The interconnected, growing threat of TB in animals and humans
Back in 2016, the death of a certain African elephant bull in the Kruger National Park (KNP) left researchers puzzled.
Rangers found the animal’s carcass near the Tshokwane camp in the southern part of the park. The elephant, estimated to be approximately 45 years old, had no external wounds or visible injuries, but its condition was poor.
Initially, scientists suspected the cause of death to have been bovine tuberculosis (TB), caused by Mycobacterium bovis. Bovine or animal TB is a significant, infectious disease affecting livestock and wildlife populations. M. bovis was diagnosed in the park in 1990 and is now endemic to this conservation area.
But this case was different. Genomic sequencing and analysis showed that the elephant had suffered from human TB caused by Mycobacterium tuberculosis, which is transmissible from humans to animals. This is the same TB bacteria that cause serious illness in humans worldwide and affect predominantly the lungs.
This was the first time researchers had identified a fatal case of human TB in a free-ranging African elephant.
The finding suggested that M. tuberculosis could be a greater threat to wildlife populations in Africa than previously realised.
“We were puzzled by this,” recalls Prof Michele Miller. “How is it possible for wild animals to get infected with human TB in the KNP?” Miller, who heads up the Animal TB Research Group at Stellenbosch University (SU), holds the DSI-NRF South African Research Chair (SARChI) in Animal TB.
To answer this and other questions, Miller and her team are investigating bovine TB in the KNP and other conservation areas, including the Hluhluwe–iMfolozi Park in KwaZulu-Natal. They are also doing research in rural communities living near or adjacent to protected areas where bovine TB has been documented.
The researchers, based in SU’s Division of Molecular Biology and Human Genetics in the Faculty of Medicine and Health Sciences, are determined to understand exactly how the transmission of TB occurs between wildlife, livestock, and people.
One of their research endeavours, for instance, is an elephant surveillance programme that involves screening the KNP’s elephants for mycobacteria. Since 2016, no more cases of human TB in elephants have been detected. Five elephants have, however, been diagnosed with animal TB.
A multi-host of concern
Not simply a single pathogen at work
TB is often thought of as a disease that affects only humans when, in fact, it is a multi-host disease [that affects many species],” Miller points out.
Also, there is not simply a single pathogen at work — several of the different species of mycobacteria that belong to the TB complex could be responsible for causing the disease in animals.
Every case of TB is caused by related bacteria within the Mycobacteria tuberculosis complex (MTBC). These pathogens include M. tuberculosis(the causative agent of human TB) and M. bovis (which causes bovine TB).
Most mammal species can be infected with bacteria in the MTBC complex, M. bovis being the primary cause of TB in wildlife and livestock.
Currently, 25 wildlife species in South Africa — including rhinos, cheetahs, lions, and buffalo — are known to be affected by animal TB.
This chronic disease has also been increasingly detected in new species and at a seemingly high prevalence rate among buffalo, wild dogs, lions, and warthogs. This can have far-reaching consequences for conservation, the livestock industry, and public health.
Miller says the KNP offers “a real-world” example of how introduced TB bacteria can impact free-ranging populations of wildlife that interact with domestic animals and humans.
“It is a global problem, however,” Miller says. “There are literally tens of thousands of TB-infected animals all over the world.”
Old problem, new challenges
Origens of TB in KNP
It is believed that bovine TB was brought to South Africa by colonial settlers importing infected cattle in the 19th century. “Primarily, it has been a disease among domestic cattle,” Miller says. “It is only since the 1990s that we’ve really been aware that it can spill over from domestic animals into wildlife in the KNP.”
Centuries ago, as high as 20% of human cases of TB were caused by transmission from cattle, primarily via unpasteurised milk. This is one of the reasons why TB is, today, an actively controlled disease among animals. “Although surveillance and monitoring programmes have reduced TB prevalence in cattle, animal TB is rearing its head again,” Miller says.
Several factors determine whether a population can maintain and spread animal TB infection. These include the presence of enough susceptible hosts, how long the bacterium infects its hosts without killing it (often dependent on the status of the host animal’s immune system), and whether the bacteria can remain viable long enough to find new hosts.
Although animals are typically infected with M. bovis and humans with M. tuberculosis, reports are increasingly noting reverse infections from animals to humans and vice versa, Miller says. “There is a gap in our knowledge as to what’s leading to these infections jumping between species.
“What we are seeing is that in a wildlife park, M. bovis crosses the border from buffalo herds to goats and cattle, to soil and water, and subsequently to humans," she says. “People with a high TB burden who, for example, care for captive elephants can also spread human TB to these animals.”
SU researchers leading the pack
Scientists in the group have reported research showing that the critically endangered African wild dog population in the KNP has a high apparent prevalence of M. bovis infection (82%), with associated mortalities.
In 2022, the group and its research collaborators showed that about one in every seven rhinos in the park was infected with M. bovis. Their findings indicated an estimated prevalence of bovine TB infection of 15,4% across black and white rhino populations.
“We are finding that TB bacteria are really, really hardy. They can remain alive in soil or around water holes for days to months, depending on the conditions.” — Prof Michelle Miller, SU
A recent surveillance study investigating antibodies against M. tuberculosis complex antigens in elephants estimated the level of infection in the KNP’s population to be between 6% and 9%, suggesting that M. bovis and possibly M. tuberculosis infection are more common than previously thought.
The Animal TB Research Group is also part of a global consortium that recently received funding from the European Commission to expand genomics surveillance to include the main pathogens affecting people and animals in South Africa, Mozambique, and Kenya.
This SU-led consortium aims to increase the use of genomic epidemiology to address public health issues such as TB, HIV-1, and antimicrobial resistance in these three countries. The consortium will use a “One Health” approach to conduct early-warning testing in wastewater, and animal surveillance to detect emerging pathogens.
The Animal TB Research Group is leading the consortium’s work on environmental and animal sources of TB. This includes investigating the role of host genetics and immunology in animals’ susceptibility to bovine TB, the genetic diversity of pathogens and their impact on wildlife and livestock, and the development of diagnostic assays for different host species.
Members of the Animal Tuberculosis Research Group preparing for field blood collection and TB testing of buffalo in the Hluhluwe-iMfolozi Park | Photo by Prof Michelle Miller
Members of the Animal Tuberculosis Research Group preparing for field blood collection and TB testing of buffalo in the Hluhluwe-iMfolozi Park | Photo by Prof Michelle Miller
One Health approach
South Africa is one of the countries experiencing significant problems around human and animal TB, starting with its diagnosis.
SU has a wide portfolio of TB expertise, ranging from that in microbiology, vaccine testing, drug development, and immunology to genetics.
The Animal TB Research Group’s specific focus is the immunology, epidemiology, management, and control of TB in animals, as well as aspects that impact the human-animal interface. The group has adopted the integrated, collaborative, multisectoral, and transdisciplinary One Health approach, which aims to sustainably balance and optimise the health of people, animals, and ecosystems.
Since both animal and human TB can spread across the species boundaries, it is crucial to employ this approach when it comes to detecting, understanding, and controlling TB.
A key area of research in the group is the development of molecular and cellular techniques for detecting infection in animals. This is important, given that bovine TB not only has a severe impact on the health of animals but also has major economic implications.
SU has a wide portfolio of TB expertise, ranging from that in microbiology, vaccine testing, drug development, and immunology to genetics.
The Animal TB Research Group’s specific focus is the immunology, epidemiology, management, and control of TB in animals, as well as aspects that impact the human-animal interface. The group has adopted the integrated, collaborative, multisectoral, and transdisciplinary One Health approach, which aims to sustainably balance and optimise the health of people, animals, and ecosystems.
Since both animal and human TB can spread across the species boundaries, it is crucial to employ this approach when it comes to detecting, understanding, and controlling TB.
A key area of research in the group is the development of molecular and cellular techniques for detecting infection in animals. This is important, given that bovine TB not only has a severe impact on the health of animals but also has major economic implications.
Animal TB researchers working with SANParks veterinary wildlife staff to process field samples in the Kruger National Park. Pictured from left to right: Dr Léanie Kleynhans, Dr Tanya Kerr, Tebogo Manamela and Dr Wynand Goosen | Photo by Prof Michelle Miller
Animal TB researchers working with SANParks veterinary wildlife staff to process field samples in the Kruger National Park. Pictured from left to right: Dr Léanie Kleynhans, Dr Tanya Kerr, Tebogo Manamela and Dr Wynand Goosen | Photo by Prof Michelle Miller
Using science to find solutions
Miller says infected animals in South Africa are slaughtered rather than treated to avoid creating drug-resistant strains of the pathogen. “M. bovisis already naturally resistant to one of the key drugs used for treatment in people.
Photo showing an immobilised African elephant in the Kruger National Park undergoing testing for tuberculosis, using respiratory endoscopy | Photo by Peter Buss
Photo showing an immobilised African elephant in the Kruger National Park undergoing testing for tuberculosis, using respiratory endoscopy | Photo by Peter Buss
“The other reason is that we don’t want to keep infected animals that can continue to transmit the disease around. So, generally, when you have a TB control programme for cattle, infected animals are culled to break the cycle of transmission."
Tuberculosis (TB) in humans cannot be eradicated unless TB in animals is also eliminated, as these two diseases are closely interconnect.
The interconnected, growing threat of TB in animals and humans
This article was originally published in Research for Impact, a digital publication produced by SU's Division for Research Development.
Built with Shorthand