Marek's disease in field chickens: correlation between incidence of Marek's disease and nuclear-inclusion formation in the feather-follicle epithelium.

The present study confirmed that Marek's disease (MD)-associated nuclear-inclusion (NI) formation in the feather-follicle epithelium (FFE) is related to mortality from MD; it also presented useful data on the epidemiology of MD in HVT-vaccinated field chickens. Incidence of NI formation in the FFE of chickens on six rearing farms varied greatly by age and flock, but most of the field flocks showed biphasic peaks of incidence of NI in chickens consisting of a small peak at an early age (usually at 2-4 weeks of age) and a large peak between 13 and 16 weeks of age. MD tended to occur in chickens over 20 weeks old, and almost all MD-affected chickens showed NI formation persistently in the FFE, usually between 13 and 20 weeks of age. Chickens that were healthy at the end of observation showed either transient NI formation, usually between 13 and 16 weeks of age, or no detectable NI formation. Incidence of NI formation in the FFE of chickens at 19-20 weeks of age was related to mortality from MD: chickens with NI formation had a MD mortality rate of 66.7%, whereas chickens without NI formation had MD mortality of only 0.8%.