FIELD INVESTIGATIONS ON MAREKS-DISEASE IN COMMERCIAL PULLET AND LAYERFLOCKS

Increase in MD losses in vaccinated commercial pullet and layer flocks has been observed since the end of 1993. Lymphoid rumours in the prov entriculus, spleen and liver are the most pronounced macroscopic chang es. The mortality rates vary according to the chicken lines between 0. 3 and 28.8%. Generally one commercial white leghorn strain was more fr equently affected than another white one and a brown one. Vaccination trials using Rispens strain either alone in single or double doses or in combination with turkey herpesvirus vaccine (HVT), or even revaccin ation has shown insufficient protection results in this white line. An increase in the virulence of the MD field isolates or a decrease in t he genetic resistance of chicken stocks are discussed as possible caus es.