It is well known that if just one virus in an IB
vaccine is administered, birds will be protected
against that particular virus but not other IB
viruses, Jackwood said.
However, it is also now known that some IB serotypes have cross-protective ability against other IB serotypes — a phenomenon that scientists call “Protectotype.” In addition, when two different IB serotypes are used, birds will develop immunity not only to those serotypes, but they will develop cross-reacting antibodies to some of the other IB serotypes. “We get a broader type of protection,” he said.
An approach toward IB control involving
Protectotype is largely based on work by Jane
Cook, PhD, a microbiologist in the UK and
recognized authority on IB. She points out that
IB variants arise frequently and that Protectotype
is less costly and makes more sense than developing
a new vaccine for every variant that arises.
Even though the IB virus is capable of frequent
mutations, she says, the genetic makeup of the
virus changes only minimally and the rest remains
the same, which is why some IB serotypes are
able to provide cross-protection.
In Europe and elsewhere, research and field experience have demonstrated that using an IB vaccine based on the Massachusetts serotype followed by one based on the IB variant 4/91 — a common variant in Europe — can provide broad protection against many IB variants.
Substitute Strain for US
In his WVPA presentation, Jackwood said, “We
don’t have 4/91 in the US and it’s not permitted
for good reason; the major problem we have
is the Arkansas IB strain. We’re asking what can
we substitute for 4/91?”
Toward this end, Jackwood spent a week in Europe with Sjaak De Wit, DVM, PhD, at the Animal Health Service, Deventer, the Netherlands. “We vaccinated specific-pathogen-free birds with Ma5 at 1 day of age and with 4/91 at 14 days of age, then challenged birds with the US Arkansas or Georgia 98 IB strains,” he continued.
Five days after challenge, birds that had received both the Ma5 and 4/91 vaccines had about 90% protection against the Arkansas IB challenge, and there was 86% protection against challenge with the Georgia 98 strain, which is a Delaware-type virus, he said.
In birds vaccinated only with Ma5, protection against the Arkansas IB challenge was also determined to be about 90%; this was higher than expected, but as it turned out, the challenge was weaker than intended. As expected, protection against the Georgia 98 IB challenge was only about 37% in birds that received only Ma5, Jackwood continued.
Protection was determined by scoring ciliary activity in the tracheal epithelium, he noted.
For the US market, Jackwood and colleagues are
going to try substituting IB 4/91 with the Delaware
072 IB strain because it is distinct and may provide
a broader set of antibodies. The studies are underway
and Jackwood hopes to have data to share in
the near future.
Jackwood explained that this work involves determining genetic similarities between viruses, particularly regarding a spike protein on the outside of viruses, and virus neutralization testing. “The spike protein is the one that induces neutralizing antibodies. If we get an amino acid sequence similarity between spike genes in the high-80% or low-90% range, there will usually be some cross-protection.”
The researchers are also carefully examining the significance of ciliostatis scores and want to find out if protection against ciliostasis correlates with protection in the field. “We know it does in Europe; we want to see if the same holds true here with US IB viruses,” he said.
Jackwood is confident that progress can be made. In an interview, he said, “We already have a lot of really good vaccines available to us. I think that we can do a better job of actually applying those vaccines and getting a little broader protection using a protocol involving Protectotype.”