Arboviral Encephalitis

West Nile virus

Epidemiological Alerts and Updates West Niles Virus Western Equine Encephalitis Protect Yourself

What are arboviral encephalitis?

Arboviral encephalitis are mosquito-borne diseases that can cause neurological symptoms in humans and animals. Examples of arboviral encephalitis include West Nile virus encephalitis (WNV), St. Louis encephalitis (SLEV), Japanese encephalitis (JEV), Eastern equine encephalitis (EEEV), and Western equine encephalitis (WEEV).

Types

These arboviruses that affect the nervous system are divided into two main groups: flaviviruses (of the genus Orthoflavivirus, part of the family Flaviviridae) and alphaviruses (of the genus Alphavirus, part of the family Togaviridae). Examples of flaviviruses known to cause neurological symptoms include West Nile virus (WNV), St. Louis encephalitis virus (SLEV), Usutu virus (USUV) and Japanese encephalitis virus (JEV). The USUV and JEV are not found in the Americas. On the other hand, alphaviruses include Eastern equine encephalitis virus (EEEV), Venezuelan equine encephalitis virus (VEEV) and Western equine encephalitis virus (WEEV), which are also known to affect the nervous system.

Dengue virus (DENV), chikungunya virus (CHIKV), and Zika virus (ZIKV) have also been linked to encephalitis and other neurological disorders.

Symptoms

Common symptoms of these diseases may include high fever, severe headache, confusion, seizures, muscle weakness, and other neurological problems.

Prevention and treatment

Prevention focuses on avoidance of mosquito bites, as well as vaccination in the case of Japanese encephalitis (JEV). Treatment mainly involves supportive care and symptom management, as there are no specific antiviral treatments for arboviral encephalitis.

Highlights

On 19 December 2023, PAHO/WHO warned about the risk to human health associated with the circulation of the western equine encephalitis virus (WEE). From that date until 9 January 2024, 374 additional outbreaks in animals were reported (338 in Argentina and 36 in Uruguay) and 21 human cases in Argentina.

Read more on the latest Epidemiological Alerts and Updates on Arboviral Encephalitis.

PAHO's Response

PAHO/WHO is providing technical cooperation for comprehensive, integrated vector surveillance and control in its member countries throughout the Americas, in cooperation with international partners.

PAHO/WHO is helping to raise awareness of vector-borne diseases, including WNV, and the need for people and communities to protect themselves against mosquito bites through the use of window screens, protective clothing, and insect repellent and by destroying mosquito breeding sites in residential areas.

West Nile Virus (WNV)

What is it?

The West Nile Virus is a member of the flavivirus genus and belongs to the Japanese encephalitis antigenic complex of the family Flaviviridae.

First isolated in the West Nile district of Uganda in 1937, WNV is today found commonly in Africa, Europe, the Middle East, North America, and West Asia. In 1999 a WNV circulating in Israel and Tunisia was imported into New York producing a large outbreak that spread across the United States and eventually across the Americas, from Canada to Venezuela. WNV outbreak sites are found along major bird migratory routes.

Transmission

It is transmitted by infected mosquitoes between and among humans and animals, including birds, which are the virus's reservoir host. Mosquitoes of the genus Culex are generally considered the principal vectors of WNV, in particular, Cx. pipiens. In addition to mosquito bites, WNV may be transmitted through contact with infected animals, their blood, or other tissues. A very small proportion of human infections have occurred through organ transplant, blood transfusions, and breast milk.

Symptoms

About 20% of infected people develop moderate symptoms, including fever, headache, fatigue, and body aches, nausea, rash, and swollen glands.

Prevention

Vaccines are available for use in horses but not yet for people.

Facts

In about 1 in 150 cases, WNV causes severe disease that can lead to encephalitis, meningitis, paralysis, and even death.
People over the age of 50, those with underlying medical conditions, and some immunocompromised persons (such as transplant patients) have the highest risk of severe illness from WNV.
In Europe, Africa, the Middle East, and Asia, mortality in birds associated with WNV is rare. In contrast, in the Americas, the virus has proven highly pathogenic for birds.

WNV Communication materials +

WNV Technical material +

Laboratory Diagnosis of West Nile Virus Encephalitis Virus Infection

Western Equine Encephalitis (WEEV)

What is it?

Western Equine Encephalitis is a disease caused by a virus called VEEO or WEEV. This virus belongs to the family of alphaviruses that are found in many parts of the world and are divided into two groups, New World and Old World, depending on the areas where they were first found. These two groups generally cause different types of disease in humans.

Old World alphaviruses, such as chikungunya virus, O'Nyong-Nyong, Ross River, Semliki Forest and Sindbis, generally cause illness with fever and joint problems.

On the other hand, New World alphaviruses, such as WEEV, Eastern Equine Encephalitis virus and Venezuelan Equine Encephalitis virus, generally cause encephalitis in horses, humans and other mammals. However, Mayaro virus, which is also from the New World, is an exception and causes joint problems.

WEEV is found mainly in parts of Canada and the United States, as well as in the southern cone of America. In Argentina, there have been outbreaks of this disease in horses, sometimes also affecting humans. At the end of November 2023, there has been an increase in cases in horses in Argentina and Uruguay, also human cases have been detected in Argentina.

Transmission and Symptoms

The period from when a person becomes infected until they show symptoms of illness is 2 to 10 days. WEEV infection in horses and humans sometimes shows no symptoms, but when it does, it can be severe and cause problems such as aseptic meningitis and encephalitis, which can leave lasting effects.

In humans, the disease begins suddenly with a headache, followed by fatigue, chills, fever, muscle aches and malaise. These symptoms may worsen over the next few days, including vomiting, drowsiness, confusion and weakness, and tremors of the hands, lips and tongue.

Prevention and treatment

There is no vaccine to prevent this disease in humans and no specific treatment with antiviral drugs. Treatment of cases is based on rest, keeping well hydrated and relieving the symptoms that occur.

WEEV Communication materials +

WEEV Technical materials +

Protect yourself and your environment

Vector-borne diseases can be prevented by:

Wearing clothing that acts as a barrier to exposure to bites.
Using mechanisms to keep vectors out of houses such as screens on doors, windows, and leaves.

Reducing breeding sites near houses or in communities by:
- Covering water storage containers,
- Eliminating puddles and drainage of places where water accumulates,
- Eliminating unusable containers where water pools, and
- Controlling garbage in yards and