2 January 2023

5 minute read

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.

Risk factors

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.

Significance

“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."