| | | Bluetongue virus is the type species for the Orbivirus genus [21] . The orbiviruses are distinguished from the reoviruses by acid lability, slight solvent sensitivity, serology (CF and HI), and capsid structure [2] , [21] . In addition, there is no nucleic acid sequence homology between Reovirus members and members of other genera [21] . At present, there are 12 serological subgroups of genus Orbivirus. The BLU subgroup consists of the 23 serotypes of bluetongue virus [34] . | | Isolation of field virus has been by inoculation into sheep, embryonated chicken egg, or less so, cell culture and suckling mice; the iv inoculation of embryonated chicken eggs has proven the most sensitive isolation system [3] , [10] . The agar gel immunodiffusion test has been the most widely used group specific test for bluetongue virus antibody and has detected all serotypes of BLU [35] , [36] . The modified direct CF test (MDCF) and IFA tests also have been used as group specific for bluetongue [35] , [36] . | | A plaque-reduction neutralization test differentiates between BLU and EHD virus antibodies; this test is currently used to identify the 23 serotypes of BLU [34] . Various serum neutralization tests have previously been used to differentiate BLU serotypes [3] , [10] | | Antigenic relationships have been demonstrated between the bluetongue serogroup and the EHD, Eubenangee and Palyam serogroups [22] , [36] . Immunological cross-reaction documented between virus-specified tubules of BLU and EHD viruses [13] . | | Two-way cross-relationships between Ibaraki virus and EHD virus, serotypes 1 and 2, demonstrated by agar gel precipitin and indirect fluorescent antibody tests [5] . By neutralization tests, Ibaraki virus was more closely related to EHD virus, serotype 2 (Alberta strain). Antigenic relationship not observed between Ibaraki virus and four serotypes of BLU then found in USA. | | |
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