September 15, 2005


 Teasing out Mycobacterium bovis' role in the tuberculosis crisis

Posted Sept. 1, 2005 

More attention needs to be paid to the role of Mycobacterium bovis in the global human tuberculosis crisis. In some countries, this pathogen may be playing an important role in a disease the World Health Organization has designated a global emergency. This was the message that Donald Enarson, MD, sent to his audience at a July 16 AVMA convention session.

"There's a great deal we don't know," said Dr. Enarson, who is director of scientific activities at the International Union Against Tuberculosis and Lung Disease.

According to the WHO, tuberculosis is the biggest killer of young people and adults in the world today. The disease kills 8,000 people a day, 2 to 3 million people each year.

Most cases of tuberculosis are caused by Mycobacterium tuberculosis, which is largely restricted to humans. Mycobacterium bovis, however, can also cause the disease, and it is sneakier. It can infect a wide range of animals, including humans. The pathogen is transmitted through contaminated milk and meat, or via aerosols from infected animals.

One striking statistic providing evidence for the impact that M bovis can have on human tuberculosis cases comes from data gathered in the Netherlands. In 1942, scientists noticed a sharp drop in the number of tuberculosis cases in that country, and they attributed the drop to the pasteurization of milk, which became compulsory that year. The drop was also accompanied by a marked reduction in the proportion of childhood cases in which M bovis was isolated. In 1933, M bovis accounted for 10 percent of isolates from pulmonary cases of tuberculosis and 17 percent of isolates from extrapulmonary cases. In 1950, these numbers were 1 percent and zero, respectively.

Currently, little is known about the distribution of M bovis or its role in the global human tuberculosis crisis. Dr. Enarson unearthed 14 reports in the past four decades from 11 countries that described the proportion of human tuberculosis cases in which M bovis was determined to be the source. The proportions ranged from 0.3 percent in Canada to 6.4 percent in Ireland. In the United States, it was 3 percent.

Although these numbers don't seem very high, Dr. Enarson suggested that such statistics may be deceiving, because the countries on the list are not the countries where M bovis is most common or where tuberculosis is striking the hardest.

Tuberculosis is not a democratic disease; it doesn't affect everybody equally. Mycobacterium bovis behaves similarly, in that it is a problem in all parts of the world, but affects Africa more than other regions.

Mycobacterium bovis is known to be enzootic in cattle in many African countries including Algeria, Burkina Faso, Burundi, Ghana, Madagascar, Malawi, Mali, and South Africa. It is also enzootic in Bahrain and Tunisia in the eastern Mediterranean, as well as Argentina, Bolivia, Chile, the Dominican Republic, Guatemala, Peru, El Salvador, and Venezuela. The pathogen is reported to cause a sporadic disease in 17 countries in Africa, 11 countries in the eastern Mediterranean, seven countries in Asia, nine countries in the Americas, and three countries in Europe.

Thus, although M bovis is widely distributed, it appears to have a predilection for Africa, where the tuberculosis epidemic is out of control. And, with worsening poverty in many areas of Africa, a coinciding epidemic of HIV, which increases tuberculosis transmission, and the emergence of drug resistance, the problem is likely to get worse.

The WHO says that most African countries with high rates of tuberculosis are losing ground against the infectious disease. Dr. Enarson wonders what role M bovis could be playing in this public health crisis.

"M bovis may have a huge role and nobody is paying attention," Dr. Enarson said. He says doctors need to test individuals with tuberculosis to determine which pathogen is the source of infection.

"We don't know where we are, and we lack the will to go out and find out," he said.