Rabies - Diagnosis

Home
Overview
Clinical Signs
Diagnosis
     Differentials
     Clinical Pathology
     Necropsy
     Histology
     Laboratory Tests
          Fluorescent Antibody Test
          Immunohistochemistry
          Mouse Inoculation Test
          ELISA
          PCR
Pathogenesis
Treatment, Control,
  and Prevention
Epidemiology
Human Health Risk
References

Differential Diagnosis
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.

Cattle/Sheep
  • lead poisoning
  • grass tetany
  • vitamin A deficiency
  • polioencephalomalacia
  • listeriosis
Horses
  • East, West, Venezuela equine encephalitis
  • West Nile virus
  • equine protozoal myeloencephalitis
  • trauma
Pigs
  • pseudorabies
  • Porcine respiratory encephalitis syndrome
  • tetanus
  • Nipah virus
Dog/Cat
  • Toxoplasmosis
  • CNS infection
  • feline infectious peritonitis
  • canine distemper
  • neoplasia
  • trauma
  • hepatic encephalopathy
  • thiamine deficiency (cats)
  • oral and pharyngeal foreign bodies
  • poisoning
  • spongiform encephalopathy (cats)
Dog/Cat list from: http://www.provet.co.uk/infectiousdiseases/5a6c9b6.htm



Clinical Pathology

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




Necropsy

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.




Histology

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)

encephalitis

inclusion






Image Credit: http://www.cdc.gov/ncidod/dvrd/rabies/diagnosis/diagnosi.htmI



Laboratory Tests



Fluorescent antibody test (FAT)

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.
(more info)




ImmunoHistoChemistry (IHC)ihc

IHC is used at WCVM to diagnose rabid 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.
(more info)





Image Credit:  http://www.defenselink.mil/news/May2001/200105103j.jpg




Mouse Inoculation Test (MIT)mouse

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 time.

Image Credit:  http://news.bbc.co.uk/olmedia/760000/images/_760964_300mice.jpg





Enzyme-Linked ImmunoSorbent Assay (ELISA)
elisa

ELISA tests are commonly used by the 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.
(more info)




Image Credit:  www.resonancepub.com/Immunassay.htm




Polymerase Chain Reaction (PCR)pcr

PCR is not used much in laboratory diagnostics. It is used exclusively for epidemiology studies.
(more info)














Image Credit:  http://fig.cox.miami.edu/ ~cmallery/150/gene/c7.20.7.pcr.jpg