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Monday, July 18, 2011
08:13 AM

FLOODING, MOSQUITOES, AND WEST NILE VIRUS…What You Need to Know

Dr. Richard Houseman, Entomologist, University of Missouri

15 July 2011

Since mosquitoes breed in standing, stagnant water, recent flooding along the Missouri River in Missouri has resulted in expanded mosquito breeding habitats and subsequent mosquito population explosions.  It has also lead to concern about the potential for increased transmission of West Nile Virus (WNV) within bird and mammal populations.  WNV life cycle is intimately linked with birds, which amplify the virus when infected, thus allowing mosquitoes who feed on them to acquire sufficient virus to transmit.  Mammals are infected but do not amplify the virus and so are considered dead-end hosts.  Humans and horses are some of the most notable mammals affected by WNV. 

While it is true that mosquito populations have increased in flooded areas during 2011, the threat of WNV transmission is not necessarily elevated because the types of mosquitoes associated with flooding are not the same as those associated with transmitting WNV.  The most common floodwater species throughout Missouri is Aedes vexans while the most common WNV vectors are Aedes albopictus, Culex tarsalis, and Culex pipiens.  The abundance of these different species also varies hugely, with the floodwater species being many times more abundant than the vector species, even during normal years where there is average flooding. 

Recent surveys in central Missouri during an entire summer using traps at 12 different locations collected over 65,000 Aedes vexans mosquitoes (floodwater species).  During the same time period and at the same locations only about 750 mosquitoes were collected from the three combined vector species.  This represents a ratio of about 99:1 floodwater to vector mosquitoes collected over an entire summer in an average year.  In a flood year, where mosquito trap collections are 3-5 times higher than normal, the ratio of floodwater to vector mosquitoes could easily increase to over 400:1.  Laboratory screening of over 30,000 individual mosquitoes from St. Louis County this summer found only 6 mosquitoes that were positive for WNV.  These surveys seem to indicate that the threat of WNV can remain low despite large increases in the number of mosquitoes associated with floodwater.

The floodwater species Aedes vexans gets its name because it such an aggressive, persistent, and ‘vexing’ biter.  It feeds on mammals, including humans, and is most active from early evening until midnight.  It lays its eggs on the soil just above standing water and they withstand drought, cold, and rain for up to four years.  It is only when eggs are flooded and floating in water with reduced oxygen levels (caused by decaying organic matter) that they hatch into larvae called ‘wigglers’ whose density can reach 5,000 to 20,000 per square foot of surface area.

The vector species Aedes albopictus, Culex tarsalis, and Culex pipiens vary in their biology and behavior.  All three species feed on birds in addition to mammals, which enables them to acquire WNV from birds and transmit it to humans or horses.  They lay their eggs in a wide variety of standing water situations that are encountered in and around our homes and other buildings.  To reduce populations of these vector mosquitoes, eliminate standing water around your home and neighborhood by emptying water from old cans, buckets, birdbaths, tires, potted plants, clogged gutters, etc. 

While it is good news that the risk of WNV is remains low despite increased mosquito populations associated with recent flooding, do not become complacent.  Seek to eliminate mosquito breeding habitats and treat standing water that is not able to be eliminated or drained.  Products called mosquito ‘dunks’ or ‘donuts’ kill mosquito larvae and are placed in standing water where larvae are found.  These products contain a bacterium called Bt and are non-toxic to humans. 

Reduce your risk of being bitten by keeping windows and doors closed, staying inside during dusk and dawn, and by wearing long sleeves/pants and using repellents if outside.  Many repellent products are shown to repel mosquitoes, but are usually effective for less than one hour.  Products containing DEET usually provide protection for up to 4 hours.  It is not necessary to apply products with more than 35% DEET concentration since they do not provide any additional repellency or longevity.  Use lower DEET concentrations on children. 

Horses have a much higher incidence of death than humans following infection and must be protected from WNV even during an average year.  A vaccine has been developed for protecting horses.  See a licensed veterinarian for more information about the vaccine. 


University Outreach and Extension David Reinbott, reinbottd@missouri.edu
Agriculture Business Specialist
Last modified: July 18, 2011