Japanese encephalitis

Japanese Encephalitis is a mosquito-borne viral infection that occurs chiefly in three areas: (1) China and Korea, (2) the Indian sub-continent consisting of India, parts of Bangladesh, southern Nepal, and Sri Lanka, and (3) the southeast Asian countries of Burma, Thailand, Cambodia, Laos, Vietnam, Malaysia, Indonesia and the Philippines. Japanese Encephalitis also may occur with a lower frequency in Japan, Taiwan, Singapore, Hong Kong, and eastern Russia.

Japanese Encephalitis is primarily a rural disease. The mosquito transmits the virus via a vertebrate host, usually pigs or wild birds. Up to 3% of mosquitoes may be infected in endemic areas. The species is found in agricultural areas where the larvae develop in ground water, especially flooded rice fields. The mosquitoes bite in the late afternoon and early evening.

Transmission

Transmission is usually seasonal, following the prevalence of mosquitoes. Epidemics usually occur towards the end of the wet season. In temperate zones of China, Japan, Korea and northern areas of Southeast Asia, Japanese encephalitis is transmitted during summer and early autumn -- May to September. In north India and Nepal transmission occurs from June to November, and in south India and Sri Lanka epidemics are found from September to January.

The total number of cases is estimated at more than 50,000 annually. China contributes more than 50% and India another 20%. The majority of infected persons develop mild symptoms or no symptoms at all. However, among persons who develop encephalitis -- a viral infection of the brain -- the consequences of the illness may be grave.

Symptoms and disease course

Japanese Encephalitis has an incubation period of from four to 14 days, and is followed by a flu-like illness with headache, fever, muscle pains, and often gastrointestinal symptoms including vomiting. The illness often resolves without further complications. Occasionally, however, the illness may progress to an infection of the brain with neck stiffness, pain in the eyes when looking at light, disturbances in behavior, seizures, loss of consciousness, and muscle weakness or paralysis. One third of cases recover without problems. One third of cases survive with serious neurologic after- effects such as paralysis or other forms of brain damage. Fatality rates are about 20% in children and more than 50% in adults.

After the onset of the infection, and until the illness has run its course, only supportive treatment is available. Correct diagnosis and treatment require hospitalization. Infections in pregnant women during the first and second trimester have been associated with miscarriages. There is no specific drug to treat Japanese Encephalitis.

Risk for travelers

The disease is not common among travelers. Those who remain unimmunized should wear mosquito repellents, sleep under bednets, and bring insecticide sprays to use in their sleeping quarters.

The chance that a traveler to Asia will develop Japanese Encephalitis is very small. Only 5 cases among Americans traveling or working in Asia are known to have occurred since 1981. Only certain mosquito species are capable of transmitting Japanese Encephalitis. In areas infested with mosquitoes, usually, only a small portion of the mosquitoes are actually infected with Japanese Encephalitis virus. Among persons who are infected by a mosquito bite, only 1 in 50 to 1 in 1000 persons will develop an illness.

Risk is very low among the vast majority of persons whose itineraries are limited to cities or who will travel to the countryside only for short periods. These travelers do not require the vaccine. Older persons (> 55 years old) may be at higher risk for disease after infection and should be carefully considered for vaccination if they travel in areas of risk.

Vaccination

CDC recommends the vaccine only to those who work or have extensive visits during the transmission season to rural areas of the previously mentioned countries. The vaccine is recommended only for persons who will travel in rural areas for four weeks or more, except under special circumstances such as a known outbreak of Japanese Encephalitis.

Vaccination is recommended for those intending to spend more than a month in a rural risk area during the rainy season, for those making repeated trips into a risk area or who are planning to stay for a year or more in a risk area, and for those visiting an area where there is an epidemic.

Diseases