Aberrant Clinical Appearance and Pathomorphology Noticed During an Outbreak of Histomonosis Indicates a Different Pathogenesis of Histomonas meleagridis Genotype 2.

The present case report describes a remarkable feature of Histomonas meleagridis characterized by aberrant clinical appearance and pathomorphologic lesions, which were mainly confined to the ceca, noticed during a field outbreak of histomonosis. In a flock of meat turkey toms, sudden death was noticed at the end of week 5 in the absence of specific clinical signs. Instead of the well-known sulfur-colored feces, some caseous cores were found in the litter. Mortality up to 17% per week was noticed in the first 2 wk of observation, after which it declined to approximately 1% per week. In the 10th week of life roughly 31% of the birds had died before the remaining birds were killed to preclude further economic losses due to insufficient growth or continuing mortality. Necropsy of affected birds on the farm and during a more detailed investigation of 15 birds prior to the killing of the flock revealed severe lesions in the ceca characterized by thickened cecal walls filled with necrotic and caseous material. Additionally, some ruptured and necrotic ceca were noticed together with localized peritonitis. Despite such severe typhlitis, only one of the sectioned birds showed pathomorphologic changes in the liver. Test tube flotation from collected fecal samples revealed only sporadic occurrence of coccidial oocysts and no nematodes. However, the presence of H. meleagridis was confirmed by PCR and/or immunohistochemistry, with specific antibodies against the parasite in a majority of the investigated ceca and in four liver samples. Remarkably, genetic characterization revealed H. meleagridis genotype 2, about which no detailed investigations have been reported so far. Although PCR detected a concurrent presence of Tetratrichomonas gallinarum, an involvement in the lesions could be excluded based upon histologic investigations. Finally, infection with Escherichia coli and Gallibacterium anatis was demonstrated by bacteriologic smears of internal organs, most likely a secondary infection. Altogether, the results demonstrate an aberrant clinical appearance and pathomorphology in turkeys suffering from histomonosis. Pathomorphologic changes were characterized by severe inflammation of the ceca with minimal liver involvement, indicating a different pathogenesis of H. meleagridis genotype 2.