Epidemiology of infectious bronchitis virus in Belgian broilers: a retrospective study, 1986 to 1995

Infectious bronchitis virus (IBV) was isolated from each of 236 broiler flocks that had respiratory infection (86%), impaired growth, enteritis and/or nephritis (14%), over a 10-year period from 1986 to 1995 in Belgium. Among them, 65% of the investigated flocks had not been vaccinated against infectious bronchitis. Type-specific reverse transcriptase polymerase chain reactions (RT-PCRs) were used after propagation of the isolates in embryonated eggs in order to detect and differentiate Massachusetts, D274, B1648 and 793/B types. The incidence of these types was approximately 50, 38, 11 and 1%, respectively. In 16% of cases, two or three types of IBV were detected, representing mostly combinations of Massachusetts and D274. The majority of the Massachusetts and D274 isolates (68 and 69%, respectively) were recovered from non-vaccinated flocks, confirming that such flocks are at greatest risk of infection by these types of IBV. Interestingly, the B1648 type was isolated from more vaccinated flocks (14%) than non-vaccinated flocks (7.6%). Most surprising was the very low incidence (1%) of the 793/B type, which was the dominant type in some neighbouring countries, during the period of investigation. The DNA derived by RT-PCR from 24 of the Massachusetts-type isolates from 12 vaccinated and 12 non-vaccinated flocks was sequenced and compared with the sequence of Massachusetts vaccines used in Belgium. This revealed that the sequence of four of the isolates (two from vaccinated and two from non-vaccinated flocks) was identical to that of a Massachusetts vaccine strain. Similar results were obtained for D274 isolates when compared with the sequence of D274 vaccines. These sequencing results demonstrate a co-circulation of vaccine and wild-type infectious bronchitis viruses in broilers, and are further justification for permanent monitoring of circulating strains in order to rationally modify vaccination strategies to make them appropriate to the field situation.