Beef & poultry dominate study of wildlife-livestock diseases

When the “Middle East Respiratory Syndrome,” or MERS, broke out in late 2012 in Saudi Arabia, many initially suspected that the coronavirus responsible for wreaking havoc on patients’ lungs, intestines, livers, and nervous systems, jumped into humans from bats. Instead, researchers would eventually discover that the virus came from a more familiar, domestic critter: the dromedary camel. Indeed, it seemed as if the virus had been floating around among Saudi Arabian camels for at least twenty years before making the leap to our species.

The role of livestock in moving diseases to and fro with humans at one end and wildlife at the other has largely been neglected by formal scientific inquiry, at least in proportion to other aspects of zoonoses. When researchers have taken a close look at diseases at the wildlife-livestock interface, the focus has often been on the role of wildlife as a source for disease, perhaps owing to our own human bias favoring livestock while neglecting the inherent value in wildlife biodiversity. After all, livestock farming constitutes some 37% of global average GDP, and losses caused by infectious diseases can not just be costly, but can also lead to food insecurity.

But the truth is that diseases pass between livestock and wildlife in both directions, both thanks to vector like insects that move pathogens back and forth, but also because in many ecosystems, wildlife and livestock share resources, like water and pastures. And when a disease moves from livestock to wildlife, it can have drastic consequences. In the worst cases, it can push a species to the brink of extinction.

Previous attempts at taking stock of the scientific efforts to understand the wildlife-livestock disease interface have revealed that a vast majority of known livestock pathogens – 77% – are known to infect multiple species, including wildlife. But nobody has attempted to take a global perspective on the problem, until now.

University of Sydney veterinary scientist Anke K. Wiethoelter and her colleagues analyzed nearly 16,000 published scientific papers on the wildlife-livestock disease interface, a compendium that covered 113 diseases going back more than a century, to 1912. While the number of relevant papers published each year increased over time, the more interesting trends only emerged once Wiethoelter and her colleagues took a closer look at their data.

For example, while early efforts focused on parasitic diseases, most research efforts since the late 1970s have focused instead on viruses. And just ten diseases accounted for half of all research efforts: avian influenza, rabies, salmonellosis, bovine tuberculosis, trichinellosis, Newcastle diseases, brucellosis, leptospirosis, echinococcosis, and of course toxoplasmosis. Most of these are zoonoses, diseases that can also infect humans.

Most researchers stuck to just five main groups of wildlife: birds, carnivorans, even-toed ungulates, rodents, and bats. They also focused on five types of livestock: poultry, cattle, pigs, horses, and small ruminants, like sheep, goats, and camels. Together, research on these groups accounted for three quarters of all research, but only a few diseases were investigated at any particular interface. For example, a quarter of all bird-poultry research concerned itself with avian flu, while a similar proportion of research at the ungulate-cattle and carnivoran-cattle interfaces were concerned with bovine TB.

There were, of course, regional differences. Most research focused on issues in North America and Europe, owing perhaps to a bias among researchers or research funding. In some areas, local concerns dominated: in Oceania, researchers worked on the marsupial-cattle interface, presumably because brushtail possums transmit bovine TB to cattle in New Zealand.

Another important trend that emerged was that interest in certain areas could be traced to key events. Research on the bird-poultry interface surged, for example, following the 2003/2004 H5N1 epidemic in Southeast Asia. And the magnitude of research output (and, ostensibly, research funding) on any particular disease or wildlife-livestock interface was not necessarily proportional to its importance or potential severity. “Many would argue that the international response to H5N1 (including wild bird surveillance) was not commensurate with the threat or scale of the problem,” write the researchers, especially because the global spread of the pathogen was facilitated by poultry movement and trade, not primarily by wild birds. And decades of funding for bovine TB research in the UK have been generally ineffective in reducing the disease’s burden there.

In other words, research efforts have scaled along with the perceptions of any given disease, rather than the actual costs associated with the diseases’ effects on human and animal health, livestock losses, or wildlife conservation. It’s perhaps not unexpected that the vast majority of research is concerned with diseases that impact poultry and beef. What would we do without our cheeseburgers and chicken nuggets? – Jason G. Goldman | 22 July 2015

Source: Anke K. Wiethoelter, Daniel Beltrán-Alcrudo, Richard Kock, and Siobhan M. Mor. (2015). Global trends in infectious diseases at the wildlife–livestock interface. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1422741112.

Header image: Domestic chickens, via shutterstock.com

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