EVALUATION OF BLUETONGUE VIRUS DIAGNOSTIC-TESTS IN FREE-RANGING BIGHORN SHEEP

Five bluetongue virus (BTV) diagnostic tests were evaluated for use in free-ranging bighorn sheep. We sampled one bighorn sheep population f our times between 1989 and 1995. The tests evaluated included virus is olation (VI), polymerase-chain reaction (PCR), serum neutralization (S N), agar-gel immunodiffusion (AGID), and competitive enzyme-linked imm unosorbent assay (c-ELISA). The c-ELISA, AGID and SN tests had high le vels of agreement in determining serogroup exposure in bighorn sheep. We used maximum-likelihood algorithms to estimate the parameters of ea ch diagnostic test used. Although the c-ELISA and AGID had high sensit ivity and specificity, the SN had perfect specificity but lower appare nt sensitivity. Due to the potential of cross-reactions among multiple serotypes, results of the SN must be interpreted with caution when as sessing serotype exposure in an area where multiple serotypes an endem ic. The PCR assay delineated convalescent antibody titers from more-re cent infections, and consequently, was pivotal in distinguishing a dif ferent exposure pattern between the bighorn sheep and cattle in an adj acent herd. Based on an increasing seroprevalence (50% to 100%), BTV c irculated through this bighorn 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 differ ent patterns of exposure to BTV between 1994 and 1995. There was no ev idence that BTV circulated through the bighorn sheep population from 1 994 to 1995. In 1995, seroprevalence to BTV decreased to 72%, none of yearling bighorn was seropositive, and all of the 39 bighorn sheep wer e PCR-negative. In contrast, all adult cattle were seropositive to BTV by c-ELISA and SN, and 4 of the calves were seropositive; 11 of the 2 4 cattle were PCR-positive, including all five calves. Overall, the pa ttern of temporal herd immunity in the bighorn sheep appeared to follo w a classic epidemic curve, with the appearance and subsequent disappe arance of herd immunity coinciding with the 1991 die-off in this popul ation. As low levels of herd immunity and high proportions of suscepti ble animals are key factors in the development of epidemics, this popu lation of bighorn sheep may be at increased risk for a BTV epidemic in the future. (C) 1998 Elsevier Science B.V.