Source:
https://www.who.int/emergencies/disease-outbreak-news/item/2022-DON365
Japanese encephalitis - Australia
28 April 2022
Outbreak at a glance
As of 28 April 2022, a cumulative of 37 human cases of Japanese encephalitis (25 laboratory-confirmed cases and 12 probable cases) have been reported in four states in Australia with symptom onset dating back to 31 December 2021. This outbreak represents the first locally-acquired cases detected on the Australian mainland since 1998. Enhanced and targeted surveillance activities are ongoing to better understand the extent of transmission and to inform control activities. Further investigations are needed to assess the ongoing risk in Australia.
Description of the outbreak
On 7 March 2022, the Australian health authorities notified WHO of three laboratory-confirmed human cases of Japanese encephalitis (JE). The first human JE case was reported on 3 March 2022 from Queensland. As of 28 April 2022, the Australian Government’s Department of Health reported 37 cumulative confirmed and probable human cases of Japanese encephalitis virus (JEV) infection, including three deaths (2 confirmed, 1 probable). Twenty-five confirmed cases were reported from four states: New South Wales (11 cases, 1 death), Queensland (2 cases), South Australia (3 cases) and Victoria (9 cases and 1 death). In addition, 12 probable cases have been reported from: New South Wales (2 cases), Queensland (2 cases), South Australia (5 cases, 1 death) and Victoria (3 cases) (Figure 1).
Figure 1. Distribution of confirmed (n=25) and probable (n=12) human cases and deaths (n=3) of Japanese encephalitis (JE) and states in which JE virus has been detected in pigs, Australia, 2022.
The number of JE cases and deaths reported in 2022 is unusually high as compared to only 15 cases notified in Australia in the ten years prior to this outbreak. Of these 15 cases, only one case was acquired in Australia in the Tiwi Islands, Northern Territory; the remaining 14 cases were acquired overseas. These are also the first known locally-acquired detections of JE in humans in these states of Australia and the first detections in mainland Australia since a single case was detected in 1998 in Cape York, Queensland.
JEV, which infects both humans and animals, has also been detected in animals in Australia. In late February 2022, JEV was confirmed in commercial pig farms in the states of New South Wales, Queensland and Victoria, then in South Australia in early March. Affected piggeries had experienced unusual levels of reproductive losses and neonatal deaths. As of 20 April, JEV has been detected in 73 pig farms across the four states. Prior to February 2022, infection with JEV has not been previously detected in animals further south in mainland Australia than the northern peninsula area of Cape York.
Epidemiology of Japanese Encephalitis
Although a rare disease in humans, JE is a serious viral infection caused by the JEV spread by infected
Culex spp
. mosquitos in Asia (e.g.
Culex tritaeniorhynchus) and parts of the West Pacific (e.g.
Culex annulirostris). Numerous wild and domestic animal species can become infected, although most do not develop clinical signs and only a few develop sufficient viraemia (when the virus is present in bloodstream) to infect the mosquito vector which can result in further transmission. In natural cycles, wading birds such as herons and egrets are the important amplifying hosts, although pigs also develop significant viraemia that can infect vectors. The species most important in the ecology of the disease in Australia are yet to be determined.
JEV cannot be transmitted from human to human, or by consuming meat from an infected animal. Prior to this outbreak, JEV was considered unusual in Australia. Previously, JE was only rarely reported in humans in the north of Australia in Queensland (Torres Strait Islands and northern parts of Cape York Peninsula), with only 15 cases notified in Australia in the past ten years prior to this outbreak; only one of these cases acquired their infection in Australia with the remainder acquired overseas. In early 2021, one human case was reported from the Northern Territory (Tiwi Islands). The source of infection for this case remains unknown. The population of Australia is unlikely to have any significant natural immunity from prior infection, including asymptomatic infection, because the virus is not endemic to mainland Australia.
The vast majority of JEV infections are asymptomatic. There are no treatments for JE and the case fatality rate among symptomatic cases can be as high as 30%. Permanent neurologic or psychiatric sequelae can occur in 30–50% of cases with encephalitis. In an immunologically naïve population, all age groups are at risk for JEV infection. There are two JE vaccines for humans registered for use in Australia only advised for risk groups. There is no vaccine for animals registered for general use in Australia. A vaccine is available for use under permit in horses intended for export, and work is underway to make this vaccine available under an emergency use permit so that horse owners can protect their animals.
Public health response
The Australian Government’s Departments of Health and Agriculture, Water and Environment are working closely with state and territory government counterparts and affected animal industries to ensure a coordinated response across human and animal health. The Australia Government has declared the JE outbreak a Communicable Disease Incident of National Significance under the Emergency Response Plan for Communicable Disease Incidents of National Significance. Epidemiological investigations are ongoing with increased and targeted surveillance activities being carried out. Australian authorities are implementing vector control activities to remove potential mosquito breeding sites, reduce vector populations and minimize individual exposures.
WHO risk assessment
Human, animal and environmental investigations are ongoing to understand the increased transmission of JEV in humans and animals in Australia in 2022 and better assess the current and future risk. Serological evidence of JEV exposure is periodically detected in animals in the Torres Strait Islands off the north coast of Queensland, but transmission on the mainland has not been previously established. The current event therefore represents a significant change in the presence of the virus in Australia. Local transmission of JEV to humans requires environmental conditions capable of sustaining an enzootic cycle, thus, the risk of infection can vary substantially within any endemic country. JE transmission intensifies during the rainy season, during which vector populations increase. Internationally, there has not yet been evidence of increased JEV transmission following major floods or tsunamis.
Thirty-seven confirmed and probable cases of infection with JEV including three deaths have been identified in Australia with symptom onsets from 31 December 2021. The latest symptom onset of the cases was 14 March 2022. The cases were reported from four different states, three of which had no history of past local JE virus transmission. Vaccination against JEV is not used for the general population in Australia and is usually only advised for people travelling to endemic regions and for people undertaking activities with increased risk of exposure, thus, the immunologically naive population could be more susceptible to severe disease. As the colder months approach in southern Australia, a reduction in mosquito populations, and with that, a reduction in transmission is expected in all susceptible species.
The risk at the regional and global level is assessed as low. The JEV does not transmit between humans, therefore the likelihood of international disease spread among humans is low. However occasional overseas acquired cases have been reported among unvaccinated individuals returning from an area with active transmission.
WHO advice
Vector control: WHO recommends increased public awareness of JEV in the affected states and the implementation of activities to remove potential mosquito breeding sites, reduce vector populations and minimize individual exposures, including vector control strategies targeting both the immature and adult stages of the mosquito. Vector control should include environmental management (eliminating stagnant pools of water including waste/polluted water) and chemical control (including larval control and adult vector control including residual spraying of walls of animal shelters with approved insecticides). Homes near pig farms or animal shelters should be protected with mosquito-proof screens on windows and doors.
Personal protective measures: The peak biting period of
Culex vectors is during the evening (after sunset) and night. Personal protective measures including use of mosquito repellent, should be encouraged. Insecticide-treated mosquito nets provide good protection during sleep at night.
Surveillance:
- Strengthened surveillance is needed to assess the burden of JE, identify cases, inform vaccination strategies, monitor vaccine safety, and monitor the impact and effectiveness of JE vaccines. All JE-endemic countries are encouraged to carry out at least sentinel surveillance with laboratory confirmation of JE.
- Seroprevalence studies in pigs and feral pigs populations in affected states are needed to assess the magnitude of the risk. If seroprevalence in pigs is high, further seroprevalence studies in at-risk human communities are needed.
- Entomological surveillance using simple ovitraps with hay infusion near farms will support vector surveillance and monitoring the impact of control methods.
Vaccination: Vaccine strategies should be designed and implemented with due consideration for the fact that, in an immunologically naïve population, all age groups are at risk for JEV infection.
- During outbreaks: The value of reactive vaccination campaigns during outbreaks of JE has not been studied. If an outbreak occurs in a country or region where JE vaccination has not been introduced, an assessment needs to be made of whether it is appropriate to implement an immediate vaccine response, including considerations such as the size of the outbreak, timeliness of the response, population affected, and programmatic capacity. Due to the need for rapid production of protective antibodies, rapid deployment of at least one dose of live attenuated or live recombinant vaccines should be used.
- Routine immunisation: JE vaccination should be integrated into national immunization schedules in all areas where JE is recognized as a public health priority. Even if the number of JE confirmed cases is low, vaccination should be considered where there is a suitable environment for JEV transmission, i.e. presence of animal reservoirs, ecological conditions supportive of virus transmission, and proximity to other countries or regions with known JEV transmission. Adjunctive interventions, such as bednets and mosquito control measures, should not divert efforts from childhood JE vaccination. As JE is not transmitted person-to-person, vaccination does not induce herd immunity and high vaccination coverage for individual protection should be achieved and sustained in populations at risk of the disease. This will allow JE disease in humans to be virtually eliminated despite ongoing virus circulation in the animal cycle.
- Endemic areas: The most effective immunization strategy in JE endemic settings is a one-time campaign (possibly wide age range) in the primary target population, followed by the introduction in routine immunization for infant birth cohorts as defined by local epidemiology.
Travel and trade: WHO advises against the application of any travel or trade restrictions based on the current information available on this event...
