The protistan phylum Apicomplexa Levine, 1970 contains many obligate intracellular parasites and includes such diverse organisms as coccidia, gregarines, piroplasms, haemogregarines and the malarias. Members of this large, heterogeneous assemblage are united, not necessarily by their biology and/or life histories, but by the presence of a unique "apical complex," composed of polar rings, rhoptries, micronemes, often a conoid, and other subcellular organelles, but visualized only by use of an electron microscope.

The coccidia, along with the gregarines, comprise the class Conoidasida Levine, 1988, characterized by the presence of a complete, hollow, truncated conoid. While the gregarines parasitize invertebrates with mature gamonts being extracellular, the coccidia mostly infect vertebrates and have intracellular gamonts. The coccidia are separated into four orders, each distinguished by the presence or absence of various asexual and sexual stages. The largest order, Eucoccidiorida Lèger & Duboscq, 1910 contains species that all undergo merogony (asexual), gamogony (sexual) and sporogony (spore formation) during their life cycle. Members of the family Eimeriidae Minchin, 1903 all are homoxenous (direct life cycle), with merogony, gamogony and the formation of oocysts occurring within the same host. Oocysts then leave the host via the feces, and are unsporulated (undeveloped, non-infective). The development of a genetically-determined number of sporocysts and sporozoites within each oocyst usually occurs outside the host if/when environmental conditions (oxygen, moisture, temperature) are appropriate.

The genus Eimeria Schneider, 1875, with more than 1300 species described to date, is the largest apicomplexan genus, and may be the most specious genus of all animal genera! Sporulated oocysts of Eimeria contain four sporocysts, each with two sporozoites.

Structure and Life History

An eimerian species was one of the first protists ever visualized when Antoni van Leeuwenhoek saw what surely were oocysts of Eimeria steidai (Lindemann, 1895) in the bile of a rabbit in 1674. Since the oocyst is the stage that leaves the host, usually in the feces, it is the structure in the life cycle that readily is available to the veterinarian, wildlife biologist, or parasitologist who wants to identify the species, often without having to kill the host. As a result, about 98% of all Eimeria species are known only from this one life-cycle stage, the sporulated oocyst.

The complete life cycle of a "typical" Eimeria species is shown in Figure 1.

When a sporulated oocyst is ingested (Fig. 1, a) by the proper host, the sporozoites must first leave the confines of the sporocyst and oocyst (i.e., they must "excyst") before infection can proceed. Both mechanical (muscular contractions) and enzymatic digestive (trypsin, bile salts) processes of the upper gastrointestinal tract of the host make the sporocyst and oocyst walls more permeable; eventually, certain parts of each may be digested, or they may collapse or are broken, releasing their sporozoites (Fig. 1, b). Once free within the milieu of the intestine, a sporozoite must penetrate a host epithelial cell before development can continue (Fig. 1, c). Invasion of the host cell is complicated, involving a sequential series of steps including recognition of a host cell, attachment to surface components, formation of a tight juction, entry into the cell (facilitated by organelles of the apical complex), and formation of a parasitophorous vacuole around the sporozoite (Sam-Yellowe, 1996). Safely inside its parasitophorous vacuole, the sporozoite initiates merogony (asexual multiple fission).

During merogony (Fig. 1, c-e), as few as two and up to as many as 100,000 merozoites may be formed by each sporozoite, depending on the species. Mature merozoites rupture and kill the host cell (Fig. 1, f), each seeking to penetrate a new epithelial cell to begin merogony again (Fig. 1, g-i). It is believed that each Eimeria is programmed genetically for a specific number of merogonous generations characteristic of that species. For the few species in which the actual number of generations is known, it varies from two to four generations. Thus, infections with Eimeria species are self-limiting as asexual reproduction does not continue indefinitely. Nonetheless, whatever the number, tremendous biological magnification of the parasite results from these developmental stages.

When the last generation of merozoites (Fig. 1, j) enter host epithelial cells, they develop not into additional meronts, but gamonts. The vast majority develop into macrogametocytes (macrogamonts) to form uninucleate macrogametes (Fig. 1, k-n), while the remaining merozoites develop into microgametocytes, each of which will undergo multiple fission to produce thousands of motile, biflagellated microgametes (Fig. 1, o-r). When they are mature, microgametes leave their host cell (Fig. 1, s), seek out and penetrate cells that have a mature macrogamete within (Fig. 1, n), and fertilization occurs, restoring the diploid (2N) condition. Soon after fertilization, a delicate membrane forms around the zygote and two types of wall-forming bodies develop in the cytoplasm; these migrate toward, and then fuse with, the surface membranes to form the resistant oocyst wall. When the oocyst wall is fully formed, the oocyst ruptures from the host cell and leaves the host (Fig. 1, t), usually in the feces. The mechanisms that regulate if a merozoite will become a macro- or microgamont, how microgametes find cells with developed macrogametes, and details of the fertilization process all are topics that warrant further study.

It has been demonstrated experimentally that at least a few bird and mammalian Eimeria may form extraintestinal tissue stages (Carpenter, 1993; Mottalei et al., 1992). Apparently, sporozoites excyst from oocysts ingested by these host, infect cells in various places in the body, and become dormant. The infected host may or may not be the "normal" host for that eimerian species; if the host with such tissue stages is eaten by the appropriate host, these dormant sporozoites become active, infect enterocytes (intestinal epithelial cells), and intitiate their typical life cycle. It is not known if such a cycle is functional in natural communities.

The time between when a suitable host first ingests a sporulated oocyst and when unsporulated oocysts leave the host in its' feces is termed the prepatent period; during this interval, which can vary from three to ten days, no oocysts are found in the feces because only merogony and the beginning of gamogony are occurring in the host. The time interval during which oocysts are discharged from an infected host is termed the patent period and lasts only until all the fertilized and unfertilized macrogametes have been released from their host cells. Both time periods vary between host and Eimeria species and are dependent upon many factors including: eimerian species, size of inoculating dose, number of endogenous stages, depth within the tissues where merogony, gamogony and fertilization occur, concurrent infection with other parasites, host age, and nutritional and immune status of the host.

Once outside the host, the oocyst must sporulate before it is infective to another host animal (Fig. 1, u-x). The presence of oxygen, moisture, shade (direct exposure to UV radiation---sunlight---will kill oocysts quickly) and, generally, a temperature less than body temperature of the host, are necessary for oocyst survival. If these conditions are met, the first nuclear division of the sporoplasm (diploid zygote) within the oocyst is meiotic; all subsequent cell divisions, which ultimately lead to the formation of four sporocysts, each containing two sporozoites, are mitotic.

Once the sporulation process is completed, the fully-formed oocyst (Fig. 2, a, c) and its sporocysts (Fig. 2, b, d) have a suite of structural characters that help the experienced taxonomist distinguish one species from the next. Unfortunately, sometimes sporulated oocysts from different host species look very nearly identical in size and structure and may not be easily or reliably differentiated by morphological features alone. Once sporulation is completed, the oocysts are resistant to environmental extremes and the sporozoites therein are immediately infective to the next suitable host animals that may ingest them.

Survival of Oocysts

Moisture, temperature, and direct exposure to sunlight all influence the ability of oocysts to sporulate in the external environment, but the interactions of these and other factors (e.g., mechanical vectors such as invertebrates) are not well understood. In general, oocysts sporulate more rapidly at higher temperatures and slower at lower temperatures; exposure to temperatures less than 10 º C or greater than 50 º C are lethal to unsporulated oocysts. Between these extremes, the sporulation of oocysts in a field-collected sample is dependent upon at least the following factors: the eimerian species, the time and temperature between collection and arrival of the sample at the lab, the medium in which the sample was stored, the amount of molecular oxygen available to the stored oocysts and the concentration of oocysts in the sample. Under optimal laboratory conditions, sporulation of oocysts from mammals occurs best between 20-23 º C, but this will vary among vertebrate classes (Duszynski and Wilber, 1997). Once sporulated, oocysts of some species remain viable and infective in 2% aqueous potassium dichromate (kills bacteria, prevents putrification) at 4-5 º C for up to four years. In their natural external environment, oocysts remain viable and infective from as little as 49 days up to 86 weeks, dependent upon the species and the interplay of abiotic and biotic environmental parameters. See our Techniques for Preservation of Oocysts page:


Eimeria species demonstrate both site and host specificity, but to somewhat different degrees. The majority of species, for which endogenous development is known, undergo development within certain cells of the gastrointestinal tract, but not all species are found in this location. Eimeria steidai undergoes develpment in epithelial cells of the bile duct and parenchymal cells of the liver of rabbits. Other species have been found to develop in cells of the gall bladder (goat), placenta (hippopotamus), epididymis (elk), uterus (impala), genitalia of both sexes (hampsters), bile duct (chamois), liver parenchyma (wallaby), and pyloric antrum (kangaroo) (Duszynski and Upton, 2001). Once within their specific organ system of choice, Eimeria species seem to be limited to specific zones within that system, specific cells within that zone, and specific locations within those cells. Thus, one species may be found only in the middle third of the small intestine and another only in the cells of the cecum. Within their specific region, one species may be found only in cells at the base of the Crypts of Lieberkühn, a second species in epithelial cell along the villi, and a third species in endothelial cells of the lacteals in the villi. Some species devlop below the striated (microvillus) border of endothelial cells, but above the nucleus, others below the nucleus, and a few within the nucleus.

The degree of host specificity seems to vary between host groups; it has been studied best in mammals, and to a lesser degree in birds, especially domesticated stock/flock animals. Eimeria species from goats cannot be transmitted to sheep and vice versa (Lindsay and Todd, 1993), but Eimeria from cattle (Bos) are found to infect American bison (Bison). Eimeria species from certain rodents (Sciuridae) seem to cross host generic boundaries easily (Wilber et al., 1998), while other rodent species (Muridae) may cross species, but not genus, boundaries (Hnida and Duszynski, 1999). Similarly, some species from gallinaceous birds can be transmitted only to congenerics, while others can be cross-transmitted between genera. One species even has been reported to cross familial lines, but this seems rare (DeVos, 1970). It also is known that Eimeria separata Becker and Hall, 1931 from rats will infect certain genetic strains of mice and that genetically altered or immunosuppressed mammals are susceptible to infection with Eimeria species to which they otherwise might be naturally refractory. Thus, numerous biotic interactions, particularly the genome of both parasite and host, must contribute to the host specificity, or lack thereof, attributed to each Eimeria species.

Literature Cited

DeVos, A.J. 1970. Studies on the host range of Eimeria chinchillae DeVos and Van der Westhuizen, 1968. Ondersteeport Journal of Veterinary Research 37: 29-36.

Duszynski, D.W. and P.G. Wilber. 1997. A guideline for the preparation of species descriptions in the Eimeriidae. Journal of Parasitology 83: 333-336.

Duszynski, D.W. and S.J. Upton. 2001. Enteric protozoans: Cyclospora, Eimeria, Isospora and Cryptosporidium (Cryptosporidiidae) spp. Chapter 16, pp. 416-459,In, Parasitic Diseases of Wild Mammals, 2nd ed. (W.M. Samuel, M.J. Pybus, A.A. Kocan, eds.) Iowa State University Press, Ames, IA.

Hnida, J.A. and D.W. Duszynski. 1999. Cross-transmission studies with Eimeria arizonensis, E. arizonensis-like oocysts and E. langebarteli: host specificity within the Muridae and other rodents. Journal of Parasitology 85: 873-877.

Lindsay, D.S. and K.S. Todd, Jr. 1993. Coccidia of mammals. In, Parasitic protozoa. Vol. 4. pp. 89-131. Academic Press, Inc., New York.

Sam-Yellowe, T.Y. 1996. Rhoptry organelles of the Apicomplexa: Their role in host cell invasion and intracellular survival. Parasitology Today 12: 308-315.

Wilber, P.G., D.W. Duszynski, S.J. Upton, R.S. Seville and J.O. Corliss. 1998. A revision of the taxomnomy and nomenclature of the eimerians (Apicomplexa: Eimeriidae) from rodents in the tribe marmotini (Sciuridae). Systematic Parasitology 39: 113-135.