Keeping More Species Around May Dilute Disease Threat

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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.”

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