Heat wave adds to West Nile danger
Published 4:00 am, Saturday, July 12, 2008
California's heat wave this week has created ideal conditions for the spread of West Nile virus, the mosquito-borne illness that killed 21 state residents last year and is emerging once again as a public health threat this summer.
Despite the ongoing drought, there are plenty of places, from irrigation ditches in the country to abandoned swimming pools in the suburbs, to serve as nurseries for fast-breeding mosquitoes. Additionally, a growing body of evidence suggests that hot weather speeds up both the breeding cycle of mosquitoes and the replication of West Nile virus in the insects' guts.
"It's not a good combination to have very hot weather and standing water, and one way or another, there always seems to be an issue of standing water," said Dr. Gil Chavez, chief of the Center for Infectious Diseases at the California Department of Public Health.
Mosquitoes acquire the virus from birds and transmit it to humans. After a human has been bitten by a mosquito carrying the virus, it can take anywhere from three to 12 days for symptoms of West Nile to show up. The virus can cause a life-threatening swelling of the brain or a polio-like syndrome of partial paralysis.
West Nile has been a significant problem in California since 2004. Summer is the season for the outbreaks, and each year the number of reported cases has started a precipitous climb during the last two weeks of July before peaking in late August.
Disease control experts are watching warily for signs of trouble - a spike in the number of dead birds found, discovery of infected mosquitoes in bug traps, or positive West Nile virus blood tests in flocks of "sentinel chickens" maintained in strategic locales throughout the state.
Some of California's driest parts - Los Angeles, Orange and San Diego counties - are reporting scores of dead, infected birds. "There appears to be more activity in Southern California than last year," Chavez said.
Cases expected to climb
So far this year, there have only been three human cases in California, the same number as this time last year. One case was reported in Tulare County, two in Stanislaus.
Because of the three- to 12-day incubation period from bite to onset of illness, and the lag in diagnosing and reporting cases to the state, many of the victims likely to be tallied in late July and early August are in fact being infected right now.
That's why the heat wave that smothered much of the state in the past week is so concerning. It hit California just as West Nile virus season typically takes off.Scientists studying West Nile virus behavior are finding that spikes in temperature play a significant role in the production of both the virus and the mosquitoes that transmit it from birds to humans.
UC Davis entomologist Bill Reisen has found that the breeding cycle of mosquitoes that carry West Nile virus will decrease from about 10 days to four days when the temperature increases to 90 degrees from 60 degrees. "This means they develop faster as eggs, larvae and pupae and therefore adult populations increase faster," he said.
Studies are also showing that the virus itself becomes more active as the temperature rises. UC Santa Cruz disease ecologist Marm Kilpatrick and a team of researchers reported last month in the journal PLoS Pathogens that an increase in temperature of only 8 degrees - to 90 degrees from 82 - effectively doubled the amount of virus transmitted by mosquitoes in the laboratory.
Hot temperatures cut the lifespan of mosquitoes, but on the whole the effect of hot weather is to boost their numbers and their ability to spread disease. Kilpatrick also found that the current strain of West Nile virus is much more likely to replicate at higher temperatures than the strain that was first detected in New York City in 1999. "The new strain, which popped out of nowhere in 2003, raced across the country," he said.
Mosquito crews out in force
For mosquito control personnel out in the field, the latest heat wave is prompting heightened vigilance. "We see variations from year to year, but in my opinion, hot summers are going to be worse years for the virus," said Steven Shutz, entomologist for the Contra Costa Mosquito and Vector Control District.
Studies by that county have found that whenever overnight temperatures stay above 55 degrees, West Nile virus activity begins to increase. "We're in another heat wave," he said, "and we've just recorded our third positive bird. I wouldn't be surprised to see more."
Even the nation's economic health may play a role in West Nile virus transmission. Eastern Contra Costa County has been hit hard by the mortgage foreclosure crisis, and that has led to a landscape dotted with abandoned swimming pools - where literally millions of mosquito larvae can be found.
The county has its own aquarium to breed and grow mosquito fish, which can eat 500 larvae a day, and residents are welcome to come with their coffee cans to bring home a batch of the 3-inch fish to put in any stagnant water sites of concern. The county itself has a full-time crew that visits abandoned swimming pools and stocks them with mosquito fish.
"A week ago, they were doing 10 pools a day," said Shutz. "Mosquitoes here are pretty well adapted to drought. They do quite well with human-provided water."