Researchers have highlighted a risk of transmission of Mycobacterium bovis via raw milk and dairy products made using such milk.

Bovine tuberculosis, caused by Mycobacterium bovis, is an animal disease and the bacteria can be transmitted to humans via the consumption of unpasteurized, raw milk.

Scientists reviewed published studies to estimate the rate of Mycobacterium bovis in on-farm bulk tank milk and individual cow’s milk. Findings were published in the journal Tuberculosis.

Mycobacterium bovis prevalence in cow’s milk, regardless of animal infection status, was estimated at 5 percent. Prevalence from tuberculin skin test positive cows was estimated at 8 percent.

Prevalence in bulk tank milk, independent of herd infection status, was estimated at 5 percent.

Such estimates can be used to help inform risk assessments on the potential risk of zoonotic tuberculosis from unpasteurized milk and dairy products made using raw milk. These assessments can help guide policy decisions relating to the prevention and control of the issue, said researchers.

Factors impacting results
Overall, 67 articles comprising 83 studies published between 1980 and 2021 were included in a meta-analysis.

Studies were principally in countries with endemic and generally uncontrolled bovine tuberculosis in cattle, and few were from regions such as Ireland or the UK where there are national eradication programs. Other countries included Egypt, Brazil, India, China, Argentina and Nigeria.

In Ireland, an average of six cases of zoonotic tuberculosis were reported annually between 2006 and 2018. It is not clear if these cases are domestic or imported, or whether there is a link between them and the prevalence of bovine tuberculosis in the Irish cattle herd, said scientists.

No work provided an estimate for the number of Mycobacterium bovis bacteria in milk. Several reported detection of Mycobacterium tuberculosis and Mycobacterium africanum.

Sampling strategies used to collect milk samples were rarely described, and when they were, the quality of reporting was poor. Also, in the selected studies, the tuberculin skin test infection status of the individual cow or herd was frequently not reported or was unclear, found the study.

Information about the infection status of a herd or animal at the time of milk sampling and the infection history of the herd over the past several years are important, given the impact of the stage of infection and disease on the shedding of Mycobacterium bovis in milk, said scientists.

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