Menus Subscribe Search

Follow us


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

November 21 • 4:00 PM

Why Are America’s Poorest Toddlers Being Over-Prescribed ADHD Drugs?

Against all medical guidelines, children who are two and three years old are getting diagnosed with ADHD and treated with Adderall and other stimulants. It may be shocking, but it’s perfectly legal.



November 21 • 2:00 PM

The Best Moms Let Mess Happen

That’s the message of a Bounty commercial that reminds this sociologist of Sharon Hays’ work on “the ideology of intensive motherhood.”


November 21 • 12:00 PM

Eating Disorders Are Not Just for Women

Men, like women, are affected by our cultural preoccupation with thinness. And refusing to recognize that only makes things worse.


November 21 • 10:00 AM

Queens of the South

Inside Asheville, North Carolina’s 7th annual Miss Gay Latina pageant.


November 21 • 9:12 AM

‘Shirtstorm’ and Sexism in Science

Following the recent T-shirt controversy, it’s clear that sexism in science persists. But the forces driving the gender gap are still being debated.


November 21 • 8:00 AM

What Makes a Film Successful in 2014?

Domestic box office earnings are no longer a reliable metric.



November 21 • 6:00 AM

What Makes a City Unhappy?

According to the National Bureau of Economic Research, Dana McMahan splits time between two of the country’s unhappiest cities. She set out to explore the causes of the happiness deficits.


November 21 • 5:04 AM

Sufferers of Social Anxiety Disorder, Your Friends Like You

The first study of friends’ perceptions suggest they know something’s off with their pals but like them just the same.


November 21 • 4:00 AM

In 2001 Study, Black Celebrities Judged Harshly in Rape Cases

When accused of rape, black celebrities were viewed more negatively than non-celebrities. The opposite was true of whites.


November 20 • 4:00 PM

Women, Kink, and Sex Addiction: It’s Not Like the Movies

The popular view is that if a woman is into BDSM she’s probably a sex addict, and vice versa. In fact, most kinky women are perfectly happy—and possibly healthier than their vanilla counterparts.


November 20 • 2:00 PM

A Majority of Middle-Class Black Children Will Be Poorer as Adults

The disturbing findings of a new study.


November 20 • 12:00 PM

Standing Up for My Group by Kicking Yours

Members of a minority ethnic group are less likely to express support for gay equality if they believe their own group suffers from discrimination.


November 20 • 10:00 AM

For Juvenile Records, It’s ‘Justice by Geography’

A new study finds an inconsistent patchwork of policies across states for how juvenile records are sealed and expunged.


November 20 • 8:00 AM

Surviving the Secret Childhood Trauma of a Parent’s Drug Addiction

As a young girl, Alana Levinson struggled with the shame of her father’s substance abuse. But when she looked more deeply into the research on children of drug-addicted parents, she realized society’s “conspiracy of silence” was keeping her—and possibly millions of others—from adequately dealing with the experience.



November 20 • 6:00 AM

Extreme Weather, Caused by Climate Change, Is Here. Can Nike Prepare You?

Following the approach we often see from companies marketing products before big storms, Nike focuses on climate change science in the promotion of its latest line of base-layer apparel. Is it a sign that more Americans are taking climate change seriously? Don’t get your hopes up.


November 20 • 5:00 AM

How Old Brains Learn New Tricks

A new study shows that the neural plasticity needed for learning doesn’t vanish as we age—it just moves.


November 20 • 4:00 AM

The FBI’s Dangerous Misrepresentation of Encryption Law

The FBI no more deserves a direct line to your data than it deserves to intercept your mail at the post office. But it doesn’t want you to know that.


November 20 • 2:00 AM

Brain Drain Is Economic Development

It may be hard to see unless you shift your focus from places to people, but both destination and source can benefit from “brain drain.”


November 19 • 9:00 PM

Gays Rights Are Great, but Ixnay on the PDAs

New research suggests both heterosexuals and gay men are uncomfortable with public same-sex kissing.


November 19 • 4:00 PM

The Red Cross’ Own Employees Doubt the Charity’s Ethics

Survey results obtained by ProPublica also show a crisis of trust in the charity’s senior leadership.



November 19 • 2:00 PM

Egg Freezing Isn’t the Feminist Issue You Think It Is

New benefits being offered by Apple and Facebook probably aren’t about discouraging women from becoming mothers at a “natural” age.


Follow us


Sufferers of Social Anxiety Disorder, Your Friends Like You

The first study of friends' perceptions suggest they know something's off with their pals but like them just the same.

Standing Up for My Group by Kicking Yours

Members of a minority ethnic group are less likely to express support for gay equality if they believe their own group suffers from discrimination.

How Old Brains Learn New Tricks

A new study shows that the neural plasticity needed for learning doesn't vanish as we age—it just moves.

Ethnic Diversity Deflates Market Bubbles

But it's not in the rainbow and sing-along way you'd hope for. We just don't trust outsiders' judgments.

Online Brain Exercises Are Probably Useless

Even under the guidance of a specialist trainer, computer-based brain exercises have only modest benefits, a new analysis shows.

The Big One

One in two United States senators and two in five House members who left office between 1998 and 2004 became lobbyists. November/December 2014

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