West Nile may be here to stay

The number of people who get West Nile fluctuates from year to year and may seem to be decreasing, but don't look for the virus to ever disappear entirely.

"The numbers will go up and down depending on the factors," said Laura Robinson, the Texas Department of State Health Services' zoonosis control veterinarian for Region 11, which consists of the state's 19 southernmost counties.

Researchers don't know all the factors that contribute to the spread of the West Nile virus, Robinson said. But they do know increases in temperature and in the availability of stagnant water in which mosquitoes breed are likely to boost the insects' population, in turn increasing the spread of the virus.

"July, August and September are really the prime times for mosquito populations to grow and the West Nile virus to occur in mosquito populations and be transmitted to other hosts," Robinson said.

In 2007, a total of 851 cases of the West Nile virus were recorded in Texas, according to the Texas Department of State Health Services. That includes cases found in humans, birds, horses and mosquitoes. Ninety cases of West Nile fever and 170 cases of the more serious West Nile neuroinvasive disease were documented in humans in the Lone Star State last year.

The West Nile virus was first detected in 1937 in Uganda. Since the virus's initial identification in the United States, in 1999 in New York, it has spread to at least 44 states and the District of Columbia.

"The big problem with West Nile is that it cycles between wild birds and wild animals and mosquitoes," said Stephen Higgs, a faculty member at the Sealy Center for Vaccine Development at the University of Texas Medical Branch in Galveston. "It's very difficult to vaccinate all the birds or all the animals to prevent transmission."

No vaccine is currently available for humans.

When birds become infected with West Nile, the virus flows through their bloodstreams. Mosquitoes become infected when they feed on infected birds.

Infected mosquitoes can then transmit the virus - carried in their salivary glands - when they bite humans and other animals, according to the state health department's Infectious Disease Control Unit.

"To date, there are 62 species of mosquito in the U.S. from which West Nile has been identified," Higgs said. "(West Nile) is unusual in that it affects a lot of different species."

In a paper Higgs and several colleagues published in June 2007, the researchers announced that three species of mosquito - all found in South Texas - are equally susceptible to the West Nile virus.

However, the number of West Nile outbreaks in any given year is impossible to predict, Higgs said.