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Clostridium perfringens are Gram-positive, spore-forming, anaerobic bacteria that are very commonly found in many environments, including soil, water, poorly preserved feeds, contaminated or improperly thawed colostrum or milk, calf-housing environments, and the normal bovine intestinal tract. C. perfringens proliferate the gut, the bacteria secrete toxins that have profound effects not only on the local intestinal environment (causing damage to the intestinal lining), but throughout the body as well. Death occurs when high levels of these bacterial toxins enter the bloodstream, leading to inflammation, shock, and cardiac arrest. C. perfringens are not spread from calf to calf, but it is not uncommon for several calves in a group to be affected at the same time, due to similar exposure and management practices.

Cryptosporidium are protozoan parasites much smaller than coccidia. After ingestion Cryptosporidium adheres to the lining of the intestine wall and causes damage to the microvilli, the finger-like projections that are important for absorption of water and nutrients. Cryptosporidium typically does not kill calves except by causing severe dehydration, electrolyte imbalance and acidosis. There is no labeled medication for Cryptosporidium so prevention is important. Treatment of affected calves consists of supportive fluid and electrolyte therapy and antimicrobial therapy to prevent secondary bacterial infection. Calves infected with Cryptosporidium can be saved with good nursing care. Cryptosporidium is also infectious to people so handle infected calves with care.

Rotavirus is one of the most common viruses associated with calf diarrhea. Diarrhea usually develops between 3 and 7 days of age. There may be a drooling of saliva and profuse watery diarrhea. The feces will vary in color from yellow to green. Rotavirus infects cells essential to the absorption of nutrients in the small intestine. The lack of small intestine nutrient absorption causes nutritional deficiencies for the calf, and interferes with the rest of the digestive tract’s ability to absorb water. Not all calves with rotavirus have diarrhea. Calves lose their appetite and the death rate may be high. Death losses are consistently associated with pronounced dehydration. The infection may be short lived but the intestinal lining has to recover from damage.

Coronavirus is a cause of diarrhea in young calves. The virus is able to infect epithelial cells of the small intestine (gut), causing them to die. Moderate to severe diarrhea will follow, lasting for 4 to 7 days. Affected calves are usually between 1 and 3 weeks of age. The calf may have a high temperature or in some cases abnormally low temperature. The calf loses the ability to digest and absorb milk. Coronavirus also damages the cells in the intestinal crypts and slows down the healing process in the intestinal lining. Furthermore, the damage caused by Coronavirus is often compounded by bacterial infections and the risk for fatal diarrhea is increased when mixed infections occur. Infected calves are severely depressed with excessive drooling and watery diarrhea; fecal color varies from yellow to green. The abdomen may be drawn in and drugs have practically no effect on the diarrhea. The calf may strain to defecate or feces are forcefully ejected.

The Salmonella species of bacteria have been associated with calf enteritis but like E. coli, Salmonella has a strong tendency to spread beyond the gut and cause widespread disease. The result of this invasion is damage to the tissue, so that water and food cannot be absorbed. Salmonella can readily invade the rest of the body, causing blood poisoning and rapid death. Salmonella produces a potent toxin or an endotoxin (poison) within its own cells. Clinical signs associated with Salmonellosis include bloody diarrhea, fibrin in the feces, depression and elevated temperature. The disease is more severe in young or debilitated calves. Infected calves can shed the organism in feces, urine, saliva and nasal secretions, contaminating everything they touch and everything that touches them.