Phylum: Apicomplexa

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

Eimeria species are most closely related to the genus Isospora. The most significant difference between Isospora and Eimeria is that the former may use a paratenic host in its life cycle, whereas the latter does not. Other closely related genera include Cryptosporidium (considered by some to be a member of Subclass Gregarisina within the Class Conoidasida, and thus a gregarine rather than a coccidian), Cyclospora, Neospora, Sarcocystis and Toxoplasma. Cattle and sheep each have their own array of Eimeria species. Clinical disease associated with Eimeria species is known as coccidiosis. In Canada, the two species of greatest significance in terms of disease in cattle are E. bovis and E. zuernii.

Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.

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 intestine, and the oocysts in the ikntestinal lumen and the feces. 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. bovis passed in feces are pear-shaped, measure up to approximately 34 µm by 24 µm, and have a thin, smooth shell and a micropyle at the narrower end.

Oocysts of E. zuernii are round to ellipsoidal, measure up to approximately 22 µm by 18 µm, and have a thin, smooth shell but no micropyle.

Freshly passed oocysts are unsporulated and contain one or two cells.After a few hours or days, depending on external environment temperatures, each oocyst 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. zuernii, there are usually two generations of merogony, the first in the lower small intestine and the second in the colon and caecum. Gametogony occurs in the large intestine. The prepatent period for E. zuernii is approximately 14 days.

With E. bovis, there are usually two generations of merogony, the first in the lower small intestine and the second in the colon and caecum.Gametogony occurs in the large intestine, although in very heavy infections it may extend into the lower small intestine. The prepatent period for E. bovis is approximately 14 days.

Eimeria bovis, but not E. zuernii, produce giant meronts in the first generation merogony that may be visible to the naked eye in tissue specimens.

Life Cycle: Eimeria species

In cattle in Canada, clinical coccidioisis is seen in young animals in feedlots or yards, usually when newly entering in the fall, and at pasture, particularly when stressed and being fed from the ground. Clinical disease can affect one, or a few, or most animals in a group.

One of the interesting questions concerning “winter” coccidiosis in cattle (see Pathology and Clinical Signs section) is the source of the infective oocysts. Have they persisted and remained infective since the previous summer and fall, or have they somehow managed to sporulate in the environment during the winter? One suggestion is that they are able to sporulate in the animal’s hair coat, where the temperature and other environmental conditions are supportive of survival and development.

Clinical coccidiosis is more common in younger animals (calves and yearlings) than in adult cattle, 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. Significant infections with E. zuernii or E. bovis often occur together with other enteric pathogens, for example bovine rotavirus. It may take some time after an attack of clinical coccidiosis for the animals to fully recover their health and productivity.

There are three different clinical presentations for coccidiosis in cattle. The first can occur at any time of the year, is usually in young animals, and is characterised by diarrhea - often with blood and mucus, dehydration, reduced appetite, and weight loss. Straining is also a common feature.Some animals may become very sick and even die. The second form occurs in winter (“winter coccidiosis”), usually in younger animals, and often follows a prolonged period of cold or a cold snap. Clinical symptoms are similar but more severe, and death may occur. The third is the least common, involves nervous signs (“nervous coccidiosis”), is characterized by muscular tremors, hyperaesthesia and convulsions, and is often fatal. Nervous coccidiosis may be caused by a product of the parasite, and does not seem to be associated with the presence of parasites within the central nervous system.

In Canada the drugs approved for the prevention of coccidiosis in cattle are the ionophores lasalocid and monensin, the thiamine analogue, amprolium, and decoquinate, a quinolone derivative .  For treatment of clinical disease, amprolium and sulfa drug combinations are approved.  All are given orally, as a feed pre-mix, in water or as a bolus.

Additional information on the product mentioned above is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

Control of coccidiosis in young cattle 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.