Evaluation of Puumala virus IgG and IgM enzyme immunoassays based on recombinant baculovirus-expressed nucleocapsid protein for early nephropathia epidemica diagnosis

Background: Puumala virus (PUU), a member of Hantavirus genus, is the causative agent of nephropathia epidemica (NE), a milder form of hemorrhagic fever with renal syndrome (HFRS). Rapid diagnosis is essential for clinical management of NE.

Objectives: To evaluate the usefulness of recombinant protein-based IgM (direct- and μ-capture) and IgG (direct- and antigen (Ag)-capture) enzyme immunoassays (EIA) in early diagnosis of NE in comparison to IgG immunofluorescence assay (IF), and to find out the time limit for PUU-specific antibody seroconversion.

Study design: The specific IgM and IgG antibody responses in serum were analyzed in 109 patients (235 serial sera) and 114 patients (233 serial sera), respectively. The serum panel used was selected from a larger material according to the availability of information concerning the date after onset of symptoms, the panel also containing NE patients who had been IgG-IF negative in their first (early) samples to find out the possible differences between sensitivities of the EIAs and IF.

Results: All NE patients tested became IgM-positive at the latest on the 6th (μ-capture EIA) or 7th (direct-IgM EIA) day after onset of symptoms. Out of a panel of very early NE-patient sera (n=38) that could not be detected by IgG-IF, 66% were already positive with both direct-IgM EIA and μ-capture EIA. When comparing IgG EIAs and IgG-IF, 98% of IF-positive sera from NE patients were also positive with direct-IgG EIA, and 99% with Ag-capture IgG EIA. Out of a panel of very early NE-patient sera (n=37) that could not be detected by IgG-IF, 57% were positive with direct-IgG EIA, and 27% with Ag-capture IgG EIA.

Conclusions: The baculovirus-expressed PUU-N-based IgG and IgM EIAs were found most suitable for NE diagnosis, giving the opportunity in some cases for earlier diagnosis as compared with PUU-IgG IF.