IS900 PCR To Detect Mycobacterium paratuberculosis in Retail Supplies of Whole Pasteurised Cows' Milk in England and Wales.

This paper was published by Hermon-Taylor et al., the discoverers of the IS900 insertion sequence, a genetic sequence which uniquely identifies Mycobacterium paratuberculosis. It describes the testing of milk bought at retail outlets across south and southwest England and south Wales. The purpose of the test is to determine if M. paratuberculosis is present in the milk, and if it has been killed by pasteurisation.

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Summary.

The main points presented in the paper are as follows.

• Overall, 7% of all milk samples tested were IS900-positive, i.e. they contained DNA from M. paratuberculosis. 312 samples were tested, and 22 were IS900-positive.
• At peak times of the year, up to 25% of samples tested postive for Mycobacterium paratuberculosis. These peak times were from January to March and from September to November. The January to March period coincides with a time of stress for cattle, since the calfing season is imminent. The September to November period is also stressful for cattle, due to the onset of Winter. At other times of the year, no samples of milk tested positive for the organism. This shows that the presence of the organism in milk follows a pattern which is seasonal.
• Eighteen samples of milk which tested positive for Mycobacterium paratuberculosis were cultured, i.e. placed in a culture dish containing an environment conducive to the growth of M. paratuberculosis. Nine of these cultures yielded active Mycobacterium paratuberculosis organisms. This proves that the organisms had not been killed by pasteurisation.
• Thirty six samples of milk which which tested negative for Mycobacterium paratuberculosis were cultured. Six of these cultures yielded active M. paratuberculosis. This demonstrates that these six samples contained M. paratuberculosis, but were not detected by the original IS900 test. This shows that the test under-reported the number of contaminated samples.
• A telephone survey of farms and veterinary practices was conducted. It was found that 1% of farms had cattle infected with BJD (Bovine Johne's Disease) and about 2% of cattle numbers had the disease. Further, a recent study using IS900 PCR on cattle at slaughter found that 3.5% were infected. This difference between the 3.5% and 2% figures is attributable to "sub-clinically" infected cows, i.e. cows that are infected with M. paratuberculosis but do not show any symptoms. Sub-clinically infected cows are known to shed M. paratuberculosis in their milk.
• To summarise all of this, the paper states: "There is a high risk, particularly at peak times, that residual M. paratuberculosis will be present in retail pasteurized cows milk in England."

Source: http://archive.crohn.ie/htmilk.htm
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