People who live in areas with a high diversity of birds could be protected from West Nile virus infections, according to a study published in PLoS ONE this week. The findings hint at maintaining biodiversity as a way to prevent various bird-related infections and experts agree that this should be part of public health planning. 

“We found that there is lower incidence of human West Nile virus in eastern US counties that have greater avian diversity,” write John Swaddle and Stavros Calos. “This pattern exists when examining diversity-disease relationships both before West Nile virus reached the US and once the epidemic was underway.” 

The virus first appeared in North America around 1999, according to the US Centers for Disease Control and Prevention. Since then, human infection patterns have fluctuated year by year. The two-year period from 2002 to 20­­­­03 was the worst recorded, with the disease affecting over 1400 people and killing 550. The viral disease is spread by mosquitoes and mainly affects birds, but at times the virus spills over from wildlife populations to infect humans. 

The researchers created spatial models after comparing data from US counties matched into 65 pairs, where one member had recorded human West Nile virus infection and the other had not. They found that areas with a wide variety of bird species were strongly associated with fewer human cases of disease.  

The findings point to a biological phenomenon seen previously only with Lyme disease, say the authors. Scientists have yet to understand fully the underlying cause of the pattern, known as the dilution effect. But they believe that a greater mix of bird species in a population reduces the number of virus-carrying birds relative to the rest of the group, and this weakens the incidence of disease. 

“The robust disease-diversity relationships confirm that the dilution effect can be observed in another emerging infectious disease and illustrate an important ecosystem service by biodiversity, further supporting the growing view that protecting biodiversity should be considered in public health and safety plans,” the authors write. 

Richard Ostfeld, from the Cary Institute of Ecosystem Studies in Millbrook, New York, says that the health benefits of protecting native biodiversity should also be considered in development decisions, even if the true costs and benefits are not entirely clear. 

“The first step is to recognise that biodiversity serves myriad functions in protecting and promoting human heath,” says Ostfeld. “The Swaddle and Calos paper shows that high avian diversity reduces human West Nile virus infection by about 50%.”

 The dilution effect has never before been seen in a disease that predominantly affects birds, and the findings could have implications for other bird-related human infections including avian influenza, say Swaddle and Calos. 

The research does not identify the mechanism responsible for the link between bird diversity and West Nile transmission, Ostfeld stresses. However, one or more of the mechanisms proposed by the team is likely to apply to other mosquito-borne diseases and avian influenza, he adds. 

The dilution effect, for example, may be relevant to avian influenza, whose spread does not involve a mosquito vector. In theory, Ostfeld explains, a wider diversity of bird types could mean that species carrying the virus have less frequent encounters with humans. 

“Decades of ecological research demonstrates that the best way to preserve high avian diversity is to prevent habitat fragmentation, degradation and destruction,” Ostfeld says. Mounting research in this field suggests a key message, he adds: preserving large tracts of intact forest close to areas where humans are at risk will reduce the burden of disease.