The distribution of Mycobacterium bovis and Mycobacterium tuberculosis, the agents of tuberculosis in animals and humans, is far more jumbled among host species than many people realize, according to participants of the symposium, "Transmission of Tuberculosis: Is TB more of a Zoonosis than We Thought?" convened as part of the 49th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC), held during September 2009 in San Francisco, Calif. Based on tuberculosis (TB) disease patterns on farms, zoos, and other settings, a substantial and growing number of species is susceptible to this disease, with both those pathogens crossing species barriers, albeit at a frequency that proves difficult to estimate.

 Throughout parts of South Africa, M. bovis appears to be moving among many wild species, including elephants, carnivores, and African buffaloes, with plenty of "spillover," according to symposium convener Paul Van Helden of the University of Stellenbosch in Tygerberg, South Africa. For instance, about 8% of local African buffaloes are infected with M. bovis, a figure that he calls "astronomical." These animals, although "big and dangerous," are "our lawn mowers," grazing savannah lands while serving as an important attraction for visitors to South African national parks, he says. However, sick animals are more vulnerable to predators, to which the disease spreads. "Predators concentrate the disease and look awful, which is not good for tourism," he says. Moreover, M. bovis-infected animals are a threat-albeit, so far, a weak one-to humans who work in the parks, he continues. "A lot of them, 80%, are skin-test positive, but most are not infected."

 The risk to animals from humans who are infected with M. tuberculosis appears to be the greater threat, according to Van Helden. The overall ratio of humans to animals is very high; some 27 million people in South Africa are infected with this pathogen, and no one can estimate what effect M. tuberculosis-HIV dual infection might have on the dynamics of this zoonosis, he says. "The risk to wildlife from mycobacterial disease is quite extreme, and we need to think about this real problem and give more protection to wildlife conservation areas."

 TB is increasingly common among a wide array of animal species at zoos, according to Michele Miller of Palm Beach Zoo in West Palm Beach, Fla. Sporadic outbreaks are difficult to handle, particularly because there are neither validated diagnostic procedures nor clinically verified therapeutic regimens for the range of TB-susceptible animals found in zoos, she points out. Skin tests, for example, are full of complications. "For elephants, the skin test doesn't work, for reindeer the results vary with season, and for other species there is anergy or nonspecific responses," she says. Drug treatments also produce complications. Treating camels for TB, for instance, led to bone marrow suppression and death. Although elephants with TB respond to drug treatments, experts now recommend treating them with three to five different drugs to avoid drug resistance.

 Agriculture is another setting where TB strikes animals and in which these pathogens move between species. M. bovis can be "highly virulent" in cattle, and can transfer from cattle to humans but rarely is transferred from one human to another, says symposium participant Glyn Hewinson of the Veterinary Laboratories Agency in Weybridge, Addlestone, United Kingdom. "Body builders who use steroids are more susceptible to M. bovis than are members of the general population, he says, noting that this path for the pathogen into humans remains "rare."

 The incidence of TB is very high in Ethiopia, where a high percentage of the labor force works in agriculture, Hewinson continues. Some farmers who follow intensive programs are now importing Holstein cattle because they yield 40-fold more milk than do indigenous species, he says. Those imported cattle, which are kept mainly indoors, have a high prevalence of tuberculosis. However, among herds kept outdoors, particularly the local variety known as Zebu, the TB prevalence is considerably lower, he says. Another worrying finding is that M. tuberculosis sometimes acts as a pathogen for cattle in Ethiopia. All in all, high-intensity farming practices and the introduction of Holstein cattle encourage a local zoonosis, in turn raising the risk of TB for humans in this part of Africa.

 Jeffrey L. Fox

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