Author: Rhyll Vallis, Nicole Byrne and Jarrad Sanderson, Department of Agriculture, Water and the Environment
Japanese encephalitis (JE) is an acute mosquito-borne viral disease of humans and animals which occurs throughout much of Asia. Infections are mainly subclinical, but can be associated with abortion in pigs and encephalitis in humans and horses. Although an effective vaccine is available, the World Health Organisation estimates approximately 68,000 human cases occur annually predominantly in South-east Asia (WHO 2019).
Clinical disease occurs most commonly in pigs, horses (and donkeys) and humans. Reports of disease in other species are rare. JE has been reported in cattle, and chicks and ducks less than 6 weeks old.
In pigs, the most common clinical signs are mummified, stillborn or weak piglets, some with neurological signs. Many cases in horses are asymptomatic and most clinical disease is mild. However, severe encephalitis can occur, which may be fatal. Reports of clinical disease in donkeys is less common.
The virus that causes JE is a single-stranded, enveloped ribonucleic acid (RNA) virus belonging to the genus Flavivirus genus in the family Flaviviridae. The virus replicates in various tissues, with tropism for neurological and lymphoid tissues.
There is one serotype of JEV and five reported genotypes. Substantial strain variation has been reported among JEV isolates. While JEV strains vary in virulence and clinical signs, all are readily inactivated by heat, photochemical treatment, pasteurisation, treatment with detergents and low pH.
Japanese encephalitis is widely dispersed throughout Asia, and its geographic range extends from maritime areas of the north-east Russia, China and Philippines in the east, to India and Pakistan at its western limits, and throughout South-East Asia.
Outbreaks of Japanese encephalitis have been reported in Papua New Guinea (PNG) and northern Australia, as well as in the Western Pacific islands of Guam and Saipan.
For the latest information on the distribution of JE, refer to the WAHIS information database website of the World Organisation for Animal Health (OIE) or the FAO EMPRESi Global Animal Disease Information System.
Japanese encephalitis emerged in the Torres Strait in 1995. A human case was reported on mainland Australia in northern Queensland in 1998. Although serological evidence of pig infection was also detected, a transmission cycle was not established.
On the Australian mainland, a human case was reported from the Mitchell River area of western Cape York in 1998. Serological evidence of pig infection was detected in the Mitchell River area and in the Northern Peninsula area of Cape York at that time, but there was no further evidence of human infections in residents of nearby communities, and JEV did not establish a transmission cycle (Hanna et al 1999). Periodically, overseas-acquired human cases of JE are detected; however, there have been no reports of associated viral transmission in Australia.
The disease was not detected in mainland Australia again until 25 February 2022. Initially the disease was confirmed by laboratory diagnosis in three eastern states – in one piggery in Victoria, 6 piggeries in NSW and in one piggery in Queensland. At the time there had been a large amount of rain across eastern Australia with high numbers of mosquitoes reported.
Effective JE vaccines are available for use in people. The Australian immunisation handbook (ATAGI 2018) provides recommendations on vaccination of humans.
In some Asian countries, pigs are vaccinated as part of control efforts, and horses in endemic areas are vaccinated to protect against disease and for international travel.
The primary mode of transmission for JEV between hosts is through the bite of infected mosquito vectors.
The virus has been isolated from over 30 mosquito species. JEV vectors are opportunistic feeders and are zoophilic, preferring animals to humans. The principal competent Australian vector mosquito is Cx. annulirostris. This vector species has been primarily responsible for JEV in the Torres Strait islands. As host availability is an important factor influencing feeding, Cx. annulirostris preferentially feed on cattle and marsupials.
JEV has also been isolated in two vector mosquito species found in Torres Strait. They are, C. gelidus and Ochlerotatus vigilax. C. gelidus was introduced into northern Queensland and is now believed to be widely distribributed throughout northern Australia.
Maintenance of the virus is believed to occur in mosquito – waterbird, or mosquito – waterbird – pig cycles. Pigs and waterbirds are important amplifying hosts.
Water birds, particularly herons and egrets, are the main natural reservoir hosts of JE. Waterbirds and pigs are amplifying hosts of JEV and can act as maintenance hosts in endemic areas. Following natural infection, pigs develop a high level viraemia for 4- 5 days capable of infecting mosquito vectors. The high birth rate and rapid turnover of pigs in commercial production systems provide a continual source of susceptible hosts, and large epidemics of Japanese encephalitis have occurred when JEV spreads to new areas where susceptible hosts and suitable vectors co-exist.
Japanese encephalitis predominantly affects rural areas where irrigated agriculture human habitation, waterbirds and pig rearing are in close proximity. Typically, two epidemiological patterns exist: endemic activity in tropical regions and epidemic activity in temperate and subtropical regions.
Humans and horses are dead-end hosts of JEV, because they do not produce adequate viral levels required to infect mosquitoes. However, the disease can be fatal to humans and horses.
The virus is relatively unstable in the environment and is not considered significant in transmission.
The incubation period is 1–3 days in pigs, 4-14 days in horses and 5–15 days in humans. Note that less than 1% of people infected with JE virus develop clinical illness.
Clinical signs, mortality and morbidity differ by species, as shown in Table 1 below.
Adult pigs generally show no overt signs of infection.
Under experimental conditions, pigs can become pyrexic (40–41 °C) after 24 hours of infection, lasting up to 5 days, with inappetence and depression.
Piglets may show central nervous signs indicative of encephalitis, such as hind limb tremor.
Infected boars may have oedematous, congested testicles, lowered motile sperm counts and abnormal spermatozoa.
Subclinical infection is most common.
Neurological sequelae can occur in surviving horses, and has manifested as incoordination, paraplegia, ataxia and incontinence.
Transient, lethargic or hyperexcitable type syndrome.
There are three clinical presentations in horses. These are:
Transient type – pyrexia of up to 40 °C for 2-4 days, with anorexia, sluggish movement, congested or jaundice mucous membranes, and rapid and an uneventful recovery.
Lethargic type – fluctuating pyrexia up to 41 °C, with lethargy, anorexia, stupor, grinding of teeth and chewing motions, difficulty in swallowing, jaundice, petechial haemorrhages in mucous membranes, incoordination, staggering and falling, transient neck rigidity, radial paralysis, impaired vision and recovery within 1 week.
Hyperexcitable type – marked pyrexia (>41°C), with aimless wandering, violent and demented behaviour, blindness, profuse sweating, muscle trembling, bruxism, collapse, coma and death.
|Mortality rates||Mortality in litters of pigs can reach 100 per cent but is rare in adult pigs.||Estimated at 5–15 per cent in endemic areas and 30–40 per cent in seasonal epidemics. JE in livestock has not been accurately quantified.|
|Morbidity rates||Reproductive losses in pig herds can be up to 50–70 per cent during epidemics.||Low (<1 per cent) and up to 1.4 per cent during epidemics.|
JE should be suspected in disease outbreaks in pigs, with clinical disease characterised by abortion, foetal mummification or stillbirth, and encephalitis in animals up to 6 months old.
JE in horses must be differentiated from other causes of neurological diseases. In Australia, this includes infections with Hendra virus and the closely-related subtype Kunjin virus and Murray Valley encephalitis viruses, which have caused outbreaks of equine encephalitis and have very similar epidemiology to Japanese encephalitis.
Geographical and temporal clustering of pigs and horses displaying the described clinical signs should raise suspicion of JE, particularly in JE high risk areas. Where JE is suspected, samples from animals euthanised in the acute stage of the disease or from animals that have been dead for less than 12 hours should be collected. This is because viremia only lasts a few days. Virus isolation is best achieved from infected brain and post-mortem tissues as the virus is rarely cultured from blood or CSF. Samples to collect:
- a range of tissues in formalin
- whole blood and serum samples (paired serum samples should be collected 2–4 weeks apart)
- placental tissues from aborted foetuses
- cerebrospinal fluid and a range of brain tissue from animals with neurological signs.
As Hendra virus infection is a differential diagnosis in cases of neurological disease in horses, please consult with the relevant jurisdictional guidelines prior to commencing sample collection. Appropriate personal protective equipment should be worn if Hendra virus is suspected.
For the latest information on specimens required for JE testing, refer to the CSIRO ACDP Japanese encephalitis Virus Diagnostic Testing Factsheet.
Management in endemic areas worldwide involves controlling disease in animal populations to support public health initiatives, limiting disease in livestock and protecting high-value animals (e.g., stud and racing horses). The primary means of control are awareness raising, use of vaccination where available, and vector control.
In Australia, the national policy is to control JE in pigs in order to support public health agencies. Strategies include:
- Early recognition and laboratory confirmation of cases
- Coordination and cooperation with public health response activities
- Epidemiological assessment to inform decisions on appropriate control measures of mosquito vectors and reservoir host species in the transmission of JEV
- Movement controls over pigs, pig semen and embryos, and other potential amplifying hosts
- Tracing and surveillance of domestic and wild animals and potential mosquito vector species.
There is no effective treatment for JE in animals. Infection with JE is reportable throughout Australia. For further information see the National list of notifiable animal diseases.
For further details on Australia’s response policy, see the AUSVETPLAN Japanese encephalitis response strategy located on Animal Health Australia’s website: https://animalhealthaustralia.com.au/ausvetplan/
Risk to Australia
JE is endemic in neighbouring Australian countries such as Papua New Guinea and in the Torres Strait.
In Australia, there are three main ways JEV could have been introduced: movement of infected waterbirds, dispersal of infective mosquitoes by wind, and movement by humans of viraemic pigs.
In the case of pigs, animal movement within Torres Strait is controlled, and few animals move south from the Thursday Island group of islands. In addition, live pigs are not imported into Australia.
However, control of movements of wild waterbirds and mosquitoes is not possible. Potential JE vector species (such as Culex annulirostris) are present in many parts of Australia. Australia also has significant populations of susceptible host animal species, including amplifying host species such as waterbirds and pigs (domestic and feral).
JE is a disease of public health importance. Social effects include human fatalities and the ongoing need for vaccination.
Given the February 2022 detections of JE in parts of Eastern Australia, it is important that Australian veterinarians maintain current knowledge and remain alert to the possibility of JE infection in horses and livestock, as early detection and laboratory confirmation are critical for a rapid and effective response.
- immediately report suspect cases of JE to a government veterinarian or via the EAD Hotline
- wear appropriate personal protective equipment (coveralls and boots, gloves, mask) when examining animals, performing post-mortem examinations, or handling tissue, carcasses or abortion materials
- isolate sick/dead animals and limit contact with the sick/dead animals
- perform personal decontamination when leaving infected premises, and thoroughly clean and then disinfect equipment before leaving the property
- protect their own health through vaccination
- advise clients that human infections can potentially occur via needlestick and mucosal exposure, or inhalation of infected samples, or through contact with aborted foetuses, bodily fluids and tissues of infected animals.
Veterinarians can also encourage clients with pigs and horses to practise good farm biosecurity and implement an effective mosquito control program. The information about controlling mosquitoes around piggeries can be found on the farm biosecurity website.
Unusual clinical presentations, particularly when symptoms are consistent with an emergency animal disease (EAD), should be reported directly to state or territory government veterinarians or through the Emergency Animal Disease Watch Hotline (1800 675 888).
Animal Health Australia (2020). Response strategy: Japanese encephalitis (version 5.0). Australian Veterinary Emergency Plan (AUSVETPLAN), edition 5, Canberra, ACT.
AVA (Australian Veterinary Association) (2022) Japanese Encephalitis detected in pigs in Queensland, NSW and Victoria, AVA, accessed 1 March 2022.
Department of Agriculture and CSIRO. 2019. Emergency animal diseases: A field guide for Australian veterinarians, Canberra, August. CC BY 4.0.
Department of Agriculture and Fisheries (27 February 2022) Japanese encephalitis detected in pigs in southern Queensland [media release], Queensland government, accessed 1 March 2022.
DPI (Department of Primary Industries) (2022) Japanese Encephalitis, DPI, accessed 1 March 2022.
WHO (World Health Organization) (2019) https://www.who.int/news-room/fact-sheets/detail/japanese-encephalitis