IS900 PCR TO DETECT MYCOBACTERIUM-PARATUBERCULOSIS IN RETAIL SUPPLIESOF WHOLE PASTEURIZED COWS MILK IN ENGLAND AND WALES

IS900 PCR for Mycobacterium paratuberculosis was applied to cream, whe y, and pellet fractions of centrifuged whole cows' milk, The test and simultaneous control reactions gave correct results for spiked milk an d for native milk samples obtained directly from M, paratuberculosis-f ree, subclinically infected, and clinically infected cows, The test wa s then applied to units of whole pasteurized cows' milk widely obtaine d from retail outlets throughout central and southern England from Sep tember 1991 to March 1993, With peak periods in January to March and i n September to November, when up to 25% of units were affected, an ove rall 22 of 312 samples (7%) tested positive for M, paratuberculosis, I n 18 of the 22 positive samples (81%), the PCR signal segregated to th e cream or pellet fractions or both, consistent with the presence of i ntact mycobacteria, Nine of 18 PCR-positive milk samples (50%) and 6 o f 36 PCR-negative milk samples (16%) yielded long-term liquid cultures which tested positive for M, paratuberculosis after 13 to 40 months o f incubation, despite overgrowth by other organisms, Taken together wi th data on the prevalence of M, paratuberculosis infection in herds in the United Kingdom, the known secretion of M. paratuberculosis in mil k from subclinically infected animals, and the inability of laboratory conditions simulating pasteurization to ensure the killing of all the se slowly growing or unculturable organisms, there is a high risk, par ticularly at peak times, that residual M. paratuberculosis mill be pre sent in retail pasteurized cows' milk in England.