Japanese encephalitis

Background

Japanese encephalitis virus (JEV) is a viral zoonotic disease spread by mosquitoes, which can cause reproductive losses and encephalitis in pigs and horses, and in rare cases it can cause disease in humans.

Australia experienced an outbreak of JEV in domestic pigs in 2022, with detections in over 80 piggeries in Queensland, New South Wales, Victoria and South Australia.

Prior to this, JEV was thought to be limited to seasonal transmission in the Torres Strait Islands and (occasionally) Far North Queensland.

Japanese encephalitis is a national notifiable disease which means an animal showing suspect signs of the disease must be reported to the Emergency Animal Disease Hotline on 1800 675 888. This number will connect you with your state or territory’s department of primary industries or agriculture.

Updated May 2023

NSW Department of Primary Industries confirmed the only case of Japanese encephalitis (JE) in pigs for the 2022-2023 season in the week commencing 14 November 2022. The piglets were from a property in the Murray Local Land Services region, a region that has been heavily flood impacted during winter and a part of the outbreak earlier this year.

JEV has been detected in samples collected from feral pigs in the Northern Territory, North Queensland and In February 2023, there were 2 feral pig detections in the Kimberly region, WA. Samples taken did not indicate active infection. JEV surveillance in feral pigs is ongoing.

Samples collected from sentinel chicken flocks in the Pilbara and Kimberly region in February 2023, tested positive for JEV. Samples from a chicken flock in Newman, WA indicated infection in the last 2 months.

Given the broad geographical distribution of detections, JEV is now considered to be established on the Australian mainland, although the risks in any given area are likely to vary seasonally and between years depending on weather and other local factors. Regardless of locality it is recommened to protect people and animals from mosquito bites.

State based web resources
Mosquito management is very important to control this disease. Useful resources include;
- Farm Biosecurity webpage: Controlling mosquito around piggeries – available at Controlling mosquitoes around piggeries - Farm Biosecurity
  - JE Guide: Integrated mosquito management principles for piggeries (V2)
  - JE Factsheet: Controlling mosquitoes around piggeries (V2)
  - Mosquito management for horses – available at Mosquito Management for Horses - Farm Biosecurity
- The national human health information relating to JE can be found at health.gov.au. The department also has free public resources that can be downloaded from their website.
How to identify the disease in animals

Pigs
- Japanese encephalitis virus is not spread directly from pigs to people and spread from pig to pig through genetic material such as semen is rare. The virus is primarily spread by mosquitoes.
- In pigs, the most common clinical signs are mummified and stillborn or weak piglets, some with neurological signs.
- Piglets infected after birth can develop encephalitis which presents as paddling or other neurological signs in the first six months of life. In other cases, wasting, depression or hindlimb paralysis may be seen in suckling piglets and weaners.
- Adult sows do not typically show overt signs of disease. If boars are present on farm, they may experience infertility and oedematous, and congested testicles.
- Pigs may also rarely become infected by direct contact between pigs or by infected pig semen.
Disease prevention in pigs
- Pig producers are asked to be highly vigilant for signs of this disease and report unexplained pig abortions or stillbirths.
- Many piggeries operate under the national Australian Pig Industry Quality Assurance program which sets high biosecurity and hygiene standards for commercial piggeries.
- People working with pigs, including those who may have a small herd or pet, should take steps to control mosquitoes, as well as of course continue to use effective biosecurity measures.
- Key measures that will help reduce the mosquito load around piggeries include:
  - Monitoring for mosquitoes at the various stages of their lifecycle. This can help determine the most effective control methods of and help break the breeding cycle.
    - To monitor, inspect bodies of water and containers for wrigglers, as well as areas where adult mosquitoes will rest like ceilings and walls. 
  - Work is under way to make a vaccine available for use in pigs. This includes trials needed to determine the effectiveness of an existing commercial vaccine against Australian JEV strains and research to develop a new vaccine locally.
Horses
- In horses many cases are subclinical, meaning that they can be infected but now show signs of the disease. Most clinical disease is mild, however more severe encephalitis can occur which may be fatal.
- The clinical presentation of infection with JEV is similar to the signs of infection with other mosquito-borne viruses such as West Nile-Kunjin virus or Murray Valley encephalitis virus, and may also present in a similar way to infection with the high-risk zoonosis of Hendra virus.
  - Signs include an elevated temperature, jaundice, lethargy, anorexia and neurological signs which can vary in severity.
  - Neurological signs can include incoordination, difficulty swallowing, impaired vision, and rarely the horse becomes over excited.
  - Due to the clinical similarity to Hendra virus infection, a sporadic but serious zoonotic disease, it is important all appropriate precautions are taken when assessing, sampling, and treating affected horses.
  - While reports of the disease in other species are rare, overseas the disease has been reported in donkeys.
  - Horses (like people) are known to be a ‘dead end host’. Unlike pigs and waterbirds, the level of virus circulating in infected horses’ blood is too low to reinfect mosquitoes.
  - Agriculture Victoria has provided a comprehensive JE Investigative Procedure for Horses at THIS LINK. It includes Sample collection, Packaging of samples, Transport, Sample submission, Diagnostic tests, Notes on testing, Disease notification and Significant Disease Investigation (SDI) Program.
Disease prevention in horses
- Horse owners can also put measures in place to help their horses avoid mosquito bites. Put a hooded rug on them, a fly mask, and if the horse allows, apply a safe insect repellent. Do not apply the repellent around or above their eyes.
- The Australian mosquito that transmits JE feeds at night and is reluctant to enter dwellings, so stabling horses between dusk and dawn is beneficial.
- Rugging and hooding along with applying an insect repellent may help protect horses that cannot be stabled. Note that permethrin treated horse fabrics which are sold with claims of insect control are viewed as unregistered agricultural chemical products under current legislation. Their supply and use in Australia have not been approved at this time.