Seroprevalence of bovine leukemia virus in dairy cattle in Argentina: comparison of sensitivity and specificity of different detection methods.

Bovine leukemia virus (BLV) is a retrovirus that induces a chronic infection in cattle, which develop in three possible pathological forms: asymptomatic course, persistent lymphocytosis (PL) and lymphosarcoma. Once infected, cattle remain virus carriers for life and start to show a serological reaction within a few weeks after infection. Eradication and control of the disease is based on early diagnostic and segregation of the carriers. The agar gel immunodiffusion (AGID) test has been the serological test of choice for routine diagnosis of serum samples. Nevertheless, in more recent years, the enzyme-linked immunosorbent assay (ELISA) has replaced the AGID for large scale testing. Although Argentina has over 60 million cattle population, no nationwide studies have been conducted yet to determine the prevalence of the infection. To estimate the rate of BLV infection in dairy cattle in Argentina, a survey for specific antibodies in >10,000 serum samples from animals over 18 months old, belonging to 363 different herds from the largest dairy production areas of the country, was carried out in our laboratory, along 1999. For this purpose, we developed an ELISA to detect serum antibodies against the BLV virus. The cut-off of the ELISA was established over 339 serum samples, using polymerase chain reaction and southern blot (PCR-SB) as confirmatory test. The sensitivity and specificity of the ELISA was of 97.2 and 97.5%, respectively, while the local official AGID test showed a sensitivity of 79.7% and specificity of 99.0%. To know the seroprevalence of BLV on dairy herds, and also the incidence of the infection within the herd, the serological survey was based on individual serum samples. The results show that the prevalence of infected individuals is of 32.85%, while the percentage of infected herds, harboring one or more infected animals, is of 84%. These results indicate a medium level of seropositive animals when taken individually, but a high prevalence of infected farms, which has been notoriously increased in the last 15 years as shown when compared with previous data from particular geographic areas, indicating that BLV constitutes a serious sanitary problem for dairy producers in Argentina. They also indicate the poor sensitivity of the official AGID test used in the country.