Diagnosing tick-borne encephalitis with a commercial enzyme-linked immunosorbent assay (ELISA) may lack specificity and need confirmatory testing, according to a study published in the Journal of Clinical Virology.

Tick-borne encephalitis is caused by the tick-borne encephalitis virus, in the flaviviridae family and flavivirus genus, and is most commonly transmitted to humans through tick bites. Although 75% of cases of tick-borne encephalitis virus infection are asymptomatic, the symptoms can vary and are typically biphasic, with influenza-like symptoms followed by an asymptomatic interval, which can lead to mild to severe neurologic symptoms. In the European Union, there have been approximately 12,500 cases of tick-borne encephalitis reported by 23 countries between 2012 and 2016. The implementation of surveillance programs and accurate diagnostics are essential for adequate clinical and public health responses. Diagnosing the infection is confirmed through detection of tick-borne encephalitis virus-specific antibodies in cerebrospinal fluid (CSF) and serum; however, the serology of the flavivirus is complex. Currently, the definition of tick-borne encephalitis virus encephalitis diagnosis is the detection of brain-derived immunoglobulin M (IgM) in the CSF.

However, cross-reactivity among flaviviruses makes tick-borne encephalitis virus serology interpretation difficult and hampers the ability to obtain reliable test results. This highlights the need for a serologic assay that is able to analyze both clinical sample types. Therefore, this study evaluated the performance of tick-borne encephalitis virus IgM and IgG ELISAs in both CSF and serum of patients with confirmed infection and discussed the role of serology in tick-borne encephalitis virus diagnostics.

Using 18 serum samples from patients with confirmed tick-borne encephalitis virus, 5 commercial ELISAs were assessed for sensitivity. The manufacturers of these 5 assays were selected based on the presence of the European Union CE certification marking and comprised the following: Serion ELISA tick-borne encephalitis virus, Immunozym FSMA (Progen), Reagena tick-borne encephalitis, Euroimmun anti-tick-borne encephalitis virus ELISA, and Enzygnost anti-tick-borne encephalitis (Siemens). Using 18 serum samples from patients with confirmed tick-borne encephalitis virus, the 5 ELISAs were assessed for sensitivity. Using 42 sera from 42 patients with a related flavivirus infection and 27 sera samples from patients with nonrelated viral infections (ie, Zika virus, Japanese encephalitis virus, Epstein-Barr virus infection, etc), the 5 ELISAs were assessed for specificity. To retrospectively evaluate the role of CSF serology, 8 serum-CSF pairs were obtained from patients infected in different areas in Europe and who presented with neurologic signs of tick-borne encephalitis.

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Results suggested that IgM testing in serum and CSF is crucial in confirming a tick-borne encephalitis virus infection. Serion, Progen, and Siemens ELISAs had a sensitivity of 94% for IgG, whereas the Reagena and Euroimmun ELISAs had a sensitivity of 83%. All the ELISAs demonstrated comparable sensitivity for IgM detection (94%-100%). For the related flavivirus panel, all commercial tick-borne encephalitis virus IgG tests scored low on specificity, ranging from 12% to 67%; of the tick-borne encephalitis virus IgM assays, only the Reagena ELISA had a specificity of 100%. For the nonflavivirus panel, all ELISAs had 100% specificity for IgM, and specificity for IgG ranged from 59% to 89%. When testing the role of CSF serology, all patients had detectable levels of IgM in both CSF and serum, which confirmed tick-borne encephalitis diagnosis, and in 7 of 8 patients, IgG levels were detected in both CSR and serum. This suggested that an IgG antibody index can support the confirmed diagnosis.

Overall, the study authors concluded that, “The lack of IgG specificity in all ELISAs emphasizes the need of confirmatory testing by virus neutralisation, depending on the patient’s background and the geographic location of exposure to [tick-borne encephalitis virus].”

Reference

Reusken C, Boonstra M, Rugebregt S, et al. An evaluation of serological methods to diagnose tick-borne encephalitis from serum and cerebrospinal fluid. J Virol. 2019;120:78-93.