Comprehensive Pathologic Evaluation of Histomonas meleagridis Field Isolates in Target (Cecum and Liver) and Primary Lymphoid (Bursa and Thymus) Organs in Turkeys.

Histomoniasis (blackhead disease) is caused by an anaerobic protozoan, Histomonas meleagridis. The pathogen affects all the gallinaceous birds, while turkeys are highly susceptible. The severity of infection and associated mortality rates of histomoniasis in turkeys vary considerably, and the virulence of H. meleagridis field isolates likely plays a critical role in determining disease outcome. Therefore, characterization of the virulence of field isolates is essential for understanding this disease. The virulence of two H. meleagridis field isolates (HMA and HMB) from the United States was evaluated in independent studies. Tissue samples of ceca, liver, bursa of Fabricius, and thymus were collected for pathologic evaluation. At necropsy, characteristic gross lesions of histomoniasis were noted in the ceca and liver of the infected birds. Histopathologic liver lesions varied from acute to subacute multifocal necrotizing hepatitis to a more chronic lymphohistiocytic hepatitis. Cecal lesions ranged from multifocal to diffuse typhlitis. Furthermore, lymphocyte depletion was documented in the bursa of Fabricius and thymus of infected birds. These findings suggest that H. meleagridis infection may impact both humoral and cellular immunity, warranting further investigation into the immunopathogenesis of the disease.