Biological Properties and Classification
Japanese encephalitis virus belongs to the family Flaviviridae and the genus Flavivirus. It has distinct antigenic cross-reactivity with other dangerous pathogens such as West Nile virus, Murray Valley encephalitis virus, and St. Louis encephalitis virus.
The pathogen is thermolabile, meaning it is extremely sensitive to heat. Its antigenic structure includes two key components: the nucleocapsid antigen, which is detected by the complement fixation test, and the envelope glycoprotein.
Various biological models are used to cultivate the virus in laboratory settings. Newborn white mice are the most susceptible—they are successfully infected even via extraneural inoculation. Chicken embryos, monkeys, and hamsters are also used. When grown in cell cultures, the virus exhibits a cytopathic effect, a specific feature of which is the formation of giant multinucleated cells, or symplasts.
Epidemiology and Natural Reservoirs
Japanese encephalitis is a classic zoonotic arboviral infection. The main range of distribution covers South and Southeast Asia. Key risk factors that facilitate the spread of the infection include rice cultivation and pig farming, as they create ideal conditions for the mass breeding of vectors and the maintenance of pathogen circulation.
Reservoirs and Sources of Infection:
- Mosquitoes: Play the role of the primary reservoir. They are characterized by transovarial transmission, allowing the pathogen to be passed on to progeny.
- Pigs: A crucial link in the development of epidemic outbreaks. The infection may be asymptomatic in pigs, yet it is accompanied by massive viremia sufficient to infect new mosquitoes.
- Birds: Especially water-associated and wading birds (e.g., herons).
- Wild mammals: Carry the disease asymptomatically, but with high levels of virus in the blood.
The transmission mechanism is vector-borne. The infection enters the human body through the bite of infected mosquitoes of the genus Culex (specifically C. tritaeniorhynchus). These insects are aggressive and actively feed on birds, domestic animals, and humans.
Pathogenesis and Clinical Presentation
The portal of entry for the virus is the skin at the site of the infected mosquito bite. Following an incubation period ranging from 8 to 14 days, the vascular phase of the disease begins. The pathogen enters the bloodstream, causing acute microcirculatory disturbances in all organs, especially in the tissues of the central nervous system.
This is followed by the visceral phase, during which the virus actively replicates in the cells of the liver, spleen, and bone marrow, maintaining a high level of viremia. Possessing marked neurotropism, the pathogen crosses the blood-brain barrier and enters neurons, causing their mass destruction.
Upon topographic evaluation, the highest concentration of the virus and lesion foci are found in the following structures:
- Hypothalamic nuclei.
- Subcortical structures.
- Motor nuclei of the brainstem.
- Cervical spinal cord.
The infection can be subclinical or mild, dominated by a general toxic syndrome. However, severe forms manifest as encephalitis or meningoencephalitis. The prognosis for the encephalitic form is extremely unfavorable—mortality can reach 90% and higher. Those who survive develop a long-lasting and robust immunity.
Laboratory Diagnostics
A complex of microbiological studies is used to confirm the diagnosis:
- Virological method: Aims to isolate the pathogen. Samples include blood (in the first 7–15 days of illness), cerebrospinal fluid (CSF), or autopsy material (brain tissue of the deceased). Isolation is performed using newborn white mice, cell cultures, and chicken embryos.
- Serological method: Detection of specific antibodies in paired serum samples and CSF. A wide range of assays is used: neutralization test (NT), hemagglutination inhibition test (HI, often using goose, rooster, or pigeon erythrocytes), complement fixation test (CFT), indirect hemagglutination assay (IHA), indirect immunofluorescence assay (IFA), and enzyme-linked immunosorbent assay (ELISA).
- Molecular genetic method: Polymerase chain reaction (PCR) for direct detection of viral genetic material.
- Rapid diagnostics: Detection of antigens using the immunofluorescence assay (IFA) and ELISA.
- Skin tests: In specific cases, an allergic intradermal test using a brain suspension from infected mice may be used.
Treatment and Specific Prophylaxis
Treatment for Japanese encephalitis includes serotherapy, which is most effective in the first days of the disease. Patients are administered convalescent serum (from individuals who have already recovered from the infection) or heterologous specific immunoglobulin.
Preventive measures are divided into passive and active:
- Emergency (passive) prophylaxis: Consists of administering immunoglobulin. Indicated for laboratory exposure or cases of massive mosquito bites in individuals located within an epidemic focus.
- Active prophylaxis (vaccination): An inactivated vaccine is used for human immunization. Because animals serve as a key reservoir, endemic areas practice widespread vaccination not only of the human population but also of domestic animals—using live vaccines for the latter.