- Short report
- Open Access
Molecular characterization and phylogenetic analysis of small ruminant lentiviruses isolated from Canadian sheep and goats
© L'Homme et al; licensee BioMed Central Ltd. 2011
- Received: 24 November 2010
- Accepted: 3 June 2011
- Published: 3 June 2011
Small Ruminant Lentiviruses (SRLV) are widespread in Canadian sheep and goats and represent an important health issue in these animals. There is however no data about the genetic diversity of Caprine Arthritis Encephalitis Virus (CAEV) or Maedi Visna Virus (MVV) in this country.
We performed a molecular and phylogenetic analysis of sheep and goat lentiviruses from a small geographic area in Canada using long sequences from the gag region of 30 infected sheep and 36 infected goats originating from 14 different flocks. Pairwise DNA distance and phylogenetic analyses revealed that all SRLV sequences obtained from sheep clustered tightly with prototypical Maedi visna sequences from America. Similarly, all SRLV strains obtained from goats clustered tightly with prototypical US CAEV-Cork strain.
The data reported in this study suggests that Canadian and US SRLV strains share common origins. In addition, the molecular data failed to bring to light any evidence of past cross species transmission between sheep and goats, which is consistent with the type of farming practiced in this part of the country where single species flocks predominate and where opportunities of cross species transmissions are proportionately low.
- Small Ruminant
- Cross Species Transmission
- Caprine Arthritis Encephalitis Virus
- Visna Virus
Caprine arthritis-encephalitis virus (CAEV) and ovine Maedi-visna virus (MVV) are members of the small ruminant lentiviruses (SRLVs) group in the retroviridae family infecting goats and sheep worldwide [1, 2]. Lentiviruses from different animal species have in common their genomic organization, the induction of slowly progressive diseases, the large spectrum of targeted organs and symptoms and the ability to persist in their hosts despite a strong immunological response. Transmission of SRLVs is thought to occur predominantly through ingestion of infected milk but, at least in sheep, horizontal transmission may also play a prominent role [3, 4]. Common clinical signs caused by SRLV infections include neurological disorders, dyspnoea, emaciation, mastitis and arthritis [2, 5, 6]. The genomic organization of SRLVs is typical of lentiviruses: positive sense RNA dimmers of approximately 9 kb in size which consist of long terminal repeats (LTRs), gag (group specific antigens), pol (polymerase) env (envelope) genes in addition to a number of regulatory genes. The gag and pol genes are relatively well conserved among SRLVs, which makes them ideal targets for PCR primer design . Originally, MVV and CAEV prototypical strains such as strain K1514 , EV-1 , SA-OMVV  and Cork-CAEV  were viewed as distinct viral species restricted to their respective hosts. Viruses isolated from sheep were closely related and referred to as MVV, and those isolated from goats were referred to as CAEV. Over the last two decades however, as more SRLV sequences became available for phylogenetic analyses, it became evident that SRLVs can cross the species barrier since some ovine and caprine strains appear on shared branches in family trees [11–13]. Additionally, molecular-epidemiological evidence suggest that SRLVs can transmit between sheep and goats under favourable conditions (Shah et al 2004; Pisoni et al 2005). In Canada, SRLV infections are widespread in small ruminants and have been associated principally with lung and mammary lesions in sheep and arthritis and emaciation in goats [14, 15]. National surveys revealed that 63% of sheep flocks and 52.9% of goat flocks had at least one infected animal (;Simard C., unpublished observations). In addition, SRLV were detected in 31.3% of Quebec sheep and 82.5% of milking goats ( and unpublished). Despite the fact that SRLVs have been known to circulate and cause disease in Canadian sheep and goats for more than three decades, molecular characterization of SRLV strains had never been carried out in either species . In this study, we report for the first time, genomic sequences and phylogenetic analyses of Canadian SRLVs from a small geographic area. Nearly complete gag sequences from both sheep and goats were obtained from animals belonging to single species flocks.
List of primers used in this study (5' to 3')
Before the mid 90s, as the first genomic sequences became available, SRLVs from distinct geographical origins were grouped according to the species they were isolated from. Two distinct but related taxonomic clusters were evident, one evolving in sheep and the other evolving in goats. In the mid 90s however, as more sequences from SRLVs were characterized, more complex distribution of strains became evident upon phylogenetic reconstructions, suggesting that SRLVs could no longer be grouped solely based on the species they were obtained from. These findings modified the perception about the faithful relationship between SRLVs and their hosts and led to the concept of cross species transmission . Numerous phylogenetic studies followed and brought additional evidence that SRLVs could no longer be classified according to the species they were recovered from [13, 24]. Nowadays, MVV and CAEV are no longer viewed as two distinct viral species infecting exclusively sheep and goats, respectively, but are rather viewed as a continuum of strains or quasispecies that can transmit between small ruminants under favourable conditions [1, 11, 25].
Results from our study somehow contrast with these recent studies which have generally reported more complex phylogenetic relationships between SRLVs and their hosts. The relatively homogeneous strain populations that were found within each host species in our study could be explained by a founder effect coupled to the single species flock type of management that prevails in this part of the country. Single species flocks type of farming obviously limits close contact between the two species and diminishes the risks of cross species transmission. SRLV strains confined to a single species are expected to be more homogeneous than if transmitted between different host species with the need to readapt each time. Alternatively, our small sample size might under represent the true variety of SRLV strains circulating in small ruminants of this country. A further explanation could come from the primers used in our study, which might have selected specific strain subgroups. Although we think of this last possibility as being highly unlikely since we used combinations of degenerate primers from conserved sequences, it cannot be completely ruled out. Future large-scale studies including more flocks from different geographical regions and breeds in addition to mixed flocks are warranted and might unveil a more thorough picture of the strain diversity present in the small ruminant population of this country.
All novel SRLV sequences reported in this study are available in [GenBank: HQ158122 - HQ158136]
This work was supported by the science division of the CFIA. The authors are thankful to the farmers who agreed to participate in this study by providing animal samples.
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