The severity of the disease depends on a number of factors including the number of oocysts eaten, the species of coccidia present, the age of the animal, or if the animal has developed immunity due to a previous infection.
Coccidiosis occurs mainly in calves that are three weeks to six months of age and is usually accompanied by diarrhea varying in severity from watery manure to one containing blood. Animals affected with coccidiosis often strain due to irritation of the lower bowel and rectum. Blood may appear in the manure after the second or third day of diarrhea. Dehydration, weight loss, depression, loss of appetite and occasionally death may also be observed.
Infections that fail to produce signs of disease may nevertheless affect the growth and health of an animal by impairing intestinal function and feed conversion. Calves with only a light infection usually show no signs of disease, but shed oocysts in manure, so the oocysts accumulate in pastures, yards, barns or on the hair coats so that severe coccidiosis may develop when new calves are placed in these areas.
Cattle that recover from coccidiosis usually become immune to later infections, but they may continue to pass oocysts in the manure, thereby providing a source of infection for susceptible calves.