Diseases that can be shared between humans and animals, like rabies or bovine tuberculosis, have an enormous impact around the world. Especially in poorer communities, where livestock often provide food and a possible economic route out of poverty, such zoonotic diseases can worsen human health while reducing food production. A new analysis published this week (July 5) by the International Livestock Research Institute (ILRI) in Kenya maps the confluence of zoonotic disease, poverty, and livestock production, primarily in resource-poor nations. The authors hope that these maps will help policy-makers and public officials allocate funds and devise strategies for alleviating animal-borne diseases and poverty in the regions of the world most in need.
“The report is important because it helps shine a light on the ‘bottom one billion’”—the poorest sector of the global population, 70 percent of who own livestock, said Lonnie King, the dean of the Department of Veterinary Medicine at The Ohio State University, who was not involved with the research. The United Nations’ Food and Agriculture Organization estimates that global food production will need to rise by 70 percent to feed the world’s estimated 9 billion people in 2050, King said, but given limited resources, reports like the ILRI’s help policy-makers decide where is “the most impactful place to put these funds.”
“Studies like these are important because they help us step back and take in the big picture,” agreed Emily Jenkins, who studies northern Canadian zoonotic diseases at the University of Saskatchewan, who also did not participate in the study.
Zoonotic diseases offer a “double whammy,” hitting both humans and animals said David Molyneux, an emeritus professor at the University of Liverpool who advises the World Health Organization (WHO) on parasitic disease control programs. In addition to infecting and possibly killing people, zoonotic disease can deprive them of livelihood and sustenance by killing livestock or sickening them to the point they can’t produce milk or eggs to bring in extra money. Funding to tackle zoonotic diseases is often lacking, a challenge that is compounded by the fact that many livestock owners are difficult for health agencies to reach, said Molyneux. “They’re often pastoralists, nomads who don’t contact government services often.”
In addition to geographically locating areas with high zoonotic disease prevalence, the study aimed to calculate the health and economic burden of zoonotic disease on local livestock farmers—many of whom live on less than $2 a day—as well as estimate how well official surveillance data reflects disease incidence. The report considered 56 different zoonotic diseases, including parasitic diseases like toxoplasmosis, bacterial pathogens such as brucellosis and bovine tuberculosis, and viral infections like rabies. It updated decade-old maps displaying the geographical patterns of livestock production and the prevalence of human and animal infection by various parasites.
“We wanted to know which diseases were where, and what harm they did,” said co-author Delia Grace of the ILRI. Agreeing with the axiom that “what can’t be measured, can’t be managed,” she and her collaborators first used official reports of livestock deaths to begin painting their picture of zoonoses prevalence and location—but quickly came up short, Grace said. Comparing study data looking at infection prevalence with actual reports of animal deaths, the researchers calculated that 99.9 percent of livestock deaths go unreported—decidedly hampering the ability to calculate the prevalence and impact of zoonotic diseases.
The researchers turned to published studies measuring disease prevalence from various regions to fill in their data gaps, eventually compiling information from over 1,000 different projects. They found that the regions with high zoonoses prevalence, such as sub-Sarahan Africa and India, were the same regions with the highest percentages of poor livestock producers. Calculating the health and economic burden posed by these diseases on poor livestock producers, Grace and her collaborators found that just 19 countries accounted for 75 percent of zoonotic disease burden. Nigeria, Ethiopia, Tanzania, Togo, and India accounted for the countries most affected by zoonotic disease and poverty.
In contrast, North America and Western Europe jumped out as hotspots for emerging zoonoses—although it’s not clear whether this could be due to better detection in these areas.
The analysis also hints at the multi-pronged approach necessary to address zoonoses. Of the 13 highest priority zoonoses, including rabies and the parasitic disease leptospirosis, nine have wildlife reservoirs, Jenkins noted. So it’s important to not only treat infected people, but to treat animals as well. For example, a great, and relatively cheap, way to prevent human rabies infections is to prophylactically vaccinate dogs, Molyneux explained, a strategy that has recently proven effective in Tanzania. “But who’s going to pay for those vaccines?” he asked. Despite the fact that researchers and public health officials realize that targeting animal diseases like rabies may be the most effective way to keep humans infection-free, implementing such strategies has been slow, as funds are tight and often dedicated to addressing human diseases.
Another important strategy will be to educate people on disease transmission, said Grace. For example, the bacterial infection brucellosis is often transmitted by drinking milk. Boiling the milk reduces the risk, but rural communities in Africa often believe that milk is pure and therefore unable to transmit infection, Grace explained.
And it may be difficult to know whether any strategy is actually working until reporting and surveillance can give more accurate information on who’s getting which diseases, said Tara Smith, an epidemiologist at the University of Iowa who was not involved in the report. “We still need improved measures of the current burden in many of the areas, so that we can have a baseline prevalence,” Smith explained. “Otherwise, we'll never know what interventions work.”