Coccidia (Eimeria bovis, E. zurnii)
A Parasite Profile

Joe Dedrickson, DVM, Ph.D.
                       
Introduction
Coccidiosis is an intestinal disease of cattle that results from infection by a single-celled protozoa called coccidia. It is estimated that coccidiosis costs American beef and dairy producers hundreds of millions of dollars each year. However, any estimate would be conservative, because it does not factor in all the losses caused by the 95%¹ of coccidial infections that are subclinical, and never diagnosed as coccidiosis.

Bovine Coccidiosis:
A common, complex and costly enteric disease.

Etiology
Almost all cattle are exposed to coccidia, especially the highly pathogenic Eimeria bovis and Eimeria zurnii. The majority of cattle exposures result only in subclinical infections that cause mild diarrhea or no clinical signs at all. However, these cattle act as carriers, which spread coccidia when they discharge oocysts in their feces.

Infections can remain at subclinical levels until reduced resistance caused by stress factors such as weaning, shipping, commingling, crowding, weather or even changes of ration, allow the coccidia populations to explode. In just a few days, millions of opportunistic coccidia infect the intestines, causing clinical signs such as hemorrhagic diarrhea, loss of weight, reduced performance and vigor, and even death.

By the time clinical signs are observed, much of the damage has already occurred.² Cattle that survive a clinical infection may never recover from the performance setback³ and may always lack the capacity to efficiently handle feed and fluids. To a much lesser extent, poor rate of gains can be observed in subclinically infected animals.


Click here for an interactive demo of the Coccidia Lifecycle.

EXTERNAL PHASE (grass, feed or water contaminated with feces)
After sporulation, the oocyst is able to withstand commercial cleansers and disinfectants
and can survive and remain in the environment for years.

Step 1
The sporulated oocyst is a mature egg containing 4 sporocysts, each with 2 sporozoites.

SUBCLINICAL PHASE (small intestine)
Subclinical coccidial infections damage the villi of the small intestine and can reduce nutrient absorption.

Step 2
After the sporulated oocyst is ingested and exposed to carbon dioxide and digestive enzymes in the host’s digestive tract, it splits open (or excysts) and releases its 8 sporozoites.

Step 3
Each highly motile sporozoite swims or glides to the small intestine.

Step 4
3 to 7 days after ingestion, sporozoites enter the small intestine and reproduce asexually through a budding process called schizogony (completed Day 5 through Day 10). Each sporozoite can produce up to 120,000 first-generation merozoites, which are released when the host cell bursts.⁴

Step 5
These merozoites undergo another asexual division in the lower small intestine and upper large intestine. Each first-generation merozoite can produce 30 second-generation merozoites.⁴

CLINICAL PHASE (large intestine)
Clinical signs of coccidiosis include bloody scours, blood-tinged feces, dehydration, anemia and general loss of body condition.

Step 6
Second-generation merozoites penetrate the large intestine, differentiating themselves as either male (microgametes) or female (macrogametes) and begin the sexual stage of the life cycle.

Step 7
A microgamete fertilizes a macrogamete to produce a zygote. The zygote forms a protective wall and becomes an oocyst, which causes the host cells to rupture.

EXTERNAL PHASE (feces, contaminated grass, feed or water)
After sporulation, the oocyst is able to withstand commercial cleansers and disinfectants and can survive and remain in the environment for years.

Step 8
The oocyst is passed, along with tissue and fluids from the ruptured cells, in the feces. At this stage the oocyst is unsporulated (immature) and is not infective.

Step 9 In the presence of oxygen, the oocyst undergoes a process called sporulation. It takes 2 to 4 days for an oocyst to become a sporulated oocyst, capable of infecting cattle. A single oocyst can produce up to 23 million oocysts⁴ during the next life cycle.

Glossary
Therefore not all terms will refer to coccidia in general but do specifically refer to bovine coccidiosis.

Excystation The release of infective sporozoites.

Gametogony After the second schizogony, merozoites enter cells and become either macrogamete or a collection of microgametes prior to sexual reproduction.

Gamont The first stage after fusion of the micro- and macrogametes.

Macrogamete A single large cell which is “female” in behavior.

Merozoite The second asexually-produced immature stage, which is the most destructive of host cells, produced by continuous replication and splitting of the second-generation sporozoite within the schizont.

Microgamete Small flagellated motile cells which are “male” in behavior.

Oocyst The sexually produced stage of coccidia as it leaves the host bovine (the egg), compromising an undifferentiated embryo within a resilient protective coat.

Schizogony The process by which each sporozoite turns into hundreds or thousands of second-generation sporozoites or merozoites within a walled vacuole in host cells and generates the next stage of the life cycle of the coccidia parasite.

Schizont The stage reached by the first- and second-generation sporozoites after it has entered a new host cell and replicated.

Sporocyst The oocyst when the embryo has developed and generated the first infective stages, still within the protective coat of the oocyst.

Sporont A one-celled zygote within the oocyst wall before it undergoes division sporogony is the process (sporulation) by which a one-celled “sporont” (zygote) within the oocyst wall undergoes a series of divisions to form sporozoites, which are contained within sporocysts.

Sporozoite The first infective stage, developed in the oocyst and released when the cyst is eaten by the host and subjected to increased concentrations of CO2, bile salts and trypsin. NOTE: In cattle it would be more correct and aid in the understanding of bovine coccidiosis if they were referred to as first- and second-generation sporozoites. The first-generation sporozoite coming from the sporulated oocyst following excystation. While the second-generation sporozoite comes from the schizont schizogony of the first-generation sporozoite.

Coccidia Reproduction
A single sporulated oocyst has the potential to turn into 23 million oocysts after just 21 days inside the host animal. During asexual division, one sporulated oocyst divides into 8 sporozoites, each of which can divide into 120,000 first-generation merozoites (a total of up to 960,000).

Each of these merozoites can asexually divide again into 30 second-generation merozoites. The resulting 48 million second-generation merozoites (microgametes “the male” and macrogametes the female”) pair up for sexual reproduction to produce as many as 23 million oocysts.

Coccidia Facts

• Coccidia are obligate intracellular parasites and must return to the host to continue their life cycle.



• The coccidia life cycle is a continuous process, with reinfection occurring daily.



• Cattle routinely ingest thousands of oocysts each day through feces, contaminated feed and water, or by preening their own coat or licking that of another animal.

                       

• As few as 50 thousand oocysts can cause severe disease in the calf.



• Cattle can develop immunity to coccidia after extended subclinical infection.



• The most effective way to manage coccidial infections is a combination of preventive and treatment measures.

References

1. Quigley J. Calf Note #17 — A review of coccidiosis in calves. Available at: http://www.calfnotes.com/pdffiles/CN017.pdf. Accessed May 15, 2009.

2. Fitzgerald PR, Mansfield ME. Economic significance of coccidiosis in calves. J Parasitol 1969;55:39 (abstract).

3. Fitzgerald PR, Mansfield ME. Effects of bovine coccidiosis on certain blood components, feed consumption, and body weight changes of calves. Am J Vet Res 1972;33(7):1391-1397.

4. Maas JJ. Fact Sheet #10: Bovine Coccidiosis. UC-Davis; 1997.