There are no tests to diagnose rabies infections in humans before the onset of clinical symptoms. Rabies can be diagnosed from euthanized animals by direct fluorescent anti body testing of brain tissue.
A. Rabies Antigens or Nucleic Acids
Tissues infected with rabies virus are currently identified most rapidly and accurately by means of immunofluorescence or immunoperoxidase staining using anti-rabies monoclonal antibodies. A biopsy specimen is usually taken from the skin of the neck at the hairline. Impression preparations of brain or cornea tissue may be used.
A definitive pathologic diagnosis of rabies can be based on the finding of Negri bodies in the brain or the spinal cord. They are sharply demarcated, more or less spherical, and 2–10 µm in diameter, and they have a distinctive internal structure with basophilic granules in an eosinophilic matrix. Negri bodies contain rabies virus antigens (Figure 1). Both Negri bodies and rabies antigen can usually be found in animals or humans infected with rabies, but they are rarely found in bats.
Fig1. Histopathologic examination of central nervous system tissue from autopsy of a decedent with suspected rabies infection, showing neuronal cytoplasmic inclusions (Negri bodies) after hematoxylin and eosin staining (A) and rabies virus antigen (red) after immunohistochemical staining (B). (Reproduced with permission from Centers for Disease Control and Prevention: Human rabies—Kentucky/ Indiana, 2009. MMWR Morb Mortal Wkly Rep 2010;59:393.)
Reverse transcription-polymerase chain reaction testing can be used to amplify parts of a rabies virus genome from fixed or unfixed brain tissue or saliva. Sequencing of amplified products can allow identification of the infecting virus strain.
B. Serology
Serum antibodies to rabies can be detected by immunofluorescence or neutralization tests. Such antibodies develop slowly in infected persons or animals during progression of the disease but promptly after vaccination with cell-derived vaccines. Antibodies in cerebrospinal fluid are produced in rabies-infected individuals but not in response to vaccination.
C. Viral Isolation
Available tissue is inoculated intracerebrally into suckling mice. Infection in mice results in encephalitis and death. The central nervous system of the inoculated animal is examined for Negri bodies and rabies antigen. In specialized laboratories, hamster and mouse cell lines can be inoculated for rapid (2- to 4-day) growth of rabies virus; this is much faster than virus isolation in mice. An isolated virus is identified by fluorescent antibody tests with specific antiserum. Virus isolation takes too long to be useful in making a decision about whether to give vaccine.
D. Animal Observation
All animals considered “rabid or suspected rabid” (Table 1) should be sacrificed immediately for laboratory examination of neural tissues. Other animals should be held for observation for 10 days. If they show any signs of encephalitis, rabies, or unusual behavior, they should be killed humanely and the tissues examined in the laboratory. If they appear normal after 10 days, decisions must be made on an individual basis in consultation with public health officials.
Table1. Rabies Postexposure Prophylaxis Guide—United States, 2008
