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Coccidiosis, What it is and How it is Transmitted

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Big Dog
Eimeria and Isospora
(coccidiosis)
Members of these two genera are often referred to as the "coccidia." The two genera contain a large number of species that infect a variety of animals throughout the world. The diseases caused by these parasites are referred to collectively as coccidiosis, and they vary tremendously in virulence. Some species cause diseases that result in mild symptoms that might go unnoticed (i.e., mild diarrhea) and eventually disappear, while other species cause highly virulent infections that are rapidly fatal.



The life cycles of both genera are similar. A host is infected when it ingests oocysts that have been passed in the feces of another host. The oocyst excysts in the host's small intestine, and the sporozoites contained within the oocyst are liberated. The sporozoites penetrate the cells of the host's small intestine and reproduce asexually. Each generation of asexual reproduction produces multiple merozoites; the merozoites are liberated from the cell and infect new cells. It is this stage of the infection that can result in destruction of massive numbers of cells in the host's small intestine and, ultimately, lead to the host's death. Some of the merozoites that enter the host's cells transform into gametocytes. The gametocytes transform into gametes, the gametes fuse, and the resulting zygote begins to develop into an oocyst. The developing oocyst escapes from the host's cell, and it is passed in the host's feces. Typically, when the oocyst is passed in the feces, it is not infective because it does not contain sporozoites; this is an unsporulated oocyst. After several days (or weeks, depending on the species) outside of the host's body, the oocyst completes development and sporozoites are found within; this is a sporulated oocyst, and it is infective to the next host (view diagram of the life cycle).

Diagnosis of the infection is based on finding oocysts in the host's feces. Differentiation of the two genera and the species within the genera is based on the internal morphology of the oocyst. Thus, while it is possible to identify an unsporulated oocyst as a coccidian oocyst, it is virtually impossible to identify the parasite that produced the oocyst until the oocyst is sporulated.

Asexual multiplication of the parasite in the cells of the host's small intestine is self limiting (although in some species the parasite actually kills the host before asexual reproduction stops). That is, after several generations of asexual multiplication, the parasite simply stops dividing, the host stops passing oocysts, and the host is effectively cured of the infection.

An unsporulated coccidian oocyst. Such oocysts typically measure between 35 and 50 µm. (Original image from the Oklahoma State University, College of Veterinary Medicine.)



Another example of an unsporulated oocyst. (Original image from Gardiner et al., 1988, An Atlas of Protozoan Parasites in Animal Tissues, USDA Handbook No. 651).



A sporulated coccidian oocyst. The oocyst contains two sporocysts, and this is typical of the genus Isospora (and Toxoplasma, although Toxoplasma oocysts are much smaller). Sporulated oocysts of the genus Eimeria contain 4 sporocysts . (Original image from the Oklahoma State University, College of Veterinary Medicine.)



A sporulated oocyst of Eimeria sp. This oocyst contains four sporocysts (only three can be seen).



A histological section showing the asexual reproductive stages of a coccidian in the tissues of the host's small intestine. Note the many developing meronts (=schizonts) (the large dark blue structures enclosed within the rectangle) in the tissues. Each meront will produce many merozoites.



A higher power magnification of a developing meront. The individual developing merozoites (*) can be seen.