In December, a tourist in the Kruger National Park saw an African wild dog attack the tyres of a car. She called park officials thinking the animal had rabies, but veterinarians have since confirmed that the dog had another, unexpected, disease: tuberculosis (TB).
Researchers have identified TB in 24 animal species, from mongooses to elephants, but this was the first time that tuberculosis in African wild dogs had been documented.
The implications of the diagnosis are far-reaching. African wild dogs are an endangered species. Park officials estimate the wild dog population in Kruger National Park to be 450 to 500 animals and the International Union for Conservation of Nature says the global population of this endangered species is a mere 1 400 mature adults.
Wild dogs are adept hunters, but depend on the pack for survival. Some packs number more than 40 and, because tuberculosis is highly contagious, one dog with the disease is likely to infect several others.
The head of the Centre for Tuberculosis Research at the South African Medical Research Council, Professor Paul van Helden, says the infection in rare species is concerning. "In the case of wild dogs, when the pack size drops too low, the remaining animals won't survive. They become very vulnerable."
Van Helden says the discovery of tuberculosis in the Kruger Park wild dog was also unusual because it occurred in a young animal.
Tuberculosis is a relatively slow-developing disease and normally manifests in older animals. "The age of this animal shows that the disease moved quickly," he says.
Animals in close contactStudies have shown that animals in close contact with each other can transmit respiratory TB through the air (probably in buffalos), grooming (baboons) or bites; extrapulmonary TB (tuberculosis found in other parts of the body) can be contracted when carnivorous animals (such as lions) eat diseased meat.
Professor Michele Miller, the research chair in animal tuberculosis at Stellenbosch University, says wildlife, particularly buffalo, in the Kruger National Park is likely to have become infected with respiratory TB from infected cattle grazing on neighbouring property, or through the environment.
"Once the host becomes infected, that animal can spread bacteria through contact or by contaminating the environment [with urine and faeces]," says Miller. "It's a significant problem." TB bacteria can be shed around water holes, and if it's in shady, moist soil, the bacteria can remain contagious for months.
Miller says the problem with tuberculosis in wildlife is that the animals do not show the typical signs of disease such as weight loss, loss of condition and lethargy until the disease is advanced.
"During this time they are continually transmitting the disease to other animals and contaminating the environment," says Miller. "Their loss of health means they are slower and less likely to keep up with the pack or herd, making them more vulnerable to predators than healthier animals."
The disease can be spread further when the carcass of the infected animal is eaten by other animals.
Van Helden says tuberculosis in wild animals is relevant to people for several reasons: from an ecosystem conservation perspective, from a tourism industry point of view and for their personal health.
TB from people to animals"Tourism attracts people looking for jobs. [Some of] these people bring their cattle to the conservation areas and this is also where cattle mix with animals as well as other people," he says. "It's especially populations with high HIV infection rates that are vulnerable, as people with HIV are very susceptible to developing TB."
According to the World Health Organisation, HIV-infected people are 30 times more likely to contract tuberculosis than those who have not acquired the virus.
A study conducted between 2013 and 2015 by researchers at the department of veterinary public health and preventive medicine at the University of Ibadan, Nigeria, and presented at the 46th Union World Conference on Lung Health in December, suggests that, in Nigeria, more than 10% of cows and 43% of herds have tuberculosis.
Research has shown that tuberculosis can be transmitted from animal to animal or animal to human through inhaling TB-contaminated air as well as from animal to human when people eat contaminated dairy products such as milk and cheese.
Tuberculosis can also be passed from people to animals. According to a 2012 study published in the journal Research in Veterinary Science, a domestic dog in a Cape Town suburb contracted respiratory TB from people who had the disease.
Cattle infected with tuberculosis are generally slaughtered. But in many low-income countries there is no financial compensation to the owner for the animals, so cases of bovine tuberculosis are often not reported, says Professor Simeon Cadmus, a veterinary public health specialist from of the University of Ibadan. He says tuberculosis spreads rapidly when pastoralists sell unpasteurised milk and when infected, undercooked meat is sold and eaten.
Resistant to first-line medicineAccording to a 2012 study conducted at Nigeria's main cattle trading market and published in the International Journal of Tuberculosis and Lung Disease, 10% of livestock traders in Nigeria are infected with some type of TB.
The strain of TB transmitted by animals and caused by Mycobacterium bovis is resistant to pyrazinamide, a key first-line medicine used in standard tuberculosis treatment for humans. According to the International Union Against TB and Lung Disease, using pyrazinamide to treat patients with bovine tuberculosis increases the risks of treatment failure and of patients developing resistance to other TB medicines.
The union estimates that one million people have been infected by animal-transmitted tuberculosis in the past 10 years.
"This number is likely to represent the tip of the iceberg," says Dr Francisco Olea-Popelka, a veterinary epidemiologist at the College of Veterinary Medicine and Biomedical Sciences at Colorado State University in the United States. "We need to broaden our concept of TB to prevent and control TB at the animal source to prevent transmission to humans."
Tuberculosis is the world's leading cause of death from infectious disease and kills more people than HIV, according to the World Health Organisation (WHO).
The organisation's 2015 Global TB Report estimates that 9.6-million people in the world fell ill with tuberculosis in 2014. Of those, 1.5-million died from the disease.
In most cases the TB bacterium, Mycobacterium tuberculosis, is spread by airborne droplets when infected people cough. The most common form of TB, according to the WHO, is respiratory (pulmonary) tuberculosis.
People can get extrapulmonary tuberculosis when the bacteria travel in the bloodstream to other sites such as the lymph nodes, bones, the brain and spine. Tuberculosis can also be spread between animals, from animals to humans, and from humans to animals.
Professor Paul van Helden, the head of the Centre for Tuberculosis at the South African Medical Research Council, says two dozen species of animals are known to have developed TB and, in theory, any mammal can contract tuberculosis, develop the disease and pass it on to others.
Animals can pass on a different, but related strain of TB, caused by the bacterium Mycobacterium bovis, to humans through airborne droplets or through products such as undercooked meat, unpasteurised milk and cheese.
TB infections, particularly in rural and wild animals, are seldom controlled and this increases the disease risk to animals and people.
The United Kingdom's public health department says there is a "significant reservoir" of TB infection among badgers in the west of England and Wales. Badgers mark their territory with urine on grass, which is then eaten by cattle.
Badgers in urban areas can also pass on the disease if they fight with domestic cats or dogs, and the pets then pass it on to people when they lick or scratch them, cough near them, or through their faeces and urine.
In Africa, according to a 2013 study in the Emerging Infectious Diseases journal, Mycobacterium bovis is estimated to cause 3% of pulmonary tuberculosis cases. The study found that Ethiopia, Nigeria and Tanzania have higher proportions of zoonotic (animal) TB, with 17%, 15% and 26% respectively of all TB cases caused by the bovine strain.
Joanne Lillie is a freelance journalist. She attended the International Union Against Tuberculosis and Lung Disease's conference in Cape Town on a conference fellowship
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