Several factors led to West Nile outbreak in Dallas County

Epidemics of West Nile virus in Dallas County, Texas, usually occur after unusually warm winters and are strongly predicted by the mosquito vector index, according to researchers from Dallas County Health and Human Services and the University of Texas Southwestern Medical Center.

Last year, there were 173 cases of West Nile neuroinvasive disease, 225 cases of West Nile fever, 17 West Nile virus–positive blood donors and 19 deaths in Dallas County. The incidence rate for neuroinvasive disease was 7.3 per 100,000 residents, the highest rate ever. The previous high rate was 2.91 per 100,000 residents in 2006.

In JAMA, researchers published data resulting from an evaluation of the outbreak and 11 years of prospective West Nile virus human and mosquito surveillance data. The first West Nile virus–positive mosquito pool in 2012 was detected in May, earlier than typical seasons. The sequential increases in the weekly vector index last year predicted the number of patients with symptoms of West Nile neuroinvasive disease in the following 1 to 2 weeks.

In the early part of the epidemic, cases were dispersed throughout the county, but by the end of the epidemic, cases were predominantly in the northern half of the county, where there were higher housing densities. Additionally, the winter preceding the outbreak last year was the mildest in the last 10 years, with above normal temperatures, no hard winter freezes and heavy winter rainfall. These factors were also associated with the increased incidence of West Nile neuroinvasive disease.

“Our findings support incorporating mosquito infection indices into response plans and closely monitoring the mosquito vector index in real time,” the researchers wrote. “The goal is to recognize significant increases above historically predictive thresholds of epidemic transmission when augmented mosquito control measures can prevent the most human illness.”

In a review of literature, Lyle Petersen, MD, MPH, of the division of vector-borne diseases at the CDC and an Infectious Disease News Editorial Board member, and colleagues found that there have been more than 780,000 illnesses related to West Nile virus since 1999, as well as 16,196 cases of neuroinvasive disease and 1,549 deaths.

Lyle Petersen

They reviewed several facets of the virus, including ecology, virology, epidemiology, clinical characteristics, diagnosis, prevention and control. Among the findings: neuroinvasive disease develops in less than 1% of the cases; diagnosis depends on antibody detection in serum or cerebrospinal fluid; and prevention should focus on surveillance and elimination of mosquito breeding sites.

“Sustainable, community-based surveillance and vector management programs are critical, particularly in metropolitan areas with a history of West Nile virus and large human populations at risk,” they wrote. “Community response plans must include provisions for rapidly implementing large-scale adult mosquito control interventions when surveillance indicates such measures are necessary.”

In an accompanying editorial, Stephen M. Ostroff, MD, formerly of the CDC and the Pennsylvania Department of Health, said since the virus emerged in the United States in 1999, funds for vector surveillance and control peaked in the early 2000s, but significantly declined since, leading to the elimination of some programs.

“Such complacency had serious ramifications with a nationwide resurgence of West Nile virus occurred [in Dallas County],” Ostroff wrote. “Periodic flares of West Nile virus, as occurred in 2012, certainly will recur. Where future outbreaks of the virus will occur and how intense they will be is difficult to predict, especially in light of declining surveillance efforts and vector monitoring programs.”

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Disclosure: The researchers report no relevant financial disclosures.