News in Science

Chicken vaccines combine to create deadly virus

Lethal mix Australian scientists have found that two different vaccines used to control an infectious disease in chickens can recombine to create new lethal virus strains.

The research, to be published today in the journal Science, has prompted authorities to review vaccine use in animals.

But the scientists, from the University of Melbourne, say the findings are not only important for vaccines in chickens, but also for any vaccine which might be able to multiply - including those used in humans.

Chicken respiratory virus ILT can lead to birds dying in a pool of bloody mucus, but vaccines that were meant to protect them have instead led to their deaths.

Dr Joanne Devlin, a lecturer in Veterinary Public Health-Epidemiology at the Asia-Pacific Centre for Animal Health, says the deaths were caused when two vaccines used to treat the virus combined.

"These new strains were formed by recombination from the different vaccine strains and that they were actually more virulent than the vaccine strains that gave rise to them," she says.

"This is something we've never before seen before in the field."

Live vaccines, where a weaker version of the virus is introduced to allow the immune system to build up its own defences, are quite commonly used for animals and humans, and include polio, measles, mumps, rubella, chicken pox and rabies.

The Australian Pesticides and Veterinary Medicines Authority is now consulting with industry to control the use of these vaccines.

'Wider implications'

Co-researcher Professor Glenn Browning says other creatures could also be under threat of vaccines combining.

"We suspect that this sort of event could potentially happen in other animal species as well and with other viruses in addition to infectious laryngotracheitis virus," he says.

"So we believe that what we've seen here has potentially wider implications than just this particular disease in poultry."

But the researchers are convinced there is no potential for humans to be affected by these chicken respiratory viruses, and current vaccines for humans are unlikely to combine.

Professor Ian Gust, from the Department of Microbiology and Immunology at the University of Melbourne, says the experience with bird flu, and fears it might recombine with human flu, bears this out.

"The one in which you would be particularly vigilant about are the possibilities of recombination is for live attenuated influenza vaccine... and measures are taken to reduce any possibility of that occurring," he says.

"The timing at which the vaccine is administered, that is out of season from the circulation of flu viruses, and the fact that it is not administered to people who might already be ill or might, or people who might shed the virus for a prolonged period of time [reduce that risk]."

Professor Gust says the failure of viruses, such as the virulent H5N1 bird flu and human flu, to combine and attack the world's population should be reassuring.

"If you take both the northern and the southern hemisphere into account, we've had 20 flu seasons since the emergence of that virus," he says.

"We've got a global population of about seven billion people and it hasn't happened.

"It is not per se that it can't happen on theoretical grounds, but you'd have to say that the chances are probably about the same as being kicked to death by a duck."