Tibetan siskins are birds native to the Himalayan region often imported into Italy for commercial purposes. Fecal examination of 45 imported subjects with clinical signs of diarrhoea revealed the presence of a large number of coccidian oocysts. After sporulation, accomplished by mixing feces with 2.5 % (w/v) acqueous K2Cr2O7 at room temperature (22 degrees C +/- 1 degrees C), exogenous stages of an Isospora species were revealed. The oocysts of this Isospora are spherical, have a bilayered colorless wall, and average 23.24 mu m x 23.05 mu m: oocyst residuum and micropyle an absent, while an oval polar granule is rarely present. The elliptical sporocysts average 18.44 mu m x 10.97 mu m and the Stieda body protrudes slightly from the end of the sporocyst. A spherical sporocyst residuum is present though it sometimes consists of scattered granules. The spindle-shaped sporozoites average 11.53 mu m x 2.86 mu m, and have two refractile bodies. The taxonomic position of the tibetan siskin is controversial. Some authors include this species in the genus Serinus, while others include it in the genus Carduelis. The coccidian species isolated from these tibetan siskins was, for this reason, compared with the Isospora species previously described both in the genus Carduelis and in the genus Serinus. As a result of this comparison a new species, Isospora thibetana, was named. In the intestine of dead subjects, oocysts were found only in the ileum where the mucosa was greatly thickened and presented a heavy leucocytic infiltration consisting mainly of lympho-monocytic cells. A similar infiltration was observed in liver and lungs as well.
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