Health authorities expand vaccinations for Japanese encephalitis

So far, 35 Australians have been infected and there have been three deaths, according to the federal Department of Health.

“Case numbers seem small but are the tip of an iceberg, with no visibility of what lies beneath,” Professor Cheng said.

The worry is that the virus has been detected in more than 50 piggeries so far. These animals act as sentinels and might represent the presence of the virus in other animals, such as feral pigs.

Typically, mosquitoes bite infected pigs and then inject the virus into humans.

According to Australian Pork Limited, there are more than 4000 commercial piggeries in Australia, housing about 2.4 million pigs at any one time.

While wild birds, particularly wading birds such as herons and egrets, are the main natural reservoirs of JEV, and other animals can spread infection too, pigs are the greatest risk because they carry very high levels of the virus in their blood when infected.

Not a food safety concern

The official advice is that JEV is not a food safety concern and commercially produced pork meat or products are safe to consume.

Professor Cheng said public health authorities were assessing the risk to decide how extensively vaccination should be recommended. Unlike COVID-19, they are not considering blanket vaccination, but are targeting groups at risk.

Traditionally, Australia has recommended a JEV jab for a few travellers returning from Asia. In March it began vaccinating workers in piggeries and abattoirs, and veterinary professionals in the industry.

The Communicable Diseases Network Australia has also prioritised jabs for families, including children aged two months and older, who live near piggeries, pork abattoirs or pork rendering plants and those who work with mosquito surveillance or control, in the field or in research/diagnostic laboratories.

It is monitoring events to see if the reach needs to be broadened.

The virus was detected in piggeries in southern parts of Australia in February. The NSW Department of Primary Industries published an alert that month about multiple piggeries in areas with reported high numbers of mosquitoes.

Emergency action was taken in March when JEV was declared a “communicable disease incident of national significance”, requiring a national approach.

How the virus travelled south

That month, the virus was found in two piggeries in Queensland and there were six cases in South Australia. This week, Victoria tallied up 19 properties and the Northern Territory reported it had detected active JEV in feral pigs.

It is not known how the virus travelled south. Some speculate it came with infected mosquitoes or migratory waterbirds. Others suggest a link with climate change and floods.

“It’s too early to say how serious this will become,” Professor Cheng said. “But over the last couple of months we’ve seen some tragic cases of brain damage, so it’s a very significant illness.

“We have a little window at the moment because mosquitoes don’t hang around southern Australia over winter. Although the risk may decrease in the next couple of months, we need to work out if vaccines are needed, and we’ll want to do that before the end of the year.”

Two vaccines are available in Australia and more supplies may be needed. While established pandemic pathways will be useful, many government resources remain tied up with COVID-19.

As the leading cause of encephalitis in south and east Asia, JEV causes up to 70,000 cases a year, with 15,000-20,000 deaths. JEV jabs are included in childhood vaccination programs in parts of Asia.

Unlike malaria, which mosquitoes can transmit from human to human, JEV cannot spread to other people.