CAPE TOWN, South Africa – One million people have been infected by mycobacterium bovis over the past decade, scientists said here this week. The zoonotic strain of tuberculosis, acquired from livestock, is clinically indistinguishable from the most common airborne strain, but results in longer, more costly treatments, more drug resistance, and more deaths than mycobacterium tuberculosis, scientists said.
Because little is known about bovine tuberculosis, countries are not actively looking for it, Michel Miller of Stellenbosch University, said. The World Health Organization conservatively estimates that 1.4 percent of tuberculosis in humans is acquired from livestock and wildlife, which means 126,000 out of nine million people who become infected with active tuberculosis each year acquire it from animals, Miller said.
Over 60 species of animals are known to get zoonotic strains of tuberculosis, Miller said, but the greatest threat to humans comes from bovine tuberculosis thanks to a “shrinking world with increased interface between humans and livestock.” Consuming unpasteurized dairy products from infected livestock can infect humans, Miller said, and people who rely on livestock for both food and income in rural areas of Africa are particularly vulnerable to bovine tuberculosis.
Researchers presented a study at the Union conference that showed higher rates of bovine tuberculosis among Nigerian cattle workers, with 5.3 percent of 266 workers screened to have positive tuberculosis cultures.
But rural African farmers aren’t the only ones at risk: in 2004, dozens of New Yorkers were infected with tuberculosis, and one infant died, after consuming contaminated unpasteurized cheese.
Infection can also occur through environmental contamination, Miller said, “As we squeeze wildlife into smaller space, we have a greater risk of encountering their pathogens.” Tuberculosis infection through contaminated feces of badgers in the United Kingdom is something that was brought up over and over again throughout the conference, often in a joking sense.
Infection from bovine TB can also occur through airborne transmission, just like mycobacterium tuberculosis, said Colleen Scott of the Centers for Disease Control and Prevention.
Inadequate knowledge of the pathogenesis and bacteriology of zoonotic tuberculosis leads to patients receiving improper diagnosis and inadequate care, especially in developing countries, Paul van Helden, director of Stellenbosch University’s medical research center, said. Most infections from bovine tuberculosis result in extrapulmonary tuberculosis in humans, he said, which “makes it more difficult to diagnose, so it can easily be missed.”
Bovine TB is “intrinsically drug resistant,” van Helden said, as it is resistant to pyrazinamide, a key first-line medicine used in the standard TB treatment regimen. Misdiagnosing mycobacterium bovis for mycobacterium tuberculosis and placing a patient on pyrazinamide will increase the risk of treatment failure and of developing resistance to other TB medicines, he said.
Treatment for bovine TB is nine months compared to six months for drug susceptible TB, and there is a “great risk for people who have bovine TB to have multidrug-resistant TB,” van Helden said. People infected with mycobacterium bovis are two times more likely to die from it than from mycobacterium tuberculosis, he said.
Zoonotic tuberculosis should be seen as a global health problem, Miller said, and more attention should be paid to addressing zoonotic infectious diseases. “Ebola came from animals,” she said. “Some of the worst diseases we’ve had in human kind came from animals.”
“When anticipating the next pandemic, we have to look closely at animals.”