Recent molecular studies using state-of-the-art technology may explain why an infectious bronchitis (IB) vaccine based on the Ma5 strain provides better protection in poultry than a vaccine with an H120 strain.
“We’ve known, based on performance data from poultry producers, that protection against an IB field challenge is better when an Ma5 vaccine is used compared to the traditional H120 vaccine strain, even though both strains are from the Massachusetts IB serotype," says Laura Villarreal, DVM, technical manager, Intervet/Schering-Plough Animal Health, Latin America.
“Our aim was to discover if there is a molecular basis for this difference in performance," says the veterinarian, who was a co-investigator in a study led by Paulo Brandaáo, DVM, and Carolina Torres, DVM, Universidade de Saão Paulo.
Using sophisticated testing methods, including RNA extraction, real-time poly- merase chain reaction and gene sequencing and analysis, the researchers found that the IB Ma5 strain in the vaccine Nobilis IB Ma5 has structural differences in the S1 subunit of the spike protein compared to an H120 IB strain.
The S1 subunit of the spike protein, Villarreal explains, is directly involved in the stimula- tion of neutralizing antibodies and, thus, protection.
Although changes in the S1 subunit can occur, she notes, the rest of an IB virus strain remains largely unchanged. This is why some strains of IB cross-protect against other IB strains.
"The differences found in the S1 subunit of the Ma5 strain are thought to be the result of amino acid mutations that might have actually helped the strain to become more im munogenic and more effective as a vaccine,â€Villarreal says.
"Even though both strains belong to the Massachusetts serotype, they had different selective pressures during their evolution," she adds.
However, the investigators found no differences in the clones of both strains used for sequence analysis, which indicates that the strains tested were stable and would remain stable in a vaccine formulation.
Moving forward, investigators will perform in vivo studies to expand on the findings of the current study, Villarreal says.