Two chicken vaccines have recombined to produce more virulent viruses in Sydney and Melbourne, research has found, prompting the regulator to examine new controls over the approval and use of veterinary vaccines.
A study by a team from the Asia-Pacific Centre for Animal Health at the University of Melbourne and NICTA found that two different vaccine viruses used to control a chicken respiratory disease — the herpesvirus infectious laryngotracheitis (ILT) — have recombined, or crossed, to form two new virulent forms of the ILT virus.
The findings are published today in the journal Science.
As a result of the discovery, the Australian Pesticides and Veterinary Medicines Authority (APVMA) is considering new measures to regulate the use of vaccines in animals, the body said.
Australian strains of ILT vaccine were developed from the 1950s. In 2008, shortly after authorities approved a European strain for use, two new ILT strains were observed in outbreaks of disease in New South Wales and Victorian flocks.
Study author Joanne Devlin, a Doctor of Veterinary Science at the University of Melbourne, said the viruses emerged “mainly around the outskirts of Melbourne and Sydney, which is where a lot of our poultry production takes place”.
The first emerged in 2008, “and that was a year that we had around 33 cases, or outbreaks, of this disease. [The second] emerged in 2008 as well, but it wasn’t until 2010 where we saw a peak in outbreaks, and around 49 outbreaks in that year,” Dr Devlin told an online briefing for the Australian Science Media Centre. “Interestingly, these new strains of the virus emerged shortly after a new vaccine strain was introduced into Australia, and that was originally a European-origin vaccine strain. Until that point we’d only used Australia-origin vaccine strain to control this disease.”
The flock mortality rate was up to 17%, which amounted to several thousand birds, she said.
While recombination was previously recognised as a risk associated with live virus vaccines, the likelihood of it happening in the field was thought to be insignificant.
The current vaccines are considered effective even against the new recombinant strains of the infectious laryngotracheitis viruses, the research team found. But they said producers should be careful only to administer one ILT vaccine at a time.
“It’s relatively common for there to be multiple live attenuated vaccines like this used in animal populations, and we suspect that this sort of event could potentially happen in other animal species as well, with other viruses,” said co-author Glenn Browning, Associate Dean of Research and Graduate Studies in the Faculty of Veterinary Science at the University of Melbourne. “So we believe that what we’ve seen here has potentially got wider implications than just this particular disease in poultry.”
John Owusu, from APVMA, said the regulatory body would carry out a risk assessment of its entire live vaccine portfolio. “Some of the things we’ll be looking at will be the family of the virus, and with other vaccines, the group they belong to,” Dr Owusu said.
“We’ll be looking at whether or not the parent virus that was used to manufacture the vaccine is exotic or local. We’ll also be looking at whether there’s any published information about the vaccine having the potential to recombine. We will also have a look at issues such as the method of administration of the vaccine, the vaccinated population and its density.
"In the long term we think we’ve got to have tools that will allow us to assess new vaccines for the potential to recombine, and in doing so we will engage government departments such as the TGA, as well as industry, and build on the experience and expertise to develop new requirements that industry must fulfil when people offer new live vaccines.”
Ian Gust, a Professorial Fellow in the Department of Microbiology and Immunology at the University of Melbourne, said the equivalent of the ILT vaccine for people was the varicella vaccine, used to treat the varicella zoster virus, one of eight herpes viruses that affects humans and commonly causes chicken pox. “That vaccine is given to children, but it’s given individually,” Professor Gust told the online briefing.
“While there are two licensed vaccines available in Australia, both of them use exactly the same starting strain … of the varicella virus. It would be extremely unusual for a vaccine session in a doctor’s office to involve vaccines from two different manufacturers sequentially. Almost invariably the material comes from the same manufacturer.
"There are a large number of live attenuated vaccines used in man. They’ve been used for the last 50 years, and they’ve got an extremely good safety record.”