Parasite distribution and early-stage encephalitis in Sarcocystis calchasi infections in domestic pigeons (Columba livia f. domestica)

Pigeon protozoal encephalitis is a biphasic, neurologic disease of domestic pigeons (Columba livia f. domestica) caused by the apicomplexan parasite Sarcocystis calchasi. Despite severe inflammatory lesions of the brain, associated parasitic stages have only rarely been identified and the cause of the lesions is still unclear. The aim of this study was therefore to characterize the tissue distribution of S. calchasi within pigeons between the two clinical phases and during the occurrence of neurological signs. For this purpose, a semi-quantitative real-time polymerase chain reaction (PCR) was developed. Forty-five domestic pigeons were infected orally (via a cannula into the crop) with 200 S. calchasi sporocysts and euthanized in groups of three pigeons at intervals of 2 to 10 days over a period of 61 days. Tissue samples including brain and skeletal muscle were examined by histology, immunohistochemistry, and PCR. Schizonts were detected in the liver of one pigeon at day 10 post infection. A mild encephalitis was detected at day 20 post infection, around 4 weeks before the onset of neurological signs. At the same time, immature sarcocysts were present in the skeletal muscle. In seven pigeons a few sarcocysts were identified in the brain, but not associated with any lesion. These results suggest that the encephalitis is induced at a very early stage of the S. calchasi lifecycle rather than in the chronic phase of pigeon protozoal encephalitis. Despite the increasing severity of lesions in the central nervous system, the amount of sarcocysts did not increase. This supports the hypothesis of a delayed-type hypersensitivity response as the cause of the encephalitis. The study also demonstrated that S. calchasi DNA is detectable in tissues negative by histological methods, indicating a higher sensitivity of the real-time PCR.