Indirect fluorescent antibody test for the diagnosis of yellow fever

The indirect fluorescent antibody (FA) test was evaluated for the serodiagnosis of yellow fever (YF) cases and for detection of antibodies after 17D YF vaccination. Results were compared with those of standard serological tests, reactions with heterologous flaviviral antigens were determined, and both IgM and IgG antibodies were measured. Spot slides of infected Vero cells were used as antigen substrate. In cases of primary YF infection, the FA test for IgG antibodies provided clear diagnostic results. Indirect FA tests with anti-IgG were more sensitive than the complement-fixation (CF) test and showed specificity comparable to the CF and neutralization (N) tests. Cases of YF infection superimposed upon pre-existing heterologous flaviviral immunity developed broadly-crossreactive IgG antibodies. IgM antibodies were highly specific in cases of both primary infection and superinfection but were not consistently present. Most individuals without previous flaviviral exposure who received 17D vaccine failed to develop detectable antibodies by the indirect FA test; of those with pre-existing immunity, 79% developed IgG antibodies. The indirect FA test provides a simple, rapid diagnostic procedure which should be especially useful in on-site epidemiological investigations of YF outbreaks.