Transfer of parental immunity to infectious laryngotracheitis in chicks.

The transfer of parental immunity to infectious laryngotracheitis was appraised by measuring serum antibody levels in 150 chicks from the day of hatch up to five weeks. The breeder flock which had received primary vaccination at eight weeks and a booster at 20 weeks transferred high antibody levels which fell markedly within two weeks and remained constant thereafter. Chicks whose parents were vaccinated at 20 weeks only, had low antibody levels throughout. These low levels, in either group of chicks, appeared to offer marginal protection only and were unlikely to inhibit the response to primary vaccination.