Why Drought Years Can Increase the Risk of Mosquito-Borne Illness

When Less Water is Worse Than More Water


It's official. California is suffering from a serious drought. The extreme weather has prompted Governor Jerry Brown to declare an official drought emergency . This comes on the heels of 2012 when California experienced  one of the hottest years on record, and according to the National Weather Service, 2013 was the driest year on record. Now, with these drought conditions, accompanied by record-breaking temperatures, businesses and residents alike face a myriad of problems. Not the least of these problems is the potential for an increasing risk of mosquito-borne illness.

In a typical non-drought year, healthy creeks and rivers flow freely and typically don't produce mosquitoes. Mosquito eggs are fragile and can't survive in these flowing or agitated waters. Standing, or still water is the culprit. Female mosquitoes only lay eggs in still waters where they won't be harmed. During drought conditions, creeks dry up and leave scattered puddles that hold still and standing water perfect for immature mosquitoes--water with no agitation nor predators. In Contra Costa County, mosquitoes that can transmit West Nile virus can lay up to 400 eggs in very little water. 

When water is scarce in drought years, the remaining puddles become critical water sources for insects, birds and animals that all gather at the same locations. This high concentration of birds, which can carry West Nile virus, and mosquitoes which can transmit the virus from birds to people, creates a greater health risk.

The risk of mosquito-borne illness also increases in a drought year when areas of water that once held mosquito-eating fish dry up, because without enough water, the fish cannot survive to eat the mosquitoes. In addition, when rain does eventually fall, the water deposits in areas that now no longer hold fish. Without these predators, mosquito activity can flourish. More mosquitoes may mean more virus.

District technicians will need to increase mosquitofish stocking to reestablish fish in those sources. Luckily, District staff are prepared for such a task. The District's mosquito rearing facility already produces more than a million fish a year dedicated to natural mosquito control in Contra Costa County and can easily accommodate a higher production.

"From large flowing streams to scattered water puddles, the landscape may be a challenge for us this year," says General Manager, Craig Downs. "Every year is different and we're used to and good at adapting, but, the real challenge will be for homeowners who might not know the significance a water puddle can make. Think exponential."

The District recommends citizens should regularly observe their own front and back yards for mosquito activity and contact the District if mosquitoes are found. Residents are cautioned to not overwater to compensate for drought conditions. Overwatering creates above ground puddles and fills underground catch basins where mosquitoes can flourish undetected. Now that drought conditions have been confirmed, less water may mean more West Nile virus.   

January, 2014

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