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

Distinct strains of Mycobacterium avium subsp. paratuberculosis with a tend ency to segregate in either sheep, or cattle and other ruminants, have been described and are known as S and C strains, respectively. These strains ca n be distinguished by a polymorphism in the IS1311 element and other DNA-ba sed methods. C strains are relatively easy to culture from tissues and faec es 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 undert aken to determine whether infection in these cases was due to S strains. Po lymerase chain reaction and restriction endonuclease analysis of the amplif ied product was used to identify the polymorphism in IS1311. Three cases of bovine paratuberculosis due to S strain were confirmed from three differen t farms. A serological survey led to the identification of a further two ca ses on one of these farms. S strains were also identified in archival tissu es from paratuberculous sheep and cattle from Iceland, confirming epidemiol ogical and microbiological evidence that paratuberculosis in Iceland was du e 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 re peated infection from sheep had occurred. Limited epidemiological evidence suggests that transmission of S strains to cattle in Australia has been unc ommon under extensive grazing conditions. In Iceland, different husbandry p ractices appear to have favoured transmission of S strains to cattle. (C) 2 001 Elsevier Science B.V. All rights reserved.