Why bovine TB is a threat to more than just cattle

Buffalo are the main wildlife carriers of Bovine TB, a disease that poses a threat not only to animals but also to humans. Thomas Mukoya/REUTERS

Bovine tuberculosis is an infectious disease that threatens the welfare of people, livestock and wildlife across the world. Originally found in cattle, many species of wildlife can also be infected, including buffalo and lion.

The disease is found in many of South Africa’s national parks and may have serious economic, ecological and public health implications. Managing the spread of bovine TB involves restricting the movement of infected animals and in some cases using test-and-cull programmes. This is easier to do with livestock but more difficult among wild animals.

Caused by Mycobacterium bovis, which is closely related to the bacterium that causes human tuberculosis, the bacterium has evolved over time to prefer animal hosts. But it is capable of infecting people and can be difficult to treat with current drug programmes.

The proportion of human TB cases caused by M. bovis varies between countries and regions, but is typically around 3% in Africa. These figures most likely under-estimate the true number of cases. Tests to distinguish between the two bacteria are seldom performed.

Humans are also at risk

The World Health Organisation lists Bovine TB as one of seven neglected zoonotic diseases. This poses a growing public health concern in areas where animals and people live in close proximity such as regions around the Kruger National Park in South Africa.

In the developed world, cases of people contracting TB from animals are rare. But health officials remain on the alert particularly following the diagnosis of the first cases of transmission from domestic cats last year in the United Kingdom.

In developing countries there is a greater chance of the disease being contracted by humans. Interactions between people, livestock and wildlife and the regular consumption of unpasteurised milk and other dairy products puts people at a greater risk of exposure to the disease. Eating game meat from illegally poached wildlife is another potential source of infection, as this meat does not undergo veterinary inspection.

Bovine TB across the world. World Animal Health Information Database Interface

Along the boundaries of many of South Africa’s national parks, local communities and their livestock are in direct contact with wildlife. This increases the possibility of transmission. The disease is most commonly spread by aerosol droplets – coughing, sneezing and spitting - as well as by eating or drinking infected animal products.

The effects of the disease on buffalo

African buffalo are considered “maintenance” hosts of bovine TB. They act as reservoirs of the disease and spread it to other species. The infection in buffalo is seldom fatal, though its effects may be more severe in other animals.

Part of the challenge of managing the disease is to limit its spread to rare or endangered species. As a result restrictions are placed on the movement of infected buffalo.

Buffalo are highly valued animals in both the eco-tourism and trophy hunting industries. The establishment of breeding programmes to stock parks and conservancies has created a constant demand for the animals.

The market value of buffalo within South Africa varies according to their disease status. Buffalo that are disease-free fetch approximately 10 times more at auction, with price tags of up to $4 million.

A problem for conservation

The disease threatens other wildlife species too. Lions, in particular, are vulnerable. They contract the disease by eating infected buffalo. It can also spread to other members of the pride during violent fights, a common feature among adult lions. As they are social animals, an increase in deaths due to the disease may have long term impacts on pride structure and survival.

Little is known about the effects of the infection on a range of other susceptible species such as baboons, kudu, warthogs, leopards making research and surveillance essential.

A costly business

The negative economic impact of bovine TB is felt by farmers as well as national parks. Cattle are often brought into game reserves during droughts to access waterholes, and buffalo sometimes break through fences and browse with cattle in surrounding areas. Running continuous cattle and wildlife testing programmes is enormously expensive, and the required slaughter of infected cattle can be devastating to local farmers.

Many game reserves and parks supplement their income with the sale of wildlife locally and abroad. If bovine TB is discovered within their borders, the sale and movement of animals is restricted. This can cause huge revenue losses, and severely limit the funds available to continue conservation efforts.

Bovine TB is particularly concerning in areas where there are communities with high rates of HIV. An immune system weakened by HIV can increase the risk of developing TB infection by up to 31 times. TB infection in people who have contracted the disease from an animal source is increasingly difficult to treat with common drugs.

Bovine TB in South Africa has serious implications for people, wildlife and livestock. There is extensive research focusing on different aspects of the disease, such as immunology and genetics. This work is essential to understanding the disease and managing it effectively.