The Occurrence of Antibody to Bluetongue Virus in New South Wales. ' I. Statewide Surveys of Cattle and Sheep

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Abstract
Two State-wide surveys were carried out in 1978 to detect bluetongue (BLU) virus antibody in cattle and sheep sera in New South Wales (NSW). The first survey showed that BLU group antibody in cattle 18-24 months old was confined to the coastal regions (east of the Great Dividing Range) and the Hunter Valley. However, in the second survey, of cattle more than 5 years old, reactors were much more widely distributed over the north-eastern third of the State and into the western division with prevalences up to 85% in some areas. In contrast, very few reactors were detected in sheep in either survey (less than 1% of the sheep sera tested). In a retrospective study of stored cattle sera, BLU group reactors were detected in the north-east of the State in each year examined since 1968, the earliest year in which samples were available from that region. Areas to the south and west were free of antibody from 1966 until the summer of 1973, but subsequently reactors were common. Examination of selection area for type-specific antibody indicated that infection of cattle with two of the three Australian BLU serotypes which were known at the time, BLU-1 and BLU-21, had occurred in NSW. No antibody to BLU-20, the original Australian isolate, we detected. A close association was observed between strong group antibody reactions and type-specific neutralizing activity against BLU-1 and BLU-21. Both were largely confined to that area of the State in which a high (75% or more) prevalance of group antibody was recognized in the older animals. In contrast, reactions from areas with a low prevalence of group antibody were almost all weak and rarely associated with neutralizing activity for BLU-1 or BLU-21. We suggest that weak reactions may be group cross-reactive and induced by BLU-related, rather than BLU, viruses.