Diagnosis of Rabies
Though the behavioral symptoms of rabies are classic, a diagnosis based solely on the symptoms can be difficult because they are similar to other diseases, conditions, and complications. The inability to swallow, for example, could be caused by an object lodged in the throat. Many factors can trigger aggressive, strange behavior and many animals are easily provoked when injured.
The only way to be 100% sure that an animal is rabid is to perform a direct fluorescent antibody test (dFA) on the brain tissue, which requires that the animal be euthanized. This is the most rapid and reliable test for rabies and it has been used for more than 40 years.
The dFA test is based on the principle that infected animals have rabies virus antigen (a protein) in brain tissue that reacts to antirabies antibodies (proteins produced by the immune system to fight disease and infection). The test involves inserting fluorescently labeled antirabies antibody molecules into the brain tissue, which bind to the rabies virus antigens and give off a bright glow when viewed through a special fluorescent microscope. A dFA test can also detect the presence of rabies antigens in the skin.
Tests that do not require euthanasia can be used to examine serum, spinal fluid, and saliva, but none of them can rule out rabies with 100% certainty. These tests are used only in humans and animals that cannot be euthanized.
Pets suspected of having rabies must be diagnosed as early as possible, especially if people have been exposed to the animal. Rabies is a fatal disease. Humans who have been exposed can be treated with safe postexposure prophylaxis, but only if it is administered before the person begins showing symptoms.