Prevalence of maedi-visna infection in culled ewes in Alberta

Maedi-visna (MV) is a relatively common chronic infection of sheep in North America resulting in economic loss to the sheep industry. The objectives of this study were to: 1) measure the prevalence of MV infection in culled ewes in Alberta, by histologic examination (lungs and udder) and serologic testing using an agar gel immunodiffusion (AGID) test, 2) examine any geographic differences in its prevalence in the province, 3) evaluate the level of agreement between histopathologic examination and serologic testing, 4) grade the lesions and correlate the serologic results with the presence of severe histological lesions, and 5) correlate the presence of histological lesions in the lungs and udder in the same animal. Based on histologic findings, the prevalence of MV was 26.8%, compared with 13.0% using serologic testing. There were no significant geographical differences in prevalence, fair agreement (kappa = 42.0%) between histopathologic and serologic results, and poor agreement (kappa = 11.5%) between the presence of lung and udder histological lesions within the same animal. This study indicates that MV is relatively common in culled ewes in Alberta, with no significant geographic variation. The poor sensitivity of the AGID test, compared with histologic examination, should be taken into consideration when interpreting serologic results.