West Nile virus
Encephalitis - West Nile; Meningitis - West Nile
West Nile virus causes a viral disease and is spread by mosquitoes. The condition ranges from mild to severe.
There are many different species of mosquito, which can carry some of the world's most common and significant infectious diseases, including West Nile, Malaria, yellow fever, viral encephalitis, and dengue fever. (Image courtesy of the Centers for Disease Control and Prevention.)
These are mosquito pupa. This is another stage in the development of the mosquito. (Image courtesy of the Centers for Disease Control and Prevention.)
Mosquitoes of the Culex species lay their eggs in the form of egg rafts that float in still or stagnant water. The mosquito lays the eggs one at a time sticking them together in the shape of a raft. An egg raft can contain from 100 to 400 eggs. The eggs go through larval and pupal stages and feed on micro-organisms before developing into flying mosquitoes. (Image courtesy of the Centers for Disease Control and Prevention.)
This illustration shows an adult southern house mosquito. This mosquito feeds on blood and is the carrier of many diseases, such as encephalitis, West Nile, dengue fever, yellow fever, and others. (Image courtesy of the Centers for Disease Control and Prevention.)
The organs of the central nervous system (brain and spinal cord) are covered by connective tissue layers collectively called the meninges. Consisting of the pia mater (closest to the CNS structures), the arachnoid and the dura mater (farthest from the CNS), the meninges also support blood vessels and contain cerebrospinal fluid. These are the structures involved in meningitis, an inflammation of the meninges, which, if severe, may become encephalitis, an inflammation of the brain.
Causes
West Nile virus was first identified in 1937 in Uganda in eastern Africa. It was first discovered in the United States in the summer of 1999 in New York. Since then, the virus has spread throughout the US.
Researchers believe West Nile virus is spread when a mosquito bites an infected bird and then bites a person.
Mosquitoes carry the highest amounts of the virus in the late summer and early fall, which is why more people get the disease in late August to early September. As the weather becomes colder and mosquitoes die off, there are fewer cases of the disease.
Although many people are bitten by mosquitoes that carry West Nile virus, most do not know they have been infected.
Risk factors for developing a more severe form of West Nile virus include:
- Conditions that weaken the immune system, such as HIV/AIDS, organ transplants, and recent chemotherapy
- Older or very young age
- Pregnancy
West Nile virus may also be spread through blood transfusions and organ transplants. It is possible for an infected mother to spread the virus to her child through breast milk.
Symptoms
Symptoms may occur 1 to 14 days after becoming infected. Mild disease, generally called West Nile fever, may cause some or all of the following symptoms:
- Abdominal pain
- Fever, headache, and sore throat
- Lack of appetite
- Muscle aches
- Nausea, vomiting, and diarrhea
- Rash
- Swollen lymph nodes
These symptoms usually last for 3 to 6 days, but may last a month.
More severe forms of disease are called West Nile encephalitis or West Nile meningitis, depending on what part of the body is affected. The following symptoms can occur, and need prompt attention:
- Confusion or change in ability to think clearly
- Loss of consciousness or coma
- Muscle weakness
- Stiff neck
- Weakness of one arm or leg
Exams and Tests
Signs of West Nile virus infection are similar to those of other viral infections. There may be no specific findings on a physical examination. About one half of people with West Nile virus infection may have a rash.
Tests to diagnose West Nile virus include:
- Blood test or a spinal tap to check for antibodies against the virus
- Head CT scan
- Head MRI scan
Treatment
Because this illness is not caused by bacteria, antibiotics do not treat West Nile virus infection. Supportive care may help decrease the risk for developing complications in severe illness.
Outlook (Prognosis)
People with mild West Nile virus infection do well after treatment.
For those with severe infection, the outlook is more uncertain. West Nile encephalitis or meningitis may lead to brain damage and death. One in ten people with brain inflammation do not survive.
Possible Complications
Complications from mild West Nile virus infection are very rare.
Complications from severe West Nile virus infection include:
- Brain damage
- Permanent muscle weakness (sometimes similar to polio)
- Death
When to Contact a Medical Professional
Contact your health care provider if you have symptoms of West Nile virus infection, particularly if you may have had contact with mosquitoes. If you are very sick, go to an emergency room.
There is no treatment to avoid getting West Nile virus infection after a mosquito bite. People in good health generally do not develop a serious West Nile infection.
Prevention
The best way to prevent West Nile virus infection is to avoid mosquito bites:
- Use mosquito-repellant products containing DEET (N,N-diethyl-meta-toluamide)
- Wear long sleeves and pants
- Drain pools of standing water, such as trash bins and plant saucers (mosquitos breed in stagnant water)
Community spraying for mosquitoes may also reduce mosquito breeding.
References
Centers for Disease Control and Prevention website. West Nile virus.
Naides SJ. Arboviruses causing fever and rash syndromes. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 358.
Thomas SJ, Endy TP, Rothman AL, Barrett AD. Flaviviruses (dengue, yellow fever, Japanese encephalitis, West Nile encephalitis, Usutu encephalitis, St. Louis encephalitis, tick-borne encephalitis, Kyasanur forest disease, Alkhurma hemorrhagic fever, Zika). In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 153.
Version Info
Version: 2.0
Last reviewed on: 12/4/2022
Reviewed by: Jatin M. Vyas, MD, PhD, Associate Professor in Medicine, Harvard Medical School; Associate in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital, Boston, MA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
