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

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Abstract
A sensitive enzyme immunoassay is described for demonstrating specific antibodies of the IgM class to tick-borne encephalitis virus (anti-TBEV IgM). Anti-m̈-coated, flat-bottomed microtiter plates are incubated with diluted patients'serum (2 hr at 37°C), then with purified TBEV, and later with pero-xidase-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-m̈. 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.