Direct evidence for natural transmission of small-ruminant lentiviruses of subtype A4 from goats to sheep and vice versa.

Small-ruminant lentiviruses (SRLV), which include the caprine arthritis-encephalitis and the maedi-visna virus, cause persistent inflammatory infections in goats and sheep. SRLV are mainly transmitted from mother to offspring through milk. Transmission after prolonged contact between adult animals has also been observed. The observation that certain SRLV subtypes are found in both goats and sheep suggests that interspecies transmission has occurred on several occasions in the past. We investigated seropositive goats and sheep that were kept together in small mixed herds. Phylogenetic analysis of long proviral sequences in gag and pol, combined with epidemiologic information, demonstrated natural sheep-to-goat transmission of the recently identified SRLV subtype A4 in two instances and goat-to-sheep transmission of the same subtype in one instance. In a further mixed cluster, the direction of the interspecies transmission could not be determined. These findings present for the first time direct evidence that natural interspecies transmission of SRLV is ongoing in both directions. The findings are of relevance to virus eradication programs in both species.

Phylogenetic analysis and reclassification of caprine and ovine lentiviruses based on 104 new isolates: evidence for regular sheep-to-goat transmission and worldwide propagation through livestock trade.

We performed a phylogenetic analysis of caprine and ovine lentiviruses using long sequences in gag and pol of 104 new Swiss isolates and six available corresponding database sequences. Forty-five isolates, forming five sequence clusters, were unclassifiable by the present classification. Pairwise DNA distance analysis indicated different categories of relatedness, requiring a new classification system. We propose four principal sequence groups, A-D, which differ by 25-37%. Groups A and B are further divided into subtypes which differ by 15-27%. Group D and four of the seven group A subtypes, A3, A4, A5 and A7, are formed by new Swiss isolates. Molecular epidemiology revealed that Swiss B1 strains differed no more from French, Brazilian or US strains than from each other, suggesting virus propagation through international livestock trade. Furthermore, infection of goats by subtypes A3 or A4 was significantly associated with documented contact with sheep, which also harbor these subtypes, thus indicating regularly occurring sheep-to-goat transmission.