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Review
14.
Advances in diagnosis of rabies -
B. P. Shankar
Vet World. 2009; 2(2): 74-78
Abstract
Rabies is a
major zoonosis for which diagnostic techniques have been
standardised internationally. Laboratory techniques are
preferably conducted on central nervous system (CNS) tissue
removed from the cranium. Agent identification is preferably
done using the fluorescent antibody test. A drop of purified
immunoglobulin previously conjugated with fluorescein
isothiocyanate is added to an acetone-fixed brain tissue smear,
preferably made from several parts of the brain, including the
hippocampus, cerebellum and medulla oblongata. For a large
number of samples, as in an epidemiological survey, the
immunoenzyme technique can provide rapid results (the rapid
rabies enzyme immunodiagnosis). FAT provides a reliable
diagnosis in 98-100% of cases for all genotypes if a potent
conjugate is used, while RREID detects only genotype 1 virus.
Infected neuronal cells have been demonstrated by histological
tests and these procedures will reveal aggregates of viral
material (the Negri bodies) in the cytoplasm of neurones.
However, the sensitivity of histological techniques is much less
than that of immunological methods, especially if there has been
some autolysis of the specimen. Consequently, histological
techniques can no longer be recommended. As a single negative
test on fresh material does not rule out the possibility of
infection, inoculation tests, or other tests, should be carried
out simultaneously. Newborn or 3-4-week-old mice are inoculated
intracerebrally with a pool of several CNS tissues, including
the brain stem, and then kept under observation for 28 days. For
any mouse that dies between 5 and 28 days, the cause of death
should be confirmed by FAT. Alternatively, a monolayer culture
of susceptible cells is inoculated with the same material as
used for mice. FAT carried out after appropriate incubation will
demonstrate the presence or absence of viral antigen. Wherever
possible, virus isolation in cell culture should replace mouse
inoculation tests. The identification of the agent can be
supplemented in specialised laboratories by identifying any
variant virus strains through the use of monoclonal antibodies,
specific nucleic acid probes, or the polymerase chain reaction
followed by DNA sequencing of genomic areas. Such techniques can
distinguish between field and vaccine strains, and possibly
identify the geographical origin of the field strains. Virus
neutralisation assays in cell cultures are the prescribed tests
for international trade.
Keywords:
Rabies, Zoonosis, Diagnosis, ELISA, CNS, DNA, Virus, Negri
bodies.
