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.
For more information please contact communications@pirbright.ac.uk Tel: +44 (0) 1483 231417.
About The Pirbright Institute
The Pirbright Institute is a world leading centre of excellence in research and surveillance of virus diseases of farm animals and viruses that spread from animals to humans. Based in the UK and receiving strategic funding from the Biotechnology and Biological Sciences Research Council (BBSRC), the Institute works to enhance capability to contain, control and eliminate these economically and medically important diseases through highly innovative fundamental and applied bioscience.
With an annual income of nearly £28.5 million from grants and commercial activity, and a total of £11.8 million strategic investment from BBSRC during 2015-2016, the Institute contributes to global food security and health, improving quality of life for animals and people.
About BBSRC
The Biotechnology and Biological Sciences Research Council (BBSRC) invests in world-class bioscience research and training on behalf of the UK public. Our aim is to further scientific knowledge, to promote economic growth, wealth and job creation and to improve quality of life in the UK and beyond.
Funded by Government, BBSRC invested £473 million in world leading bioscience, people and research infrastructure in 2015-16. We support research and training in universities and strategically funded institutes. BBSRC research and the people we fund are helping society to meet major challenges, including food security, green energy and healthier, longer lives. Our investments underpin important UK economic sectors, such as farming, food, industrial biotechnology and pharmaceuticals.
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