The largest study ever to be conducted on a free-ranging population of rhinoceros, revealed that about one in every seven rhinos in the Kruger National Park (KNP) had evidence that they had been infected with Mycobacterium bovis (M. bovis) – the pathogen that causes bovine tuberculosis (bTB).
The study, conducted by Stellenbosch University's (SU) Animal Tuberculosis Research Group, South African National Parks (SANParks), and the San Diego Zoo Wildlife Alliance, USA, tested samples of 437 rhinoceros collected from 2016 to 2020 in KNP. It revealed an estimated prevalence of M. bovis infection of 15,4% in black and white rhino populations in the park.
While the research results are worrying, the evidence provided by the study is crucial to the effective conservation of the already vulnerable rhino population. Added to this, scientists with the Animal TB Research Group, situated within SU's Faculty of Medicine and Health Sciences, developed a novel diagnostic test to detect M. bovis in rhinos, which will greatly aid conservation efforts.
Infected, but asymptomatic
The researchers emphasise that the presence of infection does not mean that the animals are diseased or dying. Prof Michele Miller, who leads the Animal TB Research Group and is the National Research Foundation (NRF) South African Research Chair in Animal TB, says their research shows that most of the rhinos can contain the infection if they are otherwise healthy.
“It can be compared to humans who are infected with Covid-19 or have latent TB but are asymptomatic. The infected rhinos are harbouring the bacteria, but their immune system is keeping it in check. They are not losing weight or coughing, and if you looked at a group of 400 rhinos, you wouldn't be able to pick out those that are infected. They can potentially live for years with infection if it is contained."
Dr Peter Buss, Veterinary Senior Manager in KNP's Veterinary Wildlife Services, adds that there is no evidence at this point to suggest that TB will have any impact on the rhino population. “The rhinos are being exposed to the organism, they are mounting an immune response, but they are not getting sick and dying from it. The same applies to other species. For example, we know that we get TB in our lions and that individuals will die of the disease. But if you look at the population level of the disease, lions seem to be doing fine and their numbers have remained fairly static."
The authors further emphasise that the findings don't really come as a surprise since TB is prevalent in at least 15 other species in KNP, but that their research has significant positive implications for SANParks' rhino conservation and management strategy.
“While this pathogen may not appear to drastically impact the health of rhinoceros individuals, the research has significant implications for conservation management decisions. For example, tuberculosis testing in KNP rhinoceros that are earmarked for translocation for conservation reasons can increase confidence of minimal risk to other susceptible individuals at their destinations," explains Rebecca Dwyer, lead author of the study and a PhD candidate in the Animal TB research team.
The study, which was published in the prestigious American scientific journal PNAS (Proceedings of the National Academy of Sciences) this week, identified proximity to buffalo herds (white rhinos) and sampling year (black rhinos) – which coincided with periods of drought – as risk factors for M. bovis infection.
A significant cluster of cases was detected near KNP's south-western border, although infection was widely distributed. The identified cluster is close to the KNP border with the surrounding Mpumalanga province, consisting primarily of farmland with livestock herds that have historically been implicated in spill-over of M. bovis to wildlife in KNP, especially to buffaloes.
“With South African rhinos being threatened by poaching, habitat loss and drought, it is key to be able to translocate them to strongholds where they can be kept safe and to preserve their genetic diversity," says Miller. “But TB is a controlled veterinary disease, so once our research group, in partnership with SANParks, found TB in Kruger rhinos in 2016, the Department of Agriculture, Land Reform, and Rural Development (DALRRD) imposed movement restrictions to prevent spreading the infection to other populations."
These restrictions created a barrier to the movement of rhinos to other national or private reserves and has a significant impact on the conservation of the species, as KNP has historically been an important population source of rhinoceros for other conservation strongholds in South Africa and other African countries.
The solution was to come up with a test to identify infected animals before they were moved to prevent disease transmission. According to Dr Wynand Goosen, Wellcome International Training Fellow in the Animal TB Research Group, the screening test that was used in their KNP study was validated by the Animal TB Research Group in 2019 and was recently approved by DALRRD for use in KNP rhinos (see info box below).
A management strategy involving a quarantine protocol and testing schedule was devised in collaboration with SANParks and has been approved. “Should we now wish to start moving rhinos out of Kruger, we have that option to quarantine them and test them, and then send them out," says Buss.
Dwyer adds: “The findings of this study are significantly important for wildlife conservation, not just of rhinoceros, but of many other species in this context. It demonstrates that the spread of pathogens in multi-host systems has important consequences for the conservation of different species and of the ecosystem as a whole."
The way forward
Dr Carmel Witte, a quantitative epidemiologist with the San Diego Zoo Wildlife Alliance and senior author of the study, emphasises that the eventual population-level health effects of bTB are currently unknown. “Tuberculosis tends to be a disease that manifests over long periods of time and when you compound an infectious disease with stochastic events such as climate change and unprecedented mortality due to poaching of endangered animals, it is cause for concern.
“Continued surveillance of rhinoceros as well as other animals can help us understand the long-term impact of this disease in wildlife and prevent catastrophic population losses and further disease spread."
Goosen highlights the importance of the further development of diagnostic tools and of a 'Tuberculosis One Health' approach. “Even though our research is very important from an animal conservation perspective, it is just as important from a human health-risk perspective. To avoid the next pandemic in people, livestock and wildlife will have to be actively monitored for various infectious pathogens with zoonotic potential. This requires appropriate diagnostic tools that are rapid and accurate. To develop these tools research in all susceptible species is of the utmost importance."
More about the diagnostic test used in the bTB study
- It was developed based on a blood test used for TB detection in humans.
- It is an improvement on traditional diagnostic methods using culture because it is based on the immune response.
- The entire system is a modified commercial test which ensures standardisation.
- It is simple for in-field use and reproducible.
The Animal TB Research Group has developed tests that can be used for African wildlife species over many years of research. Improved tests are in demand by veterinarians throughout the world since there are currently limited tests for diagnosing TB in wildlife. To name a few species: African buffaloes, warthogs, African elephants, African lions, cheetahs, leopards, wild dogs, hippos, baboons, and meerkats.
TB in other KNP species
According to the researchers, the distribution of M. bovis infection in KNP rhinoceros is similar to that reported for other species in the park. For example, a 1991-'92 survey of buffaloes showed widespread bTB in the central and southern regions of the park, with individual herd prevalence of up to 67%. A later study (2012/'13) showed an overall infection prevalence of 44% in lions in the same areas. Such extensive infection is increasingly observed in species like warthogs, wild dogs and elephants, with cases identified in more than 15 species to date. Taken together, these findings suggest that spill-over of bTB is not a new occurrence and support the need for ongoing bTB surveillance across species to continuously assess disease risk and conservation impact.
Why rhinos are translocated
KNP has been a source of rhinos for other locations starting new populations or for genetic diversity reasons – a strategy that has been adopted because rhinos were doing so well in the Park until the onset of poaching, explains Miller. “If a disaster affects an isolated population of an endangered species, that species could be lost. To avoid that risk, you don't want all the animals in one location, and they are moved to multiple places. One of those risks is poaching. It is often easier to manage the security of rhinos in smaller reserves than in a huge, complex park. Moving animals to other locations will ensure that there are future breeding populations even if the threat of poaching continues in Kruger."
Is bTB a threat to humans?
Although people can become infected with bTB, it usually only happens when they regularly handle infected (uncooked) animal organs or drink unpasteurised milk. Unlike diseases such as Covid-19, people need close prolonged contact to get TB and won't contract it from visiting KNP, stresses Miller.
Witte adds that although most humans are not interacting with wild rhinoceros in a way that would put them at risk for acquiring M. bovis, the study highlights the ongoing potential for pathogen spill-over from animals to people (and people to animals!) at the human-domestic animal-wildlife interface.
According to Goosen TB has similar consequences for humans and a broad range of animal species, yet cases in humans and animals are commonly treated as separate problems. A TB One Health approach is therefore warranted by improving the surveillance of zoonotic mycobacteria in humans, livestock, wildlife and their environment throughout South Africa using various novel technologies.
Ongoing work in KNP
According to Buss a lot of work still remains to be done on TB, especially the epidemiology, and particularly at a population level. “Although it has been around for maybe 60-70 years in Kruger, it would still be considered a relatively new disease. Because TB manifests itself so slowly, we would imagine that the disease is still expressing itself at a population level, and we still need to reach some sort of equilibrium with the disease – and we just don't know exactly where that is.
“Although we might not see much happening at the moment, it is very difficult to predict the future. It would really be helpful to have some idea of the potential impact of this disease, particularly in advance, should we be required to manage this. And that's in all animals, not only rhinos."
Prevalence in other parks and reserves?
According to Buss, KNP is currently the only national park where TB has been diagnosed in rhinos. “The problem is currently contained to the Kruger, thank goodness. But we would certainly want to keep it that way for other national parks."
Photo caption: Prof Michele Miller (second from right) collects
blood samples from a white rhino in the Kruger National Park.