Ovine lentivirus is aetiologically associated with chronic respiratory disease of sheep on the Laikipia Plateau in Kenya

A study was undertaken to investigate the occurrence of ovine lentivirus (O vLV) infection in sheep with chronic respiratory disease on the Laikipia Pl ateau, Kenya. All seven Merino crossbred sheep with chronic dyspnoea and em aciation examined for gross and microscopic lesions had lymphoid interstiti al pneumonia (LIP), and one also had pulmonary abscesses. Two of the sheep with LIP also had lesions of ovine pulmonary carcinoma (OPC, jaagsiekte). U sing in situ hybridization, OvLV DNA localized to a high proportion of pulm onary macrophages in lungs with lesions of LIP. Lung tissue samples from si x of these sheep were positive for a syncytium-inducing virus in cultures o f lamb testis cells. Thin-section electron microscopy of infected cells sho wed virions with morphogenesis typical of lentiviruses. In a western blotti ng assay, monoclonal antibodies to the OvLV capsid (CA, p27) and matrix (MA , p15) proteins of a North American OvLV isolate reacted with similar-sized bands of the virus, and serum from six of the sheep were reactive with CA from the Kenyan viral isolate. Using an OvLV agar gel immunodiffusion (AGID ) test, all seven sheep were positive for serum antiviral antibody, as were 29% of 63 clinically normal sheep from Laikipia District. However, when se ra from the healthy sheep were tested in a western blot assay, only 52% had IgG reactive to the OvLV CA, indicating a high rate of false negative reac tions with the AGID test. Serum samples from 87 Red Maasai or Dorper crossb red sheep from two farms in other parts of Kenya were OvLV seronegative by both the AGID test and the western blot assay. These results document the f irst identification of OvLV as a cause of chronic respiratory disease in sh eep in Kenya and show a high rate of infection in sheep flocks, with a high prevalence of chronic respiratory disease.