HEALTH: Predicting the next zoonotic pandemic

Photo: The Lancet

Next big pandemic most likely to break out in orange-red

zones on this map according to one projection

LONDON (IRIN) - Chances are high the world’s next pandemic will be a disease originating in animals, like 60 percent of current documented human infectious diseases. Even after hundreds of thousands of human deaths from zoonoses (diseases transmitted from animals to humans), experts say there is still limited information about how zoonoses are spread or just how to predict the next outbreak.

“There is no question of whether we will have another zoonotic pandemic,” wrote Stephen Morse, a public health professor at Columbia University in New York, in a November 2012 series on zoonoses in the UK medical journal, The Lancet. “The question is merely when, and where, the next pandemic will emerge.”

Despite virus hunters’ best efforts, no zoonotic pandemic has, thus far, been predicted before it infected humans.

“The continuing effect of the HIV/AIDS pandemic is a reminder of the risk of zoonotic pathogens spreading from their natural reservoirs to man,” wrote William Karesh from New York’s EcoHealth Alliance in the Lancet series. The NGO, formerly known as Wildlife Trust, works to prevent the outbreak of emerging diseases by preserving biodiversity. 

An estimated 1.8 million people die annually from AIDS, caused by HIV, which originated in primates. 

“What is far less broadly appreciated is that none of the approaches commonly used to search for potential new human pathogens… probably would have identified simian immunodeficiency virus (SIV) as a potential risk to man,” said Karesh.

Early warning?

The US Agency for International Development launched its Emerging Pandemic Threats Programme in late 2009 to build an early warning system to detect and reduce the impacts of zoonotic diseases.

But there are thousands of species of birds and animals that each host different diseases; where do you concentrate efforts? A virologist may focus on diseases easily spread from animal relatives, like chimpanzees, while a social scientist points out how rare contact is between humans and chimpanzees and focuses, instead, on poultry, with which people live and work in close quarters worldwide.

Between 2003 and 5 November 2012, 608 laboratory-confirmed human cases of infections from H5N1 bird flu were reported to the World Health Organization from 15 countries, of which 359 died. 

Disease hunters are homing in on emerging disease “hotspots”, mammal-rich areas with high, changing population densities. A group of experts led by Columbia University’s Morse are creating a disease map, in which Rwanda and Burundi are bright red, as is the Indonesian island of Java, one of the world’s most densely populated islands, and Egypt’s Nile Delta. Other potential sites for future outbreaks include north India and Bangladesh, northern and western China, and - to a lesser extent - more densely populated parts of western Europe and along the west African coast.

Urbanization

“Urbanization has boosted zoonoses’ outbreak risks as people get closer to animals,” Sarah Schlesinger, a scientist from New York’s Rockefeller University, told IRIN on the sidelines of a recent HIV vaccine conference.

Cities are growing, with roads and industries penetrating previously uninhabited wildlife habitats; some 3.3 billion people live in urban areas (cities and their outskirts), according to the UN. By 2030, the world’s urban population is expected to exceed five billion, with 80 percent located in the developed world. In places where animal “hosts” to disease start to disappear (as their habitats shrink), pathogens are finding a new home in human hosts.

Some 800 million people worldwide are engaged in urban agriculture, according to the World Bank, which identifies peri-urban livestock as a fast-growing sector that produces 34 percent of the world’s meat and nearly 70 percent of its eggs.

The Nairobi-headquartered International Livestock Research Institute (ILRI) has pointed out how urban livestock and agriculture can breed disease in some of the world’s most crowded places. In a recent survey in Dagoretti, one of eight districts of Kenya’s capital, Nairobi, the institute found up to 11 percent of households were affected by cryptosporidiosis, a diarrhoeal disease caused by a pathogen found in cattle, raw milk, soil, vegetables and contaminated water.

Changing harvests  may be another contributor to the spread of zoonoses. In the southwestern USA where El Niño (rising sea surface temperatures across the central and eastern Pacific Ocean) dumped more rain, vegetation growth increased, which then attracted more rats. Hantavirus is not fatal in rats - which carry the disease - but is in humans who became infected through the rats.

The interplay of biology, ecology and sociology make forecasting the next pandemic difficult, say experts in the Lancet series who call for boosting cooperation between experts (often working in silos) to meet the “huge, and rising” threat of zoonoses.