North American scientists studying West Nile virus have shown that more diverse bird populations can help to buffer people against infection. Since the virus first spread to North America it has reached epidemic proportions and claimed over 1,100 human lives. “This is an important example of the links between biodiversity and human health”, commented Dr Stuart Butchart, BirdLife's Global Research and Indicators Coordinator.

Biodiversity is increasingly being recognised as socially and economically important because of the valuable services it provides. The authors of this latest research - John Swaddle and Stavros Calos - highlighted the “increasing evidence for economically valuable ecosystem services provided by biodiversity”.

West Nile virus mainly affects birds but can be transferred to humans via mosquitoes. It first spread to North America in 2002, and since that time it has reached an epidemic scale with over 28,000 human cases – including 1,100 deaths - being reported. The cost of West Nile virus-related healthcare in the United States was estimated at $200 million in 2002 alone.

The virus is also an important threat to bird populations. Over 300 species act as hosts, although American Robin Turdus migratorius has been named as largely responsible for transmission from birds to humans. “West Nile virus may compound existing pressures - like habitat loss - to increase the risk of extinction for species”, commented Dr Butchart.

For example, Yellow-billed Magpie Pica nuttalli, which is found only in California, appears to have declined by almost 50% in the last two years as a consequence of the disease.

Scientists studying the virus looked at US counties east of the Mississippi River and compared their avian diversity with the number of human cases. They found that high bird diversity was linked with low incidence of the virus in humans. They reported that about half of the human incidences of West Nile virus could be explained by the differences in local bird populations. The study’s results also suggest that bird communities lowered human case numbers even when the epidemic was underway.

The way in which biodiversity and disease rates are linked has been dubbed the ‘dilution effect’. Although the exact mechanisms aren’t currently clear, scientists believe that increased diversity within an ecosystem reduces - or dilutes - the proportion of suitable hosts for a disease, and therefore reduces transmission rates. It has previously been studied through another infection, Lyme disease, but this new research suggests that it may be more widely applicable. If so, it could be a valuable tool for public health and safety plans.