Promising results in the search for a more efficient and effective vaccine against Marek’s disease (MD) have been claimed by scientists at The Pirbright Institute.
Using a recombinant (new combinations of genetic material) adenovirus which carries a single gene from a virulent strain of the MD virus (MDV), the Pirbright team are hopeful that further research and trials could lead to the production of an effective vaccine that will be cheaper and easier to produce and which they say will “crucially” have no possibility of reverting to a virulent strain.
Dr Susan Baigent and colleagues from the Institute’s Avian Oncogenic Virus group, led by Professor Venugopal Nair, began by examining the efficacy of using non-replicating adenovirus expressing MDV envelope glycoprotein (Ad5-gB) as a potential MD vaccine in chickens.
They then compared the experimental adenovirus with a clone of the classic ‘gold standard’ MD vaccine (pCVI988), measuring levels of protection against the disease and levels of shedding and transmission of virulent virus. A first vaccination was administered into the egg three days before hatching and a second vaccination post-hatch, before the chickens were challenged with a virulent strain of MDV.
The results, published in the Journal of Veterinary Medicine and Research, showed that the double-dose of Ad5-gB vaccine was comparable to pCVI988 in its ability to significantly reduce replication of virulent MDV, and to provide 100% protection against mortality and disease. However, although the double-dose Ad5-gB vaccine delayed the onset of shedding of virulent MDV, it did not prevent shedding and was also less effective than pCVI988 at reducing shedding and the transmission of virulent virus.
“Although it was slightly disappointing that the Ad5-gB vaccine did not significantly reduce transmission or shedding, it is very encouraging that this vectored vaccine was as effective in protecting birds against disease as the current live vaccine, and resulted in lower levels of virulent virus in infected birds’ blood when given as a double dose,” said Dr Baigent.
“What we don’t know is whether a single dose post-hatch would be as effective as a double-dose or whether using a higher dose of Ad5-gB vaccine would be more effective in reducing shedding and transmission.
“Clearly further research is needed on optimising the dose and time of vaccination in order to begin trials of Ad5-gB as a potential vectored vaccine candidate for MD.”