| The effect of viral interference on the performance of several combined Newcastle disease virus (NDV) and infectious bronchitis virus (IBV) vaccines were evaluated in live broiler chickens. Four vaccine trials were conducted with each trial evaluating a different NDV vaccine strain. The NDV strains evaluated include the more commonly used B1 and LaSota strains, as well as the highly attenuated strains VG and C2. Each of these NDV vaccine strains was evaluated in a combined vaccine containing Massachusetts (Mass) type IBV and a combined vaccine containing Arkansas (Ark) type IBV. The performance of these vaccines was compared to that of the respective monovalent NDV, and monovalent Mass and monovalent Ark vaccines. In the NDV VG trial, the vaccine containing NDV VG and Mass type IBV also contained Connecticut (Conn) type IBV.; In each trial, broiler chicks were vaccinated at two weeks of age. Viral interference between NDV and IBV was evaluated by quantification of virus in tracheal swabs taken 1, 3, and 5 days post-vaccination via real-time quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). The downstream effects of viral interference were also evaluated. At 22 days post-vaccination (22 dpv), hemagglutination inhibition (HI) was used to evaluated serum NDV antibody levels, and ELISA was used to evaluated the serum IBV antibody levels. Vaccine performance was also evaluated by challenging birds with pathogenic IBV Mass 41, IBV Ark DPI, or NDV LaSota.; Infectious bronchitis virus interference with NDV growth was detected by qRT-PCR in the NDV VG, C2, and B1 vaccine trials, but was not pronounced in the combined LaSota vaccines. The growth of the highly attenuated NDV C2 strain was the most impaired by viral interference. Neither the LaSota, VG, nor B1 vaccines showed reduced performance in serology or challenge studies as a result of being combined with Mass or Ark IBV vaccine strains. Both the C2 + Mass and C2 + Ark vaccine groups exhibited reduced serum NDV antibody titers at 22 dpv, and the C2 + Ark group was poorly protected when challenged with pathogenic NDV LaSota. Newcastle disease did not reciprocally interfere with IBV growth, nor did it alter the performance of the IBV component in any of the combined vaccine treatments. |
