Japanese Encephalitis

Summary

• Asian arbovirosis with mainly neurological symptoms

• Transmission via Culex mosquitoes

• There is a commercial vaccine.

Sometimes the name Japanese B encephalitis is used. The term “B” was used to distinguish from type A encephalitis (Von Economo encephalitis). The term "Japanese" is used since the virus was first isolated in 1935 in Japan. An earlier classification of the various arboviruses was in groups: group A, B, C according to Casals. This classification has now been abandoned, but the Japanese B encephalitis virus was then classed in group B of the arboviruses.

The area of distribution is greater than the name suggests: the Far East including the Philippines and the Mariana islands, Southeast Asia, Sri Lanka, India, Nepal and a small focus in Pakistan. Recently the virus crossed the Torres Strait between Papua New Guinea and Queensland and the virus was found in northern Australia.

Japanese Encephalitis, transmission and reservoir

Japanese Encephalitis is a serious and not so rare arbovirosis. It is a zoonosis with pigs and birds (esp. herons) as the reservoir. The infection occurs in foci in areas where pigs and people live closely together. The pigs act mainly as an amplifier of the virus for the newborn mosquito populations because they have a long viraemic period. In pigs it causes abortion and stillbirth. Bats can also be infected but their role is still unclear. The vector is a Culex mosquito (Culex tritaeniorhynchus) that breeds in rice fields. Sometimes there are epidemics.

Japanese Encephalitis, clinical presentation

In man it is usually a subclinical infection, but about 1/200 develop meningo-encephali­tis. Meningeal irritation, headache, stupor, coma and convulsions occur.

At the beginning there is leukopenia. In half of patients the cerebrospinal fluid contains raised protein and lymphocytes (<500/mm³).

Encephalitis has a high mortality rate (10 to 50%).

Neuro­logical sequelae are very frequent, such as dystonia and Parkinson-like symptoms due to the basal ganglia being affected.

The diagnosis can be made by isolating the virus from the cerebrospinal fluid early in the disease or by serology. Pregnant women run the risk of intra-uterine infection and death of the foetus during the first two trimesters. Nipah virus infection can cause a similar clinical picture.

Japanese encephalitis, vaccine

There is a vaccine (JE-Vax® from Biken), to be administered three times (e.g. on days 0, 7 and 30). To produce this vaccine the "Nakayama-Yoken" virus strain or the new "Beijing" strain are used. The virus is inoculated intracerebrally into mouse brains. The infected brains are then collected and the virus fraction is isolated, inactivated and lyophilised. Side-effects in the form of local redness at the injection site are not uncommon. Urticaria sometimes occurs after vaccination. Neurological complications occur very rarely (1-2.3/1,000,000). The entire RNA genome has been sequenced and immunogenic proteins have been expressed in yeast and bacteria. This opens new possibilities for the development of vaccines. In addition to vaccination there is the prevention of Japanese Encephalitis by vector control, impregnated mosquito nets and possibly changing the surroundings (reducing contact with the vector).

Japanese Encephalitis, biological vector control

Several biological control methods can be used to diminish problems caused by Culex mosquitoes. One of them employs a fungus-like parasite, Lagenidium giganteum. This watermould parasitizes larvae of mosquitoes, such as Culex, Aedes and Mansonia species. The parasite infects and kills most species of mosquito breeding in fresh water, at temperatures of 16-32°C (outside this temperature range it becomes dormant). L. giganteum can grow vegetatively, for example, on rotting vegetation or dead insects. It can be found in freshwater habitats which support mosquito populations. Moderate levels of salinity or organic load prevent it from sporulating, which is necessary for mosquito infection. This facultative parasite has been grown in large fermentation tanks using inexpensive culture media. Large rice fields can be sprayed from low-flying aircraft. Because the organism is relatively robust, the equipment used by many mosquito control agencies for spraying insecticides is readily adapted for use with this parasite.

Category: Microbiology Notes