July 13, 2012

Deadly Poultry Virus Caused By Gene-Swapping Vaccines

Brett Smith for - Your Universe Online

Australian scientists looking to vaccinate chicken populations against a respiratory disease may have accidentally unleashed a disease far more deadly than the one they hoped to prevent.

According to a report published this week in Science, the genomes from two different strains of the herpesvirus infectious laryngotracheitis (ILT) virus that were used in vaccines have recombined to produce more virulent ILT viruses near Sydney and Melbourne.

"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," said lead author Joanne Devlin from The University of Melbourne, Parkville.

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

Australian strains of ILT vaccine were first developed in the 1950s, but several problems were associated with their use, including the potential for the virus to lie dormant in a vaccinated bird until it can spread to unvaccinated populations.

In 2006, Australian officials purchased a European strain of the ILT vaccine, which was then used to combat the virus. Two years later, deadlier strains of ILT began showing up in flocks. While the original strains typically killed 5% of the chicken population, the two new strains were killing up to 17% of chicken populations.

At first, scientists theorized that the new vaccine´s weakened virus might have reverted back to a disease-causing form. However when the researchers sequenced the genomes of the viruses found in infected birds and the three vaccine strains; they realized that the new forms of the virus were composites of the European and Australian strains.

In their report, the research team attributed the recombination events to the fact that live viruses were used in the vaccines. Live, or attenuated, vaccines use a weaker version of the virus to cause the host´s immune system to build up its own defenses. They are commonly used for both animals and humans, and include vaccines for polio, measles, mumps, rubella, chicken pox and rabies.

“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 of 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.”

Despite the unique genetic recombination that occurred in Australian chicken flocks, scientists said there is no need for alarm regarding human immunizations.

Ian Gust, a professor from Department of Microbiology and Immunology at the University of Melbourne, said the equivalent of the ILT vaccine for people was the varicella vaccine, which is used to prevent the disease that causes chicken pox.

“That vaccine is given to children, but it´s given individually,” Gust said.

“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.”