Why the Risk of West Nile Virus Increases During a Drought


All 58 California counties, including Contra Costa County, are under a drought emergency and summer has barely begun, with its dry, hot weather. Understandably, the idea of below-average rain gauges and reservoirs may sound like fewer water sources to produce mosquitoes, but the fact is, drought years can increase the risk of mosquito-borne illnesses including West Nile virus.

While mosquitoes develop from egg to adult in water, they must swim to the surface of the water to access air. When the creeks, streams, and rivers have enough water to flow, even though there is more water, there can be fewer places where mosquitoes can successfully mature to adulthood because the rushing water prevents them from reaching the surface of the water to breathe. When water is still, however, mosquitoes can thrive.

During drought conditions, the risk of West Nile virus can increase because:

  • As water flow in creeks and streams decreases, pockets, and puddles containing standing water can appear providing mosquitoes a place to develop from egg to adult. And in the summer’s heat, the smaller amount of water in puddles can warm more quickly, actually allowing mosquitoes to develop faster.
  • Other wildlife, including birds, will look for water and will be attracted to the available puddles.
  •  As West Nile virus is a disease carried by birds, particularly those in the Corvid (crow) family, infected birds may be attracted to the same puddles that are producing mosquitoes
  • Newly emerged adult female mosquitoes can bite an infected bird and become infected.
  • The newly infected adult female mosquito can transmit West Nile virus to another animal or human through a single mosquito bite.

What can Contra Costa County residents do to reduce the risk of mosquitoes and West Nile virus during a drought?

  • Dump out any amount of water in your yard. Even the amount that can fill a bottle cap can produce mosquitoes.
  • If you’ve saved rainwater, make sure mosquitoes can’t access the water.
  • Cover the container with tight-fitting window screen to prevent mosquitoes from laying eggs in the water.
  • Use a product that can be found in home and garden stores that contains a naturally occurring bacteria that is deadly to mosquitoes but is harmless to other living things.
  • If you have a container holding 30 gallons of rainwater or more, or a swimming pool or hot tub that is holding any amount of water but is no longer being used or maintained, contact the District to request mosquitofish service. A District employee will inspect the water feature and determine the best option to prevent mosquitoes, placing fish in the water if appropriate.
  • Avoid overwatering lawns because it can lead to mosquito production from standing water in catch basins, drains, and other artificial containers.
  • If you’re going to be out where mosquitoes are present, wear repellent on exposed skin to reduce the risk of mosquito bites.

About Kevin

Manager of Mainframe Operations and Optimization – USS-UPI, Co-Founder and Board Member - Friends of Oakley A Community Foundation, Trustee RD 2137, Advisory Board – Opportunity Junction
This entry was posted in Vector Control. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s