Marek' s disease (MD), a lymphoproliferative disorder of domestic chickens is characterized by bursal-thymic atrophy and rapid onset of T-cell lymphomas that infiltrate lymphoid tissues, visceral organs, and peripheral nerves.Marek' s disease virus (MDV), the etiological agent of MD, is a highly cell-associated oncogenic(-herpesvirus that replicates in chicken lymphocytes and establishes a latent infection within CD4 + T cells.We investigated the immunosuppressive effect of MDV infection on the development of necrotic dermatitis in the combs of two MD-susceptible (72) and resistant (63) chicken lines at 21 days post infection.MDV-infected birds of line 63are relatively resistant to tumor development but exhibit an unusual necrosis of combs, wattles, and toes in addition to other classic signs associated with MD.Chickens from Line 72, on the other hand, are highly susceptible to MDV infection and tumor development.Real-Time PCR analysis revealed that IL-6, IL-8, IL-12, IL-18, iNOS,and INF-(were all up regulated in the necrotic combs of line 63.The expression levels of most of the tested genes were lower in the combs of the susceptible line with no visible necrotic damage.No MDV antigen but a massive infiltration of macrophages and lymphocytes was detected in the comb tissues of both resistant and susceptible lines.Histopathological analysis also revealed thinning and erosion of epidermis and inflammation, lympho-plasmocytic infiltration, heterophilic and histocytic cellulitis within the connective tissues of the necrotic combs.Gram stain of the sectioned frozen comb samples exposed the presence of Staphylococcus species.In conclusion, necrotic dermatitis of the combs and toes in the MD-resistant line 63is a clear example of the unintended outcome of line selection for MD resistant that has negatively affected the immunological competence of the birds to immunosuppressive agents and secondary bacterial infection.