Marek's disease (MD). XX. Incidence and epizootology of MD in broiler flocks.

The frequency of Marek's disease in broiler chickens as measured by gross evidence of tumors at processing differs markedly among flocks and between poultry farm origin. Organs affected in broilers, in order of frequency of tumors, were proventriculus (57.6%), spleen (42.3%), kidney (30.6%), lung (19.1%), liver (15.3%), gonads (12.3%), heart (7.6%). Virus isolations were performed on samples from three broiler flocks to examine the possibility that such differences in MD incidence were causally related to differences in the frequency of naturally occurring, nononcogenic serotype 2MD virus infection. Serotype 2 virus was isolated from two flocks. MD condemnations were lower in two serotype-2 infected flocks (V1) than in the third one serotype 1 and serotype 2 positive flock (Mh). Thus, these data suggest a strong positive association between MD losses, housing and a weak probable negative association between serotype 2 virus and MD losses in broilers. However, low level of leukosis condemnations in certain flocks from the same farm are not related to high serotype 2 virus exposure.