"Over the past few decades, Marek's disease has become much more virulent—which some researchers believe is the result of vaccination.
Read and researchers at the Pirbright Institute in Compton, U.K., infected chickens with Marek’s disease virus of different strains known to span the spectrum from low to high virulence. When the birds weren't vaccinated, infection with highly virulent strains killed them so fast that they shed very little virus—orders of magnitude less than when they were infected with less virulent strains. But in vaccinated birds, the opposite was true: Those infected with the most virulent strains shed more virus than birds infected with the least virulent strain."
"Over the past few decades, Marek's disease has become much more virulent—which some researchers believe is the result of vaccination.
Read and researchers at the Pirbright Institute in Compton, U.K., infected chickens with Marek’s disease virus of different strains known to span the spectrum from low to high virulence. When the birds weren't vaccinated, infection with highly virulent strains killed them so fast that they shed very little virus—orders of magnitude less than when they were infected with less virulent strains. But in vaccinated birds, the opposite was true: Those infected with the most virulent strains shed more virus than birds infected with the least virulent strain."
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