Evaluation of bluetongue virus diagnostic tests in free-ranging bighorn sheep

A bighorn sheep (Ovis canadensis) population in New Mexico, USA, was sampled 4 times between 1989 and 1995, and 5 bluetongue virus (BTV) diagnostic tests (virus isolation (VI), PCR, serum neutralization (SN), agar-gel immunodiffusion (AGID), and competitive-ELISA (c-ELISA)) evaluated. The c-ELISA, AGID and SN tests had high levels of agreement in determining serogroup exposure in bighorn sheep. The c-ELISA and AGID had high sensitivity and specificity, SN had perfect specificity but much lower sensitivity. Examining the results of seroprevalence (50-100%), it was shown that BTV circulated through the sheep population between 1989 and 1993. This increase in seroprevalence coincided with a bighorn die-off due to BTV infection in June, 1991. An adjacent cattle herd was sampled in 1995 for comparison. The bighorn sheep and adjacent cattle had different patterns of exposure to BTV between 1994 and 1995. There was no evidence that BTV circulated through the bighorn sheep population from 1994 to 1995. In 1995, seroprevalence to BTV decreased to 72%, none of yearling bighorn was seropositive, and all of the 39 bighorn sheep were PCR negative. All adult cattle were seropositive to BTV by c-ELISA and SN and 4 of the calves were seropositive; 11 of the 24 cattle were PCR-positive, including all 5 calves.