Menus Subscribe Search

Keeping More Species Around May Dilute Disease Threat

• June 23, 2009 • 11:35 PM

A study on hantavirus and Panama rats suggests another and less obvious benefit to biodiversity — it may diminish the threat humans face from zoonotic diseases.

Biodiversity provides humanity with many benefits, including clean air and water, climate stability and renewable natural resources.

But a groundbreaking experimental study has shown that species diversity is good for something else: It protects people from dangerous zoonotic (animal-borne) diseases.

Scientists investigating an outbreak of hantavirus among farmers in Panama’s Azuero Peninsula discovered the disease was harbored in two particular rodent species that thrived in areas where tropical forest had recently been cleared for cattle pasture.

In their experiment, researchers mimicked human-caused habitat degradation by removing all the native rodent species from selected plots of land at the forest-pastureland interface except for the two hantavirus-linked species, the pygmy rice rat and the cane rat. Without competition from other species, their numbers exploded — and more of the rodents became infected with hantavirus.

The study, titled “Experimental Evidence for Reduced Rodent Diversity Causing Increased Hantavirus Prevalence” and published online at PLoS One, was the first controlled test of a theoretical disease transmission mechanism known the “dilution effect,” said its lead author, Gerado Suzán.

“Diversity is producing a service of reducing the (disease) reservoirs,” said Suzán, a faculty member at the National Autonomous University of Mexico. “A healthy environment is also protecting human health.”

The idea that greater biodiversity might reduce disease transmission originated with British ecologist Charles Elton, who noticed that a greater diversity of plant species was accompanied by a lower rate of disease, according to Richard S. Ostfeld, a participant in the Panama study.

Scientists meanwhile were exploring the concept of zooprophylaxis — using animals to protect human health, Ostfeld said. They noticed that when people lived with their domestic animals, they tended to have lower rates of malaria, apparently because mosquitoes bit the livestock, rather than the people.

Ostfeld, an animal ecologist at the Cary Institute of Ecosystem Studies in upstate New York, has been studying the mechanisms underlying zoonotic disease transmission in collaboration with Felicia Keesing, a biologist at Bard College. They developed their “dilution effect” model from their research into how a host species — deer ticks — transmit Lyme disease via white-footed mice.

A fundamental principle in epidemiology says that the transmission rate of a disease and a pathogen’s reproductive success are proportional to the abundance of the host. The Ostfeld-Keesling theory proposes that with greater species diversity, there is less likelihood that susceptible members of the host species will come into contact with infected individuals, Ostfeld said.

When diversity is reduced, “These reservoir species interact with each other with higher intensity as they fight and mate, meaning that there is more opportunity for infected animals to transmit the virus to others,” he said.

Ostfeld added that having greater species diversity could also limit the numbers of the host species by increasing the competition for food and other resources. There is also the phenomenon of “spillover,” in which another species susceptible to infection with a pathogen (but incapable of transmitting it) acts as a “sink” that blocks the germ’s spread.

Although the dilution effect theory had been examined in abstract epidemiological models, the Suzán study was the first experimental test of a dilution effect in a natural ecosystem, Ostfeld said.

The study had its roots in the first Central American outbreak of rodent-borne hantavirus, a tiny strand of RNA that can cause an untreatable, lethal lung inflammation. As a zoonotic disease it usually is acquired only from close exposure to the host species. In Panama, nearly 40 people became infected with hantavirus between 1999 and 2004, and 22 percent of them died.

A strain of hantavirus caused panic in the Four Corners region of the U.S. in May and June 1993 when more than a dozen people — many of them young and otherwise healthy — died from acute respiratory failure. The late Terry Yates, a University of New Mexico biologist, led a field study showing that the main species harboring the infection was the deer mouse, a species whose numbers had skyrocketed because heavy rains had created a bumper crop of the piñon nuts the mice favored. Yates was asked to share his expertise in the Panama outbreak and brought along Suzán, who was one of his graduate students at the time.

The Azuero Peninsula, which juts southward into the Pacific Ocean, has a varied pattern of rainfall and vegetation, and much of it is covered by tropical forest. But the forest is increasingly giving way to human encroachment as trees are cleared for crops and cattle.

In 2003, Suzan’s team mapped out 24 sites in four different areas at the edge of the forest, spread across a 40-mile swath of the peninsula. At each location, they set up two control and four experimental sites. At the control sites they live-captured the nine native rodent species, took blood samples and released them. At the experimental sites they removed all rodent species except for the pygmy rice rat and the cane rat.

The field work, carried out during the rainy season, was difficult, Suzán said. In addition to collecting blood samples, researchers had to weigh, measure and ear-tag the released subjects, while conducting simple tests for hantavirus infection.

The work was complicated by tropical downpours and rain-swollen rivers, which prevented the teams from getting to the study sites, Suzán said. On one occasion, farmers even cut trees in the middle of a study plot during the ongoing research in order to create more grazing land.

Suzán notes that the hantavirus carriers thrive in proximity to humans because they are “very plastic species. They eat different kinds of food. They tolerate different climactic conditions. They are generalist species.”

Panamanian public health officials have been educating farmers about keeping the areas around their homes clean, so as to minimize the risk of encountering infected rodents, Suzán said. “The hantavirus is going to be there because it has been there for thousands of years,” he said. “If we take care of the conditions that lead to outbreaks in rodents, the more we’ll prevent outbreaks among humans.”

Ostfeld says that the study has deep policy implications. “Biodiversity loss is probably at least as important as climate change in terms of overall impacts on human society,” he said.

If biodiversity loss can be directly tied to an increase in human, plant and animal disease, it becomes a concrete factor to be taken into account when human development threatens natural habitat, he said.

“The next step is to try to change policy so that biodiversity loss is reduced.”

Sign up for our free e-newsletter.

Are you on Facebook? Become our fan.

Follow us on Twitter.

Add our news to your site.

Michael Haederle
Michael Haederle lives in New Mexico. He has written for the Los Angeles Times, People Magazine, Tricycle: The Buddhist Review and many other publications. He has also taught at Syracuse University's S.I. Newhouse School of Public Communications and is a Zen lay monk.

More From Michael Haederle

A weekly roundup of the best of Pacific Standard and PSmag.com, delivered straight to your inbox.

Recent Posts

August 22 • 10:00 AM

Turbo Paul: Art Thief Turned Art Crime Ombudsman

There’s art theft, there’s law enforcement, and, somewhere in between, there’s Turbo Paul.


August 22 • 8:00 AM

When Climate Change Denial Refutes Itself

The world is warming—and record-cold winters are just another symptom.


August 22 • 6:17 AM

The Impossibility of the Night Shift

Many night workers get “shift-work sleep disorder.” And no one knows how to treat it.


August 22 • 6:00 AM

Long Live Short Novels

Christopher Beha’s Arts & Entertainments comes in at less than 300 pages long, which—along with a plot centered on a sex-tape scandal—makes it a uniquely efficient pleasure.


August 22 • 4:00 AM

Why ‘Nature Versus Nurture’ Often Doesn’t Matter

Sometimes it just doesn’t make any sense to try to separate the social and the biological.


August 21 • 4:00 PM

Julie Chen Explains Why She Underwent Westernizing Surgery

The CBS news anchor and television personality’s story proves that cosmetic surgeries aren’t always vanity projects, even if they’re usually portrayed that way.


August 21 • 2:37 PM

How the Brains of Risk-Taking Teens Work

There’s heightened functional connectivity between the brain’s emotion regulator and reason center, according to a recent neuroscience paper.


August 21 • 2:00 PM

Cracking Down on the Use of Restraints in Schools

Federal investigators found that children at two Virginia schools were being regularly pinned down or isolated and that their education was suffering as a result.


August 21 • 12:00 PM

What Makes You So Smart, School Principal?

Noah Davis talks to Evan Glazer about why kids aren’t getting smarter and what his school’s doing in order to change that.



August 21 • 10:00 AM

Why My Neighbors Still Use Dial-Up Internet

It’s not because they want to. It’s because they have no other choice.


August 21 • 8:15 AM

When Mothers Sing, Premature Babies Thrive

Moms willing to serenade pre-term infants help their babies—and themselves.


August 21 • 8:00 AM

To Fight the Obesity Epidemic Americans Will Have to First Recognize That They’re Obese

There is a void in the medical community’s understanding of how families see themselves and understand their weight.


August 21 • 6:33 AM

One Toxic Boss Can Poison the Whole Workplace

Office leaders who bully even just one member of their team harm everyone.


August 21 • 6:00 AM

The Fox News Effect

Whatever you think of its approach, Fox News has created a more conservative Congress and a more polarized electorate, according to a series of recent studies.


August 21 • 4:00 AM

Do Children Help Care for the Family Pet?

Or does mom do it all?


August 20 • 4:00 PM

Why Can’t Conservatives See the Benefits of Affordable Child Care?

Private programs might do a better job of watching our kids than state-run programs, but they’re not accessible to everyone.


August 20 • 2:00 PM

Oil and Gas Companies Are Illegally Using Diesel Fuel in Hundreds of Fracking Operations

An analysis by an environmental group finds hundreds of cases in which drillers used diesel fuel without obtaining permits and sometimes altered records disclosing they had done so.


August 20 • 12:00 PM

The Mystery of Britain’s Alien Big Cats

In a nation where the biggest carnivorous predator is a badger, why are there so many reported sightings of large cats?


August 20 • 10:00 AM

Death Row in Arizona: Where Human Experimentation Is the Rule, Not the Exception

Recent reports show that chemical roulette is the state’s M.O.


August 20 • 9:51 AM

Diversity Is in the Eye of the Beholder

Perception of group diversity depends on the race of the observer and the extent to which they worry about discrimination.


August 20 • 8:40 AM

Psychopathic or Just Antisocial? A Key Brain Difference Tells the Tale

Though psychopaths and antisocial people may seem similar, what occurs in their brains isn’t.


August 20 • 8:00 AM

What the Cost of Raising a Child in America Tells Us About Income Inequality

You’ll spend nearly a quarter of a million dollars to raise a kid in the United States, or about five times the annual median income.


August 20 • 6:00 AM

In Praise of ‘American Greed’

While it remains semi-hidden on CNBC and can’t claim the car chases of Cops, American Greed—now with eight seasons in the books—has proven itself a worthy endeavor.


August 20 • 4:00 AM

Of Course I Behaved Like a Jerk, I Was Just Watching ‘Jersey Shore’

Researchers find watching certain types of reality TV can make viewers more aggressive.


Follow us


The Impossibility of the Night Shift

Many night workers get “shift-work sleep disorder.” And no one knows how to treat it.

How the Brains of Risk-Taking Teens Work

There's heightened functional connectivity between the brain's emotion regulator and reason center, according to a recent neuroscience paper.

When Mothers Sing, Premature Babies Thrive

Moms willing to serenade pre-term infants help their babies—and themselves.

One Toxic Boss Can Poison the Whole Workplace

Office leaders who bully even just one member of their team harm everyone.

Diversity Is in the Eye of the Beholder

Perception of group diversity depends on the race of the observer and the extent to which they worry about discrimination.

The Big One

One in two full-time American fast-food workers' families are enrolled in public assistance programs, at a cost of $7 billion per year. July/August 2014 fast-food-big-one
Subscribe Now

Copyright © 2014 by Pacific Standard and The Miller-McCune Center for Research, Media, and Public Policy. All Rights Reserved.