Tuberculosis in humans is most commonly caused by Mycobacterium tuberculosis, a slow-growing, waxy, rod-shaped bacterium. Transmitted primarily via the air when an infected individual coughs or sneezes, it’s estimated that a third of the world is infected with this agent, which causes approximately 2 million deaths every year. Though most infections are asymptomatic, infection is becoming increasingly deadly, due both to the spread of highly antibiotic-resistant strains and due to the increasing number of individuals with both HIV and TB.
While M. tuberculosis is primarily a human disease, like the MRSA I mentioned yesterday, it has the potential to be zoonotic as well–to move between animals and humans. This has been documented for a number of animal species, but one of the best-studied examples happens to be elephants, of all things–including several housed at zoos and circuses. More after the jump.
First, I’ll note that M. tuberculosis isn’t the only possible agent of tuberculosis. Though it’s by far the most common cause of the disease in humans, it’s much more rare in animals. In non-human species, other mycobacteria (such as Mycobacterium bovis) are more common causes of TB. M. bovis is also a zoonotic pathogen, occasionally infecting humans via contact with infected animals or their products (such as unpasteurized milk from M. bovis-infected cattle). So, it’s not unheard of for animals to contract tuberculosis; it’s just infrequently caused by the M. tuberculosis species, which makes the elephant reports more interesting (and, given the sad state of elephant conservation, all the more sad as well).
Almost 10 years ago, an Emerging Infectious Diseases paper came out detailing M. tuberculosis as a zoonotic disease transmitted between people and elephants. This wasn’t the first report of M. tuberculosis infection in elephants; PubMed has articles that go back to at least 1963 mentioning tuberculosis in elephants (though that one’s in Russian with no abstract). However, this was the first to document transmission between elephants and humans, during an outbreak in 1996:
In March 1996, five elephants from an exotic animal farm in Illinois were in California as part of a circus act. One elephant (with chronic, unexplained weight loss since October 1995) died under anesthesia on August 3, 1996, during a diagnostic dental work-up. Necropsy showed widespread consolidation of lung tissue with caseous necrosis of the lungs and mediastinal lymph nodes. Short, fat, relatively scant numbers of acid-fast bacilli were observed in necropsy tissues. A presumptive diagnosis of M. tuberculosis was made. The remaining four elephants were recalled to the farm in Illinois. A second elephant died en route on August 6, 1996. Necropsy revealed copious respiratory and trunk exudates and caseous necrosis of the lung.
To determine the risk for and possibility of infection among the animal trainers and caretakers, an epidemiologic investigation was initiated. The remaining elephants in the herd and the elephant handlers and trainers who were still traveling were recalled to the farm and examined for evidence of M. tuberculosis infection. All elephants were empirically begun on antituberculous therapy in early December 1996.
In all, 12 of the employees had positive PPD tests (indicating prior exposure to M. tuberculosis, and M. tuberculosis was cultured from the sputum of one handler. When it was compared genotypically with isolates taken from 3 elephants who had died of TB and an additional animal that was infected but still alive, it was found to be identical or similar to the elephant isolates, suggesting a transmission event between the two species.
A later paper looked at several different animals suspected of M. tuberculosis infection at the Los Angeles zoo: 3 goats, 2 elephants, and a black rhinoceros. Bacterial isolates from all animals were obtained, and employees were screened for infection. 55 of them were found to have evidence of TB infection (a positive skin test), but none of them appeared to have an active infection when chest x-rays were taken.
Regarding transmission to the zoo-going public, the authors suggest:
Given the public’s distance from the animals and the absence of active TB among employees with closer contact with these animals, M. tuberculosis was likely not transmitted from humans to animals at this zoo.
However, in a circus environment, things may be different. Many circuses offer elephant rides or chances to touch the animals, providing opportunity to be in close contact with the animal (and putting circus-goers at a risk of contracting TB from infected animals). Leslie Griffith reports that a number of these elephants were found to carry Mycobacterium tuberculosis, including antibiotic-resistant strains, and that handlers were also infected–and that these incidents were minimized by the circus owner.
As far as I could tell by going through the published literature, no one in the general public has been identified as having contracted TB via an elephant or other infected circus or zoo animal, but it’s difficult to really tell what the risks are if the owners aren’t forthcoming about an animal’s TB status. Of course, if an animal tests positive, they’re supposed to be kept away from other animals, and certainly away from the general public, but an animal may be infected for some time before the infection is recognized. Additionally, even when it’s diagnosed, the owners may continue to use the animal rather than risk losing money. From everything I’ve read, a circus elephant’s life is pretty terrible to begin with; it’s even more troubling that one side effect of that life could be transmission of an infection to the unaware circus goers.
Michalak et al.. 1998. Mycobacterium tuberculosis infection as a Zoonotic Disease: Transmission between Humans and Elephants. Emerging Infectious Diseases. 4:283-7. Link.
Oh et al. 2002. Human Exposure following Mycobacterium tuberculosis Infection of Multiple Animal Species in a Metropolitan Zoo. Emerging Infectious Diseases. 9: Link.
Image from http://www.mizpahcircus.com/images/IMG_2568.jpg