Shankar B.P.

Ph.D. Scholar, Department of Veterinary Pathology,

Karnataka Veterinary, Animal and Fisheries Science University

Veterinary College, Hebbal, Bangalore-24.

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.