Updating of epidemiological and pathological features of Marek's disease in laying hens and broilers

Marek's disease (MD) is a huge problem for researchers due to the significant losses in bird flocks, but more importantly, the virus's mutagenic potential. The purpose of this study was to describe non-classical gross lesions observed in broilers and laying hens that suggest the disease emergence and re-emergence. The survey was conducted on 10 broiler and 4 laying hen flocks. All of the dead birds were necropsied in order to obtain a comprehensive diagnosis of lesions, analysing both macroscopic and microscopic alterations. Marek's disease occurred in 80% of cases in broilers and 100% of cases in layer hens. The disease struck 26-day-old broilers and hens at 2 weeks of age, causing a total mortality of 6% and 15%, respectively. There were no clinical indications of the classical neurological form of the disease in either rearing type, and tumour lesions were mostly detected in the liver, spleen, and ovarian follicles in layers, and in the proventriculus in broilers. These findings demonstrated that MD is widespread and that its resurgence is primarily manifested in visceral rather than neurological manifestations. Despite MD immunization, biosecurity remains critical.