Health officials are worried that spring rains, coupled with the warm weather, will create conditions for an explosion of mosquitoes capable of spreading the West Nile virus in the Northern San Joaquin Valley.
Mosquito counts are above normal for this time of year, according to local districts conducting surveillance and control operations for the mosquito-borne illness.
The virus, capable of causing severe neurological damage, sickened 13 people in Stanislaus County last year, resulting in one death.
A larger mosquito population raises the risk that West Nile virus will afflict more people this year. Horses also are at risk, and the conditions make dogs and cats more vulnerable to heartworm, a parasite carried by a type of mosquito.
The spring rains are allowing the bugs to multiply, from Manteca to Merced, in the ditches and ponds of rural areas and the neglected swimming pools of neighborhoods pounded by foreclosures.
"To avoid West Nile virus, it is important that residents protect themselves from mosquito bites," said Aaron Devencenzi of the San Joaquin County Mosquito & Vector Control District.
In a recent advisory, the district said a barn owl found near Lathrop tested positive for West Nile, but tests showed the owl had a chronic infection carried over from last year.
It's difficult to predict how severe West Nile virus will be this spring and summer, state health officials say. But there will be more mosquito breeding habitats than typical because of the winter rains.
As the Sierra snowpack melts, rivers in the San Joaquin Valley will swell, creating new places for mosquitoes to nest.
Modesto has received more than 16 inches of rain this year, 4 inches above the annual average. The last 16-inch rainfall season was 2005, when 92 people were sickened by West Nile in Stanislaus County, including one fatality.
Officials say that in 2005 there was less immunity to the virus, first detected in California in 2002. But the natural conditions for mosquitoes are similar.
In the shaky economy, people continue to lose homes to foreclosure or can't afford to keep pool filters running. Thousands of pools are abandoned or neglected. They are left full to save them from damage, but the stagnant water creates a perfect nest for female mosquitoes.
In March, the Turlock Mosquito Abatement District had aerial pictures taken of hundreds of neglected pools in Turlock, Denair, Hickman, Grayson, Patterson and Newman. It identified 795 pools where mosquitoes likely were breeding.
The district since has told the owners to clean their pools and has treated the mosquito-infested back yards of abandoned homes, said Jerry Davis, general manager of the Turlock district. It has treated pools for owners who can't afford the upkeep.
When owners do not respond to notices left on the door, the district uses a blanket warrant to enter the property and abate the nuisance.
"A lot of the (neglected pools) were in new locations," Davis pointed out. Many of the abandoned homes treated last year have been sold, he said, but other home sites have gone into foreclosure, and those pools are neglected.
"Most of the people have been cooperative," Davis said. "Some are concerned we are going to charge for the cost of treating the pool, and when they find out we don't, they are more agreeable."
As warmer weather arrives, it will kick mosquito breeding season into high gear.
"If we get really warm weather, there is a chance we will see mosquitoes carrying West Nile virus by the middle of June," Davis said.
Already, "tree hole" mosquitoes have been busy. Those mosquitoes like to make nests in the cavities of trees. They don't carry West Nile virus, but they do carry heartworm, a parasite that is potentially deadly to dogs and cats.
Pet owners can talk with their veterinarian about a preventive medication for heartworm, Davis said.
Horses are susceptible to West Nile virus, mosquito-control officials said, and a vaccine is available for them. But there's no vaccine for humans.
People should drain standing water from flower pots and other backyard items, and wear protective clothing or mosquito repellent to ward off bites.
The Fresno Bee contributed to this story.
Bee staff writer Ken Carlson can be reached at email@example.com or 578-2321.