![]() Schematic representation of an SRLV particle. The single-stranded RNA molecules are transformed into double-stranded DNA (dsDNA) using the enzyme reverse transcriptase (RT) thanks to this mechanism, the retroviral genome (known as provirus when it is dsDNA) can be inserted into the genome of the host cell ( Figure 1). ![]() As other retroviruses, virions contain two linear molecules of RNA. The core or capsid is a protein case, which contains the viral RNA and the enzymes necessary for viral replication. The envelope is formed by a membrane bilayer of host cell-derived phospholipids, in which viral-encoded glycoproteins are inserted. MVV and CAEV particles measure between 90 and 120 nm in diameter and consist of the envelope and the core. 12 Genotype A includes strains of MVV genotype B includes strains of CAEV genotype C corresponds to SRLV isolated from Norwegian sheep and goats genotype D has been identified when analyzing pol sequences of SRLV of Spanish and Swiss sheep and genotype E has been isolated in Italy. The SRLVs are currently classified according to their phylogenetic relationships in the following five groups: A, B, C, D, and E. 1īoth MVV and CAEV belong to the family Retroviridae, genus Lentivirus. Regardless of the route of transmission, the viruses cross the mucous barriers and infect resident macrophages and dendritic cells. 1 The potential risk linked to the milking machine and the needle has not been tested. ![]() Other modes of transmission such as the transplacental and through semen are less important from an epidemiological point of view. ![]() 10, 11 It is not clear if the transmission among animals occurs through the presence of free virions and/or through infected cells. 9 The main transmission routes are airborne, favored by overcrowding (stabling), 7 and milk borne. The route of transmission is related to body fluids, mainly respiratory exudates and milk or colostrum, which may contain infected monocytes and macrophages. The prevalence is much higher in developed countries, which seems to be related to the system of feeding lambs or kids with a pool of colostrum or milk from the tank, a practice that favors transmission, 6 and to the management system. 5 So far, MVV-free (but not CAEV-free) countries are New Zealand and Australia. MVV has recently been identified in Japan, a country that was considered exempt. MVV and CAEV infections are widespread worldwide. Both diseases are slow progressing and usually subclinical. Caprine arthritis and encephalitis (CAE) is the name of the disease in goats, as the joints and nervous system are most often affected. Maedi-Visna (MV) is a sheep disease characterized by respiratory (Maedi, determined by interstitial pneumonia) and nervous (Visna, progressive inflammatory disease of the central nervous system) clinical signs and occasionally of the joints and/or mastitis. The ability of the SRLVs to produce cross-infections, jumping the interspecies barrier, is unusual, as most lentiviruses have a very limited capacity to grow in cells not from their host species. 3, 4 Nevertheless, infection by MVV is more common in sheep and infection by CAEV is more common in goats, so homologous transmission is probably favored. In addition, molecular studies have shown that both viruses represent a broad spectrum of variants that can affect both animal species 1 and certain viral variants that normally affect sheep can infect goats, and vice versa 2 or even predominate in the other species. The viruses that produce Maedi-Visna virus (MVV) and caprine arthritis-encephalitis virus (CAEV) are known as small ruminant lentiviruses (SRLVs) due to their structural, genetic, and pathogenic similarities.
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