Is Dilution the Solution?

Shannon Duerr counts excrement. She kneels on the forest floor and, between picking hungry ticks off her arm, carefully tallies a pile of at least ten pellets she has collected. Since more than half of the pellets fall inside a hundred-square-foot circle she has encompassed with a short metal pole and a long piece of string, Duerr estimates that at least one white-tailed deer has passed through here since the winter’s first freeze.

This part of upstate New York, Dutchess County, has one of the highest rates of Lyme disease in the country. Duerr is part of a research team that is trying to understand why the region is such a hotbed for the disease, which is carried by animals and can sicken humans. Clad in white suits smeared with deer feces, she and other scientists from the Cary Institute for Ecosystem Studies in nearby Millbrook, New York are busy calculating the local deer population as a prelude for conducting an unusual experiment. The researchers want to know whether the incidence of Lyme disease will change if the composition of critters in a community changes, and if the presence of certain species might “dilute” the concentration of the virus. To that end, they will trap mice, chipmunks and gray squirrels and redistribute them between small patches of forest they have zoned for the study. And then they will watch to see what happens.

“We’re shaking up the densities of Lyme disease players,” says Duerr, a senior research specialist at the Cary Institute. Of course, moving around white-tailed deer is a far more formidable challenge than collecting and transplanting white-footed mice. So instead of altering deer populations in forest tracts, the researchers rely on feces counts to estimate their numbers. These will be included in later mathematical models along with data from the redistributed animals.

Familiar faces in urban spaces

Both mice and deer belong to a growing lineup of recognized carriers of infectious disease. Hidden behind fur coats or colorful feathers, they may look innocent, yet some of these hardy co-habitants of urban and suburban landscapes — American robins, blue jays, chipmunks, shrews and many others — are spreading diseases that can be lethal to humans.

The small islands of green in our backyards and parks are stripped-down ecosystems harboring relatively few species compared to the rich variety of animals and plants in larger swaths of open land. Since scientists have found that humans are more likely to get West Nile virus or Lyme disease if they live in areas where biodiversity is low, Duerr and her colleagues want to learn which of these resilient animals — and in what combinations — have the greatest effect on disease, and in which spaces they are found. The answers could affect future land-use decisions and prompt new strategies to lower rates of infection.

Hardy species “have something in common that makes them both more resilient to human-disturbance and more permissive to pathogens,” says Rick Ostfeld, a senior scientist at the Cary Institute who is leading this summer’s field experiment in Dutchess County. “As you chop up the woods, pave things over, and put in strip malls, a lot of wildlife species will disappear. But a few, like mice, don’t.”

Why the same species we find close to our homes and cities also tend to be the most prone to passing on pathogens intrigues Ostfeld. And his current research could help explain this part of the mysterious relationship between biodiversity and disease. He thinks that certain short-lived animals like chipmunks and mice may have evolved a “live fast, die young” lifestyle that predisposes them to be less affected by habitat destruction, and more sensitive to bacteria and viruses.

“If you are going to die from predation before you ever die from infectious disease,” says Ostfeld, “then you might allocate your limited energy to higher reproductive rates and predator avoidance rather than an energetically expensive adaptive immune response.” That may be why these rapidly reproducing species have evolved into pathogen-promoting machines.

Other disease experts recognize the same patterns. “Most West Nile competent hosts thrive in human-disturbed environments,” says Brian Allan, who studies the ecology of the West Nile virus at Washington University in St. Louis. “You’re much more likely to go into Central Park and see a large number of American robins [which carry the virus] than if you went into the Catskills.”

Diluting disease

West Nile and Lyme are zoonotic diseases, meaning they emerge from non-human wildlife populations to infect people. In the case of West Nile, a mosquito acts as the viral messenger between animal and human. If a mosquito carrying the virus bites a bird — like the American robin — that is able to develop a substantial viral load in its blood, then the next mosquito to feast on that bird may acquire the pathogen. The more of these virulent pests there are flying around, the more likely one will bite a human and transmit the zoonotic pathogen. The number of humans infected has fluctuated across the country since West Nile arrived in New York State in 1999. According to the Centers for Disease Control, there were on average over 4,500 cases and 160 deaths per year in the United States since 2002.

A similar process of transmission holds true for the more common Lyme disease, with 26,739 confirmed U.S. cases in 2008. (Although Ostfeld thinks both Lyme and West Nile numbers may be significant undercounts due to budget cuts at county health departments, which are the source of the CDC’s data.) Since the first cases were recognized in the mid-1970s in Lyme, Connecticut, deer ticks have been implicated as the wildlife-to-human smugglers.

When a mosquito or tick feeds on an animal that can’t transmit the infectious agent, however, the pathogen hits a dead end. If a significant number of these weak hosts are present, then fewer mosquitoes and ticks will be carrying the pathogens, and the risk of humans contracting the disease diminishes. This watering-down of disease was coined the “dilution effect” by Ostfeld in a 2000 paper.