Japanese encephalitis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Anthony Gallo, B.S. [2]

Pathophysiology

Japanese encephalitis virus is usually transmitted via mosquitos to the human host. Japanese encephalitis virus contains positive-sense viral RNA; this RNA has its genome directly utilized as if it were mRNA, producing a single protein which is modified by host and viral proteins to form the various proteins needed for replication. The following table is a summary of the Japanese encephalitis virus:

Characteristic	Data
Nucleic acid	RNA
Sense	ssRNA(+)
Virion	Enveloped
Capsid	Spherical
Symmetry	Yes; T=3-like organization; icosahedral-like
Capsid monomers	Unknown
Envelope length (diameter)	50 nm
Additional envelope information	Mature virons contain 2 virus-encoded membrane proteins (M and E); immature virons contain a protein precursor
Genome shape	Linear
Genome length	10-11 kb
Nucleotide cap	Yes
Polyadenylated tail	No; a loop structure is formed instead
Incubation period	5-15

Japanese encephalitis is contracted by the bite of an infected mosquito, primarily Culex tritaeniorhynchus. Japanese encephalitis virus circulates between a mosquito vector and either pigs or water birds (such as herons) in Southeast Asia. Transmission to humans requires mosquito species capable of creating a "bridge" between infected animals and uninfected humans; this occurs when humans become part of the enzootic cycle. The incubation period is 5-15 day(s). Humans are dead-end hosts for the virus, meaning there is an insufficient amount of Japanese encephalitis virus in the blood stream to infect a mosquito; there is also no evidence of person to person spread.

Japanese encephalitis virus is transmitted in the following pattern:

1. Attachment of the viral envelope protein E to host receptors mediates internalization into the host cell by clathrin-mediated endocytosis, or by apoptotic mimicry.
2. Fusion of virus membrane with host endosomal membrane. RNA genome is released into the cytoplasm.
3. The positive-sense ssRNA virus is translated into a polyprotein, which is cleaved into all structural and non-structural proteins necessary for RNA synthesis (replication and transcription).
4. Replication takes place at the surface of endoplasmic reticulum in cytoplasmic viral factories. A dsRNA genome is synthesized from the genomic ssRNA(+).
5. The dsRNA genome is transcribed/replicated, thereby providing viral mRNAs/new ssRNA(+) genomes.
6. Virus assembly occurs at the endoplasmic reticulum. The virion buds via the host endosomal sorting complexes required for transport (ESCRT), and is sent to the Golgi apparatus.
7. The prM protein is cleaved in the Golgi, thereby maturing the virion which is fusion competent.
8. New virions are released by exocytosis.

On microscopic histopathological analysis, the enveloped, spherical, and icosahedral-like virion shape are characteristic findings of Japanese encephalitis.

Japanese encephalitis (JE) virus is a flavivirus which is antigenically related to St. Louis encephalitis virus.

Culex mosquito laying eggs. (Photograph by Richard G. Weber)

• In rice fields, breeding mosquitoes (primarily Mosquito-borne Culex tritaeniorhynchus group) become infected with Japanese encephalitis virus.
• 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.
• Japanese encephalitis virus is NOT transmitted from person-to-person. For example, one 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.
• Only domestic pigs and wild birds are carriers of the Japanese encephalitis virus.

• 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.

Gallery

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  Diagram illustrates the methods by which the arbovirus, Japanese encephalitis virus (JEV) reproduces and amplifies itself in the avian populations, and is subsequently transmitted to human beings as the dead end host. From Public Health Image Library (PHIL).