Phylum: Apicomplexa
Class: Conoidasida
Subclass: Coccidiasina
Order: Eucoccidiorida
Suborder: Eimeriorina

Eimeria species are coccidians within the Apicomplexa, and are most closely related to the genus Isospora. Both Eimeria and Isospora are highly host specific. The most significant difference between Eimeria and Isospora is that the latter may use a paratenic host in its life cycle, whereas the former does not.

Sheep can be infected by several species of Eimeria, among which the most important in Canada is probably E. ovinoidalis. There is, however, some uncertainty about the taxonomy of Eimeria spp. of sheep.Clinical disease associated with Eimeria species is known as coccidiosis.

The life cycle stages of Eimeria species visible by standard microscopy include those associated with asexual (merogony) and sexual (gametogony and fertilization) reproduction within the enterocytes of the large and sometimes small intestines, and the oocysts. The intestinal stages – meronts, merozoites, macrogametocytes, microgametocytes, gamonts and oocysts, are typical for the genus, and can usually be seen in histological sections.

Oocysts of E. ovinoidalis are ellipsoidal or oval, measure up to approximately 28 µm by 22 µm, and have a thin, smooth shell but no micropyle.

Freshly passed oocysts are unsporulated contain one or two cells. After a few hours or days, depending on external environment temperature, each oocysts will sporulate and become infective. Each sporulated oocyst of Eimeria contains four sporocysts, each containing two sporozoites.

The infective stage for Eimeria is the sporulated oocyst which, following ingestion by a suitable host, hatches to release eight sporozoites. These sporozoites enter cells of the intestinal mucosa and divide rapidly to form merozoites enclosed within a meront, which can come to occupy most of a host cell. Depending on the species of Eimeria, a meront can contain up to several thousand merozoites. The infected cell then bursts, releasing the meronts into the intestinal lumen, from where they penetrate new enterocytes.

Depending on the species, the merogony cycle may be repeated, greatly increasing the number of parasites and the number of infected, and damaged, host cells. Eventually, merozoites entering host cells do not divide to produce meronts, but instead form microgametocytes ("male") and macrogametocytes ("female") within the enterocytes. Each microgametocyte contains several microgametes, but each macrogametocyte contains only a single macrogamete. Next, the microgametocyte disintegrates, releasing the microgametes, which fertilize the macrogametocytes, forming gamonts which develop into unsporulated oocysts. These oocysts burst from the enterocytes and are passed in feces.

Oocyst production may persist at high levels for several weeks, after which it may decline rapidly, probably because of an immunity. Thus coccidiosis is often a self-limiting infection. In the environment, over a few days under ideal conditions, the oocysts sporulate and are then infective.

With E. ovnoidalis, there are two generations of merogony, the first in the small and the second in the large intestine, followed by gametogony in both. The pre-patent period is approximately 15 days.

Both these species produce giant meronts that may be visible to the naked eye in tissue specimens.

Basic Life Cycle of Eimeria spp.

The species of Eimeria infecting sheep can produce very large numbers of oocysts that pass in feces and develop and persist in the environment. It is these sporulated oocysts that are the source of infection for sheep. Thus transmission is enhanced by factors which are supportive of oocyst development and survival and/or increase the likelihood that sheep will ingest large numbers of infective oocysts.

In sheep in Canada, clinical coccidiosis is most common in lambs in feedlots, often shortly after entry.  Coccidiosis in sheep can be a particular problem if there is overcrowding, feeding from the ground or there are other stressors.

Many sheep infected with coccidia, even E. ovinoidalis, may show only mild or no clinical signs.  With this species the major lesions are associated with all stages in both the small and large intestines. There may be very severe damage to the mucosal epithelium, even sloughing, with semi-fluid haemorrhagic material, and sometimes partly clotted blood, in the lumen. The small intestine may also be affected by a catarrhal enteritis.

Clinical coccididosis is more common in younger animals (lambs and yearlings) than in adult sheep, perhaps because the latter have developed an immunity following exposure (the immunity is likely to be parasite species specific). Clinical disease usually requires ingestion of very large numbers of infective oocysts over a short time period. It may take some time after an attack of clinical coccidiosis for the animals to fully recover their health and productivity.

Clinically, coccidiosis in sheep associated with E. ovinoidalis is characterized by soft feces or diarrhea, sometimes with blood. Abdominal pain, loss of appetite, loss of weight and dehydration may also be seen. At post-mortem, there is inflammation of the intestine with mucosal destruction and sometimes haemorrhage into the lumen.