Complement-fixation test for reticuloendotheliosis viruses: limits of sensitivity in infected avian cells

A specific micro-complement-fixation procedure for assay of avian reticuloendotheliosis viruses (REV), designated by use as the COFAR test, was compared with an assay based on immunofluorescent (IF) antibody staining of infected chick embryo fibroblasts. Endpoint titrations in which REV strain T, chick syncytial virus, and spleen necrosis virus were used indicated that cultures infected with limiting dilutions of each strain were positive by both procedures within 6 days. Depending on cell density, infection of 2 to 9% of the cells cultured produced an unambiguous positive response (titer greater than or equal to 1:2) with COFAR test. When both tests were used in a study on the transmission and in vivo status of ducks infected with spleen necrosis virus, COFAR was no less, possibly more, sensitive than IF for detecting infection in cultures inoculated with plasma.