BIOLOGICAL AND GENETIC-DIFFERENCES BETWEEN LUNG-DERIVED AND BRAIN-DERIVED ISOLATES OF MAEDI-VISNA VIRUS

During the epidemic caused by maedi-visna virus (MVV) of sheep in Icel and, the pulmonary affection, maedi, was the predominant clinical mani festation. In some hocks, however, a central nervous system (CNS) affe ction, visna, was the main cause of morbidity and mortality. As there is only one breed of sheep in the country, host factors did apparently not play an important role in the different clinical manifestations. To obtain some information on possible viral genetic determinants of n eurotropism and neurovirulence we studied both phenotypic and genotypi c properties of two maedi-visna virus strains; a strain that was origi nally isolated from the brain of sheep with encephalitis (visna), and another strain isolated from the lungs of a sheep suffering from pneum onia (maedi). The brain isolate was found to grow faster in sheep chor oid plexus cells than the lung isolate, whereas the growth rate in mac rophages was similar for the maedi and visna virus strains. Intracereb ral inoculation indicated that the visna virus isolate induced more se vere brain lesions than the maedi isolate. In addition, a pathogenic m olecular clone derived from a visna strain (KV1772kv72/67) was tested for growth in sheep choroid plexus cells and macrophages. The molecula rly cloned virus retained the fast growth rate in choroid plexus cells . The nucleotide sequence of the env gene and the U3 of the LTR was de termined for the maedi strain and compared to that of the visna strain s. There was an 11.7% difference in deduced amino acid sequence in the Env protein and a 6% difference in the LTR. The molecular clone KV177 2kv72/67 will be a useful reagent for characterization of viral determ inants of cell tropism in vitro and possibly neurovirulence in vivo.