Mosquito Intervention Key To Stopping West Nile Virus
July 17, 2013

Mosquito Intervention Key To Stopping West Nile Virus

Lawrence LeBlond for - Your Universe Online

Since first being detected in the US in 1999, West Nile virus (WNv) has become a major North American health crisis during the summer mosquito season.

Lyle R. Petersen, MD, MPH, of the CDC, reports that since WNv was first detected, more than 16,000 cases have been confirmed with more than 1,500 deaths. He said it is likely that more than 780,000 illnesses have occurred, with highest incidence rates seen in the Midwest from mid-July to early September.

Petersen and colleagues wrote that WNv is now "endemic in all 48 contiguous United States as well as all Canadian provinces... It has produced the 3 largest arbovirai neuroinvasive disease (encephalitis, meningitis, or acute flaccid paralysis) outbreaks ever recorded in the United States, with nearly 3,000 cases of neuroinvasive disease recorded each year in 2002, 2003, and 2012."

"West Nile fever develops in approximately 25 percent of those infected, varies greatly in clinical severity, and symptoms may be prolonged. Neuroinvasive disease (meningitis, encephalitis, acute flaccid paralysis) develops in less than 1 percent but carries a fatality rate of approximately 10 percent. Encephalitis has a highly variable clinical course but often is associated with considerable long-term morbidity. Approximately two-thirds of those with paralysis remain with significant weakness in affected limbs," they add.

Currently, there is no human vaccine against WNv and the neuroinvasive diseases it produces, and prevention relies solely on an "integrated pest management approach." Pest management will be key to reducing the incidences of WNv as a resurgence of the disease came in 2012, after several years of decline in the US.

Petersen and colleagues believe WNv "will continue to produce unpredictable local and regional outbreaks" in the years to come.

To this end, it has come to the attention of researchers from UT Southwestern Medical Center to unlock the mysteries of West Nile virus outbreaks; and in doing so, they show that usage of a mosquito vector-index rating system is necessary to identify the best time for intervention.

The UTSMC team of researchers analyzed a decade of data from WNv infections, weather records and housing information. In the 2012 data, which was gleaned from Dallas County, Texas, the nation's largest outbreak region, the team worked out that a mosquito vector index was the best way to intervene on an outbreak and potentially stave off a rise in human infections. This mosquito vector index was calculated from the abundance of mosquitoes and the percentage of mosquitoes infected with WNv.

Dr. Robert Haley, chief of epidemiology at UTSMC and senior author of the study published in the latest issue of JAMA, said: "When the vector index goes above 0.5 early -- in June or July -- large numbers of people are silently infected, and this is the best time to intervene. In years when the vector index did not rise until late July or August, impending outbreaks just sputtered -- in late summer mosquito abundance declines, and mosquitoes become less active and stop biting as much."

Dr. Haley and colleagues said determining the number of WNv infections is a poor way to determine the best way to respond to an outbreak.

Once an infected mosquito bites, WNv takes about a week before it begins producing recognizable symptoms. It takes another week before sickened people begin showing up at emergency rooms and then another week before lab confirmation and a diagnosis is made, according to the researchers.

"That three-week time period is crucial," said Dr. Haley. "Acting early from the vector index rather than after human case reports and deaths mount up can nip an outbreak in the bud. However, if mosquito data are unavailable or a decision to intervene takes longer, later intervention may still be important to terminate the outbreak."

The team's analysis also concluded milder winters and unusually warm springs contribute to epidemic years for WNv, adding that a major concern will be the continuing threat of global warming.

As temperatures continue to rise and mosquito bites become more and more common, finding ways to avoid being bitten and potentially developing a deadly WNv infection is quite important.

A recent analysis of several studies showed mosquitoes utilize many different factors in determining who is targeted. People who are pregnant and drink beer are more likely than others to attract mosquitoes due to higher release of carbon dioxide, which is known to help mosquitoes hunt down human meals.

Back in the UTSMC 2012 data, Dr. Haley and his colleagues also discovered that areas of higher property values, housing density and percentages of unoccupied homes were at greater risk of WNv infection. This is likely a result of areas that foster the types of environments for mosquitoes to breed and transmit disease. The Dallas County data showed that year after year clustering was most common in Park Cities and North Dallas areas.

The team said the tools used for the Dallas County analysis may be applicable elsewhere, but due to variations in weather, mosquito populations and other factors, each region would need to conduct their own mosquito vector-index analysis to determine the best way to intervene.

Along with the JAMA study, the team also provides an instruction manual for other regions to calculate the vector index from their own mosquito infection surveillance data.

"Given the leading character of this index, epidemiologists and government officials can implement, in a more timely manner, preventative measures to reduce the impact of future West Nile Virus outbreaks," said Dr. Tom Fomby, professor of economics in the Dedman College of Humanities and Sciences at Southern Methodist University, who contributed statistical methods in the evaluation of the vector index.

Co-authors of the study were Wendy M. Chung, MD, SM, chief epidemiologist at Dallas County Health and Human Services and DCHHS colleagues Dr. Christen Buseman, Sibeso Joyner, and Sonya Hughes; and Dr. James Luby, professor of internal medicine in the division of infectious diseases at UT Southwestern.