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Analysis: Stopping disease swaps between humans and animals
Friend or foe?
Humans and animals share many diseases
- Urbanization, population growth increasing transmission
- Both people and animals sickened
- Cross-sectoral collaboration the key
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.
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
Even when people and animals are not fearful of one another, there is a common potential enemy - warming temperatures.
SLIDESHOW: Why pastoralism matters
Photo: Phuong Tran/IRIN /IRIN
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
These temperature changes kill thousands of animals annually, and can
damage surviving animals’ fertility and milk production - affecting
human nutrition - according to FAO
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
and co-researchers recommended “re-allocation of resources for ‘smart
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
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.
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.