Scientists at The Pirbright Institute have been encouraged by the results from a potential new vaccine candidate against Marek’s disease (MD). Using a recombinant (new combinations of genetic material) adenovirus which carries a single gene from a virulent strain of Marek’s disease virus, they are hopeful that further research and trials could lead to the production of an effective MD vaccine that is cheaper and easier to produce - and crucially has no possibility of reverting to a virulent strain.
Marek’s disease virus (MDV), is a highly contagious airborne pathogen that infects poultry, costing the industry around £1 billion a year. MD is currently controlled through vaccination and over 20 billion vaccine doses are administered annually worldwide.
‘Classical’ MD vaccines are live non-virulent viruses and are highly protective against mortality and disease. However, there are drawbacks due to the biological characteristics of the virus, including the requirement for a cold chain - the need for continued refrigeration in order to maintain vaccine virus viability when transported, stored and used.
The live virus vaccine, though effective in protecting against disease, does not prevent infection and replication of virulent field strains of MDV, so a vaccinated chicken will shed virulent virus, and therefore has the potential to infect other naïve (unvaccinated) birds. More worryingly, recent research supports the view that vaccination against MD could be driving the evolution of more virulent field strains.
As industry pressure increases to develop more efficient and effective vaccines, scientists are looking to vector-based vaccines that do not require a cold chain, are easier and cheaper to produce, and are more easily distinguished from the pathogenic virus. These type of vaccine also have no risk of reverting to a virulent form, which means they will not cause disease in susceptible birds.
Dr Susan Baigent and colleagues from The Pirbright Institute’s Avian Oncogenic Virus group (led by Professor Venugopal Nair), examined the efficacy of using non-replicating adenovirus expressing MDV envelope glycoprotein (Ad5-gB) as a potential Marek’s disease vaccine in chickens. Recombinant adenoviruses have already been proved effective in mammals and can be grown in a continuous cell line as opposed to primary cell cultures, making them cheaper and easier to produce.
The scientists 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 (in ovo) 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 transmission of virulent virus.
Dr Baigent said: “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.
“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 Marek’s disease.”
The recombinant adenovirus vectored vaccine was developed at the Jenner Institute Oxford, as part of the ongoing research collaboration with The Pirbright Institute.