Introduction

Attacks of clinical coccidiosis in cattle are marked by diarrhea, loss of blood, dehydration and anorexia. Even these signs are not always dramatically presented. Blood may or may not be evident in the feces. The disease results in a decline in general health, a failing appetite and eventually a loss of condition. The precise mechanism of development of these signs and the details and degree of recovery from coccidial infections are incompletely understood. It is certain that the physiologic and cellular pathologic changes which occur in infected animals significantly affect their ability to throw off the disease and return to normal. Such changes can also be expected to affect the ability of the animals with clinical coccidiosis to resist other diseases. Although little is known about the long-term effects of coccidiosis in cattle, loss of condition, reduced gains and mortality are of vital importance to dairy and beef producers.

The organisms which cause coccidiosis are tiny one-celled protozoa, chiefly of the genus Eimeria. Coccidia are very host specific—that is, coccidia which affect cattle do not affect birds, and vice versa. Another way in which coccidia differ from most other parasites is that often several species of coccidia occur in a single species of host. Twenty-one species have been described as occurring in cattle, 10 in sheep, 10 in rabbits and nine in poultry.

Although 21 species of coccidia have been described as occurring in cattle, only two species, Eimeria bovis and E. zurnii, are known regularly to cause coccidiosis accompanied by bloody diarrhea. Low-level infection with one or several species of coccidia is present in almost 100% of cattle, with no apparent damage to the host. Examinations for coccidia are made by collecting fecal samples and mixing with a concentrated sugar solution to cause the coccidia to float to the surface where they can be recovered for microscopic examination. Such an examination will permit a trained observer to identify the species of coccidia involved. This, along with other clinical signs, will enable assessment of the impact of the infection on the health of the animal.

The Parasites

Coccidia have a complex life cycle with several stages simultaneously occurring in separate host cells. Click here to see an interactive diagrammatic representation of the life cycle of E. bovis. It should be noted that the stage found in the feces is the oocyst. The oocyst, which has a protective wall resistant to physical, chemical and bacterial action, is discharged from the animal in the feces. With favorable environmental conditions of temperature and moisture, the oocyst goes through a maturation process called sporulation, which makes it infective to cattle. When the sporulated oocyst is ingested by the host, the sporozoites within it excyst and penetrate cells of the intestinal wall. They grow and develop into schizonts. Schizonts undergo a multiple dividing process that results in the formation of numerous new individuals called merozoites. After the schizont matures, the merozoites are released by the rupture of the host cell and they invade new cells and repeat the process.

For E. bovis, the second-generation merozoites enter new host cells and undergo sexual production, culminating in the formation of oocysts. All of the life cycle stages within the host are completed in a minimum of about 18 days with the peak in numbers of oocysts discharged occurring about 19 to 22 days after initiation of infection. The sexual stages are more numerous than the asexual stages—and they do more damage to their host cells. The signs of coccidiosis usually occur at the same time oocysts are passed in the feces.

E. zurnii, another pathogenic species of cattle coccidia, has been found to produce schizonts in both the small and large intestine between two and 19 days after infection. Sexual stages have also been observed at both sites. Oocysts may be discharged as early as Day 19. All of the stages are found in the epithelial cells.