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Published On Apr 22, 2015

Basic Research

Nature Bites Back

More than half of infectious diseases pass through animals first. A strain on their natural habitats may be making these ailments more dangerous.

Earth is a planet radically transformed. The march of human progress has turned more than 40% of the planet’s available land into farmland or cities, and far more is impacted by our activity. Every year, Earth Day offers an opportunity to reflect on how this new natural order affects all of the organisms found here—plant, animal and human.

Shifting patterns of infectious disease may be the newest consideration to add to a very long list of questionable human byproducts. Researchers who focus on the intersection of disease and ecology are narrowing in on a troubling question: are altered ecosystems making pathogens more virulent and likely to spread?

An estimated 60% of all infectious diseases are classified as zoonoses, which means that they travel through other vertebrate species before breaking out in humans. Ebola is zoonotic. So are many strains of influenza, including the highly lethal bird flus that—should they acquire mutations that make them more infectious to humans—could kill millions. Each of these diseases spends time in the wild. And as their host species’ habitats and life cycles are disturbed, these diseases may be given increased opportunity to thrive.

Such an understanding represents a sea change. Zoonoses had once been seen primarily as the result of untamed nature, the byproduct of humans moving into wild places and encountering animals with disease. And certainly this is true in some cases. HIV likely crossed from chimpanzees to humans during the early-twentieth-century colonization of central Africa. But while railroad-building and forest-clearing exposed people to the virus, it didn’t actually change the dynamics of the disease.

The new paradigm looks at the ways that human development may alter how those diseases arise in the first place. "It's not as simple as, 'The forest held these dangers, and now we've entered it and gotten bitten,'" explains Shannon LaDeau, a disease ecologist at the Cary Institute of Ecosystem Studies. "Rather, by entering the forest, we've changed fundamental community interactions inside the forest, before we get bitten."

One of the best-studied examples involves the Hendra virus, one of a family of viruses discovered during the past two decades in Australia and South Asia. (The viruses kill more than half the people infected by them, a fact that inspired the mega-bug in the movie Contagion. Fortunately, Hendra is currently not easily transmitted.) The first human Hendra cases occurred in people who worked with horses. The horses had picked up the disease while feeding beneath trees inhabited by fruit bats, which are Hendra’s natural host species.

The disease circulates among fruit bats, and mostly stays there. But human development tipped the balance. Habitat development disrupted the bats’ migration patterns, fragmented their populations and attracted them to food-rich urban areas. As a consequence, the bats were stressed and prone to Hendra infections, which began to erupt in boom-and-bust cycles. These booms increased the chance of spillover into other species, including humans.

North America isn’t exempt from similar examples. On the densely populated eastern seaboard, urbanization appears to have favored mice and deer, which can host the ticks that transmit Lyme disease, and birds, which can carry West Nile virus. It also favors Asian tiger mosquitos, which thrive in pool-rich, insectivore-poor urban areas, and also are carried by American robins and crows. LaDeau, whose specialty is mosquito-borne disease, worries about what this means for the spread of formerly tropical scourges such as malaria, Chikungunya virus and Dengue fever.

Why do these disease-carrying species thrive alongside humans? There’s a paucity of predators to control them. Such species also tend to be adaptable to the tumult of human development. Often, as with mice, robins and sparrows, they live briefly and breed frequently, reducing the evolutionary importance of investing heavily in their immune systems. As a consequence they may have physiologies unusually well-suited to hosting diseases that also sicken us.

"Urban and suburban environments are often the riskiest landscapes when it comes to zoonotic disease," says Rick Ostfeld, another Cary Institute disease ecologist, whose own research focuses on tick-borne afflictions. These include not just Lyme disease, but also anaplasmosis, babesiosis and Powassan encephalitis, a series of once rare but now increasingly common infections.

So what should be done? Eliminating or controlling the host species may seem like one answer, if one with limited historical success. (Just ask a rat in Manhattan.) But perhaps a different prescription is called for. Rather than keeping nature at arm’s length, more fundamental changes that promote richer, healthier ecosystems may have more lasting effects.

Mosquitoes are better controlled when mosquito-eating swallows and bats have places to live. Unpolluted waterways draw dragonflies and fish, which also eat mosquito larvae. Vulnerable shorelines can be protected from development, helping migrating waterfowl find enough to eat and making them less likely to catch and spread the flu. And rodent-eating predators such as hawks and foxes might be offered a niche in urban life.

More fundamentally, humans can foster a nature that's rich in life rather than dominated by a few plucky species, a nature connected rather than fragmented in disease-prone pockets. Healthier ecosystems may well make for healthier people.

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