Lethal Hendra Virus Outbreaks May Be Caused by Man

That the lethal Hendra virus could jump from Australia?s flying foxes to its humans might seem like nothing more than bad luck ? an unfortunate but inevitable consequence of contact with animals.

Instead, it seems that people are partly responsible for the outbreak. Flying foxes are vectors, but human alteration of their ecology may have changed how Hendra moves through the animals.

By making flying-fox populations sedentary, stressed and fragmented, development might have also made them prone to viral spikes. Hendra?s spread in people may be, in a sense, a man-made disaster.

?We?re now seeing more evidence that human-induced environmental changes may be driving this disease,? said Raina Plowright, a disease ecologist at Pennsylvania State University. ?That?s something that?s been proposed many times, but few people have been able to show a mechanism. Here?s a mechanism.?

Plowright is the lead author of a study of the Hendra virus May 11 in Proceedings of the Royal Society B. In September 1994, the virus killed 14 horses and their trainer in a suburb of Brisbane.

Thirteen more outbreaks have occurred since then, each involving transmission from a flying fox to a horse; of those, five resulted in transmission from horse to human. Fortunately, Hendra doesn?t seem to spread between people, but it?s still scary enough to merit Biosafety level 4 treatment. When researchers study it, they do so wearing moon suits, in high-security labs sealed with multiple airlocks.

?Why has the Hendra virus emerged now, when it has circulated for millions of years? This has been a great mystery.?

Hendra?s emergence made flying foxes ? the common name for the world?s largest bats, a fruit- and pollen-eating genus found throughout Asia and Australia ? the latest animal source of human disease, a fact that in itself wasn?t particularly novel: From plague to malaria to HIV to influenza, so-called zoonoses are historically common.

But with industrialization making it easier than ever for them to spread, new diseases are in many ways more threatening than ever before. They also seem to be emerging with increasing frequency, a phenomenon that may simply result from better modern surveillance techniques, but might also reflect a changing disease environment.

?Why has the Hendra virus emerged now, when it has circulated for millions of years? This has been a great mystery,? said Plowright. ?How does the virus move at the landscape scale? How do the ecological changes we?re observing change the dynamics of the virus ? where it spreads, how fast it spreads??

In the new study, Plowright and colleagues created computer models of Hendra-virus transmission in flying foxes. They used data from years of research on real-world epidemiology, experimental flying-fox infections, field observations of population structure and demography, and life histories. Together, all this allowed the researchers to simulate how Hendra ebbed and flowed in flying foxes and they were able to study how changes to populations altered virus circulation patterns.

In the researchers' model, Hendra virus infection spiked in urban flying foxes. Proceedings of the Royal Society B

Models are not proof in themselves, but the results were compelling. Outbreaks in flying foxes occurred in the same simulated seasons and patterns as outbreaks in real-world human cases, suggesting a resonance between model and reality. Most strikingly, large outbreaks occurred infrequently in rural populations of flying foxes; it was in urban and suburban animals ? those most likely to encounter humans ? that disease levels jumped.

Many of Australia?s flying foxes have moved into such areas, driven by destruction of rural habitat and the year-round availability of food from exotic plants, said Plowright. Many of those traditionally migratory animals no longer migrate. As a result, the model suggests Hendra virus no longer circulates evenly in Australia?s flying foxes, but misses some clusters; without low-level exposures to calibrate their immune systems, those animals? immunity disappears. When Hendra returns, it explodes.

?We?ve essentially created a situation in which flying foxes are more likely to undergo these massive epidemics that lead to spillover events,? said disease ecologist Richard Ostfeld of the Cary Institute for Ecosystem Studies, who wasn?t involved in the study. ?In the flying foxes, it doesn?t appear to cause terrible sickness. It may have co-evolved with them to be relatively benign. But all bets are off when the virus reaches a spillover host.?

Adding to the problem are the immediate physical stresses of habitat loss and weather extremes that have become normal in Australia.

Just as stressed humans are more vulnerable to infection, so are flying foxes.

?We?ve essentially created a situation in which flying foxes are more likely to undergo these massive epidemics that lead to spillover events.?

Asked whether the findings implied that flying foxes ought to be exterminated, Plowright said that would be impractical, unethical and potentially disastrous, because the animals pollinate many of Australia?s trees.

Instead, the results highlight the importance of conservation. Parts of New South Wales that provide winter homes for still-migratory flying foxes are threatened by development; if those are not protected, even more animals will be driven into year-round urban living.

The implications don?t end with Australia?s borders. Outbreaks of Nipah virus ? an equally lethal relative of the Hendra virus, also transmitted by flying foxes but capable of passing between humans ? now occur regularly in parts of southeast Asia. ?It spills over every year and has the potential to go pandemic,? said animal disease expert Peter Daszak, president of EcoHealth Alliance and co-author of the new study.

Other intersections of disease and development occur in western Africa, where standing water from dam building leads to mosquito-borne schistosomiasis; in the Amazon, where deforestation increases malaria rates; the U.S. southwest, where rodents can carry plague and Hanta virus; and eastern U.S. suburbs where ticks carrying Lyme disease live on deer that flourish in patchy, predator-free habitats.

?People say things like, ?A healthy environment is good for our health? or, ?damaging the environment makes us sick.? Those are platitudes in that there?s no specificity,? said Ostfeld. ?We haven?t known to what extent those platitudes represent anything real. But now we?re applying rigorous concepts from evolution, epidemiology and immunology? to studying this.

?What we?re finding repeatedly is specific mechanisms by which human impacts on the environment increase disease transmission,? Ostfeld added.

?Hopefully we can predict this, and then we can stop it,? said Daszak. ?And we stop it by changing what we do on the planet.?

A 2008 diagram of the web of relationships implicated in Hendra virus outbreaks. Raina Plowright

Top image: Fruit bat. (Antaean/Flickr)

See Also:

Citation: ?Urban habituation, ecological connectivity and epidemic dampening: the emergence of Hendra virus from flying foxes (Pteropus spp.).? By Raina K. Plowright, Patrick Foley, Hume E. Field, Andy P. Dobson, Janet E. Foley, Peggy Eby and Peter Daszak. Proceedings of the Royal Society B, published online, May 11, 2011.

Source: http://www.wired.com/wiredscience/2011/05/hendra-virus-causes/

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