most reliable tissue for finding the viral antigen, while the cerebellum often contains viral inclusions. If testing is necessary following an exposure, the animal must be euthanized to collect the required tissue samples.
Veterinary Protocols for Suspicious Exposure
When an animal is alive but potentially exposed to rabies, protocols shift to careful observation and quarantine, governed by vaccination status. If a domestic animal bites a human, public health authorities mandate a strict 10-day observation period. This is because a rabid animal shedding the virus will almost certainly show clinical signs or die within this window.
If an animal is exposed to a known or suspected rabid animal, the protocol depends on its vaccination status. A currently vaccinated dog, cat, or ferret must receive a booster vaccination within 96 hours of exposure and be monitored by the owner for 45 days. An unvaccinated animal must either be humanely euthanized or undergo a strict quarantine for four to six months, often with immediate vaccination upon entry.
Sample Collection and Submission for Analysis
When a veterinarian determines that testing is mandatory, they are responsible for collecting and submitting the appropriate specimen to the state public health laboratory. For small animals like cats, dogs, or wildlife, the entire head is typically removed after euthanasia and submitted for testing. For larger animals, such as livestock, the veterinarian may remove specific brain tissue, including the required brain stem and cerebellum, rather than submitting the entire head.
Proper preparation and packaging are critical to ensure the sample is viable for testing and safe for transport. The specimen must be kept refrigerated, not frozen, as freezing can damage the tissue and interfere with the final test results. The sample must be packaged in a triple-layer containment system, including a watertight primary container, an absorbent material, and a durable outer container, along with all necessary submission forms for the state lab.
The Definitive Laboratory Confirmation Method
The definitive laboratory technique used by public health labs to confirm rabies infection is the Direct Fluorescent Antibody (DFA) test. The DFA test is the gold standard for post-mortem rabies diagnosis due to its high sensitivity and specificity. This test involves taking impressions of the fresh brain tissue and treating them with fluorescently labeled antibodies specific to the rabies virus antigen.
If the rabies virus is present, the fluorescent antibodies bind to the antigens, making the infection visible as bright, apple-green areas under a fluorescence microscope. Results are typically available within 24 to 72 hours of the laboratory receiving the specimen. While the DFA test is the primary method, secondary tests like the Real-time Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) may occasionally be used for confirmation.