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 flo cks 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 infec tious bronchitis. Type-specific reverse transcriptase polymerase chain reac tions ( RT-PCRs) were used after propagation of the isolates in embryonated eggs in order to detect and differentiate Massachusetts, D274, B1648 and 7 93/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 recove red 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, whic h was the dominant type in some neighbouring countries, during the period o f investigation. The DNA derived by RT-PCR from 24 of the Massachusetts-typ e isolates from 12 vaccinated and 12 non-vaccinated flocks was sequenced an d compared with the sequence of Massachusetts vaccines used in Belgium. Thi s revealed that the sequence of four of the isolates (two from vaccinated a nd two from non-vaccinated flocks) was identical to that of a Massachusetts vaccine strain. Similar results were obtained for D274 isolates when compa red with the sequence of D274 vaccines. These sequencing results demonstrat e a co-circulation of vaccine and wild-type infectious bronchitis viruses i n broilers, and are further justification for permanent monitoring of circu lating strains in order to rationally modify vaccination strategies to make them appropriate to the field situation.