Backyard tiger mosquito control

West Nile virus (WNV)

West Nile virus belongs to the flaviviruses. The name for this group was influenced by the yellow fever virus, another member of the flavivirus group, as Latin “flavus” means “yellow”. Dengue fever, Japanese encephalitis, Tick-borne encephalitis (TBE), and the Zika virus, for example, also belong to this genus of pathogens.

The virus was discovered in 1937 in the West Nile district of Uganda. In the following decades it appeared in Israel, Egypt and finally also in France. Since then it has spread to all five continents and is now one of the most widespread flaviviruses worldwide. Today, it is the most commonly transmitted virus to humans by mosquitoes in the U.S.

The West Nile virus transmission cycle

The virus circulates between birds and mosquitoes. When an infected bird is bitten by a mosquito, the mosquito can transmit the virus to another bird. While WNV has been detected from 65 species of mosquitoes in 10 genera, Culex mosquitoes, Cx. pipiens (L.), Cx. quinquefasciatus, and Cx. tarsalis (Coquillett), are the primary vector species and most important in maintaining the cycle. WNV has also been isolated multiple times in field-collected Ae. aegypti and Ae. albopictus, but they appear to be of only minor importance with respect to human infections.

Sometimes a virus-carrying mosquito (vector) bites a human or a horse instead of a bird for its next blood meal. These are not the actual target of the virus, but they can also get infected with WNV (they are therefore called incidental hosts). Humans and horses are also considered “dead end” hosts because they don’t develop a high enough viremia to infect biting mosquitoes (see Figure 1). In rare cases, transmission from human to human is possible by blood transfusion, organ transplantation, breastfeeding or by transmission from an infected pregnant woman to her unborn child.

West Nile Virus Transmission Cycle
Figure 1. The West Nile Virus transmission cycle. The virus cycles between birds and mosquitoes. Humans and horses are incidental or “dead end” hosts.

The disease

Fortunately, a WNV infection is not a problem for most people, as 8 out of 10 infected don’t develop symptoms. According to Centers for Diseases Control and Prevention (CDC), about 1 in 5 infected people develop the following symptoms: fever, headache, body aches, joint pains, rash, vomiting, diarrhea. However, WNV can also lead to serious, neurologic illness in about 1 per 150 infected, and some of these cases are fatal. The main victims are people with a weakened immune system, such as elderly persons or people with pre-existing illness. The situation is aggravated by the fact that there is no specific antiviral treatment or vaccine available. A vaccine against the virus is currently only available for horses.

Therefore, the best way to avoid the disease is to protect yourself from mosquito bites: Tip & Toss, maintain pools, use EPA-approved mosquito repellents, use mosquito nets, use powerful mosquito traps.

The WNV situation in North America

The West Nile virus was first detected in the USA in New York in 1999 (Figure 2), when the inhabitants of the city noticed many dead birds in Central Park. The virus subsequently spread throughout the U.S. and into Canada, Mexico, and several Central and South American countries. Cases have been reported from every state except Alaska. It took only four years for the virus to spread across the entire country causing major epidemics along the way (Figure 3). In 2002/2003 a total of over 14,000 cases and 548 deaths were reported and in 2012, nearly 5700 WNV cases and 286 deaths were registered. On the webpages of the CDC, you find WNV statistics and maps for cases in the US.

In Canada, the virus was first registered in Ontario in 2002. Almost 1500 cases were reported in 2003 and over 2200 cases in 2007. However, due to the climate, the distribution area has so far been essentially limited to a few southern states of the country.

This map shows the incidence of human West Nile virus neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis) by state for 1999

Figure 2: This map shows the incidence of human West Nile virus neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis) by state for 1999 with shading ranging from 0.01‐0.24, 0.25‐0.49, 0.50‐0.99, and greater than 1.00 per 100,000 population. Source: CDC.

This map shows the incidence of human West Nile virus neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis) by state for 2003 with shading ranging from 0.01‐0.24, 0.25‐0.49, 0.50‐0.99, and greater than 1.00 per 100,000 population.

Figure 3: This map shows the incidence of human West Nile virus neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis) by state for 2003 with shading ranging from 0.01‐0.24, 0.25‐0.49, 0.50‐0.99, and greater than 1.00 per 100,000 population. Source: CDC.