The regime of administering the infectious bronchitis (IB) Ma5 vaccine (Massachusetts serotype) at 1 day old and the heterologous 4/91 vaccine at 2 weeks of age, was shown to be highly effective in protecting the respiratory tract of speci® ed pathogen free chickens. Protection, as measured by assessing ciliary activity of the tracheal epithelium following challenge, was excellent against challenge at 5 weeks of age with IB strains of many serotypes, isolated from disease outbreaks in different parts of the world. This vaccination protocol was more effective than revaccination with a vaccine of the same serotype as the ® rst vaccine. Furthermore, signi® cantly better protection was seen when the Ma5 vaccine was given before, rather than at the same time as or following, the 4/91 vaccine.It is suggested that the use of these two IB vaccines will frequently broaden the protection possible against challenge with IB isolates of many different serotypes, without the need to develop a new IB vaccine to combat each new IB serotype that emerges.

These results indicate that Colorado should be classi® ed as an APV, but the antigenic differences suggest that it does not belong to subgroups A or B, and represents a separate subgroup (subgroup C) or possibly a separate serotype.

The ability of the infectious bronchitis (IB) Ma5 and 4/91 live-attenuated vaccines to protect against kidney damage caused by a nephropathogenic strain of IB virus (B1648) was investigated. Protection parameters considered were gross and microscopic renal pathology, and the use of a polymerase chain reaction to detect IB RNA in kidney tissue. By each parameter, Ma5 vaccine alone provided poor protection, but 4/91 alone or the combined program both protected well.

A subgroup B isolate of turkey rhinotracheitis virus (TRTV) or avian pneumovirus (APV), obtained from a flock of commercial breeding chickens experiencing poor egg production, mortality and swollen head syndrome, was shown to cause substantial respiratory signs in both young SPF chickens and chicks with high levels of maternally derived TRT antibodies.This isolate replicated to high titre in the respiratory tract of experimentally inoculated SPF chickens for approximately 5 days after inoculation, but was recovered only occasionally after that time. It was never recovered from non-respiratory tract tissues. A detailed, sequential histological and immunoperoxidase study was performed. This revealed that, whilst TRT virus could be demonstrated consistently in the epithelium of upper respiratory tract tissue, although only for a short time after inoculation, the damage which it caused was minimal and recovery was rapid. This study, using a pathogenic TRT isolate obtained from diseased chickens, provides clear evidence that TRT virus can cause damage to the respiratory tract of chickens and that this damage is both localized and short lived.

On the basis of virus isolation and the demonstration of specific neutralising antibody in sera, infectious bronchitis virus (IBV) 4/91 (commonly called 793B) has been shown to be present in broiler, breeder and layer flocks of chickens in many parts of western Europe and also in Thailand and Mexico. These flocks had all been vaccinated against infectious bronchitis and the need for improved methods to control this new virus, still prevalent at least four years after it was first isolated, is discussed.