Recombinant IBV detection and vaccination efficacy. A case report

Infectious Bronchitis Virus (IBV), belonging to Coronaviridae family, affects the respiratory tract of poultry causing a mild respiratory syndrome, often complicated by many secondary bacterial diseases leading to mortality, great performance decrease and relevant economic losses. IBV is characterized by a high antigenic and genetic variation and recombination rate, that cause the emrgence of a lot of different strains worldwide, some of those are very persistent in some geographic area while others can appear and disappear quickly. Commonly employed control strategies involve biosecurity and vaccination. Unfortunately, the cross- protection among different IBV strains is often poor, and vaccines are not available for all IBV variants. However, the use of a combination of vaccines belonging to different IBV genotypes can broaden the protection spectrum to many different variants. Currently, in Italy the circulating genotypes are QX, 793B, Mass and Q1, but strains originating from the recombination between QX and 793B are sporadically detected. In the present study,a monitoring program was carried out in broiler farms using the following vaccination strategy: two spray vaccinations against IBV at hatchery with vaccines belonging to Massachussets and QX genotypes. Tracheal and cloacal swabs and blood samples were collected every 10 days. The swabs were processed in pool and tested using 2 real time RT-PCR specific for the two vaccines and a generic RT-PCR for IBV followed by sequencing. Titration of antibodies against Mass, QX, Q1 and 793B strains was perfomed in each sample using hemoagglutination inibition test. The real time RT-PCR for IBV QX vaccine was positive in tracheal swabs at days 12, 22 and 42 and only in cloacal ones at day 12. RT-PCR and sequencing were positive for IBV QX field strain in tracheal swabs at days 32 and 42 and in cloacal ones at days 32, 42 and 52 , while tracheal swabs were positive for a QX-793B recombinant strain at day 52. No clinical signs were observed during the whole production cycle confirming the efficacy of QX vaccine in controlling the disease caused by homologous variant. Furthermore the detection of a recombinant strain in healthy animal supports the hypothesis that the combination of vaccines applied in this farm could also protect against an enlarged spectrum of variants as already proved for the association between vaccines belonging to Mass and 793B genotype. Experimental challenges will be necessary to confirm field findings.