Vector Competence of South Dakota Mosquitoes for West Nile Virus

As of January 28, 2004, the CDC had recorded 9122 cases of West Nile Virus and 223 deaths associated with West Nile Virus nationwide.  In South Dakota alone there were 1040 cases and 14 deaths due to West Nile Virus.

Each of the 43 different species of mosquitoes located in South Dakota will play different roles in spreading West Nile virus. The virus is characterized by a bird-mosquito transmission cycle with birds serving as the primary hosts and mosquitoes serving as the primary vectors. Because the principal amplification cycle for West Nile virus appears to involve birds, a mosquito needs to feed primarily on avian hosts to serve as an efficient vector. These mosquitoes are responsible for spreading the virus from bird to bird, which will increase the distribution of West Nile virus in the United States. However, an opportunistic or more general feeding mosquito is needed to serve as a bridge vector to pass the virus from the mosquito-avian cycle to humans and domesticated animals. These mosquitoes are responsible for human infections by carrying the virus from an infected bird to a human host.
 

The following mosquitoes were identified in 2003 and 2004 as having at least one West Nile Virus infected mosquito: Culiseta inornata, Ochlerotatus fitchii, Culex tarsalis, Aedes cinereus, Aedes vexans, Culex salinarius, Ochlerotatus dorsalis, Culex pipiens.  The number of infected pools is summarized in the linked webpage listed below for each year:

Infected Mosquitoes During the Summer of 2003

Infected Mosquitoes During the Summer of 2004

Culex tarsalis

Culex tarsalis is widely distributed west of the Mississippi River.  It is a very common mosquito and made up about 13% of the mosquitoes collected in 2003, but this mosquito made up 75% of the pools that tested positive for the West Nile Virus.  The Culex tarsalis feeds mainly on birds, but has been known to feed on humans and domesticated animals.  It is most active at dusk.  It is considered the most important vector of viral encephalitis to horses and humans in western states and is the primary vector for West Nile Virus in the midwestern states.  Culex tarsalis has been known to carry viruses similar to West Nile Virus.  These include the California Encephalitis and Western Equine Encephalitis.
 
 

Aedes vexans

Aedes vexans is the most abundant mosquito in this area.  Seventy-seven percent of the mosquitoes collected in 2003 were Aedes vexans, but they made up only about 7% of the mosquito pools that tested positive for West Nile Virus.  In the laboratory this species exhibits a high transmission rate, but does not become infected easily.  Aedes vexans feed on humans and domestic animals and is most active at night.  Aedes vexans have been known to carry Eastern Equine Encephalitis, Western Equine Encephalitis, California Encephalitis, and Dengue Fever.
 
 
 
 
 
 
 
 

Ochlerotatus (Ae.) dorsalis

Ochlerotatus dorsalis is a ferocious biter that feeds mainly on humans and domestic animals, although it will feed on sizeable birds if necessary.  It made up about 4% of our population and 4.5% of the positive pools that we tested.  It normally feeds on muggy days and in the evenings. Ochlerotatus dorsalis has been known to carry Western Equine Encephalitis and California Encephalitis, both are similar to the West Nile Virus.
 
 
 
 
 
 
 

Ochlerotatus fitchii

Ochlerotatus fitchii has been known to bite at the edge of forests during the day and evening hours.  They feed on both mammals and birds.  Ochlerotatus fitchii is not a common mosquito.  It only made up 0.62% of the total population caught in 2003 and 4.5% of the positively tested mosquitoes were Ochlerotatus fitchii.
 
 
 
 
 
 
 
 

Culiseta inornata

Culiseta inornata hatch early in the spring before many of the other mosquitoes.  It mostly feeds at night.  Culiseta inornata will attack humans if no other blood source is available, but prefers to attack domestic animals.  In 2003, the Culiseta inornata made up 0.43% of the total collected mosquitoes and 2.3% of the mosquitoes that tested positive for West Nile Virus.
 
 
 
 
 
 
 
 

Aedes cinereus

Aedes cinereus are not aggressive biters.  They feed on both humans and mammals.  Aedes cinereus were not very prevalent during the summer of 2003.  We collected some during the early part of the summer, but found very few later on.  Two percent of the population collected were  Aedes cinereus, while 2.3% of the West Nile positive mosquitoes collected were Aedes cinereus.
 
 
 
 
 
 
 
 

Culex pipiens

Culex pipiens are known and the Common House Mosquito because it will readily enter houses.  It is considered a shy mosquito and prefers feeding on birds, but will feed on humans, usually if the human is motionless. Culex pipiens have been found to be the primary vector for West Nile Virus in eastern United States.  In 2003, Culex pipiens made up 0.64% of the collected population and 2.3% of the postively tested mosquitoes.
 
 
 
 
 
 
 
 
 
 
 

Culex salinarius

It is very difficult to differentiate the Culex salinarius from the Culex pipiens.  It doesn't appear to prefer a certain host.  It feeds, mostly at night, on birds, mammals and humans.  It can become a pest, but was not found in high numbers in 2003.  Culex salinarius was 0.67% of the total collected population and 2.3% of the positively tested mosquitoes.