Species diversity said to have positive and negative influences on disease transmission

According to a University of Montana researcher there is a possible association between species diversity and positive and negative impacts on disease transmission in the same host-pathogen system at the same time.

The study aims to answer the debate whether biodiversity loss leads to an increased disease risk. For a number of species, a more diverse community decreases infection risk, termed “the dilution effect,” because biodiversity dilutes infection. If this is a widespread phenomenon, then preserving biodiversity is a win-win for both animal conservation and human health. However, a debate rages about how general this phenomenon is since, for some systems, a more diverse community increases disease risk, termed the “amplification” effect. For other systems, biodiversity has no consistent effect, leaving its relationship to disease unidentified.

In the latest issue of PNAS researchers show that the Sin Nombre hantavirus among rodents displays a significant dilution effect. Areas with a more diverse rodent community have lower hantavirus prevalence among deer mice, which are the main reservoir for the disease. When the virus spills over into humans, it causes hantavirus pulmonary syndrome, which has infected more than 700 people and killed about 1 in 3 since its discovery in 1993.

The study shows deer mouse density causes the dilution effect. In more diverse communities – with more rodent species present in the same area – there tend to be fewer mice due to competition, and disease spread slows down.

However, for a given mouse density, as rodent species diversity increases, infection spreads faster in a “component amplification effect” as mice become stressed and their immunity decreases. Therefore both dilution and amplification occur in the same system at the same time. It’s not as simple as more biodiversity means less disease.