QX remains most prominent IBV strain in Europe, Africa and Middle East
A recent survey demonstrates that QX remains the most predominant field strain of infectious bronchitis virus (IBV) circulating in Europe, Africa and the Middle East.
The survey was conducted to help producers determine whether their IBV vaccination programs are adequate, according to Luuk Stooker, DVM, a technical manager for Zoetis.
“To build the right vaccination schedule, it’s key to know which IBV strains are circulating in your region,” he said.
The results are based on more than 6,500 tracheal or cloacal samples taken in nearly 30 countries from 2011 to 2014. The samples were submitted to Zoetis, which performs IBV testing as a service to veterinarians and/or poultry producers. They were collected from chickens suspected of having IBV infection and were accompanied by information including the type of chicken, the age at sampling, vaccination history and information about whether or not the bird had demonstrated signs of IBV or was sampled for routine monitoring, he said.
The first test performed by Zoetis was real-time polymerase chain reaction (RT-qPCR) utilizing specific primer-probe combinations for the N protein of the IB virus. If a sample was positive, a second RT-qPCR test was done for validation; this second test was run on the hypervariable part of the S1 protein of the virus using strain-specific primers and probes for the most common strains found in the region. In every different test, positive controls were included, Stooker said.
The samples were tested for the IBV strains Massachusetts, D274, 793b, D1466, Italy 02, Arkansas, QX and Variant 2. For the survey, samples that were positive on the first test but negative on the second test were outsourced to an independent lab for sequencing to identify the IBV strain in this specific sample, Stooker continued.
Differentiating field from vaccine strains
One of the challenges in the survey was differentiating between vaccine and field strains of IBV. Consequently, samples from birds that were likely or possibly positive due to vaccination were removed from the pool, such as samples from birds vaccinated with a strain homologous to the strain isolated, samples from birds with an unknown vaccination history as well as all breeder and layer samples since these classes of birds are routinely vaccinated against IBV and there is still a lot of unclarity on possible persistence of (vaccine) virus in these birds, Stooker explained.
Figure 1 shows the total amount of different IBV-strain isolations before the exclusion criteria were applied. Figure 2 shows the results with the exclusion criteria applied. In addition, the total number of isolations for Italy 02 and D1466 were so low they were omitted from the Figure 2 results.
Note that the absolute number of isolations between the IBV QX, 793b and Massachusetts strains in Figure 1 are nearly the same over the last two years, but in Figure 2, there are clearly more IB QX and fewer Massachusetts and 793b strains, he said.
“The survey results demonstrate that in the large majority of countries in Europe, Africa and the Middle East, protection against the QX strain should be included in IBV vaccination programs,” Stooker said.
Table 1 shows the number of samples from several of the countries in the survey and the percentage of positives for various IBV strains in each.
The reason new IBV strains emerge, Stooker explained, is because the virus can change through spontaneous mutation or genetic recombination. It is not known for certain why or how the QX strain, first identified in China, spread to other regions.
One theory is that it started after an outbreak of avian influenza in the Netherlands in the early 2000s; the outbreak resulted in extensive culling, and replacement stock may have carried IBV QX.
The best protection against IBV, the veterinarian said, is homologous protection, which means using a vaccine with the same strain as the IBV strain affecting flocks.
When a homologous vaccine isn’t available, a combination of vaccines that are heterologous (different) from the circulating strains can provide much better, though often not full, protection compared to using a single heterologous vaccine, Stooker said.
“When you’ve got more than one field strain circulating in a given region, a combination of vaccines that will help protect as well as cross-protect is needed,” he said.
For example, if the QX and 793B-type strains are circulating, a QX IBV vaccine used along with a Massachusetts (combination)-type vaccine would be a good choice. Controlled challenge studies have demonstrated that the combination of a live IB primer on day of age followed 2 weeks later by an IB QX vaccine gives “very strong cross-protection” against IB 793B, Stooker said.
The strategy for vaccination should differ depending on the type of chicken, he added. For broilers, Stooker recommended using live vaccines that are selected based on circulating field strains. However, for long-lived birds such as broiler breeders and layers, he recommended using different live strains before using an inactivated IBV vaccine to provide broad, strong protection. For flocks with a high IBV challenge, revaccination during lay should be considered.
 Stooker L, et al. Cross-protection capabilities of a combined vaccination program containing two live IBV vaccines against today’s most relevant field strains in the Europe, Africa and Middle East Region. 8th Symposium on ACOV & AMPV/2nd Meeting Cost Action 1207, Rauischholzhausen, Germany, June 2014.