The Japanese encephalitis virus (JEV) has been discovered in more than 40 piggeries across Victoria, New South Wales, Queensland and South Australia within the past month.
Human cases of Japanese encephalitis (JE), the disease caused by JEV, have emerged and one person was in intensive care on Monday. The federal health department on Friday declared Japanese encephalitis a communicable disease incident of national significance.
The agriculture minister, David Littleproud, told the ABC’s Radio National on Monday that there was no doubt climate change and extreme weather was playing a role in its spread. “The fact that it has effectively spread right across the eastern seaboard into South Australia says that this is a real threat that we need to take seriously,” he said.
How does it spread and who is at risk?
The disease is spread by mosquito bites and occurs mostly in pigs and horses, and sometimes in humans. This means pig farmers and people who work around piggeries in affected regions are most at risk.
It spreads through mosquitoes carrying it from either pigs or water birds, but it doesn’t transmit from animal to animal, animal to human (other than mosquitoes), or human to human.
You can’t catch it from eating pig or other animal products, so the virus does not present a food safety threat.
While horses and donkeys may be infected with the virus, they do not develop high enough levels of it in the blood to infect mosquitoes, so those animals are not involved in the spread.
Waterbirds, particularly wading birds such as herons and egrets, are the main source as they hold enough virus in their blood to infect mosquitoes for up to seven days. Pigs are the other major amplifiers of the virus, developing high enough viral levels in their blood to infect mosquitoes for around four days.
If any animal is suspected of having JEV in Australia, it must be reported either to a vet or the national emergency animal disease hotline. The signs to look for in animals are here.
What happens if you get the virus?
About 99% of JEV infections cause no symptoms, and of those who do get sick most will experience mild, cold-like symptoms. But in serious cases it can cause your brain to swell.
Among those who have symptoms that lead to a Japanese encephalitis diagnosis, the fatality rate is 25-30%.
The most concerning symptoms to be aware of are severe headache, neck stiffness, fever, sensitivity to light and sound, and movement difficulties, as these may indicate a brain inflammation known as encephalitis.
About 30-50% of survivors of encephalitis caused by JEV have permanent complications involving damage to the central nervous system, resulting in cognitive, sensory and movement issues. Elderly people, immunocompromised and those under five are particularly vulnerable to serious disease.
Prof Peter Collignon, who has studied the ways humans, the environment and animals interact to cause disease, said any patient presenting to hospitals in eastern Australia with encephalitis and where a cause could not be identified were having blood samples taken so they could be tested for JEV.
Collignon said authorities should also be looking for antibodies in certain populations to see how many people may have had it, since so few end up with severe disease. “We also need to find any pig with Japanese encephalitis and make sure we actually get rid of those pigs.”
There are no treatments for Japanese encephalitis.
How can I protect myself?
Protection from mosquito bites is key. The federal health minister, Greg Hunt, urged people in areas of high mosquito activity in eastern Australia to use mosquito repellent containing picaridin, oil of lemon eucalyptus, or DEET, on all exposed skin.
“Wear long, loose-fitting clothing when outside, and ensure accommodation, including tents, are properly fitted with mosquito nettings or screens,” he said.
Littleproud said anything you can do to keep mosquitoes away from you and your animals will help. “Whether that’s placing mosquito traps around pig pens or putting a hooded rug and some mosquito repellent on your horses.”
What about a vaccine?
There are two JE vaccines in Australia – one is available to children aged 9 months and older, the other to adults.
Usually, the vaccine is only advised for people travelling to Asia and the Torres Strait Island region of Australia, and those travelling in rural areas and undertaking certain activities with increased risk of exposure, or spending a month or more in endemic regions. People who work with animals in Australia can also choose to be vaccinated against the disease.
But with seven people with the virus in hospital in Victoria (including two NSW residents) on Monday and other hospitalised cases in NSW and Queensland, Littleproud said vaccines were being distributed to farmers in high-risk areas. Another order of vaccines was on the way.
A vaccine for horses and pigs is available in countries where Japanese encephalitis is endemic, but no vaccines for animals are registered for use in Australia.
How did it get here?
Japanese encephalitis is primarily a rural and agricultural disease endemic in parts of Asia, where it is the most common vaccine-preventable cause of encephalitis. It is also endemic in animals in parts of the Torres Strait region.
It is not known how the virus has come into Australia this time, the Department of Agriculture said.
JEV was first detected in northern Australia in 1995 amid a widespread outbreak on the Torres Strait Islands. It was then detected on the Australian mainland in 1998 in Cairns. Despite fears it may proliferate, only occasional Japanese encephalitis cases cropped up in subsequent years, all of them in far north Queensland. A strong vaccination program in the Torres Strait Island region meant human cases were limited there, helping to protect Australia.
The paper also said a lack of hosts, given only limited animals such as horses and pigs can infect mosquitoes, and competition with other viruses, also limited spread within Australia.
Why is it spreading in Australia now?
Littleproud said it was “because of the changing climate”. He said the waterways in which migratory birds have been able to come south is one of the reasons the chief veterinary officer, Mark Schipp, believes JEV has spread across the country.
“We’ve had extreme weather conditions again this year where obviously there’s rain that’s been falling right across the east coast,” he said. “There is a lot more water around, and these migratory birds come further because of the availability of water. And that means the risks are greater.”
A paper published in 2019 in the Annals of Global Health said: “Global warming can cause not only mosquito vectors to invade higher elevations and latitudes but also an increase in rainfall, facilitating increased breeding of JE vectors.”
Can we get rid of it?
Collignon said it’s too early to say. “If pigs are the major reservoir, we can possibly get rid of it,” he said. “The problem with other mosquito-borne viruses like Ross River virus is kangaroos and marsupials are the reservoir, and they’re everywhere.
“However, Australia’s got a lot of wild pigs. If JE is actually in those animals and not just in farm animals, that might make it difficult to stamp out.
“That’s why we need a lot more information and to work out how many people have had it, how many pigs have got it, and in which areas. With all the rain and mosquitoes around at the moment, it does also make the risk worse.”
