Infection with Eimeria is one of the most economically important diseases of sheep. Historically, some Eimeria spp were thought to be infectious and transmissible between sheep and goats, but the parasites are now considered host-specific. The names of some species of goat coccidia are still erroneously applied to species of similar appearance found in sheep. E crandallis and E ovinoidalis (ninakohlyakimovae) are pathogens of lambs (usually 1–6 mo old); E ovina appears to be somewhat less pathogenic. Older sheep serve as sources of infection for the young. All other Eimeria of sheep are essentially nonpathogenic, even when large numbers of oocysts are present in feces.
Signs include diarrhea (sometimes containing blood or mucus), dehydration, fever, inappetence, weight loss, anemia, wool breaking, and death. The ileum, cecum, and upper colon are usually most affected and may be thickened, edematous, and inflamed; sometimes, there is mucosal hemorrhage. Thick, white, opaque patches containing large numbers of E ovina oocysts may develop in the small intestine. Because oocysts are prevalent in feces of sheep of all ages, coccidiosis cannot be diagnosed based solely on finding oocysts. Peak oocyst counts of >100,000/g of feces have been reported in 8- to 12-wk-old lambs that appeared healthy. However, diarrhea with oocyst counts of a pathogenic species of >20,000/g is characteristic of coccidiosis in sheep. Immune complex glomerulonephritis has also been attributed to coccidiosis. Fly strike and secondary bacterial enteric infections may accompany coccidiosis in lambs.
Lambs 1–6 mo old in lambing pens, intensive grazing areas, and feedlots are at greatest risk as a result of shipping, ration change, crowding stress, severe weather, and contamination of the environment with oocysts from ewes or other lambs. Because occurrence of coccidiosis under these management systems often becomes so predictable, coccidiostats should be administered prophylactically for 28 consecutive days beginning a few days after lambs are introduced into the environment. A concentrated ration containing monensin at 15 g/tonne can be fed to ewes from 4 wk before lambing until weaning, and to lambs from 4–20 wk of age. The toxic level of monensin for lambs is 4 mg/kg. Lasalocid (15–70 mg/head/day, depending on body wt) may be effective. A combination of monensin and lasalocid at 22 and 100 mg/kg of diet, respectively, is an effective prophylactic against naturally occurring coccidiosis in early weaned lambs under feedlot conditions.
Treatment of affected sheep once coccidiosis has been diagnosed is not effective, but severity can be reduced if treatment is begun early. A single treatment of toltrazuril (20 mg/kg) can significantly reduce the oocyst output in naturally infected lambs for ~3 wk after administration. Diclazuril (1 mg/kg) is an effective oral anticoccidial in lambs and is administered once at ~6–8 wk of age (most common) or twice (at 3–4 wk of age and again 3 wk later). Sulfaquinoxaline in drinking water at 0.015% concentration for 3–5 days may be used to treat affected lambs. In groups of lambs at pasture, frequent rotation of pastures for parasite control also helps control coccidial infection. However, when lambs are exposed to infection early in life as a result of infection from the ewe and a contaminated lambing ground, a solid immunity usually develops and problems are seen only when the stocking density is extremely high.