An unusually mild winter and an early appearance of infected mosquitoes may have fueled a deadly outbreak of West Nile virus in Texas last summer, a new study finds.
Mosquitoes transmit West Nile virus to humans, and while most infections cause no serious problems, a small number of people suffer potentially fatal inflammation around the brain or spinal cord.
After several years of laying low, the West Nile virus resurged last summer in the United States, killing 286 people -- the most in one year since 1999. Texas accounted for one-third of all confirmed infections, with the Dallas area, where 19 people died, the hardest hit.
In the new study, published July 17 in the Journal of the American Medical Association, researchers tried to figure out why.
Using local weather data for the past decade, they found that the winters before the 2012 outbreak, and before a smaller 2006 outbreak, were unusually mild.
"Those two winters really stuck out," said senior researcher Dr. Robert Haley, of the University of Texas Southwestern Medical Center at Dallas.
Following those mild winters, West Nile infections in mosquitoes cropped up earlier in the year, and shot up at a faster rate. (Haley's team was able to track that pattern because the Dallas area has government surveillance programs that trap and test mosquitoes for the virus.)
"When you put it all together," Haley said, "you have a warmer winter and earlier spring, and more infected mosquitoes by June and July."
In 2012, the first infected mosquitoes were detected in late May, the study found. And by June and July, the number of infected mosquitoes caught in traps each night was substantially higher than in non-epidemic years.
Researchers call the average number trapped the "vector index." Until now, Haley said, it wasn't clear whether the vector index was a good predictor of a potential West Nile epidemic.
But based on what his team found, Haley said, "we're really convinced that it is."
The two epidemic years, 2006 and 2012, were the only years in which the vector index passed 0.5. In 2012, the index soared that high by the last week of June -- at which point the first 19 people with West Nile infections affecting the brain or spinal cord were already falling ill. Ultimately, 173 people contracted those serious infections.
Haley said the findings highlights the need for mosquito-testing programs, and for acting sooner rather than later when the vector index rises at an unusually fast pace.
West Nile may have slipped from many people's memories since it first hit North America in 1999, said Dr. Stephen Ostroff, a former official with the U.S. Centers for Disease Control and Prevention.
"But the 2012 outbreak shows us West Nile is not a 'has-been,'" said Ostroff, who wrote an editorial published with the study.
While the new findings are based on the situation in Dallas, the lessons can likely be applied elsewhere, Ostroff said. "If you've just had a mild winter, you may need to do more earlier in the year," he said.
That means an earlier start to mosquito-control measures, such as limiting mosquito breeding grounds -- including areas of stagnant water -- and spraying pesticides at ground level, said Ostroff. By the time the Dallas outbreak became apparent last year, officials had to use airplanes to spray pesticides on a wide scale.
Health officials say the amount of pesticide released during those aerial assaults is safe for humans. But some residents and environmental health advocates worried about the exposures. And from a budget standpoint, avoiding aerial spraying would be a good thing, Ostroff noted.
"If we act earlier," he said, "we may be able to avoid aerial application of pesticides."
In 2012, Dallas County spent an estimated $1.6 million on aerial pesticide spraying. And the cost of treating West Nile infections reached about $8 million, Haley's team noted.
That, Ostroff said, suggests that investing in local mosquito surveillance programs could end up saving money.
Haley agreed, and said that if global warming leads to more mild winters, West Nile epidemics could potentially become more common. "This is a serious disease that is going to be with us for a while," he said. "And it could get worse."