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A novel Sarcocystis species has recently been reported in the domestic pigeon (Columba livia f. domestica) as intermediate host, causing severe central nervous signs similar to Paramyxovirus-1 or Salmonella Typhimurium var. cop. infection. Transmission of the parasite via the northern goshawk (Accipiter gentilis) as definitive host has been established. Experimental infection of domestic pigeons with sporocysts excreted by experimentally infected northern goshawks reproduced the natural infection in the pigeon, proving the causative role of the parasite in the disease. Here, we describe in greater detail the course of the fulminant biphasic disease depending on the infectious dose. Pigeons infected with 10(3) or 10(4) sporocysts showed clinical signs of polyuria and apathy around 10-11 days postinfection (dpi) and sudden neurological signs 51-57 dpi as a second phase of disease. Pigeons infected with higher doses died within 7-12 dpi, also showing polyuria and apathy but without nervous signs. At necropsy, livers and spleens had multifocal necroses and infestations with parasitic stages, namely, schizonts. Moreover, lesions and schizonts were also found in the lung, bone marrow, and next to blood vessels in the connective tissue of various organs. Pigeons infected with 102 sporocysts remained symptomless until 58-65 dpi, when sudden central nervous signs occurred. Major histopathologic findings of pigeons with neurological signs were encephalitis and myositis of virtually every skeletal muscle with high infestations of sarcocysts. Only mild myocarditis and very few cysts were found in the heart muscles. Importantly, a sentinel pigeon developed identical lesions when compared to those of low-dose infected pigeons, suggesting a risk of mechanical transmission of sporocysts from freshly infected to uninfected pigeons in a flock. By contrast, chickens failed to develop any clinical signs or pathologic lesions in the same experiment. The findings further characterize the new highly pathogenic disease in domestic pigeons, which clinically mimics paramyxovirosis and salmonellosis in both phases of the disease and exclude chickens as further intermediate host species.