A Plaque-Reduction Method for the Detection of Neutralizing Antibodies for Infectious Bronchitis Virus

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Abstract
The reduction of plaques in a primary chicken embryo kidney cell culture to measure infectious-bronchitis-virus-neutralizing antibody levels in chicken serums was shown to be quite practical. The plaque-reduction method has at least 2 advantages over virus-neutralization tests performed in embryonating eggs. The former can be completed in 2 days rather than 5 to 7 days, and serums can be diluted without lowering the neutralization index of the serum, Of 55 nrmal chicken serums surveyed, no neutralization index exceeded 0.4. The neutralization indices of 54 serums from chickens that recovered from infectious bronchitis were compared using assays in embryonating eggs and plaques in embryonic chicken kidney cell cultures. The indices were generally higher in the embryonating egg system. With the plaque-reduction system, a serum with a neutralization index of 1.5 or greater would be positive for infectious bronchitis antibodies, and an index of 1,0 to 1,5 would be equivocal or suspect.