Prevalence and risk factors associated with bluetongue virus among Colorado sheep flocks.

摘要

During the summer of 2007, researchers from Colorado State University undertook a study to measure the prevalence of and identify risk factors associated with Bluetongue Virus (BTV) infection among Colorado sheep flocks. A total of 2,544 serum and whole blood samples were obtained from 1,058 ewes, 992 lambs, and 494 rams located on 108 sheep farms throughout Colorado. Flocks were recruited by the use of a questionnaire and flocks were tested for the presence of BTV antibodies utilizing cELISA, viral RNA utilizing nested RT-PCR, and the presence of clinical disease indicative of BTV based on the criteria of three or more clinical signs present in at least five animals.;Flock level seroprevalence was 28.70% (95% CI, 20.41% to 38.20%), viral RNA was detected in 22.22% (95% CI, 14.79% to 31.24%) of flocks and clinical disease was observed in 19.44% (95% CI, 12.46% to 28.17%) of flocks tested. Animal level seroprevalence within positive flocks ranged from 7.6% to 83 % with a mean of 27.09% (95%CI, 23.87% to 30.51%), viral RNA prevalence within positive flocks ranged from 4.8% to 48% with a mean of 25.62% (95%CI, 21.93% to 29.59%), and clinical disease within positive flocks ranged from 16.7% to 41.7% with a mean of 24.24% (95% CI, 20.38% to 28.43%). Animal and flock level seroprevalence was higher among the adult population whereas prevalence of viral RNA and clinical disease was higher among the lambs. Positive flocks were distributed heterogeneously throughout the state and all but three flocks that demonstrated clinical disease were identical to those with detectable viral RNA; therefore, there was a significant correlation between the detection of viral RNA and observation of clinical disease among flocks.;The two most significant clinical signs associated with detection of viral RNA were weight loss (OR, 12.366, 95% CI, 2.057-74.343) and oral ulcerations (OR, 11.756, 95% CI, 1.061-130.243). Significant risk factors associated with viral RNA detection included primary purpose of the flock being commercial (OR 3.60, 95% CI, 0.85-15.18) and administration of BTV modified live vaccination (OR, 15.95, 95%CI, 4.51-56.35). Higher maximum temperature at the time of visit, closer proximity to water, and increased cumulative precipitation over a period of thirty-five days previous to the time of flock visit were identified as environmental confounders of these estimates. Risk factors associated with seropositive flocks included administration of BTV modified live vaccine compared to animals not receiving vaccine (OR, 9.360, 95%CI, 3.046-28.764). Environmental confounders of this relationship were closer proximity to water sources and lower elevations. Risk factors significantly associated with the odds of developing clinical disease also included vaccination (OR, 8.336, 95%CI 2.486-27.959) while higher maximum temperatures at the time of flock visit and closer proximity to water confounded this relationship.