Abstract
A programme to develop resistance to Marek's disease (MD) in chickens in two non-inbred White Plymouth Rock (WPR) lines by breeding from survivors was initiated in 1968 and since then nine generations have been produced. In each generation only sires and dams which survived heavy exposure to virulent MD virus (MDV), Dutch strain K, either by inoculation (for the first two generations) or by contact-exposure (for the following seven generations) were used. In this long term trial a high level of resistance to MD was reached within five generations; thereafter no marked further increase in resistance could be obtained. Susceptibility to MD, reflected by MD mortality in each generation, was expressed as % incidence of MD. Statistical evaluation of MD susceptibility in the nine generations of the two lines revealed a significant interaction between MD mortality and two sources of variation, sex and generation. No overall line effect was found, but there was a significant line x generation interaction. The remaining interactions were not significant. Results indicate that breeding from survivors for development of resistance to MD is feasible. Exposure of birds to either artificial (by injection) or contact challenge with virulent strain K MDV changed lines of birds highly susceptible to MD into highly resistant lines in a few generations. Complete resistance to MD was not attained.
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