Etiology, Epizootiology and Control of Maedi-Visna in Dairy Sheep: A Review

SIMPLE SUMMARY: Maedi-visna is a chronic viral disease of sheep with worldwide prevalence. It is caused by a small ruminant lentivirus. Its clinical manifestation includes primarily pneumonia and mastitis, and secondarily, arthritis and encephalitis. It causes substantial economic losses associated with involuntary culling or death of infected animals and reduced productivity, challenging the sustainability of dairy sheep farms. The extensive spread of the disease and the lack of treatment or vaccines render surveillance and prevention strategies indispensable. Currently, the major obstacles in controlling the disease are (i) the absence of an effective protocol for the early and definitive diagnosis of infected animals, utilizing appropriate, universally accepted serological and molecular techniques, (ii) the long interval between infection and seroconversion, and (iii) lack of understanding whether horizontal or vertical transmission are the most important route of infection. Therefore, the most appropriate measures for the control of the disease should include more frequent serological testing with available diagnostics and isolation or culling of seropositive animals, incorporation of artificial suckling, and strengthening of hygiene and biosecurity protocols. ABSTRACT: Maedi-visna (MV) in sheep is caused by maedi-visna virus (MVV), a small ruminant lentivirus (SRLV) that causes chronic infection and inflammatory lesions in infected animals. Pneumonia and mastitis are its predominant clinical manifestations, and the tissues infected by MVV are mainly the lungs, the mammary gland, the nervous system and the joints. MV has a worldwide distribution with distinct MVV transmission patterns depending on circulating strains and regionally applied control/eradication schemes. Nevertheless, the prevalence rate of MV universally increases. Currently, gaps in understanding the epizootiology of MV, the continuous mutation of existing and the emergence of new small ruminant lentiviruses (SRLVs) strains, lack of an effective detection protocol and the inefficiency of currently applied preventive measures render elimination of MV an unrealistic target. Therefore, modifications on the existing MV surveillance and control schemes on an evidentiary basis are necessary. Updated control schemes require the development of diagnostic protocols for the early and definitive diagnosis of MVV infections. The objectives of this review are to summarize the current knowledge in the epizootiology and control of MV in dairy sheep, to describe the research framework and to cover existing gaps in understanding future challenges regarding MV.