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% of coccidial infections that are subclinical, and never diagnosed as coccidiosis.

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

Etiology
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. The immune systems of infected cattle can also be compromised, making them more susceptible to toxins and secondary infections.

By the time clinical signs are observed, much of the damage has already occurred.¹ Cattle that survive a clinical infection may never recover from the performance setback² 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 (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 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 and immune response in the host animal.

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.

Step 5
These merozoites undergo another asexual division in the lower small intestine and
upper large intestine. Each first-generation merozoite can produce 20 to 50 secondgeneration
merozoites.

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 24 million oocysts during the next life cycle.

Glossary
This glossary was developed by Merial to assist you in your goal to learn more about
bovine coccidiosis. 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 secondgeneration 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 24 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 50 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 24 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 result in death to the host animal.



• 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.

This Veterinary Bulletin is brought to you by Merial,
the maker of CORID® (amprolium), the only product labeled for both
prevention and treatment of coccidiosis.

References

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

2. 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(Ref3).