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.
The report, which was conducted by the International Livestock Research Institute (ILRI), the Institute of Zoology (UK) and the Hanoi School of Public Health in Vietnam, maps poverty, livestock-keeping and the diseases humans get from animals, and presents a "top 20" list of geographical hotspots.
"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.
Remarkably, some 60 percent of all human diseases and 75 percent 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
Today, 2.5 billion people live on less than US$2.00 per day. Nearly three-quarters of the rural poor and some one-third of the urban poor depend on livestock for their food, income, traction, manure or other services. Livestock provide poor households with up to half their income and between 6 and 35 percent of their protein consumption. The loss of a single milking animal can be devastating to such households. Worse, of course, is the loss of a family member to zoonotic disease.
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 percent.
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 percent 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 researchers initially reviewed 56 zoonoses that together are responsible for around 2.5 billion cases of human illness and 2.7 million human deaths per year. A more detailed study was made of the 13 zoonoses identified as most important, based on analysis of 1,000 surveys covering more than 10 million people, 6 million animals and 6,000 food or environment samples.
The analysis found high levels of infection with these zoonoses among livestock in poor countries. For example, 27 percent 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.