Infections caused by the small ruminant lentiviruses (SRLV) of sheep (maedi visna virus) and goats (caprine arthritis encephalitis virus) are a serious economical threat to small ruminant farming particularly in the more intensive settings like dairy farms. Revenue is ultimately negatively influenced by decreased milk production, reduced birth-rate and bodyweight and trade limitations 1, 2, 3. Chronically infected animals have a poor body condition and may display progressive dyspnoea, painful arthritis and indurative mastitis, factors that markedly affect animal welfare. However, in well managed flocks overt clinical disease is hardly seen because affected animals are culled in an early stage of this perniciously progressive disease, hence the only sign is an increased replacement rate. The infection spreads lactogenically from dam to progeny through colostrum and/or milk and horizontally on one hand via the natural close contact between the dam and her progeny 4, 5 and on the other hand between flock members via direct and indirect contacts 6, 7. Vertical -prenatal- transmission is generally considered to be of low or no importance. Infections are microscopically characterised by lymphoid infiltrations and inflammatory lesions in the lungs, the udder, the carpal joints and the central nervous system 8, 9, 10. There is no cure for this usually slowly progressive disease. Identification of infected individuals by laboratory testing and strict farm and animal sanitary management are corner-stones of programs for control and are at the same time the only tools available for reducing the prevalence of infection. Caution with respect to the reliability of laboratory testing for SRLV infections is augmented by the growing knowledge of the RNA viral genomes and their expression products. Immense genetic heterogeneity hampers protein as well as viral genome based tests. Due to sub-optimal laboratory tests generating false negative results, an early diagnosis is easily missed which will lead to spread of infection. This reduces the farmers’ support for control programs which was already under pressure because of the costs involved, the low direct market returns and the ‘invisibility’ of the long-term returns. Improvement of the diagnostic tools combined with improved test strategies will enable early detection of infection. Moreover, especially, the use of milk samples in stead of blood and in particular the use of bulk milk samples for the antibody detection as well as the molecular biological detection of SRLV infections could lead to considerable cost reduction. The small ruminant lentiviruses, a subfamily of the retroviruses, will need continuous attention because they are prone to diagnostic test escape as a result of their rapid evolving genomes 7, 11, 12 with consequences at the genomic and epitope level 13. Thus, the use of complementary tests is currently required and molecular-epidemiological surveillance is a prerequisite
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