Dr. Robert B. Tesh works with virus samples from UTMB’s World Reference Center for Emerging Viruses and Arboviruses, the most extensive such collection in the world.
Why the resurgence of West Nile virus cases in Texas this year?
Some probable predisposing factors are warm winters and hot summers, rainfall, the density of vector mosquito populations and immunity among local bird and human populations.
Why was there a lower percentage of cases in Galveston than Dallas and other parts of the state?
In Galveston, many of our local pest mosquitoes are salt marsh mosquitoes, which breed in brackish water and are not efficient transmitters of West Nile. Also, the soil on Galveston Island is sandy, so fresh rain water quickly drains away. In contrast, Houston and Dallas have less porous soils that allow fresh water to collect in storm drains, pools or ponds. These are good breeding places for the mosquitoes that transmit West Nile virus. Galveston, county also has an active summer insecticide spraying program. Until August 2012, Dallas did not use insecticides because of environmental concerns.
Is the threat over?
Cases of West Nile virus typically occur in the late summer or early fall. But, the risk of infection on Galveston Island is relatively low compared to Houston and Dallas. Galveston’s local mosquitoes are not effective transmitters of West Nile virus. A few cases have occurred in Galveston County, but I am unaware of any locally acquired cases on the island. .
What’s the status of a West Nile vaccine?
The Food and Drug Administration has very strict rules for approving virus vaccines for use in humans. Trials must be done in many groups of people, demonstrating that a candidate vaccine is effective and safe before approval is given. This is a long and costly process.
Because of the sporadic nature of West Nile virus activity and the fact that at least 80 percent of human infections are asymptomatic, it has been difficult to get a large enough group of people at high risk for serious illness to evaluate the efficiency of a vaccine. This year, Dallas would have been a good site, but Los Angeles or Denver may be a better site next year.
It is estimated by CDC that about one percent of the U.S. population has been infected with West Nile since the virus first appeared in New York in 1999. So perhaps 3.5 million people have been infected, but at least 80 percent had no obvious symptoms. So who would you vaccinate? It would be very expensive to attempt to vaccinate the entire U.S. population. Consequently, pharmaceutical and vaccine companies have not pushed vaccines because they don't see a big enough market or profit for them.