Rabies can have many differential diagnoses therefore laboratory confirmation is required. The animal is euthanized and its head is removed for shipment to the laboratory for diagnoses.
‘Negri bodies’ are intracytoplasmic inclusion bodies and are highly characteristic of rabies. They can be seen in histological preparations of the infected animal’s brain, specifically in the thalamus, hypothalamus, pons, cerebral cortex and dorsal horns of the spinal cord. These inclusion bodies are not found at certain stages of infection and so may only be found in approximately 50% of cases which tested positive on FAT . This test is no longer used as routine diagnostic confirmation.
Fluorescent antibody test (FAT)
The World Health Organization and the World Organization of Animal Health both recommend using direct FAT because it is the most common quick and reliable test. It involves having smears prepared from the infected animal’s brain tissue, and applying anti-rabies fluorescent conjugates to it. This preparation is then looked at under ultra violet light to determine is rabies specific nucleocapsid proteins are present.
Mouse Inoculation Test
Five to ten, two day old mice that are pathogen free can be injected intracerebrally with a solution of 20% (w/v) infected brain material and saline. They are watched daily for 28 days (or until they die) at which time the fluorescent antibody test is used to determine the presence of rabies in their brains. This test is long and expensive and as a consequence is not used as much anymore. Viral inoculation into tissue culture is now being done since the virulent virus can now be grown in various cell lines.
*There are other tests that can be used to help diagnose rabies in infected animals and people that are not widely used because of their low sensitivty or specificity..