Genetic Resistance to Leukosis Caused by the JM Virus in the Fowl

THE avian leukosis complex represents the greatest single cause of mortality in the domestic fowl. Estimates of the annual cost of this group of diseases to the poultry industry are as high as $200,000,000. This is largely due to condemnations of 7 to 9 week old broilers in the dressing plants and to mortality throughout the growing period and thereafter. Recent work has demonstrated that there are different types of leukosis, each resulting from different viruses or transmissible agents. At the present time there appears to be two main groups responsible for most of the field cases of leukosis. Using the classification proposed by Sevoian (1967), we refer to these as Type I (or lymphoid leukosis) and Type II (or Marek’s disease). At least two sub-types (A and B) of the “RIF-type” viruses referred to as Type I leukosis have been isolated, while a third sub-type (C) has been suggested…