Detection of human immunoglobulins G and M antibodies to Rift Valley fever virus by enzyme-linked immunosorbent assay

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Abstract
Rift Valley fever virus (RVFV) is an important human and animal pathogen in Africa and has been responsible for infections in travelers. Because of the aerosol infectivity and risk of dissemination of the virus, a need exists for simple, safe, serological tests for diagnosis. An enzyme-linked immunosorbent assay (ELISA) was developed to detect RVFV-specific Ig (IgG and IgM). In the test, a betapropiolactone-inactivated, sucrose-acetone-extracted, suckling mouse liver RVFV antigen was captured by mouse RVFV antibodies adsorbed to polystyrene plates. The test sample (human serum) was then added, and the binding of specific antibodies was indicated by alkaline phosphatase-conjugated swine anti-human IgG or IgM. A .mu.-capture IgM ELISA was also developed by using polystyrene plates coated with goat anti-human IgM incubated successively with serum sample, RVFV antigen, and indicator antibodies. The ELISA for RVFV-specific IgG proved to be more sensitive than hemagglutination-inhibition or complement fixation tests and almost as sensitive as the plaque reduction neutralization test in detecting specific antibodies in human sera after vaccination. The 2 ELISA IgM tests could detect specific IgM antibodies during the first 6 wk after RVFV vaccination. Three injections of inactivated vaccine were given on days 0, 6-8 and 32-34. ELISA IgM values for sera obtained on days 6-8 were negative or in the lower range of significance, on days 32-34 they were strongly positive and on days 42-52 they were waning. Later sera were negative. The plaque reduction neutralization test was negative on days 6-8 but rose progressively in later samples. These findings suggest that the 3 doses of RVFV vaccine induce a prolonged primary antibody response. The ELISA IgM could become an important tool for early diagnosis in acute human infection. A number of African sera, some of which were positive for RVFV by plaque reduction neutralization test, were also tested by ELISA IgG. There was good agreement between both tests.