GENETIC RESISTANCE AND SUSCEPTIBILITY TO MAREK’S DISEASE

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Abstract
Three thousand three hundred and thirty-five sire-pedigreed chicks from two commercial lines of Single Comb White Leghorns were inoculated with the Marek’s disease (MD) agent at one day of age and tested over a seven-week period for MD infection. Significant differences were observed between sexes and between lines with respect to MD mortality and total MD incidence. The females were more susceptible. Sire families in both lines showed heterogeneity in their ability to resist MD infection.A second experiment involving selection for resistance and for susceptibility to the disease was conducted on a population of University of Guelph Columbian Plymouth Rocks. Fully pedigreed chicks from this stock were inoculated and reared for 11 weeks while their full-sibs were reared separately to produce subsequent generations of selected families. Two successive generations of selection resulted in the formation of the resistant (R) and susceptible (S) lines. A random sample of the base population was propagated as a control population. Selection for MD resistance resulted in relatively greater progress than selection for MD susceptibility. After two generations of selection there was 12.5%, 54.1% and 61.9% MD mortality and 58.3%, 89.1% and 96.2% total MD incidence in the R, control and S lines, respectively. The more resistant the strain, the longer was the latent period before death. The variability between sexes and between half-sibs declined as MD mortality and total MD incidence approached high and low extremes. The results of the selection imposed showed progress despite the small population under test.