A new global study mapping human-animal diseases like tuberculosis (TB) and Rift Valley fever finds that an "unlucky" 13 zoonoses are responsible for 2.4 billion cases of human illness and 2.2 million deaths per year. The vast majority occur in low- and middle-income countries.
"From cyst-causing tapeworms to avian flu, zoonoses present a major threat to human and animal health," said Delia Grace, a veterinary epidemiologist and food safety expert with ILRI in Kenya and lead author of the study. "Targeting the diseases in the hardest hit countries is crucial to protecting global health as well as to reducing severe levels of poverty and illness among the world's one billion poor livestock keepers."
"Exploding global demand for livestock products is likely to fuel the spread of a wide range of human-animal infectious diseases," Grace added.
According to the study, Ethiopia, Nigeria, and Tanzania in Africa, as well as India in Asia, have the highest zoonotic disease burdens, with widespread illness and death. Meanwhile, the northeastern United States, Western Europe (especially the United Kingdom), Brazil and parts of Southeast Asia may be hotspots of "emerging zoonoses"—those that are newly infecting humans, are newly virulent, or have newly become drug resistant. The study examined the likely impacts of livestock intensification and climate change on the 13 zoonotic diseases currently causing the greatest harm to the world's poor.
The report, Mapping of Poverty and Likely Zoonoses Hotspots, was developed with support from the United Kingdom's Department for International Development (DFID). The goal of the research was to identify areas where better control of zoonotic diseases would most benefit poor people. It also updates a map of emerging disease events published in the science journal Nature in 2008 by Jones et al.i
Remarkably, some 60 per cent of all human diseases and 75 per cent of all emerging infectious diseases are zoonotic. Among the high-priority zoonoses studied here are "endemic zoonoses," such as brucellosis, which cause the vast majority of illness and death in poor countries; "epidemic zoonoses," which typically occur as outbreaks, such as anthrax and Rift Valley fever; and the relatively rare "emerging zoonoses," such as bird flu, a few of which, like HIV/AIDS, spread to cause global cataclysms. While zoonoses can be transmitted to people by either wild or domesticated animals, most human infections are acquired from the world's 24 billion livestock, including pigs, poultry, cattle, goats, sheep and camels.
Poverty, zoonoses and markets
Despite the danger of zoonoses, the growing global demand for meat and milk products is a big opportunity for poor livestock keepers.
"Increased demand will continue over the coming decades, driven by rising populations and incomes, urbanization and changing diets in emerging economies," noted Steve Staal, deputy director general-research at ILRI. "Greater access to global and regional meat markets could move millions of poor livestock keepers out of poverty if they can effectively participate in meeting that rising demand."
But zoonoses present a major obstacle to their efforts. The study estimates, for example, that about one in eight livestock in poor countries are affected by brucellosis; this reduces milk and meat production in cattle by around 8 per cent.
Thus, while the developing world's booming livestock markets represent a pathway out of poverty for many, the presence of zoonotic diseases can perpetuate rather than reduce poverty and hunger in livestock-keeping communities. The study found a 99 per cent correlation between country levels of protein-energy malnutrition and the burden of zoonoses.
"Many poor livestock keepers are not even meeting their own protein and energy needs," said Staal. "Too often, animal diseases, including zoonotic diseases, confound their greatest efforts to escape poverty and hunger."
Assessing the burden of zoonoses
The analysis found high levels of infection with these zoonoses among livestock in poor countries. For example, 27 per cent of livestock in developing countries showed signs of current or past infection with bacterial food-borne disease—a source of food contamination and widespread illness. The researchers attribute at least one-third of global diarrheal disease to zoonotic causes, and find this disease to be the biggest zoonotic threat to public health.
In the booming livestock sector of developing countries, by far the fastest growing sectors are poultry and pigs. "As production, processing and retail food chains intensify, there are greater risks of food-borne illnesses, especially in poorly managed systems," said John McDermott, director of the CGIAR Research Program on Agriculture for Nutrition and Health, led by the International Food Policy Research Institute (IFPRI). "Historically, high-density pig and poultry populations have been important in maintaining and mixing influenza populations. A major concern is that as new livestock systems intensify, particularly small- and medium-sized pig production, that more intensive systems will allow the maintenance and transmission of pathogens. A number of new zoonoses, such as Nipah virus infections, have emerged in that way."
Intensification and disease spread
Assessing the likely impacts of livestock intensification on the high-priority zoonoses, the study found that livestock density is associated more with disease "event emergence" than with overall disease burdens. Both the northeastern United States and Western Europe have high densities of livestock and high levels of disease emergence (e.g., BSE, or "mad cow" disease, and Lyme disease), but low numbers of people falling sick and dying from zoonotic diseases. The latter is almost certainly due to the relatively good disease reporting and health care available in these rich countries.
Bovine tuberculosis is a good example of a zoonotic disease that is now rare in both livestock and human populations in rich countries but continues to plague poor countries, where it infects about 7 per cent of cattle, reducing their production by 6 per cent. Most infected cattle have the bovine form of TB, but both the human and bovine forms of TB can infect cows and people. Results of this study suggest that the burden of zoonotic forms of TB may be underestimated, with bovine TB causing up to 10 per cent of human TB cases. Human TB remains one of the most important and common human diseases in poor countries; in 2010, 12 million people suffered from active disease, with 80 per cent of all new cases occurring in 22 developing countries.
The surveillance lacking today will be even more needed in the future, as the climate changes, she added. Previous research by ILRI and others indicates that areas with increased rainfall and flooding will have increased risk of zoonoses, particularly those diseases transmitted by insects or associated with stagnant water or flooding.
The main finding of the study is that most of the burden of zoonoses and most of the opportunities for alleviating zoonoses lie in just a few countries, notably Ethiopia, Nigeria, and India. These three countries have the highest number of poor livestock keepers, the highest number of malnourished people, and are in the top five countries for both absolute numbers affected with zoonoses and relative intensity of zoonoses infection.
"These findings allow us to focus on the hotspots of zoonoses and poverty, within which we should be able to make a difference," said Grace.