Presence of Maedi-Visna in Costa Rican sheep flocks

A total of 359 sheep serum samples from 15 farms were analyzed for the presence of antibodies against Maedi Visna Virus (MVV) by Enzyme-linked Immunosorbent Assay (ELISA). Additionally, a survey was applied to the sheep owners in order to determine management measures and presence of clinical symptoms of Maedi-Visna in the flocks. Logistic regression with random effects was performed to assess risk factors to seropositivity. Seven serum samples were positive to MVV belonging to six flocks, determining low flock seropositivity (0 to 7.1%). Only one of the five regions studied was negative, determining low overall prevalence (1.95%) and low positivity by region (0.0% to 0.84%). One seropositive animal with symptoms related to MVV was sacrificed for necropsy. An enlargement of mesenteric and celiac lymph nodes was determined, histopathology of lungs revealed acute interstitial pneumonia. Analysis of organs, fluids and blood of this sheep by Polymerase Chain Reaction (PCR) to detect MVV yielded negative results. In order to confirm the serological results, serum and blood samples from two remaining seropositive animals were analyzed six months later, one resulted seronegative and the other seropositive in ELISA, but both negative in PCR. No risk factors were associated to seropositivity. Since MVV produces latency, we conclude, that the positive serological results were due to false positive reactions. Even though, 52.0% of the participating farms had introduced animals, embryos, or semen from other farms or from abroad without any sanitary certification, MVV seems to be present in Costa Rica probably in a very low prevalence. Further studies are required to implement control measures and prevent the spread of the disease.