More rain could mean more mosquito-borne illnesses

This file photo shows an <em>Aedes aegypti</em> mosquito photographed through a microscope. More standing water could mean more mosquitos in California
This file photo shows an Aedes aegypti mosquito photographed through a microscope. More standing water could mean more mosquitos in California
Felipe Dana/AP

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All the rain we've had this winter may be good for drought-parched Southern California, but it might also mean more mosquitos and mosquito-borne illnesses, according to mosquito control agencies. 

It's not possible to predict exactly what the extra water will mean for the mosquito population, but standing water and warm spring weather tend to be ideal conditions for mosquitoes to reproduce in. That raises particular concerns about mosquito-borne illnesses like West Nile Virus.

"More water means more mosquitos, which means more concern about West Nile," said Joel Buettner with the Mosquito and Vector Control Association of California.   

California's Department of Public Health began recording West Nile Virus cases in 2003, and the virus is now considered endemic to the state. About 20 percent of infected people develop symptoms, which can include fever, headaches and vomiting. One percent of all those infected may develop a serious neurological infection which may lead to paralysis and death. Los Angeles County has the highest number of infections in the state. According to the county public health department, there were about 150 cases of the virus and 5 deaths in 2016. 

West Nile Virus is not the only concern. "The situation with mosquitos and mosquito-borne disease is actually getting worse," says Buettner. In recent years new, invasive mosquitoes have arrived in the state which are capable of carrying yellow fever, dengue and the zika virus.

The heavy rain and snow isn't all bad, said Buettner, because it could wash away standing water in storm channels and other potential mosquito breeding areas. The problem, he said, is if it leaves behind pools of water for the mosquitoes to breed in. The new invasive species only need a teaspoon of water to lay their eggs.  

Levy Sun, spokesperson for the Greater Los Angeles Vector Control District, said that the presence of invasive mosquitos meant the conditions were in place for a local disease outbreak.  He added that recent mild winters have also exacerbated the mosquito problem. "In the last few years we didn't really experience any cold snaps that would kill off a large swath of the mosquito population."

The Greater Los Angeles Vector Control District encourages residents to "dump and drain" water after rainfall to prevent mosquitoes from breeding.

Jason Farned, spokesman for the San Gabriel Valley Mosquito and Vector Control District, said the best way to deal with mosquitoes is either by removing standing water or making sure water doesn't sit for more than a few days at a time. In cases where standing water is unavoidable, mosquitofish can be introduced to feed on the larvae.  Farned said the district could change its tactics depending on the number of infections. "We might add more people to the streets. We might schedule some targeted pesticide applications," he said.