New variants of infectious bronchitis (IB) will continue to emerge, but currently available vaccine programs can control many of them, says poultry consultant Jane Cook, BSc, PhD, an authority on this highly infectious disease.

“Do we need a new vaccine for every different serotype or genotype that’s found? I think the answer to that is most definitely no, we don’t,” says Cook, who is based in the UK.

Cross-protection against the different variant strains of IB virus that arise is much broader than the results of laboratory tests suggest, she says, emphasizing that using vaccines of two different serotypes is probably the best approach for producers who want good IB control. IB causes disease and economic losses in all parts of the world that have a developed poultry industry. It affects chickens of all ages, type and breed and it is unlikely that this highly infectious disease will ever be eradicated, she points out.

In broilers, IB causes respiratory infection that is often complicated by secondary bacterial infection, leading to high morbidity and variable mortality; renal damage resulting in morbidity and mortality may also occur.

In layers and breeders, IB is associated with a drop in egg production — one that may never return to the pre-infection level. The virus also can lead to poor eggshell quality, color and internal egg quality (watery whites). Respiratory infection may occur, but its importance is minimal compared to the effect of IB on egg-laying performance, Cook says.

Different serotypes of IB appear either by spontaneous mutation or by recombination, which may occur when two different virus particles are in the same chicken cell at the same time, she says.

Connecticut, the first IB virus variant, was identified in the 1950s, and others followed. The traditional way to detect and identify IB variants was by isolating the virus, then by serotyping using crossneutralization tests, which are expensive, time-consuming and difficult to do. However, since the advent of molecular methods for genotyping, such as the reverse transcriptase polymerase chain reaction (PCR) — a technology that gives results much more quickly than previously available diagnostic methods — many more variants have been identified.

Most variants do not survive, but some, such as Arkansas or 4/91, do survive and become economically important, Cook explains. However, because the techniques are so sensitive, caution must be applied when performing the test and when interpreting results.

A potential problem with molecular methods is illustrated by experience with the IB-variant Italian 02. This variant is easily detected by PCR testing, has a wide distribution and is often impossible to isolate, but it is rarely associated with disease. In contrast, the IB-variant QX is easily detected by PCR, can frequently be isolated and is associated with disease.

“So, we have a PCR technique that is very easy and widely used to detect these viruses,” she explains, “but caution is required because sometimes a situation may arise where there is a much better correlation between isolation of the virus and disease than there is between genotyping by PCR and disease.

“PCR is a very useful technique, but interpretation needs a lot of care,” she continues. “Basically, if you can’t actually isolate the virus and you can’t associate the PCR results with disease, then is the virus important?”

Cook suggests using two different types of vaccines for IB control — a recommendation she bases on independent, controlled studies that she and colleagues conducted.¹

Investigators used an Ma5 vaccine, which is of the Massachusetts serotype and highly immunogenic, and the 4/91 vaccine, which is antigenically different from Ma5. They gave chickens either no vaccine, the Ma5 vaccine alone at 1 day of age, the 4/91 vaccine alone at about 2 weeks of age or the Ma5 vaccine at 1 day of age followed by 4/91 at 2 weeks of age. The birds were challenged at 5 weeks of age with different variants from different countries. Protection was determined by scoring ciliary activity in the tracheal epithelium.

Generally, birds that received both vaccines 2 weeks apart had the best protection against challenge with variant IB virus strains, she reports.

While it would be more convenient to give the two IB vaccines together at 1 day of age, Cook says this isn’t always the most effective.

“If the challenge is homologous, then protection is good if the two vaccines are given together at 1 day old. But if the challenge is with a different serotype or genotype of virus, then it is beneficial to leave about 14 days between the two,” she says.

However, if challenge is anticipated to occur at a very early age, or if the other vaccines included in the program make any other option difficult, application of both vaccines at 1 day old may well provide good protection.

“Sometimes,” Cook adds, “you find a challenge strain where protection is not really good, even using this combined vaccine program. But I must say, they are rare, and on most occasions, you certainly see an enormous benefit from the use of these two vaccines in the program.”

Asked why producers should use the Ma5 vaccine instead of the commonly used and less expensive H120-strain vaccine, Cook says, “H120 may control IB problems and, if so, well and good! If not, then it is worth trying Ma5, which is strongly immunogenic and provides good, broad protection. If IB is a major problem, the cost of an Ma5 vaccine becomes less of a factor to consider.”

Cook also cautions that producers vaccinating against IB virus at 1 day of age should wait 7 to 10 days before vaccinating against avian pneumovirus because IB and pneumoviruses can compete for the same receptors in the respiratory tract and interfere with one another.

_{¹ Cook, J.K.A., et al. Breadth of protection of the respiratory tract provided by different live-attenuated infectious bronchitis vaccines against challenge with infectious bronchitis viruses of heterologous serotypes. Avian Pathology 1999:28;477-485.}

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