Typing of infectious bronchitis virus (IBV) and relationship to protection in poultry

Abstract

This study deals with the detection and differentiation of infectious bronchitis virus (IBV) strains present in New Zealand, describes pathology caused by field strains and evaluates protection given by an attenuated live vaccine. Four historic (A, B, C and D) and five recent (T6, K32, K43, T75 and K87) IBV strains were used in the study.

A rapid detection system for the virus based on a reverse transcriptase polymerase chain reaction (RT-PCR) assay was developed which has proved to be at least as sensitive as virus isolation. This assay can detect virus directly from the tissues thus removing the need to culture diagnostic samples in embryonated eggs or tracheal organ cultures (TOC). Viral RNA could be detected from infected allantoic fluid at levels <100 fg and also from tissue samples where virus isolation attempts were unsuccessful.

The RT-PCR technique was then coupled with restriction fragment length polymorphism (RFLP) to differentiate the historic and recent strains of IBV present in New Zealand. The results of typing with RT-PCR/RFLP when compared with the traditional virus neutralisation test (VNT) indicate that both the assays are in agreement with each other.

The gene coding for the S1 glycoprotein was sequenced from the historic and recent field isolates. It was revealed that of the newly isolated strains, T6 was identical to historic strain D and two recent strains K43, K87 were identical to strain C. The fourth recent strain (K32) was more closely related to the historic B strain. Strains T6 and D were 99% homologous to strains C, K43 and K87. Antigenicity predictions based on the deduced amino acid sequence of the S1 glycoprotein suggested some antigenic sites are common to all the strains, while some are unique. A comparison with published S1 sequences from other countries revealed that the New Zealand strains are more closely related to Australian strains than European and North American strains. Indeed, the New Zealand A strain shared 99% nucleotide and 98% amino acid homology with the Australian Vic S (vaccine) and V5/90 strains.

The pathogenicity of some of the recent isolates was studied by the experimental infection of 10 day old chickens from a commercial source. Some differences were observed between the strains based on pathology and viral recovery from the trachea and kidney post infection, but overall only mild signs and lesions of the disease were seen.

Vaccine protection studies were done in day old commercial broilers, derived from two sources each having different vaccination histories and genetics. The New Zealand A strain vaccine when given as a spray to day old chicks with maternal antibodies provided partial protection (approximately 50%) against both the infection (virus isolation and detection by RT -PCR) and disease (reduced the degree of pathology which was indicated by tracheal lesions and ciliostasis score).