Better West Nile Surveillance May Help Reduce Infections

Monitoring mosquito traps regularly and determining quickly how many of the blood-sucking pests are infected with the West Nile virus may enable health departments to act faster in eradicating the bugs and reducing human infections, a study found.

An analysis of the 2012 West Nile outbreak in Dallas County, Texas, the largest in a U.S. metro area last year in which 19 people died, showed the virus tends to be more prevalent after a warmer-than-normal winter and if it appears early in the summer, according to research in the Journal of the American Medical Association.

The West Nile virus first surfaced in the U.S. in New York City in 1999. After spreading nationwide, the number of cases of the mosquito-borne illness declined for five years. There was a resurgence of the disease last year, particularly in Dallas County, Texas, which had the most cases of virus infections in the U.S., the authors wrote. Today’s findings show the virus isn’t disappearing from the U.S. and more steps are needed to prevent infections, which can lead to death, said Stephen Ostroff, who wrote an accompanying editorial in the journal.

“The resources are being withdrawn because the last several seasons there was not a lot of human illness that occurred in the U.S.,” Ostroff, former director of epidemiology at the Pennsylvania Department of Health in Harrisburg and deputy director of the National Center for Infectious Diseases at the U.S. Centers for Disease Control and Prevention, now retired, said in a July 15 telephone interview. “It very rapidly receded from the memories.”

Better Planning

Ostroff said resources are needed to track the virus before humans are infected and health officials must have steps in place to deal with the virus once it’s detected.

“As was so well illustrated with the episode last year in Dallas, West Nile is still with us from one coast to the other,” he said. “We certainly can anticipate that we will have additional seasons like the one that occurred in 2012. I don’t think it’s good public health to be as unprepared as we were in 1999 when it showed up in the first place.”

Researchers in the study analyzed the data available from the 2012 West Nile virus outbreak in Dallas County. From May 30 to Dec. 3, 173 patients were diagnosed with West Nile encephalitis, a brain infection, and 225 with West Nile fever. Nineteen people died.

They found that calculating the mosquito vector index, which looks at how numerous the bugs are and how many of those are infected with the West Nile virus, gives officials a quicker, up-to-date picture of a possible epidemic. Several weeks can pass from the time a person is bitten, shows up at the hospital and case of West Nile virus is confirmed.

Early Warning

“The vector index tells you right when people are getting infected. You can nip the epidemic in the bud before many people are infected,” said Robert Haley, a study author and a professor of medicine and director of the Division of Epidemiology, at the University of Texas Southwestern Medical Center in Dallas, in a telephone interview. “If you wait until you have large numbers of people sick you miss the opportunity.”

A second study in the journal showed the West Nile virus has spread to all 48 states on the U.S. mainland as well as all Canadian provinces since 1999. There have been large outbreaks in 2002, 2003 and 2012. Since 1999, 1,549 people have died.

Ostroff said late July through early September is when exposure to the West Nile virus is highest. He said more emphasis needs to be placed on finding a vaccine to prevent the infection or a treatment once it occurs, neither of which are available for humans.

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