Pathogenicity of experimental caryosporosis. International Journal for Parasitology 27: 819-824. Pathogenicity of the coccidia C. bigenetica and C. simplex was studied in experimentally inoculated pigs, goat kids (untreated and immunosuppressed) and severe combined immunodeficient (SCID) mice. The major pathological changes of caryosporosis were similar in all inoculated animals. In pigs and goat kids, caryosporosis was self-limiting, with clinical responses that included focal swelling and erythema of the muzzle, snout, jaws, cheeks, eyelids, bases of the ears, backs of the necks, scrotum, external genitalia of females, legs and footpads. Histopathological changes were characterized by involvement of the cutaneous mononuclear phagocyte system with an inflammatory exudate containing numerous macrophages, especially around the root sheaths, sensory nervous corpuscles of the hair follicles and surrounding dermal free nerve endings. The tactile hair follicles in the muzzle, snout and upper jaw were most severely changed. In SCID mice, inoculation with C. bigenetica or C. simplex caused a severe, fatal, systemic disease characterized by dissemination of numerous caryosporan developmental stages into the host mononuclear phagocyte system. This study presents evidence that both caryosporan species tested caused similar clinical signs and lesions of dermal coccidiosis in the mammalian secondary hosts.