Eimeria species — sheep

Coccidian protozoa of the genus Eimeria are common in sheep around the world.  Many species are considered to be of little or no clinical significance but in Canada E. ovinoidalis appears to be the species most often associated with disease.  This species has a typical, direct coccidian life cycle.  Infection of sheep is by ingestion of sporulated oocysts from the environment.  In the gut lumen the sporozoites released from these oocysts invade enterocytes, undergo one or more cycles of asexual reproduction, then a cycle of sexual reproduction that results in the production of unsporulated oocysts. These leave the host in the faeces and sporulate in the environment.  Under ideal conditions sporulation can occur in a few days and the prepatent period is approximately 14 days.

Sheep can usually tolerate small burdens of coccidia, but if parasite numbers become large, for example as a result of an environment heavily contaminated with viable infective oocysts, or other stressors affecting the sheep, disease can result.  Clinical signs are seen most commonly in young animals and are associated primarily with sexual reproduction by the parasite, which occurs in the enterocytes of the large and sometimes small intestines.  Typical signs include diarrhea, dysentery, dehydration and loss of appetite and body condition. 

The Eimeria of sheep are not know to be zoonotic.

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.

Various species of Eimeria infect sheep around the world and are also found in other ovids, although the host range differs with species. Details of the life cycle and pathogenicity are not known for all species.

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.

History and clinical signs are helpful, as is the recovery of oocysts from feces, although it is possible to have clinical disease before these are present in feces in large numbers. Oocysts of E. ovinoidalis can be identified to species by size and other morphological features, but this can be difficult for the inexperienced microscopist. The presence of typical post mortem changes and the identification of parasites in mucosal scrapings or in histological sections confirms the diagnosis.

A combination of sulfa drugs, given orally,  is approved for sheep in Canada as treatments for clinical coccidiosis in sheep and as preventatives.

Control of coccidiosis in young sheep depends on reducing the likelihood that animals will ingest large numbers of infective oocysts, and ensuring that they have optimal resistance to infection. The first can be achieved by measures such as ensuring that stocking densities are appropriate, particularly in feedlots and yards, and by avoiding feeding from the ground. The second depends on an adequate level of nutrition, effective control of other enteric pathogens, and the minimization of other stressors. Also, anticoccidials are sometimes administered in the feed or water to help with control of the infection.

The Eimeria species that infect sheep are not known to zoonotic.