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Mapping the genome of a cattle killer
  
By Kate Dalke

Scientists have sequenced the genome of the bacterium that causes Johne's disease. Mycobacterium paratuberculosis infects the intestines of cows and other hoofed animals. The bacteria can lie dormant for years before causing full-blown disease, during which time they can spread to healthy animals in a herd through contaminated milk or manure.


M. paratuberculosis bacterium.

"You may not know it but Johne's disease is on the rise," says Michael Collins of the University of Wisconsin-Madison, who has studied the disease for decades. Some 20 percent of dairy herds in the U.S. are infected with the bacterium, according to the US Department of Agriculture.

The rise of large cattle farms in the United States has meant that dairy cows from different herds are mixed, often with insufficient knowledge of the animals' health. "As cattle move into larger herds, they bring the infection with them," says Collins. Without another solution, infected cows must be slaughtered to prevent the spread of disease.

The University of Minnesota in Minneapolis/St. Paul and US Department of Agriculture's National Animal Disease Center collaborated on the sequencing project. They have identified 20 newly discovered bacterial genes that could be used to develop better tests for detecting the disease earlier and more accurately, says John Bannantine of the USDA.

The genes may help researchers pinpoint infection early on. Infected animals could then be separated from the herd to prevent further disease. Big and small farms alike should carefully test newly purchased animals for Johne's and other diseases, says Collins.

Less than half of all dairy farmers even know about Johne's disease, according to the USDA. "Some farmers don't want to know that their cows have the disease," says Bannantine, because bacterial infections can lower the market value of cattle.

Scientists are also using gene chips to detect genes that the bacteria 'turns on' to infect its animal host. Someday, researchers may be able to disable these genes and inhibit the bacterium. Besides cows, the bacteria infect other hoofed animals like sheep, goats and deer.


H.A. Johne

The bacterium that causes Johne's is named for H.A. Johne, the German veterinarian who first described the disease in 1895. Johne's disease is found throughout the world. It is most closely related to a bacterium that causes tuberculosis in birds, but is also related to tuberculosis in humans and cows.

The most controversial issue surrounding Johne's disease is its link, if any, to Crohn's disease, a human inflammatory bowel disease with no known cause or cure. In 1984, scientists isolated M. paratuberculosis from the tissue of a patient with Crohn's disease.

Further studies have found no conclusive evidence linking the livestock and human diseases. And no studies show that M. paratuberculosis can be passed on to humans through milk or contact with contaminated manure.

Nevertheless, because it is an intriguing lead, the US National Institutes of Health will award an estimated $5.1 million to investigate Crohn's disease in 2003. Some of the research will focus on the relationship between Crohn's disease and M. paratuberculosis.

The bacterium's genome contains roughly 4,500 genes on one circular chromosome. The researchers completed an analysis of the sequence and plan to publish their findings later this year.

Results of genome sequencing are available at
http://pathogenomics.umn.edu

To learn more about Johne's disease
www.johnes.org

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