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.
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
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.
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.