The epizootiology of Marek's disease: I. Incidence of antibody, viraemia and Marek's disease in six flocks.

Six flocks consisting of the female grandparent line of a well known broiler strain were studied in an effort to determine the important aetiological factors in Marek's disease. Each flock was hatched in the same hatchery but at intervals of six weeks. Chicks of each flock were reared to between 8 and 9 weeks of age on a rearing site after which they were moved to a production site. Mortality was recorded to house and pen in all flocks. In addition six pens of about 300 chickens on each of the production sites were selected for special examination. Blood samples were collected at intervals from birds in these pens for determination of the incidence of precipitating antibody and viraemia, and a high proportion of birds dying in these pens were examined post mortem. All six flocks were infected with Marek's disease virus. Although the levels of mortality from Marek's disease varied greatly from flock to flock (0.6 to 23.4%) and as greatly between houses used by a single flock, and between pens within a house, there was little relationship between the pathogenicity of the viruses present in a flock or group of chickens and the subsequent mortality from Marek's disease. Maternal antibody was present in 90.0% of day old chicks, 66.7% of chicks at 7 days of age, 23.9% of chicks at 14 days of age and 2.1 % of chicks at 21 days of age. Active antibodies were present in all flocks but the time at which they were detected varied from flock to flock and from pen to pen within a flock. Virus was not isolated from chicks between three and five weeks of age but was isolated from the blood of 174 of 179 chickens from 10.5 weeks of age and older. The data of this study are not adequate to determine with certainty the effect supply flock, rearing site and house, and production site and house had on mortality from Marek's disease, however they strongly suggest that 'the rearing site had the greatest influence. Because in most cases the chickens were primarily infected with Marek's disease virus at the rearing site, it is suggested that the virus with they are first infected is important in respect of subsequent mortality from Marek's disease.