Total weekly rainfall had a smaller effect on West Nile virus infections, the study found. An increase of 0.75 inch of rainfall increased the number of infections by about five percent.
Only a few mosquito species carry the West Nile virus, and each has specific habitat requirements, according to Nasci of the CDC. Warmer, wetter weather patterns will likely expand the niches of these species.
California health officials have already observed this, as some mosquito species carrying the West Nile virus have extended their ranges into higher elevations and coastal areas as temperatures warmed.
Along with mosquitoes, certain species of birds are reservoirs for the West Nile virus. Changing weather patterns also affect bird populations, which can impact the number of human infections.
For example, droughts can drive birds into urban areas, making human West Nile virus outbreaks more likely, said Kramer.
Southern states with high home foreclosure rates also face a unique West Nile virus threat, added Kramer and Landesman, since neglected swimming pools act as mosquito breeding grounds.
"The take-home message is that these systems are really complex," said Landesman. "Climate changes won't make them any easier to understand."