West Nile Virus (WNV) is a potentially life-threatening viral infection.

It can pass to animals and humans if they are bitten by an infected mosquito

WNV is a virus of the Flaviviridae family, which includes the viruses responsible for Japanese encephalitis and dengue fever.

It mainly affects birds, but it can also be experienced by mammals and reptiles.

Between 70 and 80 percent of people have no symptoms. Up to one percent of those who become ill have serious and potentially fatal complications.

West Nile Virus (WNV) used to exist only in temperate and tropical areas, but in 1999, infections appeared in New York. It has since spread across most of the United States (U.S.), and is a notifiable disease.

Fast facts on West Nile Virus

Here are some key points about West Nile Virus (WNV). More detail is in the main article.

  • Mosquitoes transmit WNV from birds to humans.
  • WNV previously did not exist in the U.S., but in 1999, some imported cases triggered an outbreak.
  • Around 80 percent of people have no symptoms, but in up to 1 percent of cases, the virus can lead to life-threatening neurological complications.
  • The best way to prevent WNV is to avoid mosquito bites.

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West Nile Virus is transferred to humans by mosquitos. They pick up the virus from deceased birds.

WNV can affect humans in three different ways:

  • Asymptomatic infection: In about 80 percent of cases, there are no signs or symptoms.
  • West Nile Fever: Around 20 percent of people experience a mild febrile syndrome.
  • Neuroinvasive disease: About 1 percent of patients develop complications in the central nervous system (CNS) that affect the brain and spine.

West Nile fever

Symptoms appear 2 to 8 days after infection. This is known as the incubation period.

They may include:

These symptoms resolve within 7 to 10 days. Fatigue may linger for several weeks, while glands may be swollen for up to 2 months.

Neuroinvasive disease

Around 1 percent of infected individuals develop more serious neurological infections, and around 10 percent of these cases are fatal.

Possible complications are:

  • Encephalitis: Inflammation of the brain
  • Meningitis: Inflammation of tissues surrounding the brain and spinal cord
  • Myelitis, or West Nile poliomyelitis: Inflammation of the spinal cord
  • Acute flaccid paralysis: Sudden weakness in the arms, legs and breathing muscles.

Signs and symptoms may include:

  • confusion and disorientation
  • convulsions
  • high fever
  • muscle jerking
  • pain
  • Parkinson's disease-like symptoms, including tremors
  • sudden weakness, poor coordination, and partial paralysis
  • severe headache
  • stiff neck
  • stupor
  • coma

Most at risk are people aged over 60 years and those with existing conditions, such as kidney disease, diabetes, cancer, and conditions that weaken the immune system.

Some neurological effects can be permanent.

The American robin is one of the key culprits for the presence of WNV in the U.S.Share on Pinterest
The American robin is one of the key culprits for the presence of WNV in the United States.

Infected birds have high levels of the virus. In the U.S., the American robin and the American crow are common carriers.

If a mosquito bites an infected bird, and it then bites a person, the virus will enter the bloodstream of that individual.

The Culex Pipiens mosquito is known to pass on WNV in the U.S.

It remains unknown exactly how the virus works. WNV enters the bloodstream and reproduces, and sometimes it can cross the blood-brain barrier to cause inflammation in the brain.

Transmission is also possible through:

  • Blood transfusions: Health authorities now screen patients for WNV before accepting a transfusion.
  • Organ transplants: According to the Centers for Disease Control and Prevention, some centers test organ donors for WNV, whereas others do not.
  • Pregnancy: An infected mother can infect her fetus, but the risk is very low.
  • Breast-feeding: There is a very small chance of passing on the virus through breast milk, but the risk is so small that the Centers for Disease Control and Prevention (CDC) advise mothers to continue breast-feeding.

Risk factors

Certain factors increase the risk of infection.

Seasons: In temperate areas, WNV begins to appear in early spring. Infections peak in late summer and early fall. In tropical and some sub-tropical areas, there is a year-round risk of infection.

Location: Living in or visiting an area known to have WNV increases the risk of infection. In the U.S., this includes all states except Alaska and Hawaii.

Exposure to mosquitoes: Spending more time outdoors increases the chance of infection after being bitten by an infected mosquito.

Laboratory work: Infection can occur in laboratories where WNV is present.

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West Nile Virus is a notifiable disease, meaning that the authorities must be informed if its presence is detected.

In the U.S., WNV is a notifiable disease. This means that all cases must be reported to the authorities.

In 1999, WNV was imported to the U.S., triggering a "large and dramatic outbreak," according to the World Health Organization (WHO).

Since then, nearly 44,000 cases have been reported. Of these, over 20,000 people with WNV have have developed neuroinvasive complications and over 1,900 have died.

In 1999, a total of 62 cases were reported, including 7 fatalities, a mortality rate of 11 percent. By 2015, there were 2,175 cases and 146 deaths, or a fatality rate of 7 percent.

The highest number of cases was in 2003, at 9,862, and the highest mortality rate was in 2001, at 15 percent.

From 2016 to January 17, 2017, 2,038 people were reported to have had WNV. 56 percent of these developed neuroinvasive diseases, such as meningitis.

The doctor will ask about symptoms and carry out a physical examination.

The following diagnostic tests may be ordered:

  • Blood test: This may reveal higher-than-normal levels of antibodies to WNV. A complete blood count may be done.
  • CT or MRI scan of the head: This can sometimes reveal brain inflammation and swelling.
  • Lumbar puncture, or spinal tap: This can diagnose meningitis.

In a lumbar puncture, cerebrospinal fluid from around the brain and spinal cord is extracted. A needle is inserted between the lower vertebrae of the spine. A high level of white cells suggests an infection.

Most patients make a full recovery without medical treatment. Over-the-counter (OTC) treatments can help with symptom relief.

Severe symptoms will require hospitalization and supportive treatment, such as assistance with breathing and intravenous fluids.

There is currently no vaccine available to protect against WNV.

Humans cannot pass on the disease, so the best prevention is to avoid being bitten by mosquitoes.

Things to consider include:

  • Clothing: Cover as much skin as possible. Wear long-sleeved shirts, long pants, high socks, and a hat. Some people tuck the bottom of their pants into their socks.
  • Mosquito repellents: Use one with at least 10 percent concentration of DEET. DEET should not be used on young children, and insect repellent should not be used on infants under 2 months.
  • Mosquito traps, nets, and screens: Maintain screens on doors and windows, and have nets over beds and children's strollers. Ensure that there are no holes.
  • Smell: Avoid heavily scented soaps and perfumes, as these can attract mosquitoes.
  • Camping: Treat clothes, shoes and camping gear beforehand with permethrin. Specially treated clothes are available from some stores.
  • The time of day: Mosquitoes are more plentiful at dawn and dusk.
  • Stagnant water: Mosquitoes breed in clean, stagnant water.

To reduce the risk of WNV due to stagnant water:

  • Check and remove stagnant water from around the home, and avoid camping near lakes and ponds.
  • Turning over pails and watering cans and storing them under shelter can prevent them filling with water.
  • Remove the water from plant pot plates, or avoid using them, if possible. Loosen hard soil from potted plants to prevent puddles from developing on the surface.
  • Change the water in flower vases every two days, and scrub and rinse the inside of the vase thoroughly each time.
  • Keep leaves from causing puddles.

To remove mosquito eggs, thoroughly clean and scrub pot plates, pails, and other containers.

Keep scupper drains unblocked and do not cover them with plants or other items.

Cover rarely used gully traps, use non-perforated traps, and install anti-mosquito valves.

Do not place receptacles under or on top of air-conditioning units.

Along with reporting any dead birds to the authorities, these measures can help to prevent WNV in the wider community.