Sentinel chooks: why was Japanese encephalitis such a surprise? – Preview+

VICTORIAN doctors who treated an infant with Japanese encephalitis in one of the state’s first cases shared their experience, urging other clinicians to be aware of differential diagnoses. This begs the question: is mosquito-borne disease here to stay?

Doctors at Royal Children’s Hospital Melbourne describes the case of a previously healthy 4-month-old boy who presented with febrile seizures after a 2-day prodrome of fevers, reduced energy and nutrition, which progressed to aseptic meningoencephalitis.

No causative pathogen was initially found in cerebrospinal fluid (CSF) tests and the boy was treated for sepsis and seizures.

It wasn’t until a public health alert was issued for the mosquito-borne Japanese encephalitis virus (JEV), previously unknown in Australia’s southern states, that it was considered a potential cause of the child’s symptoms.

The child’s CSF, collected on day 1, was tested for JEV and found to be positive. Another story revealed he had traveled to a town on the Victoria-New South Wales border 15 days before the onset of symptoms, close to where the virus had been detected in pigs.

Dr. Andrea Zhu and colleagues said that although 99% of JEV infections are considered asymptomatic, the case described a typical symptomatic presentation.

Japanese encephalitis should “now be considered in all patients with meningoencephalitis in whom another causative pathogen has not been identified, particularly when epidemiological risk factors are present,” they wrote.

They reported good results for their patient, who returned to close to baseline neurological function with some residual limb weakness, but was improving.

Why was JEV such a surprise?

Since the first human case was reported in March 2022 in Queensland, there have been 42 confirmed and probable cases of Japanese encephalitis in Australia, including four deaths. Two-thirds of the cases have been reported in New South Wales and Victoria.

In a exclusive podcastDr David Williams, head of the emergency disease laboratory diagnostics group at CSIRO’s Australian Center for Disease Preparedness, said the discovery of Japanese encephalitis this year was “unprecedented” so experts were “caught off guard”.

Dr Williams explained that JEV had not been seen in Australia since outbreaks in the far north of the country and in the Torres Strait in the 1990s.

“There was a certain sense of complacency that Japanese encephalitis was not going to be transmitted further south,” Dr Williams said. “It all took us by surprise, I think, and also goes under the radar.”

Surveillance activities in recent years have mainly focused on Ross River fever, Murray Valley encephalitis and West Nile/Kunjin virus, he said: “Many surveillance systems mosquitoes didn’t have Japanese encephalitis on the target list.”

Dr Williams also said it was also surprising that the first case was detected in a piggery in Queensland (the virus causes stillborn and weak piglets and infertility in wild boar) and that in the following days, cases have been found in piggeries in NSW, Victoria and South Australia as well.

“It wasn’t in just one targeted area,” he said. “It was everywhere.”

Moreover, the disease had been present in the country since at least the beginning of November 2021, given that infection in sows must have occurred before 60 to 70 days of gestation to affect piglets.

Associate Professor Cameron Webb, a medical entomologist at NSW Health Pathology, said the occurrence of JEV in a very large area of ​​southern Australia was “incredibly significant”.

“In New South Wales specifically, this is the first time people have died from mosquito bites since the 1970s, when there was a severe outbreak of Murray Valley encephalitis virus.” , did he declare.

Associate Professor Webb said the strain of JEV currently circulating was different from that found in northern Australia in the 1990s.

“The best explanation is that the virus made its way to Australia through infected birds, or potentially wind-infected mosquitoes,” said Associate Professor Webb.

Will climate change make JEV more mainstream?

Although extreme weather events associated with climate change are clearly part of the equation, predicting the impact of climate change on diseases such as Japanese encephalitis is not straightforward.

Associate Professor Webb explained that the virus most likely made its way from northern Australia to southern regions by a “cascade effect” through populations of waterfowl and mosquitoes, activated by weather conditions. humid conditions associated with La Niña weather conditions.

“But if we were to go back to extreme drought in many parts of Australia, this virus could disappear and we might not see it again for another decade,” he said.

“So while a changing climate may explain the emergence of JEV across Australia in 2021-2022, it does not necessarily mean it will be an annual problem in many of those same areas then.”

As the first day of winter approaches this week, Associate Professor Webb said there was little evidence that JEV was actively circulating among mosquitoes in areas affected by the outbreak.

However, the virus could still persist in mosquito eggs through the winter, he said, paving the way for its reintroduction next summer.

Good news for next summer

Along the waterways of the Riverina, sentinel chickens will be waiting for you.

Professor Dominic Dwyer, a medical virologist and infectious disease physician at NSW Health, said the labs would test their sentinel flocks of chickens for JEV next summer in addition to their usual tests for Murray Valley encephalitis and Kunjin virus.

“Now that we know it’s there, it becomes easier to find it and order the appropriate tests,” he said.

Professor Dwyer said that for clinicians the most important red flag for JEV was encephalitis, along with anything that might be relevant to the patient’s history, such as having been in an endemic area or having worked with it. pigs.

“You can do a JEV [polymerase chain reaction (PCR) test] on cerebrospinal fluid, but it is often negative because the period of viremia is short, so serological tests become important on both CSF and blood,” he said.

There are no antivirals for JEV, but there are two vaccines available in Australia – Imojeva live attenuated vaccine, and I respectan inactivated vaccine – usually given to travelers to endemic countries in Asia.

Given the limited supply of vaccines, the Australian government has prioritized at-risk populations for local vaccination, including slaughterhouse and piggery workers and some entomologists and virologists.

Professor Dwyer said it was too early to say whether the vaccine should be given routinely in Australia as it is in parts of Asia.

“We don’t know if this is a one-time incursion into Australia or if it’s happening next summer or the summer after and if so what parts of the country will be affected,” he said. . “You need to know all of this before deploying a vaccine strategy.

“We are lucky to have a break to work through this before next summer,” he added.

CSIRO’s Dr. Williams said states and the federal government responded quickly to the detection of JEV, forming task forces to address various aspects of the response.

“There have been quite good levels of communication in the animal health and human health sectors, but also in the pig industry,” he said.

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