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Several viral outbreaks have caused all the alarms to go off in Australia. These are outbreaks of the virus japanese encephalitis (JEV) in pig farms in New South Wales, Queensland and Victoria, listed by the Australian government as “nationally important”.
They have reason to be scared. The Japanese encephalitis virus represents the most important cause of viral encephalitis in the world and causes about 20,000 annual deaths. It causes direct and indirect damage to the central nervous system, therefore, of those who survive the infection, almost half suffer from irreversible neurological deterioration.
A virus that returns with force every ten years
Genetic studies suggest that Japanese encephalitis virus (JEV) originally evolved from an ancestral flavivirus, probably several millennia ago, in the Malay Archipelago region.
However, clinical records of the virus only date back to the early 1870s, when regular summer outbreaks of encephalitis were described in Japan, with substantial incidents occurring periodically at a rate of approximately once every 10 years.
The first major outbreak of the disease dates back to 1904 and was called “Yoshiwara cold” or “Yoshiwara cold”. Epidemics of encephalitis followed in 1924, 1935, and 1948. The 1924 outbreak caused more than 6,000 cases and 3,000 deaths in 6 weeks. More recently, in 2005, large outbreaks of Japanese encephalitis occurred in northern India and Nepal, causing 5,000 cases and 1,300 deaths.
Four billion people at risk of infection
Since its discovery, the Japanese encephalitis virus has spread considerably and is now endemic to most of Asia, putting nearly 4 billion people at tangible risk of infection. The disease is endemic in 24 countries in South and Southeast Asia, from Pakistan to Japan, northern Australia and Oceania, meaning that almost half of the world’s population is at risk of infection.
The annual incidence of Japanese encephalitis is estimated at around 69,000 cases, although the figure is most likely underestimated due to the lack of accurate diagnostic tools. It mainly affects children. Some data suggest that the impact of the disease may be greater than that of dengue.
To add insult to injury, although outbreaks in the Western Hemisphere remain rare, the situation could change soon. Rapid globalization and climate change are respectively facilitating international travel and expanding the habitats of vector mosquitoes of Japanese encephalitis virus. That increases the likelihood of the disease emerging in geographic regions that were previously unaffected by the virus.
Transmitted by rice paddy mosquitoes
The virus that causes the disease is an arbovirus belonging to the Flavivirus genus and the Flaviviridae family. The main vectors of the virus are mosquitoes of the genus Culex, especially Culex tritaeniorhynchus. The mosquito vectors that transmit the disease thrive regularly in Asian rice fields. If we go to endemic areas it is advisable to use a registered insect repellent, wear long-sleeved shirts, long pants and get vaccinated.
The virus has a wide vertebrate host range and appears to pigs and wading birds act as amplifying hosts. The virus remains in circulation in an enzootic cycle of transmission between mosquitoes, pigs and/or lake birds. Disease transmission is more intense during the rainy season, because the vector population increases.
Humans and other mammalian species, such as horses, serve as dead-end hosts, as the viremia is not believed to reach levels that are infectious to mosquitoes. Only 1 in 25 to 1 in 1,000 human infections produce symptoms. However, the mortality rate of symptomatic cases is high, around 20-30%, and around 30-50% of survivors experience significant neurological and psychiatric sequelae (motor paresis, spasticity, movement disorders, chronic seizures, and developmental delay).
Cases of severe illness are characterized by the sudden onset of high fever, headache, stiff neck, disorientation, coma, seizures, spastic paralysis, and death.
There are no antivirals but there are vaccines
There is no antiviral treatment for patients with Japanese encephalitis, but there are safe and effective vaccines to prevent the disease. Four main types of Japanese encephalitis vaccines are currently available: inactivated mouse brain-derived vaccines, inactivated Vero cell culture-derived vaccines, live attenuated vaccines, and live recombinant vaccines.
The first vaccine was an inactivated mouse brain vaccine produced in Japan and used worldwide for 50 years. Although production of such vaccines declined in 2006, similar inactivated mouse brain vaccines are still produced locally in South Korea, Taiwan, Thailand, and Vietnam.
In recent years, in countries where the disease is endemic, the live attenuated vaccine SA14-14-2, manufactured in China, has become the most widely used. In Europe, only the Ixiaro vaccine is available, which contains the inactivated SA14-14-2 strain of the Japanese encephalitis virus and is produced in Vero cells and subsequently purified and inactivated.
Estimates suggest that, between 2000 and 2019, vaccination programs prevented 315,000 cases of Japanese encephalitis and 115,000 deaths from the disease worldwide.
Unfortunately, global climate change affects our present and future in very different ways. Not least because it offers an opportunity to increase the distribution of Japanese encephalitis as the habitats of vector mosquitoes expand into previously non-endemic areas.
Raul Rivas Gonzalez. Professor of Microbiology, University of Salamanca.
This article was originally published on The Conversation.
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