13.07.2012

Lee et al (2012) Science 337:188

Research from the University of Melbourne has shown that two different vaccine viruses- used simultaneously to control the same condition in chickens- have combined to produce new infectious viruses, prompting early response from Australia's veterinary medicines regulator. The research found that when two different ILT vaccines strains were used in the same populations, they combined into two new strains (a process known as recombination), resulting in disease outbreaks. combining of live vaccine virus strains outside of the laboratory was previously thought to be highly unlikely, but this study shows that it is possible and has led to disease outbreaks in poultry flocks.
To combat ILTV, chicken flocks are vaccinated with attenuated vaccines that can still infect and replicate but do not lead to disease. Australia has used two vaccines, which are produced by Pfizer and called SA2 and A20. In 2006, however, the country introduced a new vaccine from Intervet that contains an European strain: Serva. Two years later, new strains of ILTV, called class 8 and 9, appeared and were found to be responsible for most of the outbreaks of disease in New South Wales and Victoria (1). The new viruses were actually stitched together from the European and Australian vaccines.   Australian Pesticide and Veterinary Medicines Authority (APVMA) are now to determine both short and long term regulatory actions.
The rapid emergence of two virulent recombinants suggests that recombination between attenuated herpesvirus vaccines and resultant restoration of virulence can be rare but can bring about a fitness advantage, with severe consequences. The findings from this study raise concerns about the use of multiple distinct attenuated herpesvirus vaccines under conditions that favor recombination. These findings have implications for the use of herpesvirus, and possibly other DNA viruses as attenuated vaccines or vaccine vectors.

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 1. Blacker et al (2011) Epidemiology of recent outbreaks of infectious laryngotracheitis in poultry in Australia. Aust Vet J 89:89