Can improper vaccination make IBV worse?
Improper vaccination not only fails to protect flocks from infectious bronchitis virus (IBV), but it can also make the problem worse, a leading poultry health expert says.
Speaking at the 2017 World Veterinary Poultry Association meeting, the University of Georgia’s Mark Jackwood, PhD, stressed that vaccination is critical to stopping the cycle of IBV infection, replication and mutation that leads to the emergence of new strains. By failing to fully block infection, however, improper vaccination can worsen matters by enabling the development of new variants, for which no vaccines may be available.1
“The bottom line is to vaccinate properly. With good surveillance and proper application, IBV vaccines work very well,” Jackwood said. “But if the vaccine is applied improperly — or if it’s ineffective against the dominant field strains or induces only partial protection — it can actually allow new strains to emerge and make IB even harder to control.”
Vast genetic diversity
Infectious bronchitis is a highly contagious disease of chickens and a severe economic burden on the poultry industry worldwide. The coronavirus that causes it mainly affects the upper-respiratory tract, but some strains also cause kidney lesions that lead to nephritis. In layers, the virus can replicate in the reproductive tract, resulting in egg quality and production losses.2
According to Jackwood, the vast genetic diversity of IBV is largely due to the imprecise manner in which RNA viruses replicate. Among RNA viruses, IBV is a particularly poor genetic “proofreader,” resulting in frequent mutations that can accumulate and lead to the evolution of new and more virulent strains.
“The strategy of RNA viruses is to create a lot of genetic diversity so that in any host environment they encounter, some part of the population can survive and flourish,” he said. “For a new IBV variant to become predominant in the field, it must have a selective advantage that allows it to evade immune response and replicate.”
To help producers control infections by new strains, researchers and vaccine manufacturers are constantly testing vaccine combinations that may provide cross-protection, a strategy that has proven effective with a number of IBV variants, such as Italian-02.3 When combinations of existing vaccines fail to cross-protect, new vaccines may be developed for especially prevalent or virulent strains, such as the QX variant in Europe, the Middle East and China, and the Georgia-08 variant in the US.
However, with new strains emerging all the time — many of which never lead to widespread problems — it is impossible to develop vaccines for each new variant. For this reason, Jackwood said, the best policy is to limit factors that contribute to the development of new strains.
IBV can only replicate inside the cells of hosts, so the best way to prevent the emergence of new variants is to stop the virus from infecting birds in the first place, Jackwood emphasized. Vaccination plays a key role in preventing infection, but the efficacy of vaccine programs depends on careful planning and administration, he stressed.
“Successful vaccination programs start with surveillance,” he said. “You need to know what’s out there so you can choose vaccines based on what’s circulating right now.”
Surveillance programs should include both passive and active techniques, Jackwood advised. “Passive techniques, such as routine diagnostics on diseased flocks, are telling because if we see a particular virus coming in on numerous sick birds, it’s a clue to pay attention to it. Then we can use active techniques to go out into the field and see how widespread it is.”
It’s also important to monitor for when viral populations start shifting, he added. “New viruses start to emerge because of vaccine failures. Outbreaks in the face of vaccination serve as a warning to make sure we’re doing a good job with our vaccine programs and to pay attention to new strains that might arise.”
If field strains are present that are not covered by commercially available, homologous vaccines, the first step is to identify which of those vaccines cover phylogenetically similar viruses, Jackwood said, and then test combinations of those vaccines for efficacy against the new strains.
“Sometimes, vaccine combinations work well enough in the field to achieve good control, but other times not,” he said. “For example, the DMV-1639 strain — which is causing lots of problems in certain areas of the US — isn’t responsive to any of the combinations of commercially available vaccines that we tested.”
He also recommended using multiple vaccine antigens to achieve broader protection. The total number recommended for each bird will vary based on the IBV strains prevalent in the field, as well as the bird’s lifespan. While two or three vaccine antigens may be sufficient to protect broilers, replacement pullets may require a total of four or more antigens to protect them against the wider range of strains they may be exposed to during their longer lives.
To ensure an adequate immune response, however, Jackwood advised administering no more than three vaccine antigens at a time.
“When you administer four or more vaccine antigens at once, you can get a less robust immune response against one or more of the vaccines,” he warned. “If the antigens are too similar, the bird’s immune system may not be able to distinguish them. If there are too many, the bird’s immune system doesn’t know which ones to respond to, so it tries to respond to all of them and the response is diluted.
“If more than three vaccine antigens are required, it is better to space them apart a few weeks and follow up with an inactivated booster.”
Dosing and administration
Correct vaccine dosing is also paramount, Jackwood added.
“A lot of people cut the dose to avoid strong vaccine reactions, but low vaccine reaction usually also means weak immune response,” he said. “If we want a strong immune response, we have to put up with some vaccine reaction. When the dose is cut, that allows field viruses to continue to replicate and mutate, leading to more genetic diversity and new viruses.”
Once the vaccine is prepared, proper administration is critical to ensuring adequate protection, he emphasized.
“Vaccine temperature is very important, as the vaccine is fragile and must be kept cold to maintain full titer,” he said.
“The vaccine can also be damaged in spray cabinets if they aren’t well maintained and working properly.
“Nozzle selection and droplet size affect the uniformity of coverage in the chick basket. Humidity in the hatchery should also be carefully monitored, as it affects the rate of evaporation when the vaccine is administered.”
In conclusion, Jackwood underlined the importance of following vaccination protocols carefully.
“Commercially available vaccines are rigorously tested for safety and efficacy against specific field strains of IBV. But even the best vaccine can fail if vaccination methods aren’t strictly followed. And when vaccines for IBV fail to prevent replication of field viruses, it can result in the emergence of new strains capable of causing disease.”
1. “Infectious bronchitis virus: Genetic diversity and challenges for control.” XXth WVPA Abstract Book, 2017; 43