Utah Pest News Fall 2010

click here for pdf version

Mosquitos and West Nile Virus in Utah

culex mosquito egg raft
Fig. 1.  Culex mosquito eggs are deposited vertically forming a raft of eggs that floats on water.
culex mosquito pupa
Figs. 2 and 3.  Mosquito larvae breathe air at the water surface using an anal tube (top). The pupa (bottom) develops into an adult and uses siphons to breathe at the water surface. 

Mosquitoes are one of the most medically important insects because several species are capable of transmitting disease-causing organisms to humans and animals.  West Nile Virus (WNV), a disease most commonly transmitted by Culex mosquitoes, was first detected in North America in 1999 in New York City.  Since then WNV has moved west and was recently detected in Utah.

Mosquitoes are small, slender flies that have four distinct life stages (egg, larva, pupa, and adult) associated with aquatic habitats.  Female Culex mosquitoes will lay eggs in rows directly on the surface of pooled, slow-moving or stagnant water.  Interestingly, the eggs hold together by surface tension and form a raft that floats on the water (Fig. 1).  Mosquito larvae hatch from eggs and live in the water (Fig. 2), feeding on micro-organisms and decaying plant and animal matter. Larvae then develop into non-feeding pupae that also live in water (Fig. 3).  The adult mosquito emerges from the pupal case at the water surface and prepares itself to begin flying.

It is important to note that only the adult female mosquito bites, as she requires a blood meal to lay eggs.  Male mosquitoes, on the other hand, feed on plant nectar.  Female mosquitoes may live for several weeks biting every few days.

Mosquitoes become carriers of WNV after biting infected birds that serve as a reservoir for the virus.  Birds infected with WNV maintain viable virus particles in their blood for 1-4 days.  Some bird species are highly susceptible to the virus and die, while other bird species develop immunity. A mosquito that has bitten an infected bird has the ability to transmit the virus to humans and other mammals.  In particular, horses are highly susceptible to WNV.  Symptoms of WNV develop 3-14 days after being bitten by an infected mosquito.  WNV particles, however, never build up to high enough levels in the blood of infected humans or horses to be taken up by mosquitoes.

Most people with healthy immune systems that are infected with WNV will never show symptoms.  Mild symptoms similar to the flu occur in approximately 20% of people infected with WNV.  One in 150 people infected with the virus develop severe symptoms (including high fever, stiffness, tremors, disorientation, and vision loss) and may also experience serious complications (such as swelling of the brain or spinal cord), requiring hospitalization. People over the age of 50 and those with compromised immune systems have the highest risk of severe illness from WNV.

Local and state agencies work together to detect the activity of WNV.  The surveillance of WNV in Utah involves testing mosquitoes collected from traps and surrounding water sources, taking oral swabs of live and dead birds, and monitoring blood samples of humans, horses, and chickens. WNV was first detected in Utah in 2003.  Utah had its highest activity of WNV in 2006 which spanned the Wasatch Front in populated areas of Salt Lake and Utah Counties. Unfortunately, in 2006 there were 158 human cases of WNV detected and 5 resulted in death.  Activity of WNV appears to have decreased since 2006 with only 2 human cases reported in 2009.  Although no human cases of WNV have been reported to date in 2010, four counties (Davis, Salt Lake, Uintah, and Washington) have tested positive for WNV from mosquito and horse samples.  As a result, people should remain vigilant and take the proper precautions to reduce their exposure to mosquitoes.

In order to reduce the risk of getting WNV, use protective methods to prevent mosquito bites.  Make sure the seals and screens for windows and doors around the home are secure.  Avoid using incandescent lights outside of the home that attract mosquitoes.  Stay indoors during peak mosquito activity at dawn, dusk, and early evening.  When outdoors, discourage mosquitoes from biting by wearing long-sleeve shirts and long pants, or use mosquito repellent. Finally, reduce mosquito breeding grounds around homes and neighborhoods by not allowing water to stagnate in places like birdbaths and pools and eliminate standing water from sources like containers and tires left outside and clogged roof gutters.

-Ricardo Ramirez, Entomologist


Hodgson, E.  2007. West Nile Virus in Utah. Utah State University Extension Fact Sheet. ENT-105-07.
Utah Department of Health, Bureau of Epidemiology http://health.utah.gov/els/index.html