Summary
Bluetongue virus (BTV) is the prototype member of the genus Orbivirus, family Reoviridae. The BTV serogroup contains 24 serotypes. Diagnostic tests currently used for the detection of BTV involve the isolation and growth of virus isolates in eggs or mice, followed by passaging in tissue culture. The virus is subsequently characterised using serological tests to detect reaction with reference antisera, such as the agar gel immunodiffusion test or serum neutralisation test. These procedures are time-consuming and may fail to detect low levels of infectious virus or strains of BTV that do not replicate in eggs, mice or cell culture. The use of the enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies to BTV in infected animals is faster but does not necessarily confirm recent infection. Similarly, ELISA may be used to detect virus directly in tissue samples but the sensitivity is relatively low. A number of procedures have been developed to detect the presence of BTV antigens or nucleic acids. The polymerase chain reaction (PCR) technique has proved to be a powerful tool for BTV diagnosis. PCR techniques may be used not only to detect the presence of viral nucleic acid but also to ‘serogroup’ orbiviruses and provide information on the serotype and possible geographic source (topotype or genotype) of BTV isolates within a few days of receipt of clinical samples, such as infected sheep blood. Traditional approaches, which rely on virus isolation followed by virus identification, may require at least three to four weeks to generate information on serogroup and serotype and yield no data on the possible origin of the virus isolated. Moreover, PCR enables differentiation between field isolates and vaccine strains.

Keywords
Bluetongue, Diagnosis, Polymerase chain reaction, Reverse transcriptase, Virus.