Experience with GA 08 provides valuable information about IBV variants in poultry
Experience with Georgia 2008 (GA 08) — the infectious bronchitis variant that’s caused major economic losses for poultry producers in the Southeast — has provided valuable information that will be useful when new variants emerge in the future, speakers said during a recent webinar sponsored by Zoetis Inc.
Reports of significant economic losses in Georgia broilers due to infectious bronchitis virus (IBV) started in January 2008, said Holly Sellers, PhD, professor, Poultry Diagnostic and Research Center, University of Georgia, Athens. Losses were due to secondary Escherichia coli mortality and, especially, high airsacculitis condemnations.
Testing showed the virus circulating was not serologically related to any known IBVs. The closest match was to Massachusetts (Mass) 41, but the similarity was only about 80%, she said.
Sellers and colleagues initiated several controlled studies to determine if existing commercial vaccines administered at day of age, with a boost at day 14, would protect flocks against GA 08. Protection was “at best” incomplete, she reported. In addition, partial immunity was not enough to offset field losses due to airsacculitis.
The prevalence of GA 08 was increasing in birds, including those vaccinated against IBV, and involved multiple companies in multiple states (Figure 1). The exception was a poultry company in 2009 that initially had the highest incidence of GA 08 but started using a live, attenuated autogenous vaccine that helped manage the disease, Sellers said.
The experience with GA 08 demonstrated that IBV variants have varying degrees of fitness in the field. “Many don’t hang around long, but every once in a while, one like GA 08 does and it causes problems,” Sellers said.
For some of these viruses, heterologous vaccine programs — vaccines from different serotypes — might provide some cross-protection, depending on the field situation. However, homologous vaccines — vaccines from the same serotypes — provide the best protection. Whether a new homologous vaccine should be developed for each new IBV variant that arises should depend on the geographic distribution, prevalence and economic impact — and after determining that current commercial vaccines or combinations of commercial vaccines afford little to no protection, she said.
The situation with GA 08 prompted Zoetis Inc. to develop a commercial vaccine for GA 08 that is now fully licensed by the USDA. Kalen Cookson, DVM, MAM, director of clinical research for the company, said producers have reported the vaccine has been instrumental in reducing airsacculitis condemnations and that when the vaccine went into use, the effect was “like flipping a light switch.”
GA 08 genetic drift
However, Cookson pointed out that new variants like GA 08 don’t always stay the same. They can change too, due to genetic drift. This raised the question of whether the Zoetis GA 08 vaccine would be as effective against a novel GA 08-like field isolate that emerged in Mississippi.
The GA 08-like isolate was 94% similar to the original GA 08 IBV S1 spike protein — the main part of these viruses that help characterize them. When two IBVs are less than 90% identical in that spike protein, they tend to be from different serotypes, Cookson explained.
To explore vaccine protection against the GA 08-like IBV, investigators at the Mississippi Veterinary Research and Diagnostic Laboratory, with support from Zoetis,
conducted an IBV challenge study in commercial broilers. They tested various vaccine combinations against the GA 08-like virus (Figure 2). In addition, birds vaccinated with
GA 08 were compared for their ability to protect against both the original GA 08 and the “drifted” GA 08-like isolate.
|Group||IBV vaccines administered on day of hatch||Day 20 challenge||Day 21 challenge||Number of birds|
|1||Mild Massachusetts + Arkansas combination||n/a||GA 08-like||10|
|2||Mild Massachusetts + Arkansas combination + GA 08||n/a||GA 08-like||10|
|3||GA 08||n/a||GA 08-like||10|
|4||Holland + Arkansas||n/a||GA 08-like||10|
|6||None||No challenge||No challenge||10|
|7||GA 08||GA 08||n/a||10|
Fig 2. Study design
In the study, birds were vaccinated at day of age by eye drop and challenged with the GA 08-like virus at 21 days of age. Five days after challenge, researchers determined the efficacy of the vaccine programs based on clinical signs, airsacculitis and the ciliostasis test. With real-time polymerase chain reaction testing, they also determined cycle threshold values, which is an indication of viral load.
The GA 08 vaccine given by itself resulted in the most consistent reduction in pathology and viral load after challenge. “What does this say about serotype competition for immunity and how many IBVs we can or should use?” Cookson asked. “Obviously, in most cases of high field challenge, it’s not practical to vaccinate with only one serotype, but we should try not to present more serotypes than we need.”