Nature of Immunity to Newcastle Disease in Vaccinated Chickens I. Influence of Residual Resistance upon the Level and Duration of Immunity Following Revaccination

Abstract

The influence of a residual immunity in chickens, obtained from immune and susceptible dams, vaccinated intranasally at 5 days of age with the Bl strain of Newcastle disease (ND) virus, followed by revaccination with TCND at 32 and 39 days of age, demonstrated the importance of this factor upon the quality and duration of resistance to challenge with the GB strain of ND virus. A residual immunity was detectable at 32 days in chicks obtained from susceptible dams following Bl vaccination at 5 days of age, but not in chicks obtained from immune dams or in chicks from susceptible dams, provided revaccination was delayed to 39 days of age. One dose of TCND given at 32 or 39 days of age to chickens vaccinated at 5 days of age with Bl strain, was capable of inducing a substantial degree of immunity, as shown by the resistance of 39 of 40 chickens challenged intramuscularly at 55 weeks of age. A lesser resistance following challenge by contact with GB virus, expressed by signs of the respiratory tract, depression, and drop in egg production was more evident in chickens revaccinated at 32 days of age when a residual immunity at the time of revaccination was demonstrable. The influence of a residual resistance in 32-day-old chickens upon revaccination was detectable when the clock was challenged at 59 and 93 weeks of age.