Japanese encephalitis in horses

Japanese encephalitis is a mosquito-borne viral disease that can cause reproductive losses in pigs and encephalitis in other susceptible animal species. Horses are a susceptible species. It is caused by Japanese encephalitis virus which is a member of the Flavivirus genus.

The virus is primarily spread by mosquitoes feeding on infected animals and does not usually spread directly from animal to animal (i.e. it is not considered contagious). Whilst horses can become infected and occasionally show symptoms, they cannot infect mosquitoes and therefore cannot infect any other animals or humans. Certain types of mosquitoes can become infected with JE such as Culex sp. and these tend to be most active at dawn and dusk.

Animals with the infection do not transmit the infection to other animals but may infect mosquitoes if bitten while they still have the virus in their blood. However, Horses do not gain enough virus to infect mosquitoes so are called an end host.

In general, spread is through the movement of the primary hosts of waterfowl, and waders such as herons and egrets, and through the movement of infected mosquitoes, often over long distances due to wind dispersal.

Clinical signs of Japanese encephalitis in horses

In horses many cases are asymptomatic and most clinical disease is mild, however more severe encephalitis can occur which may be fatal. Signs include:

  • fever
  • jaundice
  • lethargy
  • anorexia and neurological signs which vary with severity of the clinical disease.

Neurological signs can include incoordination, difficulty swallowing, impaired vision, and rarely a hyperexcitable form occurs. Disease has also been reported in donkeys.

I suspect Japanese encephalitis in my horse

Japanese encephalitis is a notifiable exotic disease in Victoria. If you suspect Japanese encephalitis in your horse, immediately contact your local Agriculture Victoria staff or phone the all-hours Emergency Animal Disease Watch Hotline 1800 675 888.

Learn about the investigation procedure for diagnosis of Japanese encephalitis in horses.

Reducing the risk of infection

Japanese encephalitis virus is spread primarily by mosquitoes.

Integrated mosquito management is critical to reducing the risk to both people and animals. This includes controlling mosquitoes in various ways, such as reducing opportunities for breeding and attacking both the larval and adult life stages.

Resources on mosquito management for horses can be found on the Farm Biosecurity website at Mosquito management for horses.

Chemicals for mosquito control

Chemicals used for larvae or adult mosquito control or used as a direct application to horses as a repellent

  • should be used in accordance with the product label
  • must be used only by people authorised to use chemicals
  • should be restricted to being applied only to areas required to be treated e.g. mosquito harbouring or breeding sites.

Chemical users must also keep records of their chemical use in accordance with relevant Victorian law.

Horse owners should seek the advice of a veterinarian before applying chemicals to horses and must not apply chemicals in an off-label manner (i.e. a manner that is not consistent with the label instructions) to horses without the authorisation of a veterinarian.

Some repellents registered for use on horses do not allow treated horses to ever be used for human consumption.  These products must not be used if there is any likelihood the horse may enter the food chain at some time in the future.

Management practices

The virus can also be spread through management practices such as reuse of needles. To minimise the risk of spreading the virus through management practices avoid reusing needles between animals when injecting and thoroughly wash and decontaminate other equipment between animals.

Stable horses between dusk and dawn

If available, stabling horses between dusk and dawn will help, as the mosquito most likely to carry diseases of concern is a night-time feeder that stays outdoors. If stabling horses, consider using the following measures:

  • installing appropriate physical barriers at entry and exit points, e.g. a double door entry-exit system
  • turning off lights inside stables during the night
  • using fluorescent lights in stables that do not attract mosquitoes
  • placing incandescent bulbs around the stable perimeter to attract mosquitoes away from horses
  • screening building openings e.g. windows with shade cloth mesh that have been treated with an insecticide registered for treatment of that shade cloth material. Reapply product weekly by applying the chemical to screens as per the label instructions.
  • fogging, fans and automatic overhead misting systems to eliminate mosquitoes within stables
  • spraying of stable walls with insecticides registered for use as a structural treatment around agricultural buildings.  In order to prevent possible adverse reactions DO NOT apply to areas where horses may come into direct contact with the sprayed area.

In addition to stabling horses overnight there are other techniques to prevent mosquito bites. They include:

  • topical treatment of horses using chemicals registered for direct application to horses as mosquito repellents
  • physical barriers - rugging and hooding horses in lightweight permethrin treated material (if climatically appropriate). Horse owners can treat their own horse rugs using agricultural chemicals registered for the treatment of horse rugs.
  • where possible reducing mosquito breeding sites on the premises such as dripping or leaking taps and water troughs, as well as discarding old tires which are a common breeding ground.
  • where it is not possible to fully eliminate all potential breeding sites, treat the breeding sites with an appropriately registered larvicide.
  • treating all interiors of horse vehicles with an aerosol insecticide registered for the control of mosquitoes.

Treatments for horses

There is no specific treatment for JE in horses; your veterinarian will provide supportive care according to the signs observed.

You are not at risk of JE infection from your horse; however, other diseases that are transmissible to people (such as Hendra virus in areas where flying foxes occur) could present with similar disease signs. Horses can be dangerous to people if they are unsteady on their feet so these risks should be kept in mind.

Horses are not a significant host of JE virus in terms of disease transmission, so there is generally no requirement for official controls to be put in place such as quarantine.

If your horse becomes infected, you may be at risk of being infected by mosquitoes in the area and should take precautions to avoid being bitten.

Vaccines

Vaccines for horses and pigs are used in some countries where the disease is endemic. No vaccines for animals are currently registered for general use in Australia. Expanded availability of animal vaccines is under consideration by the Australian Government Department of Agriculture, Fisheries and Forestry.

Horse movements

Horses are not a significant host in terms of disease transmission, so there is generally no requirement for official controls on horses to be put in place such as quarantine and movement controls. People movements are not affected.

Horse events

Horses are considered an incidental host of the virus, so there is no need to prevent horse movements or congregations at events from a disease control perspective.

Event organisers are encouraged to disseminate human health and animal health messaging and advice on protection from mosquito bites before an event.

For events in proximity to active mosquito breeding grounds, organisers should consider seeking professional advice regarding integrated mosquito management options.

Human symptoms of Japanese encephalitis

A small number of people infected with JE virus may become seriously ill with encephalitis (inflammation of the brain). Other people may develop an illness with fevers, headaches and aches and pains.

Symptoms of encephalitis may include rapid onset of fever, headache, sensitivity to light, neck stiffness, vomiting, confusion or disorientation and sometimes seizures, progressing to coma. Anyone experiencing symptoms, particularly if they’ve visited regions near the border of Victoria and New South Wales or been in contact with pigs or with mosquitoes should seek urgent medical attention.

Any person who is experiencing any of the symptoms should seek urgent medical attention.

Please visit the Department of Health website for public health advice.

Fact sheets

Information for horse owners

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Protecting your horses from mosquito bites

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Page last updated: 05 Dec 2022