Indexed on: 01 Oct '74Published on: 01 Oct '74Published in: Avian pathology : journal of the W.V.P.A
Virus clones from 2 Marek's disease virus (MDV) isolates from flocks with low natural incidences of Marek's disease (MD) were, at most, only mildly pathogenic compared to clones from the highly virulent JM isolate. Clones from a single donor bird appeared to vary in pathogenicity. The infectivity and pathogenicity of CU-2 (low virulence) and JM-10 (high virulence) were compared with age, genetic constitution and virus dosage as variables. In JM-10 infection, S-strain birds had only neural and gonadal involvement, while P-line birds exhibited extensive visceral lesions and commercial-strain birds had primarily neural involvement. In contrast, with CU-2 infection, commercial-strain birds were free of lesions but neural involvement alone was seen frequently in S-strain birds and occasionally in P-line birds. Microscopic neural lesions were seen in CU-2 infection at 8 d and 10 d after infection but not later. With JM-10 infection, they were not observed until 14 d but then progressively increased in intensity and distribution. Virus dosage did not affect the pathogenicity of either virus. Day-old chicks were more susceptible than 4-week-old birds to JM-10, but not to CU-2 infection. Viral antigen was detected in thymus, spleen, and bursa equally with both infections. Levels of cell-associated infectivity were either higher in early infection than in later infection with CU-2 or remained at a very low level, while infectivity levels increased more slowly but progressively in the case of JM-10 infection.