Japanese encephalitis is generally thought to be limited to rural. Scientists are now questioning this belief, to wit the publication on 23 August of an article in the journal PLOS Neglected Tropical Diseases. The study in question showed that in Cambodia, the virus circulates as intensely on the fringes of the capital, Phnom Penh, as it does in rural areas.
Knowing more about how Japanese encephalitis spreads...
This result arose from a series of studies by CIRAD researchers and their partners*. The studies have produced new knowledge of how the disease is transmitted, persists and spreads. These new results are particularly important since the disease's distribution area may well continue to grow.
The virus that causes Japanese encephalitis is transmitted to man by mosquitoes of the genus Culex, which are found in wetlands and rice paddies. "This is why Japanese encephalitis has commonly been associated with rural areas", says Julien Cappelle, a veterinary researcher with CIRAD. "However, the disease also circulates within peri-urban zones."
The zoonosis can also affect pigs, which are the main amplifiers of the virus. Water birds are seen as the main wild reservoir of the virus, although the researchers involved in the study also observed that in Cambodia and Vietnam, chickens and ducks were also highly exposed to it. It may be that domestic poultry is involved in the epidemiological cycle of the disease. The presence of poultry may therefore account for the maintenance of the virus in zones with few, if any, pigs. The hypothesis of existence of direct transmission between pigs, which has been demonstrated experimentally by a Swiss team and backed up by modelling work by CIRAD researchers, could contribute to virus persistence and transmission in temperate regions with few, if any, mosquitoes.... in order to control it more effectively
Vaccinating children is still the best way of protecting humans against the disease, which will be impossible to eradicate entirely. However, Japanese encephalitis raises another problem for pig farmers, since it can cause serious reproductive issues in contaminated sows. According to researchers, various control methods could prove effective against Japanese encephalitis. Their work showed that vaccinating sows, combined with measures to stop mosquitoes breeding could substantially reduce virus circulation. This knowledge should prove valuable for regions where mass vaccination is difficult to implement.
Is Japanese encephalitis spreading?
For Véronique Chevalier, an epidemiologist with CIRAD, "the Japanese encephalitis virus could be exported to other continents!" This is backed up by two recent discoveries. RNA of the virus has been found in wild birds in Italy, suggesting that the disease may have spread to Europe. In 2017, an indigenous human case of Japanese encephalitis was declared in Angola. After introduction, the virus could become established and cause epidemics in areas with favourable climatic conditions where both hosts and vectors are present. As Véronique Chevalier says, "the Indian Ocean is a "good candidate", since it has extensive air and sea transport links with Asia". This risk of the disease spreading and becoming endemic is due to be studied shortly as part of a PhD thesis co-funded by CIRAD.
More than three billion people are exposed to Japanese encephalitis
Despite the existence of a vaccine, Japanese encephalitis is the main form of encephalitis in Southeast Asia. Transmission is endemic in 24 countries, which means that more than three billion people are exposed to it. For 2012, the WHO counted almost 68?000 clinical cases and as many as 20?400 deaths worldwide, primarily in children.
*Institut Pasteur (Paris), Institut Pasteur (Cambodia), ANSES (Maisons-Alfort animal health laboratory) and National Institute of Health and Epidemiology (Vietnam).