Studies leading to sustainable strategies for the control of Marek's disease: Is vaccination responsible for virulence evolution in Marek's disease?

Marek's disease virus is a cancer-causing virus of chickens which causes global losses of $US1 billion/year, considerable animal suffering, and which puts at risk free-range poultry operations and possibly wildlife. In commercial poultry operations, the virus has evolved substantially greater virulence over the last fifty years, with previously unseen hyperpathogenic strains now circulating. These rapidly kill unvaccinated birds. It seems extremely likely that something people did to the MDV-chicken interaction prompted this virus evolution; here we seek to find out what. This is an important issue for the poultry industry, which needs to assess the likely direction of future MDV virulence and the evolutionary risk associated with alternative control strategies such new vaccines and genetically enhanced chickens. Preventing further virulence evolution, and even reversing it, would be highly desirable on commercial, ethical and possibly wildlife conservation grounds. While there is little doubt that MDV virulence evolution has occurred, there is very little understanding of which of environmental conditions changed by the poultry industry has been responsible for this evolution. A leading possibility is that it is vaccination. MDV has been subject to vaccination since the late 1960's. MDV vaccines are not sterilising, so that vaccinated birds transmit virus they acquire from their environment. Such 'leaky' vaccination makes viral evolution in immunised populations possible. Theoretical models show that leaky vaccines can prompt virulence increases; the intuition here is that by protecting hosts, vaccines keep virulent pathogens from killing their hosts and therefore themselves. Thus, where they would previously have been removed by natural selection, they can circulate in vaccinated populations. In this proposal, we aim to determine whether this scenario can explain MDV virulence increases. The experimental work will involve comparisons of the evolutionary fitness of MDV strains of varying virulence in vaccinated and unvaccinated birds, and experimental evolution of viral strains through vaccinated and unvaccinated birds. The subsequent data will enable direct testing of the vaccine hypothesis, and other possible explanations of MDV virulence evolution. This will directly contribute to MDV management, but more broadly, will determine whether there are other diseases - agricultural and human - where widespread use of leaky vaccines could lead to the evolution of pathogen strains which put the unvaccinated at greater risk.

We propose a combination of single-generation and multi-generation experiments to test the hypothesis that the prolonged and widespread use of imperfect (leaky) vaccines has been responsible for the increasing virulence of Marek's disease virus over the last half century. Viral strains inducing the symptoms originally described by Marek can no longer be isolated in intensified commercial poultry operations in the US and Europe, having been replaced with hyperpathogenic strains which induce very rapid morbidity and mortality. The fundamental data required to determine whether vaccination is responsible for this evolution are the aim of this proposal. We will (a) determine the most appropriate measures of evolutionary fitness for MDV, using a combination of transmission experiments and quantitative PCR of feather dust, feathers, skin and blood, (b) compare the fitness of strains varying in virulence in vaccinated and unvaccinated birds, and (c) experimentally evolve an MDV strain through vaccinated and unvaccinated birds. The data we obtain from these experiments would also enable the testing a number of explanations for virulence evolution. If the vaccine hypothesis is correct, we will find that vaccination reverses the relative fitness of mild and virulent strains of MDV, and that virulence evolves upwards more rapidly in vaccinated birds. This will directly contribute to MDV management, but more broadly, will determine whether there are other diseases - agricultural and human - where widespread use of leaky vaccines could lead to the evolution of pathogen strains which put the unvaccinated at greater risk.