Victorian doctors who treated an infant with Japanese encephalitis in one of the state’s first cases shared their experiences, urging fellow physicians to be mindful of the differential diagnosis. This raises the question: is mosquito-borne disease here to stay?
Doctors at Royal Children’s Hospital in Melbourne case described A 4-month-old boy who was already well, presented with a febrile seizure followed by a 2-day outbreak of fever, reduced energy and food, which turned into aseptic meningoencephalitis.
No causative pathogen was initially found in cerebral spinal 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), first seen in Australia’s southern states, that it was considered as a possible cause of the child’s symptoms.
The child’s CSF taken on the first day was tested for JEV and found positive. Another history showed he had traveled to a town on the Victoria-New South Wales border 15 days before symptoms began, where the virus was detected in pigs.
Dr Andrea Zhu and colleagues noted that although 99% of JEV infections were thought to be asymptomatic, a typical symptomatic presentation was described in this case.
Japanese encephalitis “should now be considered in all patients with meningoencephalitis in whom an alternative causative pathogen has not been identified, particularly where epidemiological risk factors are present”, they wrote.
He reported good outcomes for his patient, who returned to close to baseline neurological function with some residual but improvement in limb weakness.
Why was JEV so surprised?
Since the first human case was reported from Queensland in March 2022, Australia has had 42 confirmed and probable Japanese encephalitis cases, including four deaths. Two-thirds of the cases have been reported in NSW and Victoria.
in one special podcastDr David Williams, leader of the Emergency Disease Laboratory Diagnostics Group at CSIRO’s Australian Center for Disease Preparedness, said the discovery of Japanese encephalitis this year has been “unprecedented”, so that experts have “caught the ignorant”.
Dr Williams explained that JEV had not been seen in Australia since its outbreak in the far north of the country and in the Torres Strait in the 1990s.
“There was a certain complacency that Japanese encephalitis was not going to spread further south,” Dr Williams said. “I think it all takes us by surprise, and also falls 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 mosquito surveillance systems did not have Japanese encephalitis on their list of targets.”
Dr Williams said it was also surprising that the first case was found in Queensland pig farming (the virus causes infertility in dead and vulnerable pigs and swine) and, later, pig farming in NSW, Victoria and the South. cases were found in Australia too.
“It was not in a concentrated area,” he said. “It was everywhere.”
What’s more, the disease had been in the country since at least early November 2021, infecting pigs must have occurred before 60-70 days of gestation in sows.
Associate Professor Cameron Webb, a medical entomologist at NSW Health Pathology, said the occurrence of JEV over a very wide area of southern Australia was “incredibly significant”.
“In NSW, in particular, this is the first time people have died from mosquito bites since the 1970s, when there was a serious outbreak of Murray Valley encephalitis virus,” he said.
Associate Professor Webb said the strain of JEV now running was different from the strain found in the north of Australia in the 1990s.
“The best explanation is that the virus may have made its way into Australia through infected birds or possibly through infected mosquitoes flying through the air,” Associate Professor Webb said.
Will climate change make JEV more common?
While 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 made its way from northern Australia to southern regions by a “cascade effect” through waterbird and mosquito populations, enabled by wet weather conditions associated with La Nia weather patterns.
“But if we return to extreme drought in many areas of Australia, this virus may disappear and we may not see its re-emergence for a decade or so,” he said.
“Therefore, while climate change may explain the emergence of JEV across Australia in 2021-22, it does not mean that it is going to be an annual problem in many of these similar regions.”
As the first day of winter approaches this week, Associate Professor Webb said there is little evidence that JEV was actively circulating among mosquitoes in areas affected by the outbreak.
However, the virus can still persist in mosquito eggs over the winter, he said, paving the way for it to be reintroduced the following summer.
good news for next summer
Along the Riverina’s waterway, watchdog chickens will be waiting.
Professor Dominic Dwyer, a medical virologist and infectious disease physician at NSW Health, said laboratories will test their sentinel herd of chickens for JEV next summer in addition to their usual tests for Murray Valley encephalitis and Kunzin virus.
“Now that we know it’s there, it’s easier to look for it and order the appropriate test,” he said.
Professor Dwyer said that for clinicians, the most important red flag for JEV was encephalitis, along with anything relevant in the patient’s history, such as being in an endemic area or working with pigs.
“You can do a JEV” [polymerase chain reaction (PCR) test] on cerebrospinal fluid but this is often negative as the duration of viremia is short, so serology tests on both CSF and blood become important,” he said.
There is no antiviral for JEV, but there are two vaccines available in Australia – Imojave, a live attenuated vaccine, and JESpect, an inactivated vaccine – usually given to travelers to endemic countries in Asia.
Given the limited vaccine supply, 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 tell whether the vaccine should be given as 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 occurs next summer or after summer and if so, which parts of the country will be affected,” he said. “This is all you need to know before formulating a vaccine strategy.
“We’re lucky that we’ve got the breathing space to work through this before next summer,” he said.
CSIRO’s Dr Williams said states and the federal government have responded rapidly to JEV detection, setting up working groups to deal with different aspects of the response.
“There have been fairly good levels of communication in the animal health and human health sectors, but the pork industry is also involved,” he said.