Frequency of serological cross-reactions between Ibaraki and bluetongue viruses using the agar gel immunodiffusion test.

The frequency of serological cross-reactions between Ibaraki (IB), and bluetongue (BT) viruses using the agar gel immunodiffusion (AGID) test was investigated. The percentage of IB neutralisation-positive bovine serum samples that were positive to the BT AGID test was 42.5%; 12.2% of the BT AGID-positive serum samples and 2.5% of the BT AGID-negative serum samples were positive to the IB AGID test. When the BT competitive ELISA (c-ELISA) was used, these cross-reactions disappeared. These results indicate that serum samples from areas in which IB is epidemic are often positive against the BT AGID test, but negative against the BT virus neutralisation test (VNT). To obtain specific BT surveillance results in these IB endemic areas, the AGID-positive results should be confirmed using the c-ELISA or VNT.