Quantitative determination of IgM antibodies reduces the pitfalls in the serodiagnosis of tick-borne encephalitis

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Abstract

Background

Tick-borne encephalitis (TBE) is the most important arbovirus disease in parts of Europe and Asia. Its laboratory diagnosis depends on the detection of specific IgM antibodies which can be impeded by (1) long-time persistence of IgM antibodies after infection, (2) vaccine-induced IgM antibodies, and (3) cross-reactive IgM antibodies from other flavivirus infections.

Objectives

To assess the extent of interference factors in the serodiagnosis of TBE that might lead to the false positive assignment of a recent infection.

Study design

We quantified TBE virus-specific IgM and IgG antibodies in sera collected at different time points from cohorts of (1) 61 TBE patients, (2) 131 TBE vaccinees, and (3) 42 patients with recent dengue or West Nile virus infections.

Results

All of the TBE patients were IgM- and IgG-positive upon hospitalization and 87% of acute TBE sera had IgM antibody titers of >500 Arbitrary Units (AU). These titers rapidly declined and only 16% of TBE patients had low IgM titers ≥9 months after infection. Vaccine-induced as well as flavivirus cross-reactive IgM antibodies were rarely detectable and of low titer.

Conclusions

Most of the potential problems of TBE serodiagnosis can be resolved by the quantification of IgM antibodies in a single serum sample taken upon hospitalization. High IgM values (>500 AU in our assay) are indicative of a recent infection. Lower IgM values, however, may require the analysis of a follow-up sample and/or a specific neutralization assay to exclude the possibilities of IgM persistence, vaccine-induced IgM antibodies or heterologous flavivirus infections.

Section snippets

Background

Tick-borne encephalitis virus (TBEV), like the closely related yellow fever (YF), Japanese encephalitis (JE), West Nile (WN) and dengue (DEN) viruses, is a member of the genus Flavivirus in the family Flaviviridae.1 It is endemic in large parts of Europe as well as Central and Eastern Asia and >10,000 clinical cases of TBE are reported annually.2

In Europe, TBE usually takes a biphasic clinical course and starts with a primary phase of an uncharacteristic febrile illness ∼7–14 days after

Objectives

It was the goal of our study to assess the extent of IgM antibody persistence after TBEV infection and TBE vaccination as well as the degree of cross-reactions of antibodies induced by other flavivirus infectionsas problems for IgM-based TBE serodiagnosis. For this purpose, we quantified TBE IgM and IgG antibodies at different time points after acute infections and vaccination and determined the amount of TBEV cross-reactive antibodies in sera from recent DENV and WNV infections.

Human sera

TBEV post-infection sera were obtained from 24 TBE patients admitted to the Hospital Ceske Budejovice at hospitalization as well as 3 and 9 months later (cohort 1). Sera from a second cohort of 37 TBE patients were collected ≥1.5 years (median 21 months; range 18–46 months) after admission to the same hospital (cohort 2).

TBE post-vaccination sera were collected after the two doses of the primary vaccination and after the 1st booster vaccination (3rd dose) from 131 healthy adults enrolled in two TBE

Persistence of IgM antibodies after TBEV infection

To perform a quantitative analysis of TBEV-specific IgM antibody decline after infection, we tested sera obtained upon hospitalization (1st samples) as well as 3 months and 9 months later from a cohort of 24 TBE patients. IgM antibodies were found in all 1st samples, although the quantities observed were highly variable (Fig. 1A and Table 1) and three of the acute sera were only in the lower positive range of 250–500 AU (Fig. 1A). IgM antibody concentrations were below the cut-off (100 AU) in most

Discussion

The routine laboratory diagnosis of acute infections with TBEV relies on the demonstration of TBEV-specific IgM antibodies in serum.6 There is usually no support from PCR techniques because the virus is only detectable in the first febrile phase of the disease and has disappeared upon hospitalization in most of the cases.7 The relative ease of TBE serodiagnosis is reassured by our findings that all of the acute TBEV post-infection sera were both IgM- and IgG-positive. However, three potential

Funding

This work was supported by intramural funds of the Medical University of Vienna and by the Swiss National Science Foundation (PP0033-110737 to UK).

Competing interests

None declared.

Ethical approval

All serum samples were collected and tested with the approval of the local ethics committees. All subjects enrolled in clinical studies gave their written informed consent.

Acknowledgements

We thank Cornelia Stoeckl, Jutta Hutecek and Lea Häberli for their excellent technical assistance and Walter Holzer for help with virus production and inactivation.

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    Present address: Cantonal Hospital Winterthur, Winterthur, Switzerland.

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