RELATIONSHIP BETWEEN THE IMMUNOSUPPRESSIVE POTENTIAL AND THE PATHOTYPE OF MAREKS-DISEASE VIRUS ISOLATES

Isolates of Marek's disease virus (MDV) representing three pathotypes of differing virulence were compared for relative immunosuppressive pr operties in generically susceptible P2a-strain and genetically resista nt N2a-strain chickens. Criteria of immunosuppression were 1) persiste nce of early cytolytic infection (i.e., a delay or failure to enter la tency) in lymphoid organs, 2) atrophy of the bursa of Fabricius and th ymus as measured by organ weight proportional to body weight at 8 and 14 days postinfection (DPI), and 3) histopathologic evidence of necros is and atrophy in lymphoid organs. No significant differences in infec tion level were observed among the pathotypes during the early (4-5 DP I) period of infection. However, the extent of persistent cytolytic in fection at 7-8 DPI, based on numbers of tissues positive and mean scor es in immunofluorescence tests, was greater (P < 0.05) for three isola tes (RK1, 584A, 648A) in the highest virulence pathotype (very virulen t-plus MDV [vv+MDV]) than for two isolates (JM16, GA5) in a lower viru lence (virulent MDV [vMDV]) pathotype. Results from two isolates (RB1B , Md5) classified in the intermediate very virulent pathotype (very vi rulent MDV [vvMDV]) fell between those from the other two pathotypes. Similarly, there was a stepwise effect of viral pathotype in which the vv+MDV isolates caused the most severe damage to lymphoid organs in t erms of atrophy (relative organ weights) and histopathologic changes. Organs from chickens infected with vv+MDVs showed little recovery betw een 8 and 14 DPI. The vMDV isolates caused the least severe damage, an d lymphoid organs showed a significant return toward normal by 14 DPI; vvMDV isolates induced intermediate degrees of atrophy and recovery. The same pattern of relationship between virulence pathotype and degre e of bursal and thymic atrophy was also observed in genetically resist ant N2a chickens. These results suggest that the degree of immunosuppr ession is linked to virulence and that a simple measure of atrophic ch anges (relative organ weights) in the bursa of Fabricius and thymus mi ght be useful in determining the pathotype classification of new MDV i solates. The basis for differences in immunosuppressive potential of M DV isolates needs further clarification.