May 6 - TB Strain May Be Linked to Unpasteurized Dairy

The incidence of a strain of tuberculosis (TB) called Mycobacterium bovis, or M. bovis, associated more often with cattle than humans, is increasing in San Diego and is concentrated mostly in Hispanics of Mexican origin, according to a study conducted by researchers at the University of California, San Diego School of Medicine in collaboration with San Diego County public health officials. Their analysis shows that changing patterns of TB in the United States are increasingly being driven by conditions outside of the country, especially in binational communities.

Lead author Timothy C. Rodwell, MD, PhD, MPH, an associate physician and a fellow in the division of international health and cross cultural medicine at the University of California, San Diego, and his colleagues analyzed regional data for TB cases in San Diego County obtained from the Tuberculosis Information Management System database maintained by the San Diego County TB Control Program. In their review of 3,291 culture-positive cases of TB covering 1994 to 2005, M. bovis was isolated in only 8% of cases, but the strain accounted for 45% of TB cases in children under the age of 15, with almost all M. bovis cases from 2001 to 2005 found in persons of Hispanic ethnicity.

“This strain of TB is thought to be primarily spread to humans through consumption of raw dairy products from infected cattle, with only minimal human-to-human contagion,” Rodwell says. “Some raw dairy products from Mexico, for instance, unpasteurized cheese like the popular queso fresco, have been found to contain M. bovis and should be considered unsafe.”

Because of the widespread adoption of pasteurization of all commercially available dairy products in the United States, along with aggressive programs designed to keep dairy cattle in this country free of the disease, the threat of M. bovis in U.S. dairy products was largely eliminated in the mid-20th century. The San Diego-Tijuana binational region, however, shares one of the busiest border crossings in the United States with the Mexican state of Baja California, where M. bovis is prevalent in cattle and consumption of unpasteurized dairy products is common.

The researchers found that more than 90% of M. bovis cases in San Diego occurred in Hispanics, most born in Mexico, Rodwell says. He adds that collaborations with Mexico on prevention strategies including education and regulation of unpasteurized dairy products, along with elimination of the disease from dairy cattle would be required long term to ensure that this mode of transmission of TB is stopped.

“The changing face of TB in San Diego County may reflect a new pattern of the disease in the United States,” Rodwell says.

Source: University of California, San Diego Health Sciences

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