West Nile Infection: Causes, Symptoms, and Treatment5 min read

What Is West Nile Virus?

West Nile virus (WNV) is a microorganism (virus) transmitted by mosquitoes and, more rarely, ticks. It is commonly found in Africa, West Asia, the Middle East, and the Caribbean.

Most humans who contract the virus have been bitten by an infected mosquito, often the common house mosquito (Culex pipiens). This mosquito, found in most U.S. states, is a known vector of WNV. (A “vector” is an animal host that transmits an infectious disease).

This disease can infect humans, birds, mosquitoes, horses, and some other mammals. West Nile virus infection can cause West Nile fever (WNF) and West Nile Neuroinvasive Disease (WNND).

History of West Nile Virus in the U.S.

West Nile virus was discovered in 1937 in the West Nile district of Uganda. West Nile emerged in the United States for the first time in the New York City area in Aug. 1999. There were 62 confirmed human cases and seven deaths during this outbreak, creating widespread concern.

In 2012, the CDC confirmed an outbreak of 1,118 cases of West Nile virus, the highest number of reported cases since the virus was first detected in the U.S. Of those, 56% of patients from 47 states developed neuroinvasive disease. Approximately 75% of the cases were reported from five states (Texas, Mississippi, Louisiana, South Dakota, and Oklahoma), and almost half of all cases were reported from Texas, many in the Dallas area.

According to the CDC, since 1999 more than 50,000 people have been reported with West Nile virus in the U.S. More than 2,300 of those people died. Cases of West Nile infection spiked in 2018, with 25% more cases reported that year than average.

West Nile Virus in the U.S. Today

As of August 2019, every state but Hawaii has reported West Nile virus infections according to the Centers for Disease Control. More than 2,600 cases were reported in 2018, resulting in 167 deaths. More than 1,600 of these cases had potential to infect the nervous system, marking them as more serious and potentially deadly. Indeed, 10% of the patients reported to the CDC with neuroinvasive WNV died in 2018.

How Does West Nile Virus Infect Mosquitoes?

Birds are vectors (intermediate carriers) of the virus. As such they are important to the virus’s life cycle and transmission cycle. For female mosquitos that carry WNV, a typical blood meal comes from birds. When mosquitoes feed on the blood of an infected bird, the virus can infect these mosquitoes.

Birds make ideal vectors for WNV. That’s because they can circulate high levels of the infection in their blood. The infected birds themselves may or may not show signs of illness. Crows and Jays are some of the species most likely to die from infection, whereas most other bird species survive.

How Do People Get Infected With West Nile Virus?

Almost all people infected with West Nile virus contract the disease through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which circulate the virus in their blood. To date, more than 300 different species of bird have been found to carry the virus in the U.S. However, there is no evidence the virus can be spread directly from a bird to a human.

The virus eventually finds its way into the mosquito’s salivary glands. During subsequent blood meals, the virus may be injected into humans and animals, where it can multiply and possibly cause illness.

Is West Nile Virus Contagious?

It is important to remember that West Nile virus is not contagious and cannot be transmitted from person to person. A person cannot get the virus, for example, from touching or kissing someone who has the disease. Likewise, a health care worker who has treated someone with the disease cannot spread the infection to others.

Humans are called “dead-end” hosts for the virus, meaning we can be infected but our immune systems prevent the virus from multiplying enough to be passed back to mosquitoes and then to other hosts. Horses can also catch West Nile virus, but are also dead-end hosts and cannot infect humans.

Rare Cases in Which West Nile Virus Is Spread

In a tiny number of cases, WNV has also been spread through these unusual means:

• Blood transfusion
• Organ transplant
• Breastfeeding
• Pregnancy (mother to baby)
• Laboratory exposure

What Are the Risks of West Nile Virus During Pregnancy?

Pregnant mothers have a very low risk of spreading WNV to their newborns. However, only a small number of cases have been studied so far.

Related: What you must know about Zika Virus 

That means it is not yet possible to determine what percentage of WNV infections during pregnancy result in infection of the fetus or medical problems in newborns.

One small study of infected mothers found that all 17 children were born healthy. In fact, there has been only one documented instance of a newborn becoming infected during pregnancy.

Even so, concerns remain that mother-to-child WNV transmission can occur with possible adverse health effects. For this reason, pregnant women should take precautions to reduce their risk for WNV and other mosquito-borne infections by avoiding mosquitoes. They can do this by following three safety precautions:

• Stay away from mosquitos and mosquito-prone areas
• Wear mosquito-protective clothing, including long sleeves
• Use repellents that have DEET. FDA-registered repellents with DEET are deemed safe for pregnant women.

How Does West Nile Virus Actually Cause Severe Illness and Death in Humans?

Following transmission by an infected mosquito, West Nile virus can multiply in the blood system and cross the blood-brain barrier to reach the brain.

When this happens, it causes West Nile Neuroinvasive Disease. During WNND the virus interferes with normal central nervous system functioning and causes inflammation of brain tissue.

Why some people develop WNND and not others remains unclear. A weak immune response may be responsible, though. Some researchers believe the virus develops into WNND when a person’s immune system fails to adequately control the virus, allowing the infection to develop deeper into the nervous system.

Treatment for West Nile Virus

The diagnosis of West Nile virus infection is confirmed with a blood or cerebrospinal fluid test. There is no specific treatment for West Nile virus infection. Mild illness does not require therapy other than medication to reduce fever and pain.

When West Nile infects the brain, intensive supportive therapy is needed. Anti-inflammatory medications, intravenous fluids, and intensive medical monitoring may be required in severe cases. There is no specific antibiotic or antidote used to treat this viral infection.

Is There a West Nile Virus Vaccine?

There is no current vaccine to prevent the virus. A vaccine for horses was first introduced in 2005, and four other horse vaccines have been released since then. Several US laboratories are currently using various approaches to research and develop a human vaccine, though to date none have moved past the trial phase of development.

Similar Viruses

West Nile virus is closely related to Japanese encephalitis virus and St. Louis encephalitis virus. These viruses are found in the US in the southeast, west, and Midwest. These are all mosquito-borne viruses, and have a similar life cycle in birds and mosquitoes to WNV. They are even found in some of the same mosquitos that carry WNV.

A major difference is that St. Louis encephalitis is “silent” in birds, generally not killing them, so there is usually no warning before a human case occurs.

With West Nile virus (at least the American strain), some birds, particularly jays and crows, become ill or die from the disease and therefore offer an early warning to researchers.

Source: WHO