Genomic heterogeneity of small ruminant lentiviruses: existenceof heterogeneous populations in sheep and of the samelentiviral genotypes in sheep and goats

Abstract

We have recently shown that French small ruminant lentiviruses (SRLV) from sheep are more similar to Caprine Arthritis Encephalitis Virus (CAEV) than to visna maedi virus (VMV) in a conserved region of the pol gene. To extend these results, we have examined sequences from a variable region of the env gene in French SRLV. We found that they were nearly equally distant from both CAEV and VMV strains, suggesting a considerable divergence since the initial introduction of the virus. Analysis of separate clones from individual animals showed that some carry a population of variant viruses. The study of further pol gene sequences from both goats and sheep suggests that viral variants show little or no host species specificity. A phylogenetic tree of pol gene sequences confirmed the presence of a novel genotype of SRLV in France.

Direct Evidence for Natural Transmission of Small-Ruminant Lentiviruses of Subtype A4 from Goats to Sheep and Vice Versa

“…We demonstrate a much closer genetic relationship among these epidemiologically linked sequences, which differ by less than 1% from each other, than that found among viruses from epidemiologically unlinked animals, which differ by 4 to 8%, as is evident from Fig. 3 and has been reported previously (12,26,32). We thus present the first direct evidence that the recently identified SRLV subtype A4 is transmitted across the species barrier in both directions by the natural horizontal infection of adult animals.…”

“…Although some subtypes of the large and diverse SRLV group are found among both goats and sheep, thus presenting evidence for more than one event of interspecies transmission in the past (12,26,32), and although interspecies transmission has been achieved experimentally (1), the natural transmission of these viruses across the species barrier has never been documented directly. Here we have phylogenetically analyzed viruses isolated from goats or sheep that seroconverted after they were newly exposed to SRLV-infected animals of the other species.…”

“…These new facts should be considered in all SRLV eradication programs. Whether subtype A4 may be particularly prone to interspecies transmission is unknown, but the fact that subtypes A3, A6, and B2 can be found in both sheep and goats is as compatible with ongoing interspecies transmission as with a single event in the past (12,26,32).…”

“…MVV can, indeed, be experimentally transmitted to goats and, vice versa, CAEV can be transmitted to sheep (1). The fact that some SRLV subtypes have been isolated from both sheep and goats furthermore indicates that interspecies transmission must have occurred at least once in each of these subtypes, although the frequency and direction of transmission remain unknown (12,21,22,32). While interspecies transmission from goats to sheep under natural conditions has never been observed (1,16,23,27), we have recently forwarded statistical evidence suggesting that transmission of the newly discovered SRLV subtype A4 from sheep to goats must occur regularly, though rarely (26).…”

“…(14,17,18,28). Earlier phylogenetic analysis, mostly based on short sequences, has suggested that these very diverse viruses can be divided into six different sequence clades, I to VI (12,21,22,32). Recent work based on long sequences in gag and pol of more than 100 new isolates from Switzerland and all available database sequences has, however, demonstrated that the SRLV should rather be divided into four principal sequence groups A to D, which differ by 25 to 37% in gag and pol sequences.…”