Fallibility and flaviviruses: a diagnostic lesson in Japanese encephalitis

Clinical record

We present the case of a patient with Japanese encephalitis who was identified by a retrospective audit. The patient had initially been diagnosed and notified as a case of Murray Valley encephalitis (MVE).

In May 2021, a 77‐year‐old Victorian man was caravanning through the Northern Territory, where he and his partner recalled that he was extensively bitten by mosquitoes at a creek camp stop, 130km south of Darwin.

Ten days later he became increasingly confused, with fever and headache, progressing within 24 hours to ataxia and expressive dysphasia. He continued to deteriorate, and by 48 hours he was completely aphasic and required urgent transfer to the Royal Darwin Hospital and admission to the intensive care unit, with intubation and ventilation. A brain magnetic resonance imaging scan was normal and cerebrospinal fluid showed 39 white blood cells/μL (reference interval [RI], <5 white blood cells/μL), 100% monocytes, and protein 0.77g/L (RI, 0.1–0.5g/L). He made a slow but steady recovery and was extubated after 28 days; he then spent over four months in rehabilitation. At 12 months after illness onset, he was independent in daily activities but with some residual cognitive impairment.

The initial serology samples were negative for endemic NT flaviviruses (Japanese encephalitis, MVE and Kunjin viruses) (Box). Eleven days later, repeat serology revealed a positive result for MVE virus (MVEV) IgM, and a positive MVEV haemagglutination inhibition of 10 (reference interval, <10). He was formally notified as a case of MVE. Meanwhile, a second laboratory reported total antibodies for Japanese encephalitis virus (JEV) and MVEV as negative.

Six weeks after presentation, MVEV IgM was again positive, with MVEV haemagglutination inhibition of 20. As this was not a diagnostic fourfold titre rise, he was formally de‐notified as a case of MVE. At that stage, positive JEV IgM and IgG were also reported. A diagnosis of encephalitis of uncertain aetiology was made on discharge from the Royal Darwin Hospital, with recognition of the propensity for serological cross‐reactivity between flaviviruses.

Following the 2022 national outbreak of Japanese encephalitis across the southern and eastern states of Australia,1,2 the NT conducted a retrospective audit of NT patients with encephalitis presenting between October 2020 and March 2022. October 2020 was selected as the beginning of the wet season before the sentinel Japanese encephalitis case of the Australian outbreak that was reported in the NT.3 Thirty‐five patient files were reviewed in detail, identifying nine patients with encephalitis, including this patient. After a review of the conflicting 2021 flavivirus serology results, the patient and his family were contacted in May 2022, one year after his initial presentation, and convalescent flavivirus serology was organised.

This 2022 serology identified the likely diagnosis as Japanese encephalitis, due to the loss of JEV IgM but with JEV IgG remaining positive. Further testing across laboratories was also consistent with past JEV infection and not infection with MVEV or Kunjin virus (Box). Of note, the patient had never received a Japanese encephalitis vaccine and had not spent significant time overseas in locations where Japanese encephalitis is endemic. This case was formally notified as Japanese encephalitis in June 2022, 13 months after his initial presentation to hospital.

Discussion

This case represents the second case in the Australian Japanese encephalitis outbreak, with the sentinel case from February 2021 having also occurred in the NT.3,4 A flavivirus diagnosis can be done by serological, nucleic acid amplification test (NAAT), or viral culture methods. Serology is most widely used. NAAT assays, although very specific, can be negative, as the period of flavivirus viraemia is limited. The major endemic flaviviruses of Australia (MVEV, JEV and Kunjin virus) are closely structurally linked, which can lead to the development of cross‐reactive antibodies. Since the national Japanese encephalitis outbreak was declared, the serological and NAAT diagnostics for flaviviruses have improved in Australia.5 Accurate diagnosis requires careful interpretation, with repeat serology over time and using a range of assays. In challenging cases, diagnostic consensus may require comparing results from several laboratories experienced in flavivirus testing.

Accurate flavivirus diagnosis is critical to map the extent of national spread of JEV in Australia and to monitor future endemicity (predicted for northern Australia) and seasonal summer outbreaks (predicted as likely in southern states).2 Public health implications include strategies for prevention by human Japanese encephalitis vaccination, public messaging around avoiding mosquito exposure, and mitigation of JEV infections in commercial piggeries. Current widespread MVEV activity in southern Australia adds further complexity to ongoing flavivirus surveillance in Australia.