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Loss of biodiversity can increase disease transmission

BLACKSBURG, Va., Dec. 3, 2010 – Current species extinction rates are estimated to be at least 100 times higher than normal background levels. This loss of biodiversity has many costs, including the alteration of many “ecosystem services,” such as flood control, water purification, pollination of crop plants, and soil production. A paper in the Dec. 2 issue of Nature, co-authored by a Virginia Tech faculty member, provides a review of the scientific literature relating to yet another important impact of biodiversity loss, namely how this loss can result in the increase of the transmission and spread of infectious diseases, such as Lyme disease and West Nile.

Felicia Keesing, associate professor of biology at Bard College in Annandale, N.Y. and Richard Ostfeld, disease ecologist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y. instigated the study and invited 11 others to contribute, including Lisa Belden, assistant professor of biological sciences in the College of Science at Virginia Tech.

"The consequences of losing biodiversity have been discussed for decades," said Belden. "In recent years, people have begun talking about increased disease transmission as a consequence of loss of biodiversity."

As a result, there has been an expansion from the traditional focus of understanding only host and pathogen interactions to the inclusion of ecological and environmental factors. "A lot of emerging diseases come from animals or are transmitted by insect vectors – mosquitoes, ticks. We need to think about how species interact in these complex disease systems, and about environmental conditions, such as rainfall, climate change, and changes in land use. People have only recently begun to do that," said Belden.

The article in Nature cited research revealing that several mechanisms can be responsible for the protective effect of biodiversity. For example, the spread of pathogens can be limited in diverse systems because there are more “dead-end” hosts that don’t support pathogen growth or transmission. In addition, loss of species often increases encounter rates between pathogens and hosts. The classic example of this is with Lyme disease in the Northeastern United States. When there are abundant species in the system, the ticks that vector the pathogen end up biting a lot of different host species, such as opossums, that either kill most of the ticks that attach to them or are poor hosts of the pathogen. In contrast, as species are lost from these systems. White-footed mice, which are very good pathogen hosts, become more abundant and more commonly used by the ticks, which increases the human risk of disease in these low diversity systems.

Another component of pathogen transmission is diversity within individual hosts, Belden said. "Macro systems, such as Lyme disease, are what are best understood. But there are diverse microbial systems within animals that can influence disease outcomes as well, such as the microbes within humans. Based on cell number alone, we humans are mostly bacterial, and most of that microbial diversity resides within our guts. We know that the composition of these microbial communities can affect many characteristics, including whether we are lean or obese," she said. "We also know that some disease outcomes may be influenced by these microbial communities. I would argue we should be careful about how we treat our microbes. A single dose of antibiotics can have long-term impacts, so they shouldn’t be overused; they shouldn’t be prescribed or taken for viral infections like the common cold.”

The article authors have specific recommendations for additional research and management approaches, such as targeted surveillance against wildlife pathogens where there has been land-use change; preserving intact habitats in these areas to reduce human-animal contact; and reducing contact between wildlife and domestic animals in areas of high-density animal husbandry. They conclude, "We must increase the number of disease systems for which we understand the effects of biodiversity, … implement specific policies informed by this science, (and) monitor changes in epidemiology in regions in which conservation measures are imposed."

Belden and her graduate students work in wildlife systems. They study how biodiversity and community-level species interactions affect disease outcomes in two different systems. One system she studies involves parasites with complex lifecycles that have multiple hosts, including snails, amphibians and muskrats. She is also examining how microbial diversity on the skin of frogs can prevent infection with a lethal fungus that is devastating many amphibian populations around the world.

“Ecologists have an important role to play in advancing our knowledge of disease dynamics,” Belden said. “We think about complex systems and how species interact and what the consequences are of those interactions, and also, what happens when we start removing those species and interactions. Biodiversity loss has clear consequences. We’ve known about many of these consequences for a long time, and now we can add an increased risk of many infectious diseases to that list.”

Belden is an affiliated faculty member with the Fralin Life Science Institute at Virginia Tech.

The article, "Impacts of Biodiversity on the Emergence and Transmission of infectious Disease," appeared in the Dec. 2, 2010, issue of Nature. Authors are Keesing; Belden; Peter Daszak of the Wildlife Trust, New York, N.Y.; Andrew Dobson of the Department of Ecology and Evolutionary Biology, Princeton University; C. Drew Harvell of the Department of Ecology and Evolutionary Biology, Cornell University; Robert D. Holt of the Department of Biology, University of Florida; Peter Hudson of the Center for Infectious Disease Dynamics, Pennsylvania State University; Anna Jolles of the College of Veterinary Medicine, Oregon State University; Kate E. Jones of the Institute of Zoology, Zoological Society of London; Charles Mitchell of the Department of Biology, The University of North Carolina at Chapel Hill; Samuel S. Myers of Harvard Medical School; Tiffany Bogich of the Wildlife Trust; and Ostfeld. The lead authors' news release was posted by the National Science Foundation.

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