Molecular epidemiological confirmation and circumstances of occurrence of sheep (S) strains of Mycobacterium avium subsp. paratuberculosis in cases of paratuberculosis in cattle in Australia and sheep and cattle in Iceland.

Distinct strains of Mycobacterium avium subsp. paratuberculosis with a tendency to segregate in either sheep, or cattle and other ruminants, have been described and are known as S and C strains, respectively. These strains can be distinguished by a polymorphism in the IS1311 element and other DNA-based methods. C strains are relatively easy to culture from tissues and faeces of animals with paratuberculosis but S strains are difficult to culture. A retrospective survey of archival formalin-fixed paraffin-embedded tissue samples from culture negative Australian paratuberculous cattle was undertaken to determine whether infection in these cases was due to S strains. Polymerase chain reaction and restriction endonuclease analysis of the amplified product was used to identify the polymorphism in IS1311. Three cases of bovine paratuberculosis due to S strain were confirmed from three different farms. A serological survey led to the identification of a further two cases on one of these farms. S strains were also identified in archival tissues from paratuberculous sheep and cattle from Iceland, confirming epidemiological and microbiological evidence that paratuberculosis in Iceland was due to S strain following importation of infected sheep from Europe. In each bovine case in both Iceland and Australia there had been direct or indirect contact of calves with paratuberculous sheep. We were unable to determine whether S strains had established endemic infection in cattle or whether repeated infection from sheep had occurred. Limited epidemiological evidence suggests that transmission of S strains to cattle in Australia has been uncommon under extensive grazing conditions. In Iceland, different husbandry practices appear to have favoured transmission of S strains to cattle.