Ken Kalthoff, NBC 5 News
Experts believe that the highly developed Park Cities and North Dallas area may have been more attractive to the Southern House Mosquito - a big carrier of West Nile virus in summer 2012.
Researchers at the University of Texas Southwestern Medical Center are shedding light on what contributed to last year's West Nile virus outbreak and there are fewer cases this mosquito season.
The study, which was released Tuesday, found three key factors were present in advance of both the severe 2006 and 2012 West Nile virus seasons in North Texas.
The winter and spring preceding the summer outbreaks were exceedingly warm, with few nights that had freezing temperatures lower than 28 degrees. And during an ongoing drought, there was sufficient rainfall at certain times that proved ideal for breeding large numbers of mosquitoes.
"That seems to have been the combination that amplified this into such a huge bird epidemic and then spilled over into people," said Dr. Robert Haley, UTSW chief of epidemiology.
A vector index tracked over the past several years combines both the number of mosquitoes with the number of infected mosquitoes.
Comparing that index to the number of human cases has provided health authorities with a new way to predict a spike in the number of human cases of the disease, Haley said.
"We can monitor that and then know very early that we're going to have an epidemic so that we can intervene before a lot of people get sick," he said.
Aerial spraying used late in last summer's North Texas outbreak was found to be safe and very effective, according to the study.
"Had we known what we know now, we could have intervened several weeks earlier and prevented a lot more cases," Haley said.
Haley gives North Texas public health experts praise for expanding prevention efforts this year, including the use of ground spraying in areas with a high number of mosquitoes but no detected West Nile virus.
"They're experimenting with different options, which is what we ought to be doing because, with global warming, we're going to have more years with mild winters," Haley said.
The hardest hit areas in North Texas were the Park Cities and North Dallas. The style of development in those areas may be part of the reason, Haley said.
More heavily developed areas are more attractive to Southern house mosquitoes, the primary carrier of the disease, than open spaces and woodlands, Haley said.
"Think about Oak Cliff and West Dallas -- that's where you have lots of woods and fields and open places and parks," he said.
The conditions that existed last summer have not reappeared this year in North Texas.
"Interestingly, the mosquitoes are more abundant this year than they were last year, but there's no virus -- or such little virus that it's not causing an outbreak," Haley said.
Haley said there is still more research to do, including a review of migratory bird patterns, which could help show whether the disease moves from South America through North Texas.
But the findings in the new UTSMC student provide insight that was not available during last year's outbreak.
The research was based on information from many sources, including the Dallas County health department, the U.S. Centers for Disease Control and Prevention and weather data.
The study appears in the current edition of the Journal of the American Medical Association.