Diarrhea in cattle stems from infections, diet problems, or chronic disease, and the specific cause depends heavily on the animal’s age. In calves under a month old, the culprits are almost always infectious: bacteria, viruses, and parasites that target the immature gut. In older cattle, dietary disruptions like grain overload and chronic conditions like Johne’s disease become more likely. Understanding which causes hit at which life stage is the fastest way to narrow down what you’re dealing with.
Age Determines the Most Likely Cause
In neonatal calves, the pathogens responsible for scours follow a remarkably predictable timeline. Enterotoxigenic E. coli (commonly called K99 E. coli) strikes earliest, typically causing profuse, watery diarrhea in calves just 1 to 4 days old. It can progress to shock and death quickly. Rotavirus and coronavirus overlap next, hitting calves from about 4 to 14 days and 4 to 30 days of age, respectively. Cryptosporidium, a parasitic infection, fills the 1- to 4-week window. Salmonella can cause severe illness in calves under a month old, though it also affects adults.
This timeline matters practically. If you’re losing calves in their first few days, bacterial causes top the list. If diarrhea appears in the second or third week of life, viruses and parasites become more likely. Multiple pathogens often overlap, and mixed infections are common.
Viral Causes
Bovine rotavirus and bovine coronavirus are the two most significant viral causes of acute calf diarrhea worldwide. Rotavirus targets the lining of the small intestine, destroying the cells responsible for absorbing nutrients. The result is malabsorption diarrhea: fluid and nutrients pass straight through the gut instead of being taken up. This damage also makes the calf more vulnerable to secondary infections, compounding the problem.
Coronavirus affects both calves and adults, though it presents differently by age. In young calves it causes acute diarrhea. In adult cattle, particularly during cold months, it causes winter dysentery, a contagious outbreak of diarrhea that can sweep through an entire herd. The virus has significant genetic diversity, which makes it a persistent challenge across regions and seasons.
Bovine viral diarrhea virus (BVDV) is a separate and less common cause of neonatal scours, but it deserves attention because persistently infected calves, those infected in the womb, can shed the virus continuously and serve as a reservoir for the rest of the herd.
Bacterial Causes
Enterotoxigenic E. coli is the most common bacterial cause of neonatal calf diarrhea. These strains carry a surface protein called K99 that lets them attach to the intestinal wall, where they release toxins that force the gut to secrete massive amounts of fluid. The diarrhea is watery and can be fatal within hours if fluids aren’t replaced. A second type, attaching and effacing E. coli, produces diarrhea in calves from 2 days to about 6 weeks of age through a different mechanism: it damages the intestinal lining directly rather than relying on toxin secretion.
Salmonella is the other major bacterial player. The two serotypes most commonly isolated from cattle are Salmonella typhimurium and Salmonella dublin. They behave differently. S. typhimurium tends to cause acute diarrheal disease, often with blood and mucus in the stool. S. dublin is more likely to cause systemic illness, spreading beyond the gut into the bloodstream, particularly in young calves. Salmonella infections can also affect adult cattle and are a zoonotic risk, meaning they can spread to people handling sick animals.
Parasitic Causes
Cryptosporidium is the most important parasitic cause of calf diarrhea. It infects the cells lining the gastrointestinal tract and causes acute, profuse, watery diarrhea along with depression, weakness, and loss of appetite. In neonatal calves, dehydration from cryptosporidiosis can be fatal. The parasite spreads through oocysts shed in feces, which are extremely hardy in the environment and resistant to many common disinfectants. This makes it difficult to eliminate from calving areas once it’s established.
Coccidiosis, caused by Eimeria species, is another protozoal disease that primarily affects calves and young cattle. It tends to hit slightly older animals than cryptosporidiosis, typically around 3 weeks to 6 months of age, and causes diarrhea that can range from mild to bloody. Coccidiosis thrives in crowded, contaminated environments. In Estonian dairy herds, calf mortality from coccidiosis alone accounted for 8% to 9% annual losses.
Dietary Causes and Ruminal Acidosis
In older cattle, especially feedlot animals, diet is a leading cause of diarrhea. Ruminal acidosis occurs when cattle consume too much grain or other rapidly fermentable carbohydrates without enough fiber to balance digestion. The rumen becomes excessively acidic, which disrupts normal gut motility and leads to diarrhea along with dehydration.
Acidosis ranks second only to respiratory disease as a cause of reduced performance in feedlot cattle. The triggers are straightforward: high-grain diets, finely processed grain, sudden diet changes, inadequate fiber, and interruptions in normal feeding patterns. Warning signs before full-blown acidosis develops include reduced chewing activity, decreased feed intake, weight loss, and rising rates of lameness. In severe cases, the damage extends well beyond the gut. Chronic acidosis can lead to liver abscesses, heart and kidney infections, and neurological symptoms.
Prevention centers on gradual diet transitions, adequate fiber in the ration, and consistent feeding schedules. Any abrupt shift toward higher-energy feeds, whether intentional or accidental (such as cattle breaking into a grain bin), can trigger an episode.
Johne’s Disease in Adult Cattle
Johne’s disease is a chronic, progressive infection caused by the bacterium Mycobacterium avium subspecies paratuberculosis. Cattle are typically infected as calves through ingestion of contaminated feces or milk, but clinical signs don’t appear until adulthood, often years later. The hallmark is persistent diarrhea that does not respond to treatment, accompanied by steady weight loss despite a normal appetite. The disease is always fatal once clinical signs develop.
One of the biggest challenges with Johne’s disease is diagnosis. Tests for this bacterium have notoriously poor sensitivity at the individual animal level. Blood-based ELISA tests detect only about 7% to 27% of infected animals, depending on the test and stage of disease. Fecal culture is more reliable but still misses a majority of light shedders. PCR testing of feces picks up roughly 76% of heavy shedders but only about 4% of animals shedding lower levels of bacteria. This means many infected cattle go undetected, silently spreading the organism through the herd for years before anyone realizes there’s a problem.
Herd-level testing improves these odds somewhat. Testing bulk milk tanks or pooling fecal samples from multiple animals increases the chance of catching at least one positive, but no single test is definitive. Control programs rely on repeated testing, culling of clinical animals, and strict hygiene in calving areas to reduce transmission from cow to calf.
Dehydration Is the Immediate Threat
Regardless of the underlying cause, dehydration is what kills cattle with diarrhea, especially calves. A scouring calf can lose fluid equivalent to several percent of its body weight in a single day. Oral electrolyte solutions are the first line of defense for calves that are still able to stand and suckle. These solutions work by pairing sodium with glucose to drive water absorption across the intestinal wall, even when the gut is damaged. The recommended glucose-to-sodium ratio in these solutions, borrowed from research in human infants, falls between 1.0 and 3.0, with an optimum around 1.4.
Calves that are too weak to drink, or that have lost so much fluid they can no longer stand, need intravenous fluids. The speed of intervention matters enormously. A calf with E. coli scours at 2 days old can go from healthy to critical in under 12 hours, while a calf with rotavirus at 10 days old may have a slightly wider window but still deteriorates fast without support.
Environmental and Management Factors
Crowding, poor sanitation, and inadequate colostrum intake amplify every infectious cause on this list. Calves that don’t receive enough quality colostrum in the first hours of life lack the circulating antibodies that protect them during their most vulnerable weeks. Wet, muddy calving areas concentrate pathogens where newborns are most exposed. Mixing age groups allows older, subclinically infected animals to shed organisms that overwhelm younger ones.
Stress also plays a measurable role. Transport, weaning, weather extremes, and changes in social grouping all suppress immune function and can trigger diarrhea outbreaks even when pathogen loads haven’t changed. In feedlot settings, the combination of transport stress, diet change, and commingling of animals from different sources creates a perfect setup for both infectious and nutritional diarrhea simultaneously.