The potential damage to the intestinal cells is obvious. It is estimated that as few as 50,000 infective oocysts ingested by a young susceptible calf can cause severe disease. The replication of the coccidia within the host's intestinal cells and the subsequent rupture of the cells is responsible for the disease and the clinical signs that develop.
The severity of the disease is directly related to the dose of infective oocysts that are ingested. The more oocysts ingested, the more severe the subsequent disease. With light infections, the damage to the gut cells is minimal and because the cells in the gastrointestinal tract are replaced rapidly the damage is quickly repaired. In the case of heavy infections, about two weeks after the oocysts are ingested, most of the epithelial cells at the base of the intestinal glands are occupied by meronts or gametocytes. As these cells rupture, damage is severe and there is loss of blood into the feces. Also, fluid, electrolytes, and blood proteins (albumin) are lost.
Most animals infected with coccidia do not show signs of illness. This is due to the normally low dose and after a course of infection the animal is immune to that particular Eimeria species. However, this does not mean they are immune to all Eimeria species. Therefore, coccidiosis is primarily a disease of the young where there is crowding, stress, and/or nonimmune animals. Older cows certainly act as a reservoir and shed oocysts into the environment. Stress such as shipping, weaning, dietary changes, steroid therapy, and other problems can precipitate an outbreak of coccidiosis. Older cattle immune to their own endemic species of coccidia can become infected and/or ill when moved to a new herd and exposed to a different species.