Causes of Japanese encephalitis


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Cause of Japanese encephalitis: By the bite of mosquitoes infected with the Japanese encephalitis virus. 1

Japanese encephalitis (JE) virus: flavivirus antigenically related to St. Louis encephalitis virus 2

Cause details for Japanese encephalitis: Aedes japonicus is an Asian species of mosquito generally found in Japan, Korea, the Ryukyu Archipelago (Okinawa and associated islands), Taiwan, South China, and Hong Kong.  In 1998, the subspecies Aedes japonicus japonicus was first detected in the United States in New York and New Jersey. Since that time, Aedes japonicus has been found in six other states: Ohio, Maryland, Connecticut, Massachusetts, Pennsylvania, and Virginia.3

Larvae are found in a wide variety of natural and artificial containers, including rock holes and used tires. Preferred sites usually are shaded and contain water rich in organic matter. The similarity of breeding habitats used by Aedes japonicus to those of other Aedes species suggests that the transport of eggs, larvae, and pupae in used tires may be an important mechanism for introducing the species into previously uninfested areas. Eggs are resistant to desiccation and can survive several weeks or months under dry conditions. Aedes japonicus overwinters as eggs in the more northern parts of its range. However, it is found throughout the winter as larvae as far north as Tokyo (37° N), which is equal in latitude to Norfolk, Virginia. 3

Although few studies have been done to assess the public and veterinary health importance of Aedes japonicus, this species is suspected of being a vector of Japanese encephalitis (JE) virus to swine in northern Japan. Under experimental conditions it has been shown to transmit JE virus to mice and also to transmit the virus to its progeny through the eggs. Unpublished studies conducted at the United States Army Medical Research Institute of Infectious Diseases in Fort Detrick, MD, indicate that Aedes japonicus is also a competent experimental vector of West Nile virus, a flavivirus closely related to JE and St. Louis encephalitis viruses. 3

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). 1

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. 1

All flaviviruses (e.g., West Nile [shown here], St. Louis encephalitis, dengue, Japanese encephalitis, tick-borne encephalitis, yellow fever, etc.) share a common size (40-60nm), symmetry (enveloped, icosahedral nucleocapsid), nucleic acid (positive-sense, single stranded RNA approximately 10,000-11,000 bases), and appearance in the electron microscope. Therefore, images of West Nile virus are representative for this group of viruses. 4

Related information for causes of Japanese encephalitis: Further relevant information on causes of Japanese encephalitis may be found in the risk factors for Japanese encephalitis and underlying causes of Japanese encephalitis.

Footnotes:
1. excerpt from Questions and Answers About Japanese Encephalitis: DVBID
2. excerpt from Japanese Encephalitis Fact Sheet: DVBID
3. excerpt from Information on Aedes japonicus: DVBID
4. excerpt from West Nile Virus Virology: DVBID

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