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Analysis: Stopping disease swaps between humans and animals

By Dana MacLean
HONG KONG, 13 December 2013 (IRIN) - With more than half of all human infections originating in animals, experts say a multi-sectoral, global response to zoonoses - diseases passing between animals and humans - is urgently needed. IRIN talked to a panel of experts to learn just how deadly humans and animals can be to one another, and ultimately how each can save the other.

“By neglecting the health of animals and ecosystems, we fail to recognize that human health is inextricably linked with animal and ecosystem health,” said Laura H. Kahn, a physician and researcher at Princeton University in the US. Khan co-founded the One Health Initiative, which links human health to how well animals and the ecosystem fare.

With almost half of the some 1,000 pathogen species found in livestock and animals kept as pets able to cross over into humans, poor animal health undoubtedly increases the risk of poor human health, experts warn.

Known zoonoses cause an estimated 2.3 billion cases of sickness and 1.7 million human deaths annually, reported the Nairobi-headquartered International Livestock Research Institute (ILRI) in 2012.

But then there are the unknown viruses, which are estimated to number at least 320,000, according to calculations by Simon Anthony and co-researchers published in 2013.

Scientists say preventing and containing zoonoses requires improved human and animal health surveillance systems, food safety and biodiversity conservation and - equally difficult, if not more so - collaboration among biologists, veterinarians and doctors for people.

Changing world, changing risk

A good part of zoonoses (some 70 percent of which come from wildlife) are directly attributable to human actions that have vastly changed animal environments, decreasing animals’ resilience against infection and boosting the risk of humans falling ill.

“Changes in farming and marketing systems have led to more and more pathogens present in society that human beings have never previously been exposed to,” Yi Guan, the medical doctor and virologist based in Hong Kong who first traced the 2003 outbreak of Severe Acute Respiratory Syndrome (SARS) to live poultry markets in eastern China, told IRIN.

The planet’s population is expected to exceed nine billion people by 2050, leading to more pressure on environmental resources and food systems. By that time, the UN Food and Agriculture Organization (FAO) estimates global meat consumption per person will increase 27 percent (with most of that growth in China and Brazil).

“Urbanization is [linked] to the intensification of animal systems [in cities], leading to an increased risk of zoonosis,” said Fred Unger, a veterinary scientist with ILRI.

From the 1960s until 2010, due to urbanization, especially in developing countries, FAO calculated the global consumption of milk doubled, while that of meat tripled and eggs grew fivefold.

These days, each city in eastern China hosts at least one dozen retailers selling different species of live poultry in open markets. Birds taken from different regions of China to urban markets led to the outbreaks of SARS, H1N1 bird flu and the recently diagnosed bird flu strain H7N9, said Yi.

“There are direct impacts and consequences of the change in the farming and marketing systems,” said Yi. “These also provide increased opportunities and chances for human and animal interaction.”

While there are health benefits to moving animal husbandry into cities and slums, there are also zoonotic pathogens brewing in the unhealthy physical conditions animals are kept in, increasing humans’ exposure to those pathogens.

But it is not just people casting a wary eye at animals; animals would be justified in being suspicious of us.

Decreased animal immunity

Expanding industrialization, such as extractive industries, frequently establish worker camps in virgin forests, exposing wildlife to people for the first time, said Kaia Tombak, a conservation programme assistant at Wildlife Conservation Society Canada.

“Increased human disturbance and habitat loss cause elevated levels of stress in many animals. This reduces their immune function and causes diseases [in animals] to become more prevalent,” said Tombak.

Development-induced habitat loss is another contributing factor to zoonoses, say biologists.

The Nipah virus - a potentially fatal disease with respiratory symptoms that can infect the brain - emerged in Malaysia the late 1990s, when people destroyed massive sections of rainforest, the natural habitat of fruit bats, in order to build pig farms and cultivate fruit orchards.

Researchers theorize
that as bats, carriers of the virus, came into closer contact for the first time with densely populated pig farms, they infected a number of pigs, which then infected humans.

On average, some 75 percent of people infected died.

Yet, cross-infection can be prevented in these cases, said Tombak. Camps for extractive industries based in forested areas can install screens to prevent bats from roosting indoors and establish responsible waste disposal practices to prevent attracting wild animals to camps, she suggested.

Even when people and animals are not fearful of one another, there is a common potential enemy - warming temperatures.

Edward Allen, a research scientist at the Laos Institute for Renewable Energy, based in Vientiane, has noted that even moderate temperature dips and rises within 10 degrees Celsius can lead to more deaths in both groups.

These temperature changes kill thousands of animals annually, and can damage surviving animals’ fertility and milk production - affecting human nutrition - according to FAO.

Stumbling blocks

Half the world’s surface area has some type of disease surveillance, but most national programmes are based in places with the least number of outbreaks, according to the UK medical journal The Lancet. The journal noted in 2012 that countries with high wildlife biodiversity and population density are “hotspots for emerging infectious disease”, yet almost none of the major surveillance systems were based in these lower-latitude regions.

In an oft-cited meta-study published in 2008 in the journal Nature, Kate Jones and co-researchers recommended “re-allocation of resources for ‘smart surveillance’”.

Between 1996 and 2009, more than half of all emerging infectious disease outbreaks were in Africa - a continent that lags behind in early warning systems and disease surveillance - according to a 2012 report by the UK-based Royal Society of Biological Sciences.

And even where there is adequate surveillance, politics - including powerful agriculture lobbies - can prevent zoonoses from being identified and rapidly treated, said Kahn of the One Health Initiative.

When Q fever, an infectious disease causing stillbirths and miscarriages in sheep, cattle and goats, broke out in the Netherlands between 2007 and 2009, “the Dutch Ministries of Agriculture and Health were at odds with each other, hindering an effective response,” said Kahn, explaining that the Ministry of Agriculture initially denied the livestock origins of the disease, which eventually spread to more than 2,000 people, killing about 1 percent, by 2009. The initial denial, which lasted until June 2008, of Q fever's origins meant “nothing was done to prevent further outbreaks... The disease continued to spread,” said Kahn.

Harnessing survival


But while humans and animals can spell each other’s doom, they can also support each other’s survival.

In 2000, the prevalence of fully immunized nomadic pastoralist children and women in eastern Chad’s Chari-Baguirmi and Kanem regions was near zero. In the same nomadic camps, however, livestock were compulsorily vaccinated by circulating veterinary teams.

Chad’s Ministries of Livestock and Health decided to join forces to carry out vaccination campaigns for pastoralists and their animals, resulting, for the first time, in 10 percent of nomadic children under the age of one being fully immunized wherever the joint campaign was conducted.

But while calls for such joint campaigns, which cut costs while reaching more people and livestock, have been proposed by FAO and the World Health Organization (WHO) for at least two decades, experts say there is still not enough collaboration.

Part of the problem is academic “silos”, said disease ecologist Peter Daszak, who heads the New York-based EcoHealth Alliance, at a recent standing-room-only session on One Health at the American Society of Tropical Medicine and Health annual conference held in Washington, DC.

“Working with animals - that’s the easy part. But for human experts, we need to create a dictionary just for each side to understand one another,” he wryly noted.

“Nobody has the knowledge and expertise to do everything from beginning to end [of disease identification and containment],” added Yi.

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Efforts to track zoonotic diseases
Monitoring health and disease in the animal kingdom is the first step towards predicting and preventing the spread of zoonotic disease, say scientists.

WHO’s Global Outbreak Alert and Response Network (GOARN), launched in 2000 by WHO, FAO and the World Organization for Animal Health (OIE), aims to assist countries with rapid response to new disease outbreaks.

Other regional and state organizations include:

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Theme (s): Environment, Health & Nutrition,

[This report does not necessarily reflect the views of the United Nations]

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