Tracking Disease

Protection of the environment often seems like a low-priority issue when stacked up against more immediate concerns. But a healthy environment is no mere luxury, says Mary Pearl, president of the Wildlife Trust. It is a prerequisite for human health. Pearl and her colleagues spearheaded the development of "conservation medicine"--a scientific exploration of the links between the health of humans, wildlife and ecosystems. Among the trust's current projects: a collaboration to monitor the spread of avian flu among wild birds. The trust's Consortium for Conservation Medicine has also been making headlines. Last month Science magazine published research by an international team of scientists, including Peter Daszak and Jonathan Epstein at the consortium, showing that severe acute respiratory syndrome, or SARS, originated in Chinese horseshoe bats.

Pearl spoke in New York recently with NEWSWEEK's Anne Underwood. Excerpts:

PEARL: It's an approach that evolved from the recognition of a crisis--namely, unprecedented levels of disease, driven by human-induced environmental degradation. Since the mid-1970s, more than 30 new diseases have emerged, including AIDS, Ebola, Lyme disease and SARS. Most of these are believed to have moved from wildlife to human populations. Yet no one was getting a grip on the totality of the picture. Damaged ecosystems--characterized by toxins, degradation of habitat, removal of species and climate change--create conditions for pathogens to move in ways they wouldn't normally move.

The destruction of the Peruvian rain forest, for example, has led to an explosion of malaria-bearing mosquitoes that thrive in sunlit ponds created by logging operations. Even a 1 percent increase in deforestation leads to an 8 percent increase in mosquitoes, according to Jonathan Patz at the University of Wisconsin.

In our own backyard, Lyme disease is a good example. The [bacterium in tick saliva] that causes Lyme disease has been around a very long time. But only recently has it emerged as a cause of disease in humans. That's because we have chopped up forests into suburbs. White-footed mice are happy living in little clusters of rhododendrons in the suburbs, so they proliferate. But they also carry Lyme disease, which passes from mice to the ticks that suck their blood.

In a healthy forest, there would be many more species for the ticks to feed on, including chipmunks, weasels and foxes, which are generally poor reservoirs for Lyme disease. These species also outcompete or prey on mice, reducing their numbers. As a result, a tick in the Adirondacks is less likely to carry Lyme disease than a tick in a suburb like Scarsdale.

The range of certain diseases expands. As oceans warm, sea turtles are moving farther north and south, bringing fibropapilloma virus with them and spreading it to new populations. FPV is related to human papilloma virus, which causes cervical cancer. Now, for the first time, it's been seen in manatees.

Another example is mosquito-borne diseases. A United Nations report released last week predicts the movement of malaria into southern Europe and the United States.

You never import just one species. You import the animal and all its parasites and pathogens. A few weeks ago, the British government identified the deadly H5N1 strain of bird flu in an imported parrot that was being held in quarantine. But half the wildlife trade is illegal, so it's not even being monitored.

Live-animal markets are also a concern. We recently learned that SARS originated in the Chinese horseshoe bat. The bats live in remote caves, but merchants in China brought them into live-animal markets, chopped them up on the same cutting boards used for poultry and didn't wear gloves or masks. That was a recipe for the disease to spread to humans, who normally would not come into intimate contact with bats. The good news is that once you know that, you can break the link, because there is no need for humans to eat horseshoe bats. They're not a significant source of protein, because they're so small.

When we sampled a variety of bats, there were different versions of the virus in them. It had been in them long enough to evolve into different forms. That wasn't true of civets. Also, civets in the market carried the virus, but farmed and wild civets did not. It all added up. The civet was just another victim of the SARS virus. It picked it up in the marketplace from the bats.

Jonathan Epstein, our field scientist on the case, was looking at bat populations because of Nipah virus--a deadly virus that emerged in humans in Malaysia in 1999. Nipah virus is harmless in fruit bats, but in people it causes a high fever, brain inflammation, seizures and death. It emerged in pig farms in formerly forested areas when it passed from bats to pigs and then to farmers. The Malaysian government moved quickly to shut down those pig farms and put an end to the spread of the disease in Malaysia. Subsequently there have been outbreaks in Bangladesh, so it will be important to continue monitoring fruit bats for this virus wherever the bats are found.

Our need for bats is greater than our need not to have them. About 40-50 percent of all tropical trees are pollinated by bats. Bats consume agricultural pests, and they do it without pesticides. In Mexico, bats pollinate agave, from which we get tequila. It's not bats that are to blame. Disease agents are all around us; they're part of the natural world. It's our behavior that creates the circumstances for these disease agents to move from animals into human populations.

Again, it's a situation where wildlife disease has moved into the human sphere, this time through wild birds sharing habitat with domestic birds. We're increasingly substituting ourselves and our livestock for the animals that would be the natural reservoirs of disease-causing agents.

The avian flu is not new. Our vice president, Alonso Aguirre, has been studying it for over a decade. Most forms of the disease are harmless to us and no more than a nuisance to birds. But it's a highly evolving group of viruses--and the wild birds that carry it are migratory, unlike most bats, so when a deadly flu emerges in these birds, it has the potential to spread rapidly. That's why you need good surveillance, to identify points of transmission. Then you can be hypervigilant in looking at poultry farms in those areas for signs of disease.

In October the Senate introduced an amendment on avian-flu preparedness. I was really happy to see, for the first time, money in those appropriations for migratory-bird surveillance. This is the first time Congress has recognized the importance of wildlife-disease dynamics in the spread of human disease.

Marine mammals are getting diseases that used to be strictly terrestrial, like distemper and chlamydia. There's a lot of disease transmission through densely populated coastal zones. You know that innocent walk with the dog along the beach, where you don't clean up after the dog? That can be the source of problems.

Absolutely. I foresee the time when departments of public health will have conservation-medicine specialists. It's rare for diseases to jump between species. But we can look for hot spots in landscapes, where problems are likely to arise.

Tracking Disease | News