Serological diagnosis of acute tick-borne encephalitis by demonstration of antibodies of the IgM class.

A sensitive enzyme immunoassay is described for demonstrating specific antibodies of the IgM class to tick-borne encephalitis virus (anti-TBEV IgM). Anti-mu-coated, flat-bottomed microtiter plates are incubated with diluted patients' serum (2 hr at 37 degrees C), then with purified TBEV, and later with peroxidase-coupled anti-TBEV immunoglobulin for a further 2 hr. After washing the plates, orthophenylenediamine is added and the optical density is measured at 510 nm. At an early stage after onset of illness anti-TBEV IgM could be demonstrated up to serum dilutions of 10(-4). The specificity of the test system was shown by total inhibition of the positive reaction by preincubation of patients' sera with anti-mu. Tick-borne encephalitis (TBE) could be diagnosed serologically in 54 patients by the described method, and anti-TBEV IgM could be demonstrated in sera drawn up to 9 months after onset of illness. A peak in the number of infections occurs in the age group of 31-40 years, and most infections occur from June to October with a small peak in July.