A clearance test to assess protection in chickens vaccinated against avian infectious bronchitis virus

A clearance test is described that is designed as a model for the quantitative assessment of protection in chickens vaccinated against avian infectious bronchitis virus (IBV). This is based on the ability of a vaccinating strain to have induced within 3 weeks an immunity sufficient to inhibit the replication of a challenge strain in the upper respiratory tract by the 4th day subsequent to challenge. A median protective dose (PD50) was determined for each of 4 vaccinating strains (H120, H52, D41 and Doorn 274), and chickens were vaccinated with 20 PD50 of one of these intranasally. Challenge strains were administered intratracheally 3 weeks later and assays of residual challenge virus in the trachea as well as the kidneys and oviducts were made after a further 4 days. Observations were also made on tracheal ciliary activity and histopathological changes. The H120 and H52 strains were efficient in clearing antigenically related challenge strains, but the H120 strain was less so in the case of the Doom 274 strain and D41 strain. The Doom 274 strain was effective against the heterologous strains examined with the exception of the T strain. The D41 strain was generally effective in protecting against all the strains selected for challenge, and its candidature as a broad-spectrum vaccine strain is endorsed accordingly. A small number (6%) of unvaccinated chickens had virus in low titre in the kidneys or oviducts after challenge with some strains. Virus was not detected in these organs of vaccinated birds.