Rabies Virus

 
Diagnosis of rabies infection is generally done post mortem.  Given the imprecise nature of the clinical signs tissues must always be examined for the presence of the virus. In human cases of rabies contact and subsequent infection the need is critical to determine the diagnosis.  The risk of death is real and rapid identification becomes paramount so effective treatment can begin as soon as possible.  Rabies infection is referred to as a case of medical “urgency” rather than emergency.  That being said a Center for Disease Control (CDC) study in 2009 reported that 55,000 people die from rabies World Wide every year. This analysis is achieved with the evaluation of multiple tissues, salivary samples and using PCR technology. In the case of veterinary medicine, definitively diagnosing rabies in a patient is done post mortem.  Given the tropism of rabies for the brain, the diagnostic tests are focused on these tissues.
 
CNS tissues can be sent to histopathology using H and E staining procedures.  The pathologist will investigate for histological signs of encephalitis and will also be examining for viral inclusion bodies (Negri bodies --> see arrow). Given the facts that myelitis can result from other etiological agents and that Negri bodies have been found in non-rabies cases, the histological method has fallen out of favor.

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Immunofluorescence on the fresh brain can be performed in order to determine the presence of this disease. This technique uses fluorescein tagged rabies antibodies which bind to the viral components in the brain. Specifically they target the nucleoprotein in the host’s cytoplasm.  The excess is washed off and if viral components are present, they fluoresce under a special microscope. This highlights the rabies nucleoprotein in the CNS. According to the CDC website, the direct fluorescence method is considered the gold standard for rabies testing due to the high sensitivity and specificity of the technique.  Once the lab receives the tissue samples it is to test, a diagnosis can be made within a few hours. The medulla oblongata and the cerebellum are the primary tissues for evaluation in the lab. Although the virus makes its way throughout the brain, given the pattern in which the virus spreads, looking at the brainstem is most reliable to detect the infection. Further, rabies can affect the brain unilaterally, especially in large animals. Therefore cross section analysis of the brainstem helps to make definitive diagnoses in these cases and is the only technique that can give negative diagnoses. It also seems that the virus is easily found in the large neurons of the cerebellum, easing identification. However, lack of detection in the cerebellum is not equivalent to lack of infection. As previously mentioned, this can only be achieved by investigation of the medulla or other components of the brainstem.