Coccidiosis is a problem that almost all shepherds experience one time or another.  Here is a great article that brings out some of the facts of coccidiosis.

Treatment And Control of Coccidia in Sheep
by Helen A. Swartz
State Sheep, Goat and Livestock Specialist
Lincoln University Cooperative Extension

Definition

Coccidia are tiny internal parasites (Elmeriaspp) that live in the cells of the sheep's intestines. The tiny cocysts containing the infective stages are passed in the feces and are picked up by other sheep through contaminated feed and water. The number of parasites that invade the intestinal tract determine the severity of disease that results in the condition called coccidiosis.

Symptoms

Coccidia are considered internal parasites but are resistant and non-responsive to dewormers used for internal parasites in sheep. Practically all sheep tolerate a low level of coccidia with no adverse effects. A high level of coccidia, especially in lambs, damages the intestinal lining resulting in improper or reduced absorption of nutrients and weight loss. This damage can also result in bloody and dark diarrhea, causing dehydration and death. In some cases, very little diarrhea, if any, has been observed. Stress induced from changes in weather or sudden changes in feeding, such as from pasture to dry lot, will often result in a severe coccidia outbreak.

Causes

Coccidiosis is generally seen in intensified sheep management operations. Sheep can be managed profitably in an intensive operation by practicing proper sanitation and sufficient space allotment for ewes and lambs. If coccidia are suspect in your flock, collect some fresh fecal samples and take them to your veterinarian for analysis of call for advice.

Treatment

Outbreaks of coccidiosis are usually treated with sulfa drugs and amprolium. These drugs must be prescribed by your veterinarian. The treatment for the outbreak of the disease should not be confused with the drugs used for control. Control Bovatec (lasalocid) has been approved for sheep as a control coccidia infestation. However, the veterinarian can prescribe drugs that are not approved for use in sheep by the Food and Drug Administration, such as monensin (Rumensin) and decoquinate (Deccox) if you have a good client/patient relationship. These drugs are used as a preventative not a treatment.
Bovatec and monensin have been used to slightly improve feed efficiency in lambs.

TREATMENT OF COCCIDIOSIS
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PREVENTION OF COCCIDIOSIS

Summary

Sheep and especially lambs may be infected with coccidia that may be confused with internal parasites. Contact your local veterinarian if your sheep have symptoms of coccidia that may cause sudden death.