Viral encephalitis


An in-depth report on the causes, diagnosis, treatment, and prevention of viral encephalitis.

Alternative Names

West Nile virus



Encephalitis, an inflammation of the brain, is rare but can be caused by many different viruses. The main causes of viral encephalitis are:

  • Herpes viruses, among which herpes simplex virus is particulary important.
  • Arboviruses, which are transmitted by blood-sucking insects such as mosquitoes. In the U.S., mosquito-borne encephalitis infections include West Nile encephalitis, Eastern equine encephalitis, Western equine encephalitis, St. Louis encephalitis, La Crosse encephalitis.


Encephalitis symptoms can appear within 2 days to 2 weeks of exposure to the virus. Many people who are infected do not develop any symptoms. In milder cases, symptoms may resemble the flu. In severe cases of encephalitis, symptoms may include:

  • Fever
  • Headache
  • Vomiting
  • Lethargy and reduced consciousness
  • Seizures
  • Memory loss
  • Stiff neck and back
  • Confusion
  • Speech, hearing, and vision problems
  • Muscle weakness
  • Partial paralysis
  • Loss of consciousness
  • Coma


Because encephalitis can be dangerous, it needs to be diagnosed promptly. Patients are treated immediately, even before diagnostic tests identify the specific virus that caused the illness. If herpes is a possible cause, the standard treatment is the antiviral drug acyclovir. Once the cause has been determined, other drugs may be administered. Unfortunately, however, many types of encephalitis, such as the ones caused by West Nile virus and other arboviruses, do not respond to antiviral drugs.


The best way to prevent becoming infected with a mosquito-borne virus is to avoid being bitten by a mosquito. Use insect repellant when you go outside, especially during the peak mosquito hours of dusk and dawn. Remove mosquito-breeding environments from your property.

2012 Record Year for West Nile Virus

According to the U.S. Centers for Disease Control, more cases of West Nile virus were reported in 2012 than in any other year in nearly a decade. A third of all cases occurred in Texas. Researchers are studying the virus and trying to understand what makes it so unpredictable and difficult to control. Fortunately, only a small percentage of West Nile virus infections cause encephalitis. Unfortunately, there is currently no treatment available. 


Encephalitis is a rare but potentially life-threatening inflammation of the brain that can occur in people of all ages. The most common cause of encephalitis is infection by a virus. In very rare cases, encephalitis can also be caused by bacterial infection, parasites, or complications from other infectious diseases. This report focuses on viral encephalitis.

Encephalitis: Viral Infection of the Brain

Many viruses can cause encephalitis. The West Nile virus, for example, has been responsible for well-publicized outbreaks in the U.S. Most people exposed to encephalitis-causing viruses have no symptoms. Others may experience a mild flu-like illness, but do not develop full-blown encephalitis.

In severe cases, the infection can have devastating effects, including:

  • Swelling of the brain (cerebral edema)
  • Bleeding within the brain (intercerebral hemorrhage)
  • Nerve damage (neuropathy)

The damage may cause long-term mental or physical problems, depending on the specific areas of the brain affected.

Other Viral Infections of the Central Nervous System. Viral infection and inflammation can affect multiple areas of the central nervous system, and is categorized by its location:

  • Meningitis: infection of the meninges (the membranes that surround the brain and spinal cord)
  • Meningoencephalitis: infection of both the brain and meninges
  • Encephalomyelitis: infection of the brain and spinal cord

Specific Viruses Associated with Encephalitis

Encephalitis caused by viruses in the United States generally fall into the following groups:

  • Arboviruses are the primary cause of acute encephalitis (sudden-onset encephalitis caused by direct infection). Arboviruses, short for "arthropod-borne viruses," are spread by mosquitoes and ticks. There is no treatment for encephalitis caused by arboviruses.
  • Herpes viruses are the other major cause of encephalitis in the U.S. This virus family includes herpes simplex, Epstein-Barr, cytomegalovirus, and varicella-zoster. Herpes simplex is the most common type of herpes-associated encephalitis. It can cause severe brain damage, but can be treated with antiviral medication..
  • Less common viral causes of encephalitis include enteroviruses, adenoviruses, and viruses associated with childhood diseases such as measles, mumps, and rubella.

[For more information, see the Causes section in this report.]

Primary and Secondary Encephalitis

Encephalitis can develop shortly after an initial viral infection, or it can develop when a virus that was lying dormant in the body suddenly reactivates.

There are two ways that viruses can infect brain cells and cause encephalitis:

  • Primary encephalitis is when the virus directly affects the brain or spinal cord. The resulting inflammation can occur in one area (focal) or can occur throughout the brain (diffuse).
  • Secondary encephalitis, also called post-infectious encephalitis, is when the virus first attacks another part of the body and the infection then spreads to the brain.


Herpes Viruses

The herpes virus family includes at least 8 distinct viruses that cause infections in humans. These viruses and infections include varicella-zoster virus (the cause of chickenpox and shingles), Epstein-Barr virus (the cause of mononucleosis), cytomegalovirus, and herpes virus 6. Although any herpes virus can cause encephalitis, the herpes simplex virus is the most important cause of encephalitis.

Herpes simplex virus (HSV) is responsible for 5 – 10% of encephalitis cases worldwide. Herpes simplex encephalitis (HSE) tends to be most severe when it affects children and older people. There are two distinct types of the herpes simplex virus:

  • Herpes simplex virus 1 (HSV-1) causes most cases of adult herpes encephalitis. HSV-1 is the main cause of oral herpes infections but it can also cause genital herpes.
  • Herpes simplex virus 2 (HSV-2) causes most cases of encephalitis in newborn infants. HSV-2 is the main cause of genital herpes.

Unlike arbovirus encephalitis, herpes simplex encephalitis is treatable, but treatment (typically intravenous acyclovir) must be administered within the first few days of symptom onset. If left untreated, herpes simplex encephalitis can be fatal. [For more information, see In-Depth Report #52: Herpes simplex.]


Arboviruses, including the West Nile virus, are transmitted by blood-sucking insects such as mosquitoes and ticks. Most of the time, these viral infections initially develop in birds, which function as the reservoir of infection. Insects that feed on the infected blood from a diseased bird  pick up the virus, and transmit it when they bite a susceptible host (such as an animal or a human). The insects that play a role in this disease-transmission process are referred to as vectors.

Arboviruses multiply in blood-sucking vectors. In fact, the word arbovirus is an acronym for arthropod-borne virus. Mosquitoes, ticks, and many other insects are classified as arthropods. Mosquitoes are the most common vector for arboviruses.

In general, the virus first passes through an insect before infecting a person. These infections are not transmitted through casual contact from one person (or animal) to another. (However, a small number of West Nile virus cases have occurred through blood transfusions, organ transplantation, and possibly breastfeeding.) Only a small percentage of people who are infected by an arbovirus develop encephalitis.

Arboviruses that cause encephalitis are primarily found in three virus families: Togaviridae, Bunyaviridae, and Flaviviridae.

In the United States, the main mosquito-borne encephalitis strains are Eastern equine, Western equine, St. Louis, La Crosse, and West Nile. Equine encephalitis causes disease in both humans and, as its name implies, horses. Powassan encephalitis is a less common tick-borne flavivirus that occurs primarily in the northern United States.

Japanese encephalitis, which is also transmitted by mosquito, is the most common form of viral encephalitis to occur outside of the United States. It is endemic in rural areas in east, south, and southwest Asia, especially China and Korea. Venezuelan equine encephalitis is found in South and Central America.

Different arboviruses cause different forms of encephalitis. Although the overall disease is the same, there are subtle differences in symptoms and the type of brain damage they produce.

Common Forms of Mosquito-Borne Arbovirus Encephalitis

Eastern Equine Encephalitis

Virus Family

Togaviridae (genus Alphavirus)

U.S. Geographic Areas

Atlantic and Gulf coasts, in New England, and around the Great Lakes. States most affected are Florida, Georgia, Massachusetts, and New Jersey.

Symptom Onset

Symptoms appear 4 - 10 days following infection and can range from mild flu-like symptoms to full-blown encephalitis.

Incidence and Mortality Rates

About 6 cases are reported each year. About a third of people who contract EEE die from it. Children are more likely to survive but also to suffer complications afterward.

Age Risk Groups

Adults over age 50 and children under age 15.

Western Equine Encephalitis

Virus Family

Togaviridae (genus Alphavirus)

U.S. Geographic Areas

Farming areas in western and central Plains and Rocky Mountain states west of the Mississippi.

Symptom Onset

5 - 10 days following infection.

Incidence and Mortality Rates

Very rare. Mortality rate is 3 - 4%; 30% of survivors have complications afterward. Most severe in children, especially those younger than 1 year. Infants may suffer permanent neurological damage.

Age Risk Groups

Infants younger than 12 months.

St. Louis Encephalitis

Virus Family

Flaviviridae (genus Flavivirus)

U.S. Geographic Areas

Takes its name from an epidemic in St. Louis, but has occurred throughout the U.S., especially central and southern states, as well as parts of Canada, Caribbean, and South America.

Symptom Onset

5 - 15 days following infection.

Incidence and Mortality Rates

Mortality rate range between 5 - 30%, with highest rates among elderly. About 5% of survivors suffer complications afterward.

Age Risk Groups

Elderly adults (over age 60) are at highest risk, and the disease is most severe in this age group. Younger people usually experience mild, flu-like symptoms.

La Crosse Encephalitis

Virus Family

Bunyaviridae (genus Bunyavirus)

U.S. Geographic Areas

Occurs most frequently in upper Midwestern, southeastern (Appalachia), and mid-Atlantic states. Most cases have occurred in Ohio and Wisconsin. Unlike other encephalitis viruses which originate in birds, La Crosse encephalitis is transmitted to mosquitoes from infected chipmunks and squirrels.

Symptom Onset

5 - 15 days following infection.

Incidence and Mortality Rates

Mortality rates are less than 1%. More common and severe in children under age 16.

Age Risk Groups

Children younger than 16 years.

West Nile Encephalitis

Virus Family

Flaviviridae (genus Flavivirus).

U.S. Geographic Areas

Cases have been reported throughout the mainland United States.

Symptom Onset

3 - 14 days following infection.

Incidence and Mortality Rates

In 2012, 5,387 cases of WNV were reported to the CDC, with 243 deaths. Of all the reported cases, 49% took the form of West Nile fever, while 51% were meningitis and encephalitis. However, most cases of West Nile virus do not produce symptoms, and are not reported, so these numbers imply a more worrisome picture than actually exists. In fact, fewer than 1% of people who are infected with WNV go on to develop neurological disease.

Age Risk Groups

Adults over age 50.

West Nile Virus (WNV). Until 1999, the West Nile virus was generally restricted to Africa, the Middle East, southwestern Asia, eastern Europe, and Australia. It emerged in the United States with the first outbreak in New York City in 1999. WNV is now found in birds and mosquitoes in every state except Alaska and Hawaii.

Human cases of West Nile encephalitis have been reported throughout the continental United States. In 2012, Texas had by far the greatest number of reported cases of WNV (1739 cases) followed by California (451), Louisiana (335), and Illinois (282). Throughout the United States, 2012 was a record year for West Nile virus, with the largest number of cases in nearly a decade.

How WNV Is Transmitted. WNV, discovered in Uganda in 1937, circulates primarily between birds and mosquitoes and can be carried long distances by migrating birds. In a given geographic area, the appearance of the virus among birds and mosquitoes generally precedes infection in humans. WNV has infected over 110 species of birds. In addition to mosquito-to-human transmission, other causes of human infection have included blood transfusions and organ transplantation. The U.S. now uses screening tests to detect West Nile virus in donated blood and organs. There have also been cases of mother-to-child transmission during pregnancy, and one confirmed case of transmission through breastfeeding.

Severity of WNV. About 80% of people infected with WNV will not develop symptoms. Twenty percent will develop West Nile fever (which includes fever, headache, and occasional skin rash). Less than 1% of infected people will develop neuroinvasive disease, the most severe form of WNV. It is still not clear if the physical and mental symptoms of West Nile virus persist long term.

Neuroinvasive disease affects the nervous system and includes encephalitis, meningitis, and spinal cord infection. People over age 50 and those with weakened immune systems are at the greatest risk for neuroinvasive disease. In older adults, neuroinvasive disease usually manifests as encephalitis. In children and younger adults, meningitis is more common. The fatality rate for those afflicted ranges from 3 - 15%. Neuroinvasive disease symptoms include high fever, headache, stiff neck, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. There are currently no vaccines to prevent WNV or specific antiviral drugs to treat it.

Tick-Borne Encephalitis Viruses

Tick-borne encephalitis (TBE) is commonly found in many countries throughout Europe, Asia, and the former Soviet Union, but it is reported only rarely in the U.S. Powassan encephalitis is the main tick-borne encephalitis found in the United States and Canada. The first human encephalitis fatality caused by deer tick virus, which is closely related to Powasson virus, was reported in 2009. Cases of tick-borne encephalitis have also been reported from Rocky Mountain spotted fever, but this is a bacterial (not viral) infection.

Other Viral Causes of Encephalitis

Rabies. The rabies virus is transmitted from the saliva of an infected animal. The encephalitis it causes is virtually always fatal but is very rare in the U.S. Only one or two cases are typically reported each year, usually from contact with raccoons, bats, or other wild animals.

Encephalitis Associated with Childhood Diseases. Vaccines have virtually eliminated encephalitis caused by common childhood infections such as measles, mumps, rubella, and chickenpox. Encephalitis can still occur in rare cases, particularly with immunocompromised children.

Adenoviruses.Adenoviruses typically cause respiratory or eye infections, but in rare cases they can cause encephalitis.

Enteroviruses. Enteroviruses include various viruses that enter the body through the gastrointestinal tract. They account for about 5% of viral encephalitis cases, usually caused by types of coxsackievirus.

Parasitic Causes of Encephalitis

Toxoplasmosis. Toxoplasmosis is a parasitic disease that can be transmitted through a cat's feces or by eating contaminated food. The condition causes mild symptoms in most people but toxoplasmosis can occasionally cause encephalitis. Adults with weakened immune systems (such as people with HIV/AIDS) are at highest risk for developing toxoplasmosis encephalitis. In addition, in pregnant women toxoplasmosis can cause severe problems in the fetus’ central nervous system and eyes. .

Raccoon Roundworm. Raccoon roundworm (Baylisascaris procyonis) lives in the intestines of raccoons. Humans usually become infected by ingesting the worm's eggs through accidental contact with soil, wood chips, or tree bark contaminated with raccoon feces. The worm is harmless in raccoons but can produce severe central nervous system disease, including encephalitis, in people. (Raccoons should not be kept as pets.) .

Other Parasitic Infections. Encephalitis may be caused by other parasitic infections such as toxocariasis (from roundworms found in dogs and cats) or cysticercosi (from food or water contaminated with pork tapeworm eggs). People with weakened immune systems are at highest risk for these forms of encephalitis.

Bacterial and Fungal Organisms

In very rare circumstances, encephalitis may be caused by bacterial or fungal organisms.

Risk Factors

Encephalitis is a relatively rare disease. People   at highest risk for encephalitis, and its complications, include the very young, the very old, and people with weakened immune systems..


Encephalitis can occur at any age. Age-associated risks depend on the type of encephalitis virus. Newborn infants are particularly at risk for herpes simplex encephalitis. For arboviruses, infants are most vulnerable to Western equine encephalitis. Older children and teenagers are more susceptible to Eastern equine and La Crosse encephalitis. Older and elderly adults are at higher risk for Eastern equine, St. Louis, and West Nile encephalitis.

Weakened Immune System and Other Medical Conditions

Patients whose immune systems are compromised by conditions such as HIV-AIDS, cancer therapies, or organ transplantation are more susceptible than other individuals to any form of encephalitis.

Other medical conditions that may increase the risk for viral encephalitis include chronic kidney disease, diabetes, and alcohol abuse and dependence.

Risk Factors for Herpes Simplex Encephalitis

Herpes simplex encephalitis (HSE) can be caused by either:

  • A new infection through person-to-person contact or
  • Reactivation of a latent pre-existing herpes infection.

The herpes simplex virus is very common and most people have been infected with at least one of its two types:

  • Most cases of herpes simplex encephalitis occur in adults and are caused by herpes simplex virus 1 (HSV-1), which is transmitted through skin or oral contact. Half of all adults who contract HSE are older than age 50.
  • Encephalitis caused by herpes simplex 2 virus (HSV-2) usually occurs in newborn infants and is transmitted from an infected mother during delivery. HSV-2 is usually spread through sexual contact.  .

[For more information, see In-Depth Report #52: Herpes simplex .]

Risk Factors for Arboviruses

Geography. The primary risk factor for arbovirus encephalitis is living in areas of possible exposure to virus-carrying mosquitoes. Most arbovirus outbreaks occur in rural or farming areas, but they can also occur in cities. While some forms of arbovirus are limited to specific geographical regions, the West Nile virus has become endemic throughout the mainland United States. However, encephalitis only occurs in a small percentage of West Nile infections. [See Common Forms of Mosquito-Borne Encephalitis table for more detailed regional information.]

Season. Transmission of arboviruses correlates with the mosquito season and is highest during the months of July through September (late summer through early fall). The ideal conditions for mosquito breeding are a wet spring followed by a hot, dry summer.


Mild Encephalitis

Mild cases of encephalitis can resemble the flu. Most people who have mild cases of encephalitis make a full recovery within 2 – 4 weeks.

Severe Encephalitis

Prognosis for severe encephalitis depends on many factors, including:

  • Age of the patient -- worse outcomes for infants under age 12 months and adults over age 55
  • Immune status
  • Preexisting neurological conditions
  • Virulence of the virus

In very severe cases of encephalitis, the swelling of the brain inside the skull places downward pressure on the brain stem. The brain stem controls vital functions, such as respiration and heartbeat. If the pressure becomes too severe, these vital functions can cease and cause death.

Complications from Brain Damage

Survivors of encephalitis often experience neurologic consequences, which can be long-term and even permanent. The degree and type of brain damage can vary from mild-to-severe and from focal (in one part of the brain) to multifocal (several parts of the brain) to diffuse (throughout the brain).

While coma can occur in patients with severe encephalitis, it does not necessarily predict a fatal or severe outcome. Some patients experience no or mild-to-moderate complications after awakening from an encephalitis-associated coma.

The location and severity of the infection largely determines the pattern of brain damage and its effects, which can be:

  • Physical (muscle control)
  • Behavioral and emotional (personality changes)
  • Cognitive (memory, speech)
  • Sensory (vision, hearing)


Symptoms of encephalitis usually appear within 2 days to 2 weeks of exposure to the virus. In milder cases, symptoms may resemble the flu. In severe cases of full-blown encephalitis, symptoms may include:

  • Behavioral and personality changes
  • Sensitivity to light
  • Fever
  • Headache
  • Vomiting
  • Lethargy and reduced consciousness
  • Memory loss
  • Stiff neck and back -- accompanied by fever and headache would indicate meningitis
  • Confusion
  • Speech, hearing, and vision problems
  • Muscle weakness
  • Seizures
  • Partial paralysis
  • Loss of consciousness
  • Coma

Patients experiencing these types of symptoms (especially if they may have recently been bitten by a mosquito or tick or if they have lesions on the lips or genitals) should immediately seek medical treatment.

Symptoms in Infants. Infants with herpes virus encephalitis may develop lesions in the mouth, in the eye, or on the skin 1 - 45 days after birth. Other symptoms include lethargy, seizures, and changes in temperature. The fontanels, the soft spots on their head where the skull has not yet closed, may bulge outward.


Because the various types of encephalitis produce similar symptoms, doctors cannot rely on clinical features to differentiate among the many causes of brain inflammation. The primary objective in diagnosing viral encephalitis is to determine if it is caused by:

  • Herpes simplex or other conditions that can be treated with specific medications
  • Arboviruses or other viruses that can be managed only by targeting symptoms

Imaging Techniques

If the doctor suspects encephalitis, a scanning technique is often the first diagnostic step. Computerized tomography (CT) or magnetic resonance imaging (MRI) scans can show the extent of the inflammation in the brain and help differentiate encephalitis from other conditions. MRIs are recommended over CT scans because they can detect injuries in parts of the brain that suggest infection with herpes virus at the onset of the disease, while CT scans cannot.

Electroencephalogram (EEG), which records brain waves, may reveal abnormalities in the temporal lobe that are indicative of herpes simplex encephalitis.

Cerebrospinal Fluid Tests

When encephalitis is suspected, a sample of cerebrospinal fluid is taken using a lumbar puncture, which involves inserting a needle between two vertebrae in the patient's lower back. The sample is taken to count white blood cells and identify specific blood cell types, to measure proteins and blood sugar levels, and to determine spinal fluid pressure.

Doctors can use cerebrospinal fluid to test for herpes viruses, and to look for the presence of antibodies to the West Nile virus or other virus types. While cerebrospinal fluid tests may help diagnose encephalitis, they cannot provide information on how severe the disease will be.

Blood Tests

Blood tests may be used to test for West Nile virus and other arbovirus infections.

Brain Biopsy

If necessary, tiny samples of brain tissue are surgically removed for examination and testing for the presence of the virus. Tissue is prepared using staining techniques and then viewed under an electron microscope. In a few cases, the viruses in brain cells are able to be cultured; that is, the viruses can actually be made to replicate in samples. A brain biopsy is the gold standard for diagnosing rabies.


With the exception of herpes simplex and varicella-zoster encephalitis, the viral forms of encephalitis are not treatable. The primary objective is to diagnose the patient as soon as possible so they receive the right medicines to treat the symptoms. It is very important to lower fever and ease the pressure caused by swelling of the brain.

Patients with very severe encephalitis are at risk for body-wide (systemic) complications including shock, low oxygen, low blood pressure, and low sodium levels. Any potentially life-threatening complication should be addressed immediately with the appropriate treatments.

Antiviral Drug Treatment

Although it is difficult to quickly determine the cause of encephalitis, rapid treatment is essential. Clinical guidelines recommend immediately administering intravenously the antiviral drug acyclovir without waiting to determine the cause of the illness. Ganciclovir is another antiviral drug that is used to treat some types of herpes encephalitis.

Additional Treatments

Other encephalitis treatments are aimed at reducing symptoms.

  • Seizures may be prevented by using oral anticonvulsant drugs or intravenous lorazepam (Ativan).
  • Corticosteroids may be given to reduce brain swelling.
  • Sedatives may be prescribed for irritability or restlessness.
  • Mild cases of encephalitis can be treated with simple pain relievers (ibuprofen, acetaminophen) for fever and headache, fluids, and bed rest.

Investigational Treatments

No specific drugs have been effective for treating arboviruses, including West Nile virus, although a number of drugs used to treat other virus infections are being investigated. They include interferon alfa 2a (Roferon-A) and other interferons.


Certain vaccinations can help prevent the diseases that can lead to encephalitis.


Measles used to be a very common childhood disease. In about 1 in 1,000 patients it can lead to encephalitis or death. The risk for these severe complications is highest in the very young and very old. Aggressive vaccination programs have reduced the incidence of measles in the U.S. to fewer than 100 cases a year. Rarely, patients who receive the live-measles vaccine develop encephalopathy (brain damage), but the risk is far lower than brain problems occurring from the disease itself.

Vaccine for Varicella Virus (Chickenpox and Shingles)

Herpes zoster, or shingles, is a reactivation of the varicella virus, which causes chickenpox. Children (and adults who do not have a history of infection and who lack evidence of immunity) should receive 2 doses of the chickenpox vaccine. A vaccine for shingles (Zostavax) is available for adults age 50 years and older. [For more information, see In-Depth Report #82: Shingles and Chickenpox.]

Vaccines for Arboviruses

A vaccine (Ixiaro) is currently available for adults traveling for a month or longer to Asian regions where Japanese encephalitis is endemic. (An older vaccine, JE-VAX, is no longer manufactured, but limited quantities are available for vaccinating children.) Countries and regions with high rates of Japanese encephalitis include Viet Nam, Cambodia, Myanmar (Burma), southern India, Pakistan, Nepal, Malaysia, Korea, northern Thailand, Malaysia, Sri Lanka, and the Philippines.

Another type of vaccine (FSME-IMMUN) is used to prevent tick-borne encephalitis (TBE) in travelers visiting regions where this type of encephalitis is prevalent. TBE is found mainly in Eastern and Central Europe. This vaccine is available in Canada and many European countries, but it is not yet approved in the United States.

Several types of vaccines are under investigation for West Nile virus, but it will be several years before these vaccines could become commercially available.

Rabies Vaccine and Immune Globulin

Anyone exposed to the secretions of an animal suspected of having rabies, should be evaluated for post-exposure rabies vaccine. Exposed individuals may also receive immune globulin unless they were previously vaccinated. The regimen is one shot of immune globulin and four shots of rabies vaccine given over a period of two weeks. The new types of rabies vaccines cause much less discomfort and many fewer adverse effects than the older ones. Side effects may include mild reactions such as pain, redness, or swelling at the injection site. Patients may experience pain at the injection site and low-grade fever following the immune globulin shot.


The risk for mosquito-borne infections is highest between dusk and dawn, when mosquitoes feed. A good insect repellent is very helpful in reducing the risk for vector-borne disease. The most complete personal protection program for adults and most children is to apply the insect repellant DEET to the skin, and also permethrin to clothing and similar surfaces.

DEET and Other Insect Repellant Skin Products

DEET. Most insect repellents contain the chemical DEET (N,N-diethyl-meta-toluamide), which remains the gold standard of currently available mosquito and tick repellents. DEET has been used for more than 40 years and is safe for most children when used as directed. Comparison studies suggest that DEET preparations are the most effective insect repellents now available.

Concentrations range from 4% to almost 100%. The concentration determines the duration of protection. Most adults and children over 12 years old should use preparations containing a DEET concentration of 20 - 35% (such as Ultrathon), which provides complete protection for an average of 5 hours. (Higher DEET concentrations may be necessary for adults who are in high-risk regions for prolonged periods.)

DEET products should never be used on infants younger than 2 months. According to the Environmental Protection Agency (EPA), DEET products can safely be used on all children age 2 months and older. The EPA recommends that parents check insect repellant product labels for age restrictions. If there is no age restriction listed, the product is safe for any age. The American Academy of Pediatrics recommends that children use concentrations of 10% or less; 30% DEET is the maximum concentration that should be used for children. When deciding what concentration is most appropriate, parents should consider the amount of time that children will be spending outside, and the risk of mosquito bites and mosquito-borne disease.

When applying DEET, take the following precautions:

  • Do not use on the face, and apply only enough to cover exposed skin on other areas.
  • Do not over apply, and do not use under clothing.
  • Do not apply over any cuts, wounds, or irritated skin.
  • Only parents or an adult should apply repellent to a child. They should first put DEET on their own hands and then apply it to the child. They should avoid putting DEET not only near the child's eyes and mouth but also on the hands (since children frequently touch their faces).
  • Wash any treated skin after going back inside.
  • If using a spray, apply DEET outdoors -- never indoors. Spray repellents should not be applied directly on anyone's face.

Other Insect Repellent Products. The U.S. Centers for Disease Control (CDC) also recommends the mosquito repellents picaridin and oil of lemon eucalyptus.

Picaridin, also known as KBR 3023 or Bayrepel, is an ingredient that has been used for many years in repellents sold in Europe, Latin America, and Asia. A product containing 7% picaridin is now available in the United States. Picaridin can safely be applied to young children and is also safe for women who are pregnant or breastfeeding. According to the CDC, insect repellents containing DEET or picaridin work better than other products.

In scientific tests, oil of lemon eucalyptus, also known as PMD, worked as well as low concentrations of DEET. However, oil of lemon eucalyptus is not recommended for children under the age of 3 years.

Permethrin for Clothing and Surfaces

Permethrin is an insect repellent used as a spray for clothing and bed nets, which can repel insects for weeks when applied correctly. Electric vaporizing mats containing permethrin may be very helpful. A permethrin solution is also available for soaking items, but it should never be applied to the skin. Side effects from direct exposure may include mild burning, stinging, itching, and rash. In general, however, permethrin is very safe and its use may even reduce child mortality rates from malaria. People allergic to chrysanthemum flowers or who are allergic to head-lice scabicides should avoid using permethrin.

Controlling Mosquitoes around the House

Eliminate Sources of Standing Water. The best way for homeowners to reduce mosquito populations is to eliminate sources of standing water.

  • Look for any source of standing water, where mosquitoes can breed. For example, discard any rubbish with standing water, such as old tires, cans, and bottles. (Even bottle caps can breed mosquitoes.) Do not let water accumulate in outdoor flower pot basins or pet bowls. Turn over wading pools and wheelbarrows when not in use. Change bird bath water every 3 - 4 days. A product such as Mosquito Dunk can be used to prevent breeding in standing water.
  • Swimming pools and hot tubs should be clean and chlorinated or drained and covered if not in use.
  • Clean vegetation and debris from the edges of ponds.
  • Keep gutters clean and unclogged.

Mosquito Traps and Bug Zappers. Mosquito traps use various methods for repelling or attracting and trapping female mosquitoes, which are the primary transmitters of arboviruses. These methods include electricity or propane. However, there is little evidence to support their effectiveness.

Insect light traps (commonly called bug zappers), which attract and electrocute insects, may actually spread viruses and bacteria that are on the insects. They are also not very effective for killing female mosquitoes.

Citronella Candles. Burning citronella candles reduces the likelihood of bites. (Indeed, burning any candle helps to some extent, perhaps because the generation of carbon dioxide diverts mosquitoes toward the flame.)

Other Preventive Measures

Your home environment, personal hygiene, and what you wear can also help reduce your risk for mosquito bites:

  • Wear trousers and long-sleeved shirts, particularly at dusk.
  • Sleep only in screened areas.
  • Air-conditioning may reduce mosquito infiltration. Where air-conditioning is not available, fans may be helpful. Mosquitoes appear to be reluctant to fly in windy air.
  • Don't wear perfumes.
  • Cover up bare skin after dusk.
  • Wash your hair at least twice a week.

Community Mosquito Control Programs

Spraying. Public health measures are the most effective methods for controlling mosquitoes. Local communities that experience outbreaks of encephalitis or West Nile virus from mosquitoes often have public spraying programs that target mosquito larvae during breeding season as well as adult mosquitoes. The U.S. Environmental Protection Agency (EPA) approves the safety of the insecticides used. While these pesticides are generally considered safe for humans, people with asthma or other respiratory problems should avoid exposure by staying indoors while spraying takes place.

Report Dead Birds. Dead birds may be indicators that the West Nile virus has reached a specific region. Report any dead birds to your local public health authorities. You should never touch a dead bird with your bare hands.



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  • Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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