The higher temperatures, humidity and rainfall associated with climate change have intensified outbreaks of West Nile virus infections across the United States in recent years, according to a study published this week.

One of the largest surveys of West Nile virus cases to date links warming weather patterns and increasing rainfall--both projected to accelerate with global warming--to outbreaks of the mosquito-borne disease across 17 states from 2001 to 2005. 

The authors predict the pattern will only get worse. "If temperature and precipitation are influential in determining West Nile virus infection risk, such changes would likely increase the burden of this disease in coming decades," the authors note in the study, published online Monday in the journal Environmental Health Perspectives.

In the study, Jonathan Soverow of the Beth Israel Deaconess Medical Center in Boston and colleagues at Toronto's Hospital for Sick Children and the Harvard School of Public Health matched more than 16,000 confirmed West Nile cases in 17 states to local meteorological data.

The team found that warmer temperatures had the greatest effect on the virus' transmission to humans.  Higher humidity, heavier rainstorms and increased precipitation were also tied to higher rates of West Nile virus infection, according to the study.

"A lot of the trends we see depend on local conditions," said Roger Nasci, an entomologist at the U.S. Centers for Disease Control and Prevention who studies vector-borne diseases but was not involved with the study.  "West Nile virus is a very focal disease.  It's not uniformly distributed across the U.S."

West Nile virus led to 43 deaths in 2008 in the United States.  More than 1,300 infections were diagnosed last year, according to the CDC.

Humans can become infected if bitten by a mosquito carrying West Nile virus.  Around 20 percent of infected people show symptoms of the disease, such as fever, headache and nausea.  Of those, about one percent develop neurological symptoms such as numbness, convulsions and paralysis.

Warmer weather helps spread West Nile virus because it extends the length of the mosquito season, said Vicki Kramer, chief of the vector-borne disease section at the California Department of Public Health.

Higher temperatures also let mosquitoes reach biting age sooner and speed multiplication of the virus within insects, said Kramer.  Thus in a warmer climate not only are there more biting mosquitoes, but those mosquitoes carry more copies of the West Nile virus, making them more likely to infect their human targets.

"It takes a while for the disease to build up," says Kramer.  "That's why we see more cases in August than in June."

Rainfall's effects on mosquitoes and West Nile virus are more complicated, cautioned Bill Landesman, an ecologist at Rutgers University.  For example, although their eggs need standing water to hatch, mosquito populations often flourish after a drought because mosquitoes can re-colonize faster than other insects.

"We're wrestling with this interplay of abiotic (physical) factors, mosquito populations and the West Nile virus," said Landesman, "and that sometimes makes things difficult to understand."
The new study by Soverow's team may help researchers make sense of some of these complex interactions.

For example, the study found that a single rainstorm resulting in at least two inches of rain could increase infection rates by 33 percent, while smaller storms did not.  Heavy rainfall increases humidity, which can stimulate mosquitoes to bite; it also creates pools of water in which mosquitoes can breed.

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."

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.