By Kate Kelland
LONDON (Reuters) - A global study mapping human diseases that come from animals like tuberculosis, AIDS, bird flu or Rift Valley fever has found that just 13 such diseases are responsible for 2.4 billion cases of human illness and 2.2 million deaths a year.
The vast majority of infections and deaths from so-called zoonotic diseases are in poor or middle-income countries, but "hotspots" are also cropping up in the United States and Europe where diseases are newly infecting humans, becoming particularly virulent, or are developing drug resistance.
And exploding global demand for livestock products means the problem is likely to get worse, researchers said.
"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 the International Livestock Research Institute (ILRI) in Kenya and lead author of the study.
She said targeting these diseases in the hardest hit countries is crucial to protecting global health, and failing to tackle them would allow demand for meat products to "fuel the spread of a wide range of human-animal infectious diseases."
The study, conducted by the ILRI, the Institute of Zoology in Britain and the Hanoi School of Public Health in Vietnam, mapped livestock-keeping and diseases humans get from animals, and drew up a list of the top 20 geographical hotspots.
It found that Ethiopia, Nigeria, and Tanzania, as well as India have the highest zoonotic disease burdens, with widespread illness and death.
It also found the United States and Europe - especially Britain - Brazil and parts of Southeast Asia may be becoming hotspots of "emerging zoonoses", which are infecting humans for the first time, are especially virulent or are becoming drug resistant.
The report studied so-called endemic zoonoses which cause the vast majority of illness and death in poor countries.
One such disease is brucellosis, also known as Bang's diseases or Mediterranean fever, which is a highly contagious zoonosis people catch by consuming unsterilized milk or meat from infected animals.
The researchers estimated that about one in eight livestock in poor countries are affected by brucellosis. As well as threatening people with disease, this also reduces milk and meat production in cattle by around 8 percent.
The study also looked at epidemic zoonoses, which typically occur as outbreaks - such as anthrax and Rift Valley fever - and at the relatively rarer emerging zoonoses like bird flu. A few of these, like HIV/AIDS and H1N1 swine flu, have shown the ability to spread to cause pandemics.
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.
The study initially looked at 56 zoonoses that together are responsible for around 2.5 billion cases of human illness and 2.7 million human deaths per year.
It then zoomed in on the 13 most important, and found high levels of infection with these in livestock in poor countries.
Some 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 estimated at least a third of the world's cases of diarrhoeal disease are caused by animal-human diseases and said this was the biggest zoonotic threat to public health.
John McDermott, director of the CGIAR research program on agriculture for nutrition and health led by the International Food Policy Research Institute (IFPRI), said that in booming livestock sectors in developing nations the fastest growing areas are poultry and pigs - putting the potential disease risk emphasis on flu.
"Historically, high-density pig and poultry populations have been important in maintaining and mixing influenza populations," he said in a statement accompanying the study.
"A major concern is that as new livestock systems intensify, particularly small- and medium-sized pig production ... more intensive systems will allow the maintenance and transmission of pathogens. A number of new zoonoses ... have emerged in that way."
(Editing by Mark Potter)