June 2, 2009, Science Line: Scientists study how biodiversity affects the spread of animal-borne disease.


Is Dilution the Solution?

American robins are one of many wildlife species that can thrive in developed areas near humans, and spread diseases to them. [Credit: Colin Purrington, flickr.com] Below left, Researchers survey scat to estimate the local population of deer. [Credit: Lynne Peeples]

American robins are one of many wildlife species that can thrive in developed areas near humans, and spread diseases to them. [Credit: Colin Purrington, flickr.com] Below left, Researchers survey scat to estimate the local population of deer. [Credit: Lynne Peeples]

biology | environment

Scientists study how biodiversity affects the spread of animal-borne disease.

Posted By Lynne Peeples on June 4th, 2009 in biology | environment

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, [1] Dutchess County, has one of the highest rates of [2] 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 [3] 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 [4] West Nile virus or [5] 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 [6] 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 [7] 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 [8] 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 [9] 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.

Several studies published over the last few months have bolstered the case for the dilution effect in both West Nile and Lyme diseases. Washington University’s Allan looked at birds and mosquitoes along a gradient of rural-to-urban sites in the St. Louis area. As he moved from habitats with few humans to those with higher densities he found a simultaneous drop in biodiversity and rise in West Nile prevalence. “As diversity goes up, disease goes down,” says Allan, whose study was published in January 2009 in the journal Oecologica.

His findings align with work by [10] John Swaddle, a biologist at the College of William and Mary in Virginia, published just a few months earlier in the journal PLoS ONE. Rather than counting species over tracts of land, Swaddle used public bird counts and health department reports to compare bird diversity between pairs of adjacent counties with and without human cases of West Nile. This way, he was able to factor out other potential explanations, such as differences in weather. Again, less biodiversity meant more disease.

Not all West Nile research supports such a direct relationship, however. A recent study in the Chicago area found no association between the number of species and cases of West Nile. Published in Oecologica in March 2009, it addressed the dilution effect at a finer scale than previous state or countywide analyses.

The lack of perfect consistency among the studies hints at some unanswered questions. “We have good patterns,” says[11] A. Marm Kilpatrick, an infectious disease ecologist at the University of California, Santa Cruz. “But we don’t fully understand the mechanisms for those patterns.”

Many ecologists believe the dilution effect is more complicated than simply saying greater diversity leads to less disease. Growing evidence suggests that individual species do play a significant role. American robins, for example, appear to be one of the key hosts for West Nile. “Even if they only make up 1 to 10 percent of the birds in a site, they’re often 40 to 60 percent of the blood meals,” says Kilpatrick.

In the Lyme disease model, Ostfeld of the Cary Institute recently found that the total number of species was less predictive of disease than the actual identities and abundance of those species. The presence of white-footed mice was most vital to transmission, according to a study he co-authored in Ecology in October 2008.

This ambiguity inspired Ostfeld to continue digging deeper into disease dynamics, which is why he and his team are working in Dutchess County this year. Until the researchers are ready to begin trapping and transporting forest animals this summer, they remain busy counting creatures — and droppings.

Manipulating species, and counting feces

On this warm spring afternoon, Ostfeld’s colleague Shannon Duerr has found an unusually large number of droppings on Tract Number 36, including one circle boasting seven piles. This likely means “lots of deer, lots of mice, lots of ticks,” she says. The tally also means she can put her name on the Deer Poop Count Leader Board back at the Institute — a humorous “competition” that helps keep this messy and monotonous field work bearable.

The team’s latest research suggests that in addition to some species being unusually good hosts, others are really bad ones. Originally, a species was thought to “dilute” simply by providing a meal for ticks, but not infecting them. Now, certain animals have been found to act as especially powerful diluters because they are inhospitable, or even poisonous, hosts for ticks. “It looks like some species actually go out there and kill lots of ticks,” says Ostfeld, highlighting the opossum as one of these deadly hosts. At the other extreme, he says, white-footed mice are great for both boosting and infecting tick populations.

Just as Santa Cruz’s Kilpatrick doesn’t recommend hunting American robins — his research suggests that when robins disperse, mosquitoes’ next choice is people — Ostfeld’s work doesn’t mean that ecologists will be introducing furry “ecological traps” into urban parks anytime soon. “Imagine air-dropping opossums,” says Ostfeld. “I don’t think that would fly.”

On an experimental scale, however, Ostfeld is doing his own species manipulation in Dutchess County. His team hopes that a better understanding of the connections between habitat size, biodiversity and the spread of pathogens will eventually lead to actions that could dilute not only West Nile and Lyme disease, but also new diseases before they arise.

While people know more about animal-borne diseases now than they did several decades ago, Ostfeld sees a continuing need to alert the public to the important role biodiversity plays in keeping disease in check. “People care about their health a lot,” he says. “The notion that biodiversity might influence their probability of getting sick is a strong motivator.”

Public awareness could not only help prevent disease, Ostfeld says, it could also promote conservation — especially because the species involved are charismatic animals that are attractive to humans. “We’re talking about furry, feathered, colorful, warm fuzzy creatures,” he says. “People care about those as well.”

Related on Scienceline:

Preserving [12] amphibian biodiversity.

What species give you [13] Lyme disease?

Why are [14] mosquitoes selective in who they bite?

Location: http://scienceline.org/2009/06/04/peeples-environment-animal-borne-disease-lyme-west-nile-virus/

URLs in this post:
[1] Dutchess County: http://www.dutchesstourism.com/maps.asp#here
[2] Lyme disease: http://www.cdc.gov/ncidod/dvbid/Lyme/
[3] Cary Institute for Ecosystem Studies: http://www.ecostudies.org/
[4] West Nile virus: http://www.cdc.gov/ncidod/dvbid/westnile/index.htm
[5] Lyme disease: http://www.cdc.gov/ncidod/dvbid/Lyme/
[6] Rick Ostfeld: http://www.ecostudies.org/people_sci_ostfeld.html
[7] Brian Allan: http://bigballan.googlepages.com/
[8] zoonotic diseases: http://www.vetmed.wisc.edu/pbs/zoonoses/
[9] Lyme, Connecticut: http://www.ct.gov/dph/cwp/view.asp?a=3136&q=388506
[10] John Swaddle: http://jpswad.people.wm.edu/
[11] A. Marm Kilpatrick: http://www.eeb.ucsc.edu/faculty/kilpatrick.html
[12] amphibian biodiversity: http://scienceline.org/2009/05/22/konkel-bio-amphibians-decline/
[13] Lyme disease: http://scienceline.org/2008/07/14/ask-hamalainen-tickbites/
[14] mosquitoes selective: http://scienceline.org/2007/09/10/ask-knight-mosquitoes/

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