Eimeria leuckarti

Eimeria leuckarti is a coccidian protozoan that infects horses around the world, including in Canada.


Summary

Eimeria leuckarti is a coccidian protozoan that infects horses around the world, including in Canada.  The life cycle is typical for a coccidian.  The oocysts passed in the faeces of horses are very distinctive: oval or pear-shaped with a thick, dark brown wall and a micropyle at the thinner end.  Eimeria leuckarti is very rarely associated with distinct clinical signs in horses and is not known to be zoonotic.

Taxonomy

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 species 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.

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.

Morphology

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 small intestine, 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. leuckarti passed in feces are very characteristic. They are oval to pear-shaped, measure up to approximately 88 µm by 60 µm, and have a very thick, dark brown shell and a micropyle at the narrower end.

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

Host range and geographic distribution

Eimeria leuckarti is found in horses and other equids around the world. Among the several species of Eimeria infecting horses, E. leuckarti is the most common worldwide and is thought to be the only one in North America. It is thought to be pathogenic only very rarely.

Life cycle - direct

Eimeria species undergo asexual (merogony) and then sexual (gametogony and fertilization) reproduction in the enterocytes of the small intestine. The infective stage is the sporulated oocyst which, following ingestion by a suitable host, hatches to release eight sporozoites. These sporozoites enter enterocytes and divide rapidly to form merozoites enclosed within a meront, which can come to occupy most of a host cell. The infected cell then bursts, releasing the meronts into the intestinal lumen, from where they penetrate new enterocytes. The number of generations of merogony for E. leuckarti is not known.

Eventually, merozoites entering host cells do not divide to produce meronts, but instead form microgametocytes ("male") and microgametocytes ("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.

The oocysts burst from the enterocytes and are passed in feces. In the environment the oocysts sporulate and are then infective. For E. leuckarti, sporulation under ideal conditions takes approximately three weeks. Infection of horses is by ingestion of sporulated oocysts.

Life Cycle: Eimeria leuckarti

Epidemiology

Generally, Eimeria leuckarti produces relatively few oocysts and they take a long time, relative to other coccidial species, to sporulate. It is the sporulated oocysts that are the source of infection for horses. Thus transmission is enhanced by factors which are supportive of oocyst development and survival and/or increase the likelihood that horses will ingest large numbers of infective oocysts.

Pathology and clinical signs

Eimeria leuckarti is generally believed to be only very rarely pathogenic in horses. The clinical sign most often reported is diarrhea.

Diagnosis

The recovery of oocysts of E. leuckarti from feces by flotation is the most commonly used method for diagnosis. As the oocysts are very large and heavy they may not be easily recovered by routine flotation techniques. Diagnosis at post mortem is confirmed by the presence of typical post mortem changes and the identification of parasites in mucosal scrapings or in histological sections.

Treatment and control

Eimeria leuckarti is rarely if ever a cauase of clinical disease in horses and there are no products approved in Canada for this parasite.  Control of E. leuckarti is not usually an issue.

Public health significance

Eimeria leuckarti is not known to zoonotic.
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