Coccidiosis in Cows: Bloody Diarrhea in Calves and Prevention
- Coccidiosis is an intestinal parasite disease, usually caused by Eimeria species, that most often affects calves and recently weaned young cattle.
- Common signs include watery or bloody diarrhea, straining, dehydration, weight loss, weakness, and reduced growth after stress, crowding, or manure exposure.
- See your vet promptly if a calf has blood in the stool, severe diarrhea, weakness, or is not drinking. Young calves can decline fast.
- Treatment often combines an anticoccidial drug with supportive care such as oral or IV fluids, nursing care, and management changes for the whole group.
- Prevention focuses on clean, dry housing, lower stocking density, reduced fecal contamination of feed and water, and strategic use of labeled coccidiostats when your vet recommends them.
What Is Coccidiosis in Cows?
Coccidiosis is a parasitic disease of the intestinal tract caused by Eimeria organisms. In cattle, the most important species linked with clinical disease are Eimeria bovis and Eimeria zuernii. Calves and recently weaned young stock are affected most often, especially when they are grouped, stressed, or exposed to manure-contaminated environments.
The parasite damages the lining of the intestine as it multiplies. That damage can lead to diarrhea, dehydration, pain with defecation, and sometimes bloody manure. Even when calves survive, they may lose weight, grow more slowly, and become more vulnerable to other illnesses.
Not every calf that sheds coccidia looks sick. Some animals carry and spread oocysts in manure without obvious signs, which is one reason outbreaks can move through a pen or group quickly. Because of that, coccidiosis is often both an individual calf problem and a herd-management problem.
Symptoms of Coccidiosis in Cows
- Watery diarrhea, often starting mild and becoming persistent
- Mucus or frank blood in the manure
- Straining to pass stool, sometimes with frequent small amounts
- Dehydration, sunken eyes, or tacky gums
- Poor appetite or reduced milk/feed intake
- Weight loss, poor weight gain, or a rough hair coat
- Weakness, depression, or lagging behind the group
- Tail and hindquarters soiled with manure
- In severe cases, anemia, collapse, or death
- Rarely, nervous system signs may occur in severe outbreaks
Mild cases may look like ordinary calf scours at first, but blood in the stool, repeated straining, weakness, or dehydration are more concerning. See your vet immediately if a calf is down, very weak, not nursing or eating, or passing large amounts of blood. Even calves with milder signs should be checked promptly, because early treatment and group-level management can limit losses.
What Causes Coccidiosis in Cows?
Coccidiosis starts when calves swallow infective coccidia oocysts from contaminated manure, bedding, feed, water, or pen surfaces. Once inside the intestine, the parasites multiply and damage the gut lining. Heavy exposure raises the risk of clinical disease, but stress often plays a major role in whether a calf becomes visibly ill.
Common risk factors include overcrowding, wet or dirty bedding, poor drainage, manure buildup, contaminated feed bunks or waterers, transport, weaning, weather stress, and commingling. Outbreaks are especially common when calves are grouped in conditions that allow manure to accumulate and spread.
Older cattle may shed oocysts without looking sick, which can contaminate calving and rearing areas for younger animals. That means prevention is not only about treating sick calves. It also involves reducing environmental contamination and lowering stress across the group.
How Is Coccidiosis in Cows Diagnosed?
Your vet will usually start with the calf’s age, housing history, stress events, manure appearance, and hydration status. Bloody diarrhea and straining in a recently weaned or grouped calf can raise suspicion, but those signs are not specific to coccidiosis. Other causes of diarrhea, including salmonellosis, bovine viral diarrhea, dietary problems, and other intestinal disease, may need to be considered.
Diagnosis often includes a fecal exam or fecal flotation to look for coccidia oocysts. A positive fecal test can support the diagnosis, but timing matters. Some calves with severe intestinal damage may not be shedding large numbers at the exact moment they are tested, while others may shed oocysts without being clinically ill.
Because of that, your vet may make a presumptive diagnosis based on the full picture rather than one test result alone. In severe or unclear cases, additional testing, bloodwork, or postmortem evaluation may be recommended to rule out other diseases and guide herd-level treatment and prevention.
Treatment Options for Coccidiosis in Cows
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Farm call or herd consult if needed, or exam of the affected calf
- Fecal testing when available
- Labeled oral anticoccidial selected by your vet, often group-based when multiple calves are exposed
- Oral fluids, electrolytes, nursing care, and close monitoring
- Immediate pen cleanup, fresh bedding, and reduced manure exposure
Recommended Standard Treatment
- Veterinary exam and herd-level treatment plan
- Fecal testing and additional diagnostics as needed
- Prescription or labeled anticoccidial therapy chosen by your vet
- Oral or injectable supportive care based on hydration and condition
- Treatment of secondary problems when indicated by your vet
- Written prevention plan for bedding, drainage, stocking density, and feed/water hygiene
Advanced / Critical Care
- Urgent veterinary assessment for severe bloody diarrhea, collapse, or marked dehydration
- IV fluids and intensive supportive care
- Bloodwork and broader diagnostics to assess anemia, dehydration, and competing diagnoses
- Possible blood transfusion in life-threatening anemia or severe dysentery
- Hospitalization or repeated on-farm rechecks
- Expanded herd investigation and prevention planning after the crisis
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Coccidiosis in Cows
Bring these questions to your vet appointment to get the most out of your visit.
- Does this calf’s history and manure pattern fit coccidiosis, or should we also test for salmonella, BVD, or other causes of diarrhea?
- Which calves should be treated right now, and should the whole pen or group be managed as exposed?
- What fecal test do you recommend, and how should I collect and store samples before they are submitted?
- Which labeled anticoccidial options are appropriate for these calves, and what meat or milk withdrawal times apply?
- How dehydrated is this calf, and can we manage with oral fluids or do you recommend IV fluids?
- What changes in bedding, drainage, stocking density, and feeder or waterer setup would lower reinfection risk on this farm?
- Should we use a preventive coccidiostat during weaning, transport, weather stress, or other high-risk periods?
- What signs mean this calf needs to be rechecked immediately or moved to more intensive care?
How to Prevent Coccidiosis in Cows
Prevention starts with manure control and stress reduction. Keep calf areas as clean and dry as possible, provide generous bedding, improve drainage, and avoid overcrowding. Feed bunks and water sources should be positioned and maintained to reduce fecal contamination. Group movement works best when calves enter clean pens together and stay with the same group rather than being mixed repeatedly.
High-risk periods include weaning, transport, severe weather, feedlot arrival, and any time calves are crowded or commingled. During those times, your vet may recommend a labeled preventive coccidiostat for the group. Products used in cattle prevention programs can include amprolium, decoquinate, or certain ionophores in appropriate classes of cattle, depending on label directions and your operation’s goals.
Good prevention also means watching for early signs. A calf with mild diarrhea today may be the first clue that many penmates have already been exposed. Prompt veterinary guidance, fast cleanup, and a practical herd plan can reduce illness, limit environmental contamination, and protect future groups of calves.