Fluorescent Antibody Test
Mouse Inoculation Test
Human Health Risk
Animals with changes in behaviour and varius neurological signs should always be suspected of being rabid. However, other diseases, problem, or toxins can cause similar neurological signs as rabies. The following are some differential diagnosis' that have clinical signs similar to rabies.
Typically there are no significant clinical pathology findings associated with rabies. However, there are a few clinical findings that have been associated with rabies. For example, the cerebral spinal fluid may contain increased protein levels, monocytes, and neutrophils
There are no gross lesions evident at necropsy. A mixture of neurological clinical signs and no necropsy findings makes rabies a significant differential diagnosis. Therefore, it is important to remove the brain and perform diagnostic testing to make a diagnosis. For diagnostic testing, slice the brain in sagital plain. Fix half of the brain in formalin and the other half leave fresh or frozen (it will be shipped to CFIA Lethbridge for testing). It is important to take proper precautions during the necropsy. These precautions include wearing double gloves, protective clothing (coveralls, rubber boats, and disposable sleeves), respiratory protectors, and cut resistant gloves.
On histological examination there will be a non-suppurative encephalitis. There also may be brain edema, meningeal congestion, perivascular cuffing and focal areas of hemorrhage. Intraneural cytoplasmic inclusion bodies (Negri bodies), which are highly characteristic of rabies, may also be seen. An inclusion body can be seen (bottom left) and a encephalitis (bottom right)
The direct FAT is the most common
test used for confirmation rabies according to World Health
Organization. It requires
the brain to be removed from the animal, processed and put under a
microscope with ultra violet light. The quality of the FAT depend on
the experience of the examiner, the quality of the conjugate, and the
quality of the fluorescent microscope.
IHC is used at WCVM to diagnose
animals that have not come into contact with humans; however, most
academic institutions do not have the resources to use IHC to make a
diagnosis. Typically polyclonal rather than monoclonal antibodies are
used. IHC is that is not as sensitive as other tests available.
Image Credit: http://www.defenselink.mil/news/May2001/200105103j.jpg
Mice are injected with a solution
infected with rabies from a specific animal and observed for 28 days
or until they die. The solution consists of 20% w/v infected brain
material and saline. 5-10 specific pathogen free mice are
anesthetized and then inoculated intracerebrally with the 20%
solution. Brains are harvested from the mice when they die, and rabies
is confirmed using the
fluorescent antibody test. Mice typically die at day 9. MIT is not
used much anymore, due to SPF mice being expensive and diagnosis
Image Credit: http://news.bbc.co.uk/olmedia/760000/images/_760964_300mice.jpg
ELISA tests are commonly used by
CFIA to confirm a positive diagnosis of rabies, since they are highly
sensitive. A BioRad test called Platelia Rabies II kit is often
used. It uses immunoglobulin G antirabies virus glycoprotein from
human and animal serum and plasma to detect the rabies virus. The
titer of the animal can then be determined and along with an
appropriate history, a diagnosis can be made. It is important to
note, a history is crucial to a diagnosis, because a vaccinated
animal will also have a titer.
Image Credit: www.resonancepub.com/Immunassay.htm
PCR is not used much in
diagnostics. It is used exclusively for epidemiology studies.