Histomonas meleagridis (Sarcomastigophora, Monocercomonadidae)

Histomonas meleagridis is a pleomorphic flagellate transmitted in the eggs of the caecal nematode, Heterakis gallinarum. The protozoan cannot exist by itself in the external environment. It is commonly found within gallinaceous birds and may cause a serious disease termed "blackhead." This disease occurs most commonly in turkey poults, although other birds sometimes become diseased. The amoeboid form of the parasite invades the intestinal mucosa, causing ulcerations, peritonitis, and intestinal perforations. Disease is associated with several species of pathogenic bacteria, including Clostridium perfringens and Escherichia coli, and the protozoan appears incapable of causing pathology in the absence of these bacterial pathogens. When pathogenic, the parasite may invade the liver where necrotic lesions form (left panel). Infected birds develop ruffled feathers, dark skin pigmentation, and hanging wings and tail (right panel). Control involves the use of mebendazole and related anti-nematode drugs to prevent transmission of the protozoan.