Staying One Step Ahead of West Nile

West Nile outbreak in Dallas Texas shows potential ways to predict infections

(RxWiki News) Mosquito bites are itchy and annoying, a fact that is hard to forget. It is more difficult to remember that they can pose a real health threat.West Nile virus is spread through mosquito bites, and can cause serious illness.

A new study examining cases of West Nile in Dallas, Texas found that a variety of factors, such as weather patterns and geography, can help predict outbreaks.

"Use screens on windows to keep mosquitoes out."

Though West Nile infections had previously been on the decline, the number of cases of the virus surged across the country last summer. The highest rates of cases in 2012 occurred in Dallas, Texas, where this new study took place.

According to the Centers for Disease Control and Prevention (CDC), around 70 to 80 percent of people who become infected with West Nile don't experience any symptoms. However, around one in five patients develop symptoms like fever, vomiting, fatigue and rash that can linger for weeks.

Major neurologic issues can be seen when the infection develops into West Nile neuroinvasive disease (WNND). Problems like swelling of the brain, coma and paralysis can occur, though these issues are rare and only develop in around one percent of West Nile cases.

This new study, led by Wendy M. Chung, MD, SM, of Dallas County Health and Human Services, calculated the number of cases of West Nile virus infection in Dallas County, which has a population of 2.4 million people.

The researchers also examined weather data, trapped and mapped mosquitos and kept track of emergency department visits associated with West Nile symptoms between May 30, 2012 and December 3, 2012.

Results from this time period showed a total of 173 cases of WNND and 225 cases of West Nile fever in the county. The researchers also found 17 cases of blood donors with the West Nile virus, as well as 19 deaths.

The patients who experienced neurological symptoms were more likely to be older in age, white and male. Of this group, 96 percent were hospitalized, 35 percent required intensive care and 18 percent required help breathing through ventilation procedures.

Dr. Chung and team determined that the rate of WNND in Dallas County during 2012 was 7.30 cases per 100,000 residents. This was up from the previous high in 2006, when the rate was 2.91 cases per 100,000 residents.

This study found that watching trends in the mosquito population could help predict infection trends in humans.

The researchers began to discover West Nile infections in mosquitos in late May, which was earlier than usual. As the authors described, "An unusually rapid and early escalation of large numbers of human cases closely followed increasing infection trends in mosquitoes."

The 2012 season began with WNND cases spread out geographically across Dallas County, but infections began to cluster into certain neighborhoods as the season went on. The areas with the highest case numbers were located in the north central part of the county and had a high housing density.

The researchers also looked back at weather patterns since the first West Nile case was identified in Dallas 11 years earlier. The results showed that higher rates of WNND were associated with unusually warm winters.  
In both 2012 and 2006 (when another outbreak was seen in Dallas), a mild winter was seen prior to the outbreak. Mild winters included trends like fewer hard winter freezes, warmer average temperatures and high amounts of winter rainfall.

According to the study authors, these findings could help minimize future outbreaks.

"Areas such as Dallas with wide variations in West Nile virus activity between seasons should consider analysis of local West Nile virus history to identify predisposing weather patterns and perennial high risk geographical areas to efficiently direct preseason prevention measures and surveillance resources," the study authors concluded.

Further research over more diverse regions is needed to confirm and expand on these findings.

This study was published July 17 by the Journal of the American Medical Association (JAMA).

No conflicts of interest were reported.

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Review Date: 
July 16, 2013