Abstract & Commentary
Synopsis: Mycobacterium tuberculosis caused an outbreak of disease and death in free-ranging wildlife in Botswana and South Africa. Such infections may serve as a reservoir for possible transmission to humans.
Source: Alexander, KA et al. Mycobacterium tuberculosis: An emerging disease in free-ranging wildlife. Emerg Infect Dis. 2002;8:598-601.
Alexander and colleagues report an outbreak of Mycobacterium tuberculosis in free-ranging banded mongooses (Mungos mungo) and suricates (Suricata suricatta), are small, burrowing mammals more familiarly known as meerkats.
Behavioral ecologists began monitoring 12 troops of meerkats along the dry bed of the Kuruman River in South Africa from October 1998 to December 1999. This epizootic began when an unknown infected male meerkat with enlarged cervical lymph nodes joined the study group of 5 adults and 15 pups. A month later, the lymph nodes of the infected animal ruptured and began draining pus, eventually becoming a persistent nonhealing wound. Progressive cachexia and debilitation occurred until the animal eventually disappeared. Within 2 months, signs of disease such as lymphadenopathy, weakness, and emaciation appeared among other animals in the group. Eventually all members of the troop either died, disappeared (presumed dead), or were euthanized. One human case of tuberculosis (TB) was known to have occurred in the vicinity of the meerkat burrows. The affected animals were seen foraging around roads and "investigating" human sputum.
Another group of scientists identified an epizootic in banded mongooses at the northern extreme of Chobe National Park in Botswana from June to September 1999. Disease spread quickly to 6 different troops. The last case occurred in September 1999. No new cases appeared during monitoring through January 2001. Human garbage pits and a human TB case were near the initial outbreak sites. Banded mongooses were seen feeding regularly at these garbage pits.
Gross postmortem examination of the animals (1 meerkat, 7 mongooses) revealed lymphadenopathy and miliary masses in various organs including liver, spleen, lungs, and kidneys. Histopathologic examination showed granulomas in many organs as well as acid-fast organisms in the cytoplasm of macrophages.
Specimens from 1 mongoose and 1 meerkat grew acid-fast colonies on Lowenstein-Jensen media after 5-6 weeks. The isolates produced niacin and reduced nitrates, characteristics that are typical of M tuberculosis. Mycobacterium bovis is usually niacin-negative and does not reduce nitrates. In addition, these specimens were PCR amplified using a protocol of de Witt et al to differentiate M tuberculosis from M bovis.1 Findings were consistent with M tuberculosis.
Comment by Mary-Louise Scully, MD
Humans are the only reservoir for M tuberculosis whereas M bovis is widespread in domestic, captive and free-ranging wildlife populations.2 Previous reports have shown possible transmission of M tuberculosis in situations of captivity or close, prolonged contact between animals and humans, such as the 4 elephants and their trainer at an exotic animal farm in Illinois in 1996. The DNA fingerprint comparisons showed that the isolates were the same strain, suggesting transmission of M tuberculosis between humans and elephants.3
These epizootics reported in mongooses and meerkats occurred in the vicinity of human cases of TB though no further details on the human cases are available. The researchers suspected transmission occurred via an oral route via animal exposure to human excretions and secretions in the surrounding environment. An increase in these exposures seems inevitable. In 1999 alone, more than 89,000 visitors to Chobe National Park were recorded.4
Death and disease due to TB continue to increase, especially in developing countries where HIV is prevalent as well. In Botswana, the TB infection rate increased from 202 per 100,000 in 1989 to 537 per 100,000 in 1999. In addition, 36% of women receiving routine antenatal care in Botswana in 1999 were seropositive for HIV.5 Coexisting HIV and TB can influence the severity of TB infection and shorten the time from initial TB infection to development of overt disease, potentially increasing the amount of TB shed into the environment. Some evidence suggests that concurrent helminthic infections may decrease the host immune response to TB, further increasing the burden of TB in Africa and developing countries.6
Transmission of human disease to animals is not new, although public attention is often more focused on the reverse situation. In 1998, evidence strongly linked the death of 6 endangered mountain gorillas in Rwanda to human measles. The epidemic was abruptly stopped by the administration of measles vaccine to the remaining 65 healthy gorillas.7 As humans and animals share the world’s dwindling resources and habitats, more disease overlap will invariably occur. Even more unsettling is whether these infected animals could become new reservoirs for pathogens previously confined to humans. Clearly this would pose a new challenge in the attempt at TB and other disease eradication.
Dr. Scully is part of the Group Health Cooperative of Puget Sound, Seattle, Wash.
References
1. De wit D, et al. Direct detection of Mycobacterium tuberculosis in clinical specimens by DNA amplification. J Clin Microbiol. 1990;28:2437-2441.
2. Cosivi O, et al. Zoonotic tuberculosis due to Mycobacterium bovis in developing countries. Emerg Infect Dis. 1998;4:1-14.
3. Michalak K, et al. Mycobacterium tuberculosis infection as a zoonotic disease: Transmission between humans and elephants. Emerg Infect Dis. 1998;4: 283-287.
4. Visitor statistics for national parks and game reserves. Gaborone, Botswana: Department of Wildlife National Parks; 1999.
5. AIDS/STD Unit, Botswana Ministry of Health. Sentinel Surveillance Report, AIDS/STD Unit. Gaborone, Botswana: Ministry of Health; 1999.
6. Bentwich Z, et al. Can eradication of helminthic infections change the face of AIDS and tuberculosis? Immunol Today. 1999;20:485-487.
7. Ferber D. Human diseases threaten great apes. Science. 2000;289:1277-1278.
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