mosquito biology

What is West Nile Virus and How Does it Spread?

West Nile virus (WNV) is a mosquito-borne arbovirus that was discovered in 1927 in the West Nile sub-region of Uganda. The first serious outbreaks of WNV occurred in the mid-1990s in Algeria and Romania. The virus was introduced in the United States in 1999, with the first case being identified in New York City. That year, 62 human cases, 25 horse cases, and countless bird diagnoses were reported in New York State. The CDC has since received more than 40,000 reports of people affected with WNV in the lower 48 states, making it the most common virus transmitted by mosquitoes to humans in the U.S. Because only a portion of all cases are reported, the CDC believes the actual number of illnesses may be as high as 700,000.

virus-life-cycle_mosquito-bird-mosquito.pngWNV is typically transmitted to humans by mosquitoes that have previously fed upon an infected bird. While over 150 species of mosquitoes have been known to carry the virus, the main vector species in the U.S. are Culex pipiensCulex tarsalis, and Culex quinquefasciatus. These mosquitoes are all active at night, and most cases of infection occur during the summer months. Transmission of the virus has occurred in a very small number of cases through blood transfusion, organ transplant, and breastfeeding; however, WNV is not transmitted from person-to-person or from animal-to-person through casual contact.

icon-thermometer Symptoms of West Nile Virus Infection
  • The majority (4 out of 5) of people infected with West Nile virus will experience no symptoms.
  • Approximately 20% of people affected by the virus will experience flu-like symptoms including fever, headache, nausea, muscle pain, and swollen lymph glands.
  • Other symptoms may include a stiff neck, rash, sleepiness or disorientation.
  • Less than 1% of those infected will develop West Nile Encephalitis or Meningitis, which can lead to coma, tremors, convulsions, paralysis, and even death.
icon-treatment Treatment of West Nile Virus Infection
  • Rest and acetaminophen can be used to treat the symptoms associated with West Nile virus infection.
  • There is currently no WNV vaccine available.
  • People with mild symptoms typically recover completely within several weeks.
  • Those with severe symptoms are often hospitalized to receive supportive treatment and close care.
  • If you believe that you or a family member might have a West Nile virus infection, consult a physician immediately.

Severe Neurological Diseases Caused by West Nile Virus

ICON-risk_easter_equine_encephalitis_EEE.pngSevere cases of WNV infection can affect the central nervous system and result in a number of serious neurological diseases.

West Nile encephalitis, West Nile meningitis, and West Nile poliomyelitis are all diseases that cause inflammation of the brain and/or spinal cord and the surrounding tissues. These diseases can be debilitating or, in some cases, even deadly.

West Nile Virus and the United States

As of December 14, nearly every state has reported West Nile virus infections in people, birds, or mosquitoes in 2021. Arizona is reporting the highest number of human cases (1,426).

In 2021, 49 out of 50 states and the District of Columbia reported West Nile virus infections in people, birds, or mosquitoes. Overall, 2,445 cases of WNv were reported in humans, and there were 165 (6.8%) confirmed deaths in 2021. This data shows an increase in the number of human cases reported in 2020 (731). It also brings attention to the increase in lives lost, when compared to the 66 (9%) confirmed deaths in 2020 and the 60 (6.2%) confirmed deaths from WNv in 2019.

CDC WNV 12.14.21

This map from CDC shows West Nile virus activity across the U.S. in 2021.

A Global View of West Nile Virus

West Nile virus is the cause of the largest outbreak of neuro-invasive disease ever observed. The virus can be found throughout Africa, regions of Europe and the Middle East, West Asia, Australia, Canada, Venezuela, and the United States. Outbreak areas are typically found along major bird migratory routes, with the largest outbreaks having occurred in Greece, Israel, Russia, Romania, and the United States. Since there is no vaccine for humans, the only way to reduce the spread of this dangerous disease is to raise awareness of prevention measures through public education and to implement mosquito surveillance and control programs in at-risk areas.

Know Your Culex Mosquitoes

West Nile virus is spread by the Culex species of mosquito. The main vector species in the U.S. are Culex pipiens, Culex tarsalis, and Culex quinquefasciatus. These mosquitoes typically feed from evening to morning.

Culex pipiens, commonly known as the Northern House Mosquito, is found mainly in the Eastern U.S. It is a medium-sized mosquito with a brownish or grayish body and brown wings. Their larvae thrive in containers of stagnant, polluted water, and thus these mosquitoes are the primary target of the campaign to drain backyard sources of standing water as a way to reduce the incidence of WNV. In addition to WNV, Culex pipiens is a known carrier of St. Louis encephalitis, Western Equine encephalitis, and heartworm in dogs.

Culex tarsalis lives throughout the U.S., but is found mainly in the Midwest and West. It is a black mosquito distinguished by a white band on its proboscis, as well as white stripes along its middle and hind legs. Culex tarsalis is most active in the few hours after sunset.

Culex quinquefasciatus, the Southern House Mosquito, can be found in the southeastern region of the U.S. It is a medium-sized mosquito that is brown in color, and is a vector of WNV, St. Louis encephalitis, Western Equine encephalitis, and avian malaria.

Controlling Culex Mosquitoes and West Nile Virus

An Integrated Mosquito Management (IMM) program is essential to helping prevent mosquito bites and transmission of serious vector diseases in the United States.  As part of an effective IMM program, VDCI recommends a 4-pronged approach to target all phases of the mosquito’s life cycle.

1:  Public Education
Mosquito control professionals can only do so much, and this is why we rely on a well-educated public in order to have a successful mosquito control program. Educating the public empowers people to take control of the mosquitoes breeding in their back yard and gives them the tools needed to reduce mosquito annoyance.

2:  Surveillance 
Surveillance allows us to detect mosquito species in a given area as well as any changes in populations. With this valuable data, we are able to more effectively time larvicide applications and more accurately target adulticide activities.

3:  Larval Mosquito Control
Our trained field technicians inspect both known sources of standing water and any newly discovered sites for the presence of mosquito larvae. Eliminating mosquitoes prior to their becoming adults is an important element of controlling West Nile virus and other mosquito-borne diseases because it stops mosquitoes before they acquire the virus and have the opportunity to transmit it to people.

4:  Adult Mosquito Control
When necessary, adulticide applications are conducted with EPA-approved pesticides that are used in the safest and most environmentally sound way possible. Additionally, VDCI regularly tests adult mosquitoes and takes all appropriate measures to prevent them from developing resistance; thereby, minimizing the number of applications needed to control the population.


VDCI is a company built on the foundations of public health, ethics, professionalism, and technical expertise. We establish vector management programs that are based on an understanding of the underlying vector’s ecology and rooted in the current science of environmentally sound control measures.