Dengue:

We would like to acknowledge and thank the Centers for Disease Control for allowing
the use of the following information and links concerning mosquito-borne illness.

Dengue Facts & Information:

CLINICAL FEATURES:

• Sudden onset of fever, severe headache, myalgias and arthralgias, leukopenia, thrombocytopenia and hemorrhagic manifestations
• Occasionally produces shock and hemorrhage, leading to death

ETIOLOGIC AGENT:

• Dengue viruses (DEN-1, DEN-2, DEN-3 and DEN-4) - flaviviruses

INCIDENCE:

• Variable, depending on epidemic activity
• Globally, there are an estimated 50 to 100 million cases of dengue fever (DF) and several hundred thousand cases of dengue hemorrhagic fever (DHF) per year
• Average case fatality rate of DHF is about 5%
• In 1995, 250,000 cases of DF and 7,000 cases of DHF reported in Americas
• Between 100 to 200 suspected cases introduced into U.S. each year by travelers

SEQUELAE:

• None

COSTS:

• $250 million estimated in Puerto Rico in past 10 years

TRANSMISSION:

• Mosquito-borne (Aedes aegypti)

RISK GROUPS:

• Residents of or visitors to tropical urban areas
• Increased severe and fatal disease in children under 15 years
• No cross-immunity from each serotype
• A person can theoretically experience four dengue infections

SURVEILLANCE:

• Active, laboratory-based surveillance in Puerto Rico and the U.S. Virgin Islands
• In U.S., passive surveillance of imported cases reported to CDC and other reference laboratories
• Laboratory-based, passive surveillance in endemic areas

TRENDS:

• Resurgent disease worldwide in the tropics
• Epidemics are larger and more frequent
• Transmission in continental U.S. in 1995; first since 1986
• Since first epidemic in 1981, DHF now reported from 18 countries in the Americas
• Evolution of disease pattern in Americas similar to SE Asia in 1950s and 1960s

CHALLENGES:

• Increased incidence associated with increased urbanization
• Rapid dispersal of viruses via air travel
• Emergency control methods ineffective
• Severe hemorrhagic disease poorly understood by physicians in Americas
• Change emphasis from emergency response to prevention of epidemics
• Develop better government-based programs
• Encourage community participation in prevention and control programs

(Source: http://www.cdc.gov/ncidod/dvbid/dengue/facts.htm)

Dengue and Dengue Hemorrhagic Fever: Questions and Answers

Q. What is dengue?

A. Dengue (pronounced den' gee) is a disease caused by any one of four closely related viruses (DEN-1, DEN-2, DEN-3, or DEN-4). The viruses are transmitted to humans by the bite of an infected mosquito. In the Western Hemisphere, the Aedes aegypti mosquito is the most important transmitter or vector of dengue viruses, although a 2001 outbreak in Hawaii was transmitted by Aedes albopictus. It is estimated that there are over 100 million cases of dengue worldwide each year.

Q. What is dengue hemorrhagic fever (DHF)?

A. DHF is a more severe form of dengue. It can be fatal if unrecognized and not properly treated. DHF is caused by infection with the same viruses that cause dengue. With good medical management, mortality due to DHF can be less than 1%.

Q. How are dengue and dengue hemorrhagic fever (DHF) spread?

A. Dengue is transmitted to people by the bite of an Aedes mosquito that is infected with a dengue virus. The mosquito becomes infected with dengue virus when it bites a person who has dengue or DHF and after about a week can transmit the virus while biting a healthy person. Dengue cannot be spread directly from person to person.

Q. What are the symptoms of the disease?

A. The principal symptoms of dengue are high fever, severe headache, backache, joint pains, nausea and vomiting, eye pain, and rash. Generally, younger children have a milder illness than older children and adults. Dengue hemorrhagic fever is characterized by a fever that lasts from 2 to 7 days, with general signs and symptoms that could occur with many other illnesses (e.g., nausea, vomiting, abdominal pain, and headache). This stage is followed by hemorrhagic manifestations, tendency to bruise easily or other types of skin hemorrhages, bleeding nose or gums, and possibly internal bleeding. The smallest blood vessels (capillaries) become excessively permeable (“leaky”), allowing the fluid component to escape from the blood vessels. This may lead to failure of the circulatory system and shock, followed by death, if circulatory failure is not corrected.

Q. What is the treatment for dengue?

A. There is no specific medication for treatment of a dengue infection. Persons who think they have dengue should use analgesics (pain relievers) with acetaminophen and avoid those containing aspirin. They should also rest, drink plenty of fluids, and consult a physician.

Q. Is there an effective treatment for dengue hemorrhagic fever (DHF)?

A. As with dengue, there is no specific medication for DHF. It can however be effectively treated by fluid replacement therapy if an early clinical diagnosis is made. Hospitalization is frequently required in order to adequately manage DHF. Physicians who suspect that a patient has DHF may want to consult the Dengue Branch at CDC, for more information.

Q. Where can outbreaks of dengue occur?

A. Outbreaks of dengue occur primarily in areas where Aedes aegypti (sometimes also Aedes albopictus) mosquitoes live. This includes most tropical urban areas of the world. Dengue viruses may be introduced into areas by travelers who become infected while visiting other areas of the tropics where dengue commonly exists. In the America region, all dengue virus serotypes are now present. DEN-3 was reintroduced into Central America in 1994 and is now found in several countries in the region. Since this serotype has been absent from the Americas for almost 20 years, the population has a low level of immunity and the virus is expected to spread rapidly.

Q. What can be done to reduce the risk of acquiring dengue?

A. There is no vaccine for preventing dengue. The best preventive measure for residents living in areas infested with Aedes aegypti is to eliminate the places where the mosquito lays her eggs, primarily artificial containers that hold water. Items that collect rainwater or are used to store water (for example, plastic containers, 55-gallon drums, buckets, or used automobile tires) should be covered or properly discarded. Pet and animal watering containers and vases with fresh flowers should be emptied and scoured at least once a week. This will eliminate the mosquito eggs and larvae and reduce the number of mosquitoes present in these areas. For travelers to areas with dengue, a well as people living in areas with dengue, the risk of being bitten by mosquitoes indoors is reduced by utilization of air conditioning or windows and doors that are screened. Proper application of mosquito repellents containing 20% to 30% DEET as the active ingredient on exposed skin and clothing decreases the risk of being bitten by mosquitoes. The risk of dengue infection for international travelers appears to be small, unless an epidemic is in progress.

Q. How can we prevent epidemics of dengue hemorrhagic fever (DHF)?

A. The emphasis for dengue prevention is on sustainable, community-based, integrated mosquito control, with limited reliance on insecticides (chemical larvicides and adulticides). Preventing epidemic disease requires a coordinated community effort to increase awareness about dengue/DHF, how to recognize it, and how to control the mosquito that transmits it. Residents are responsible for keeping their yards and patios free of sites where mosquitoes can be produced.

Dog Heartworm:

Our thanks to the American Heartworm Society, who provided the following
information. Please visit www.heartwormsociety.org for more information

Dog Heartworm Information:

Heartworm Disease - A Serious Threat

Since the first diagnosis of canine heartworm disease over one hundred years ago, heartworm infection has been widely recognized throughout the world as one of the major health problems affecting pets today.

Heartworm infection is caused by worms (Dirofilaria immitis) that may grow to be 14-inch-long adults. These adult worms live in the right side of the heart and the arteries of the lungs. Heartworm infection can cause serious damage to these arteries, eventually leading to heart failure, and in severe cases, damage to other organs such as the liver and kidneys. Dogs of any age and breed are susceptible to contracting heartworm disease.

While cats appear to be more resistant to heartworm infection than dogs, with fewer worms surviving into adulthood, they are still susceptible to infection and can also suffer from the effects of heartworm disease. However, they do not contribute significantly to spreading the infection.

Although the risk of heartworm infection varies from state-to-state, heartworm disease has been identified in all of the contiguous 48 states and Hawaii.

Heartworm Life Cycle

Heartworm infection is spread from animal to animal by mosquitoes. Dogs, cats, ferrets, coyotes, foxes, wolves, sea lions and even humans have all been found to be infected by heartworm. Adult female heartworms release their young, called microfilariae, into the animal's bloodstream. Mosquitoes then become infected with microfilariae while taking a blood meal from an infected animal. During the next 10 to 14 days, the microfilariae mature to the infective larval stage within the mosquito. When the mosquito bites another dog, cat, or other susceptible animal, the infective larvae enter through the bite wound. In dogs, it then takes a little over six months for the infective larvae to mature into adult worms that may live for five to seven years in dogs. In cats, it takes about eight months to mature into adult worms that live from two to three years. Microfilariae cannot mature into adult heartworms without first passing through a mosquito.

Heartworm Life Cycle

For both dogs and cats, clinical signs of heartworm disease may not be recognized in the early stages, as heartworms tend to accumulate gradually over a period of months and sometimes years and repeated mosquito bites. In dogs, recently infected animals may exhibit no signs of the disease, while heavily infected animals may eventually show clinical signs, including mild, persistent cough, reluctance to move or exercise, tiredness after only moderate exercise, reduced appetite, and weight loss.

Severe cases of the disease in dogs may lead to heart failure, most often recognized by a 'swollen belly' caused by accumulation of fluid in the abdomen. 'Caval Syndrome,' a form of liver failure, is also a potential serious complication, causing dogs to become weak very rapidly and turning their urine dark brown. This is a life-threatening situation that prompts surgical removal of the worms.

Cats may exhibit clinic signs that are very non-specific, mimicking many other feline diseases. Chronic clinical signs include vomiting, gagging, difficulty breathing or rapid breathing, lethargy and weight loss. Infected cats may die acutely without allowing time for diagnosis or proper treatment.

Detecting Heartworm Infection

Detection of heartworm infection in apparently healthy animals is usually made with blood tests for microfilariae or a heartworm substance called an 'antigen,' although neither test is consistently positive until about seven months after infection has occurred.

Diagnosis of heartworm infection may also be detected through x-ray and/or ultrasound images of the heart and lungs, although these tests are usually used in animals that are known to be infected.

Treatment

In dogs, most cases of advanced heartworm disease can be successfully treated with a drug called an adulticide that is injected into the muscle. A series of injections are given to dogs who have received a thorough examination to assess the risk of the treatment. Hospitalization is usually recommended during treatment, but treatment can also be performed on an outpatient basis. During the duration of the recovery period lasting one to two months, it is essential that exercise for the pet be limited to leash walking, decreasing the risk of partial or complete blockage of blood flow through the lungs by dead worms.

Dogs in heart failure and those with caval syndrome (a form of liver failure) require special attention. Reinfection is prevented by administering a heartworm preventative. Some also eliminate microfilariae if they are present.

Currently, there are no products in the United States approved for the treatment of heartworm infection in cats. Cats have proven to be more resistant hosts to heartworm than dogs, and often appear to be able to rid themselves of infection spontaneously. In severe cases of the disease, veterinarians will treat an infected cat with supportive therapy measures.

Prevention

Although heartworm infection can cause serious complications in pets, it can be prevented. For dogs, there are a variety of options for preventing heartworm infection, including an injectable administered by your veterinarian that provides protection for six months, daily and monthly tablets and chewables, and monthly topicals. All of these methods are extremely effective and when the drugs are administered properly on a timely schedule, heartworm infection can be completely prevented.

In cats, there are three products currently approved to prevent heartworm infection, two oral and one topical medication. All three are virtually completely effective in preventing the development of adult heartworms when administered correctly.

Heartworm medications work by halting heartworm development before the adult worms reach the lungs and cause disease. Compared to the cost of treating an animal with mature adult heartworms, heartworm prevention is safe, easy and inexpensive.

Retesting

Periodic retesting is important for monitoring the success of any heartworm prevention program. It is recommended that pet owners discuss with their veterinarian what testing procedures are appropriate for their pet.

Founded during the Heartworm Symposium of 1974, the American Heartworm Society was formed to facilitate and encourage the generation and dissemination of information about heartworm disease and encourage adoption of standardized procedures for its diagnosis, treatment and prevention. The Society stimulates and financially supports research, which furthers knowledge and understanding of the disease. Its headquarters is located in Batavia, Illinois.

Eastern Equine Encephalitis:

We would like to acknowledge and thank the Centers for Disease Control for allowing
the use of the following information and links concerning mosquito-borne illness.

Eastern Equine Encephalitis

Eastern Equine Encephalitis (EEE) is a mosquito-borne viral disease. As the name suggests, EEE occurs in the eastern half of the US. Because of the high case fatality rate, it is regarded as one of the more serious mosquito-borne diseases in the United States.

TRANSMISSION: What is the EEE transmission cycle? How do people become infected with EEE virus?

• EEE virus is transmitted to humans through the bite of an infected mosquito
• The main EEE transmission cycle is between birds and mosquitoes
• Several species of mosquitoes can become infected with EEE virus. The most important mosquito in maintaining the enzootic (animal-based, in this case bird-mosquito-bird) transmission cycle is Culiseta melanura
• Horses can become infected with, and die from, EEE virus infection

ETIOLOGIC AGENT: What causes EEE?

• Eastern equine encephalitis virus is a member of the family Togaviridae, genus Alphavirus
• Closely related to Western and Venezuelan equine encephalitis viruses

HUMAN CLINICAL FEATURES: What type of illness can occur?

• Symptoms range from mild flu-like illness to encephalitis (inflammation of the brain), coma and death
• The EEE case fatality rate (the % of persons who develop the disease who will die) is 35%, making it one of the most pathogenic mosquito-borne diseases in the US
• It is estimated that 35% of people who survive EEE will have mild to severe neurologic deficits

INCIDENCE: How many and where have human disease cases occurred?

• 200 confirmed cases in the US 1964-present
• Average of 4 cases/year, with a range from 0-14 cases
• States with largest number of cases are Florida, Georgia, Massachusetts, and New Jersey
• The enzootic (animal-based) transmission cycle is most common to coastal areas and freshwater swamps
• Human cases occur relatively infrequently, largely because the primary transmission cycle takes place in swamp areas where populations tend to be limited

RISK GROUPS: Who is at risk for developing EEE?

• Residents of and visitors to endemic areas (areas with an established presence of the virus)
• People who engage in outdoor work and recreational activities
• Persons over age 50 and younger than age 15 seem to be at greatest risk for developing severe disease

PREVENTION: How can people avoid infection with EEE virus?

• A vaccine is available to protect equines
• People should avoid mosquito bites by employing personal and household protection measures, such as using insect repellent containing DEET, wearing protective clothing, taking precautions from dusk to dawn when mosquitoes are most likely to bite, and controlling standing water that can provide mosquito breeding sites. For more information about preventing mosquito-borne disease see our protection information.

SURVEILLANCE: How is EEE monitored?

• EEE is reportable under the National Notifiable Diseases Surveillance System
• In 2003, EEE cases are being reported to ArboNet for the first time. ArboNet is the national, electronic surveillance system established by CDC to assist states in tracking West Nile virus and other mosquito-borne viruses.
• Data for 1964-2000 is posted on this website, by state, as is a map of cases from 1964-1997

TRENDS:

• Risk of exposure to infected mosquitoes increases as population expands into areas with an established presence of the virus

CHALLENGES:

• There is no licensed vaccine for human use
• There are no effective therapeutic drugs
• Unknown overwintering cycle
• Control measures expensive
• Limited financial support of surveillance and prevention

Filariasis:

We would like to acknowledge and thank the Centers for Disease Control for allowing
the use of the following information and links concerning mosquito-borne illness.

Filariasis (Lymphatic):

Q. What is lymphatic filariasis?

A. Lymphatic filariasis is a parasitic disease caused by microscopic, thread-like worms. The adult worms only live in the human lymph system. The lymph system maintains the body's fluid balance and fights infections.

Lymphatic filariasis affects over 120 million people in 80 countries throughout the tropics and sub-tropics of Asia, Africa, the Western Pacific, and parts of the Caribbean and South America. You cannot get infected with the worms in the United States.

Q. How is lymphatic filariasis spread?

A. The disease spreads from person to person by mosquito bites. When a mosquito bites a person who has lymphatic filariasis, microscopic worms circulating in the person's blood enter and infect the mosquito. People get lymphatic filariasis from the bite of an infected mosquito. The microscopic worms pass from the mosquito through the skin, and travel to the lymph vessels. In the lymph vessels they grow into adults. An adult worm lives for about 5-7 years. The adult worms mate and release millions of microscopic worms into the blood. People with the worms in their blood can give the infection to others through mosquitoes.

Q. Who is at risk for infection?

A. Many mosquito bites over several months to years are needed to get lymphatic filariasis. People living for a long time in tropical or sub-tropical areas where the disease is common are at the greatest risk for infection. Short-term tourists have a very low risk. An infection will show up on a blood test.

Q. What are the symptoms of lymphatic filariasis?

A. At first, most people don't know they have lymphatic filariasis. They usually don't feel any symptoms until after the adult worms die. The disease usually is not life threatening, but it can permanently damage the lymph system and kidneys. Because the lymph system does not work right, fluid collects and causes swelling in the arms, breasts, and legs. The name for this swelling is lymphedema (limf-ah-DE-ma). For men, the genital area also becomes swollen, a condition known as hydrocele. The entire leg, arm, or genital area may swell to several times its normal size. Also, the swelling and the decreased function of the lymph system make it difficult for the body to fight germs and infections. These people will have more bacterial infections in the skin and lymph system. This causes hardening and thickening of the skin, which is called elephantiasis (el-ah-fan-TIE-ah-sis).

Q. What is the impact of this disease?

A. Lymphatic filariasis is a leading cause of permanent and long-term disability worldwide. People with the disease can suffer pain, disfigurement, and sexual disability. Communities frequently shun women and men disfigured by the disease. Many women with visible signs of the disease will never marry, or their spouses and families will reject them. Affected people frequently are unable to work because of their disability. This hurts their families and their communities. Poor sanitation in tropical and subtropical areas of the world, where the disease is common, has created more places for mosquitoes to breed and has led to more people becoming infected.

Q. How can I prevent infection?

A. Prevention includes giving entire communities medicine that kills the microscopic worms and controlling mosquitoes. Avoiding mosquito bites is another form of prevention. The mosquitoes that carry the microscopic worms usually bite between the hours of dusk and dawn. If you live in an area with lymphatic filariasis you can reduce your risk of infection by sleeping under a mosquito net and using mosquito repellent on exposed skin between dusk and dawn

Q. What is the treatment for lymphatic filariasis?

A. People infected with adult worms can take a yearly dose of medicine that kills the microscopic worms circulating in the blood. While this does not kill the adult worms, it does prevent infected people from giving the disease to someone else. Even after the adult worms die, lymphedema can develop. To prevent the lymphedema from getting worse by getting care from lymphedema therapist and by following several basic principles:

 • Carefully wash the swollen area with soap and water every day

 • Use anti-bacterial cream on any wounds to help precent bacterial infections

 • Elevate and exercise the swollen arm or leg to move the fluid and improve the lymph flow

Japanese Encephalitis:

We would like to acknowledge and thank the Centers for Disease Control for allowing
the use of the following information and links concerning mosquito-borne illness.

Japanese Encephalitis Facts & Information:

CLINICAL FEATURES:

• Acute encephalitis; can progress to paralysis, seizures, coma and death
• The majority of infections are subclinical

ETIOLOGIC AGENT:

• Japanese encephalitis (JE) virus: flavivirus antigenically related to St. Louis encephalitis virus

INCIDENCE:

• Leading cause of viral encephalitis in Asia with 30-50,000 cases reported annually
• Fewer than 1 case/year in U.S. civilians and military personnel traveling to and living in Asia
• Rare outbreaks in U.S. territories in Western Pacific

SEQUELAE:

• Case-fatality ratio: 30%
• Serious neurologic sequela: 30%

COST:

• Domestic: < $1 million/year - largely cost of immunizing travelers and military personnel
• International: no data, probably tens of millions of dollars

TRANSMISSION:

• Mosquito-borne Culex tritaeniorhynchus group

RISK GROUPS:

• Residents of rural areas in endemic locations
• Active duty military deployed to endemic areas
• Expatriates in rural areas
• Disease risk extremely low in travelers

SURVEILLANCE:

• Passive system based on domestic imported cases referred to CDC and other reference laboratories
• Laboratory-based passive surveillance in endemic areas

TRENDS:

• Expanding range of JE viral transmission to northern Australia
• Inactivated JE vaccine

CHALLENGES:

• Currently available killed vaccine expensive and occasionally reactogenic

(Source: http://www.cdc.gov/ncidod/dvbid/jencephalitis/facts.htm)

Questions and Answers About Japanese Encephalitis

Q. How is Japanese encephalitis transmitted?

A. By rice field breeding mosquitoes (primarily the Culex tritaeniorhynchus group) that become infected with Japanese encephalitis virus (a flavivirus antigenically related to St. Louis encephalitis virus).

Q. How do people get Japanese encephalitis?

A. By the bite of mosquitoes infected with the Japanese encephalitis virus.

Q. What is the basic transmission cycle?

A. Mosquitoes become infected by feeding on domestic pigs and wild birds infected with the Japanese encephalitis virus. Infected mosquitoes then transmit the Japanese encephalitis virus to humans and animals during the feeding process. The Japanese encephalitis virus is amplified in the blood systems of domestic pigs and wild birds.

Q. Could you get the Japanese encephalitis from another person?

A. No, Japanese encephalitis virus is NOT transmitted from person-to-person. For example, you cannot get the virus from touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease.

Q. Could you get Japanese encephalitis from animals other than domestic pigs, or from insects other than mosquitoes?

A. No. Only domestic pigs and wild birds are carriers of the Japanese encephalitis virus.

Q. What are the symptoms of Japanese encephalitis?

A. Mild infections occur without apparent symptoms other than fever with headache. More severe infection is marked by quick onset, headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions (especially in infants) and spastic (but rarely flaccid) paralysis.

Q. What is the incubation period for Japanese encephalitis?

A. Usually 5 to 15 days.

Q. What is the mortality rate of Japanese encephalitis?

A. Case-fatality rates range from 0.3% to 60%.

Q. How many cases of Japanese encephalitis occur in the world and the U.S.?

A. Japanese encephalitis is the leading cause of viral encephalitis in Asia with 30-50,000 cases reported annually. Fewer than 1 case/year is reported in U.S. civilians and military personnel traveling to and living in Asia. Rare outbreaks in U.S. territories in Western Pacific have occurred.

Q. How is Japanese encephalitis treated?

A. There is no specific therapy. Intensive supportive therapy is indicated.

Q. Is the disease seasonal in its occurrence?

A. Seasonality of the illness varies by country (see table).

Q. Who is at risk for getting Japanese encephalitis?

A. Residents of rural areas in endemic locations, active duty military deployed to endemic areas, and expatriates who visit rural areas. Japanese encephalitis does not usually occur in urban areas (see table).

Q. Where is Japanese encephalitis endemic?

A. See map below.

Q. Where do Japanese encephalitis outbreaks occur?

A. Japanese encephalitis outbreaks are usually circumscribed and do not cover large areas. They usually do not last more than a couple of months, dying out after the majority of the pig amplifying hosts have become infected. Birds are the natural hosts for Japanese encephalitis. Epidemics occur when the virus is brought into the peridomestic environment by mosquito bridge vectors where there are pigs, which serve as amplification hosts, infecting more mosquitoes which then may infect humans. Countries which have had major epidemics in the past, but which have controlled the disease primarily by vaccination, include China, Korea, Japan, Taiwan and Thailand. Other countries that still have periodic epidemics include Viet Nam, Cambodia, Myanmar, India, Nepal, and Malaysia.

Q. Who should be vaccinated against Japanese encephalitis?

A. According to the Advisory Committee on Immunization Practices (ACIP) (Centers for Disease Control and Prevention. Inactivated Japanese Encephalitis Virus Vaccine Recommendations of the Advisory Committee on Immunization Practices [ACIP]. MMWR. Jan 8, 1993;42:11):

Expatriates: Japanese encephalitis vaccine is recommended for persons who plan to reside in areas where Japanese encephalitis is endemic or epidemic (residence during a transmission season). Risk for acquiring Japanese encephalitis is highly variable within the endemic regions (see table; see map). The incidence of Japanese encephalitis in the location of intended residence, the conditions of housing, nature of activities, and the possibility of unexpected travel to high-risk areas are factors that should be considered in the decision to seek vaccination.Travelers: Japanese encephalitis vaccine is NOT recommended for all travelers to Asia. In general, vaccine should be offered to persons spending a month or longer in endemic areas during the trans-mission season, especially if travel will include rural areas. Under specific circumstances, vaccine should be considered for persons spending <30 days in endemic areas, e.g., travelers to areas experiencing epidemic transmission and persons whose activities, such as extensive outdoor activities in rural areas, place them at high risk for exposure. In all instances, travelers should be advised to take personal precautions; e.g., to reduce exposure to mosquito bites. The decision to use Japanese encephalitis vaccine should balance the risks for exposure to the virus (see table; see map) and for developing illness, the availability and acceptability of repellents and other alternative protective measures, and the side effects of vaccination. Risk assessments should be interpreted cautiously (see table; see map) since risk can vary within areas and from year to year and available data are incomplete. Estimates suggest that risk of Japanese encephalitis in highly endemic areas during the transmission season can reach 1 per 5,000 per month of exposure; risk for most short-term travelers may be 1 per million. Although Japanese encephalitis vaccine is reactogenic, rates of serious allergic reactions (generalized urticaria or angioedema) are low (1 to 104 per 10,000). Advanced age may be a risk factor for developing symptomatic illness after infection. Japanese encephalitis acquired during pregnancy carries the potential for intrauterine infection and fetal death. These special factors should be considered when advising elderly persons and pregnant women who plan visits to areas where Japanese encephalitis is endemic.