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En-Min(Eric) Shou, MD, PhD and Julie Ann Jarvinen, PhD, DVM
Iowa State University
To estimate the prevalence of infection with bovine viral diarrhea virus (BVDV) in US alpaca herds and the incidence of infection (number of new BVDV infections occurring in a given time period), we conducted a national survey based on detection of serum antibodies against BVDV. In 2006 a request to participate in the study was mailed to 551 randomly selected herd owners representing 10% of alpaca herds in each state. Owners were asked to submit blood samples from up to five alpacas in their herd for testing in the virus neutralization (VN) assay. A positive test result indicates the alpaca has been exposed to the virus and has developed BVDV antibodies, but it does not distinguish between past and present infections. The VN assay also will not detect alpacas infected very recently and sampled before antibodies are produced or alpacas with persistent BVDV infection that do not produce BVDV antibodies.
Samples were received from 191 alpacas representing 39 herds in 20 states. Three participating herds (7.7%) had at least one alpaca with antibodies to BVDV and eight individual alpacas (4.2%) tested positive. Antibody titers ranged from 32 to 1024 against BVDV Type 1 and 16 to 128 against Type 2. Titers against BVDV Type 1 were consistently greater than titers against Type 2 indicating the alpacas were most likely exposed to Type 1. A second set of samples obtained from the same alpacas in 27 herds was collected an average of 5.6 months (range: 2.5 to 9.5 months) after the first sample set. Antibodies in previously negative alpacas would result from new BVDV infection acquired between sample dates. None of the herds or individual alpacas initially negative for antibodies against BVDV was positive at the second sampling. One of three previously positive herds submitted a second set of samples from 3 of the 5 alpacas initially sampled. Of these, one alpaca was negative at both samplings and a second alpaca with initial titers of 128 and 64 against BVDV Types 1 and 2, respectively, was negative at the second sampling. The third alpaca with initial titers of 1024 (Type 1) and 128 (Type 2) remained positive with titers of 1024 and 256 against BVDV Types 1 and 2, respectively. These results suggest that little or no transmission of BVDV infection occurred in the herds during the sampling interval.
Herd owners were asked to provide age, sex and breed of alpacas sampled and to complete questionnaires with details regarding herd size and composition, biosecurity practices, movement of alpacas on/off premises, occurrences of reproductive loss, ill heath or death, vaccination practices and presence of other animal species on the premises. Statistical analyses were performed to determine if positive VN tests were correlated with any of these factors, but no associations were found. This result should not be interpreted to mean there are no associations between exposure to BVDV and the various risk factors evaluated because the small sample size likely did not provide the statistical power necessary to detect them.