Fig. 5

Life cycle of Caryospora bigenetica. The parasite infects the intestinal epithelium of snakes, where asexual (merogony) and sexual (gametogony and oocyst formation) reproduction take place. (1) Unsporulated oocysts are shed in the feces of the definitive reptilian host and become infective through sporulation. Ingestion of the sporulated oocysts by the definitive host (snake-to-snake infection) results in intestinal caryosporosis with asexual and sexual multiplications. However, heteroxenous predator–prey cycle (snake-to-mouse-to-snake) may play a vital role in the life cycle in nature. Ingestion of sporulated oocysts by the prey results in extraintestinal asexual and sexual multiplication through merogony, gametogony, fertilization, sporulation, and formation of caryocysts. In the secondary or intermediate host, parasitic stages are found mainly in the skin and mucocutaneous epithelium. In the present case, the kitten played a role as a secondary host. In histopathology, all stages of C. bigenetica described in experimental infections could be identified. (2) Sporozoite entering a macrophage, (3) type 1 meront, (4) type 2 meront with tachyzoites (tachyzoic merozoites), (5) gamonts, (6) unsporulated oocyst, (7) sporulated oocyst, (8) sporozoite, and (9) caryocyst. Caryocysts and the abundance of unsporulated oocysts were specific findings in this case. The heteroxenous life cycle is completed when a snake ingests an infected secondary host. As the secondary hosts may get the infection not only from oocysts voided by the snake (primary host) but also by consuming other secondary hosts bearing caryocysts, the kitten may have gained the infection feco-orally (snake-to-kitten route) or by ingesting a secondary host (snake-to-mouse-to-kitten route). Also, an unlikely transplacental and galactogenic infection from the queen that got the infection and transmitted the infection to the kitten must be considered