The vast majority of new, emerging or re-emerging human diseases are caused by pathogens from animals, according to the World Health Organization. But a widely accepted theory of risk reduction for these pathogens is likely wrong, according to a new study by researchers from the Stanford Woods Institute for the Environment.
The dilution effect theorizes that disease risk for humans decreases as the variety of species in an area increases. For example, it postulates that a tick has a higher chance of infecting a human with Lyme disease if the tick has few animal host options. If many other animal hosts had been available to the tick, the tick’s likelihood of being infected and spreading that infection to a human host would go down, according to the theory.
In the first study to formally assess the dilution effect, Jones, Stanford colleague Dan Salkeld and California Department of Public Health researcher Kerry Padgett tested the hypothesis through a meta-analysis of studies that evaluate links between host biodiversity and disease risk for disease agents that infect humans. They found "very weak support, at best" for the dilution effect.