Coccidiosis in Horses: Eimeria Infection and Foal Diarrhea
- Coccidiosis in horses is usually linked to the protozoan parasite *Eimeria leuckarti*, most often in foals and young horses.
- Many infected horses have no obvious signs, but some foals develop diarrhea, poor weight gain, dehydration, or a rough hair coat.
- Diagnosis usually starts with a physical exam and fecal testing, but routine fecal flotation can miss this parasite because the oocysts are large and heavy.
- Treatment is often supportive first: fluids, nursing care, and monitoring. Your vet may also discuss anti-protozoal medication in selected cases.
- Repeated diarrhea, weakness, fever, or signs of dehydration in a foal should prompt a same-day call to your vet.
What Is Coccidiosis in Horses?
Coccidiosis is an intestinal infection caused by microscopic protozoal parasites called coccidia. In horses, the species most often discussed is Eimeria leuckarti. It infects the lining of the small intestine and is found in horses and other equids worldwide. Most reported infections are mild or even incidental, but some foals and young horses can develop diarrhea and intestinal inflammation.
This condition matters most when a foal has diarrhea and your vet is working through a long list of possible causes. Foal diarrhea can come from parasites, viruses like rotavirus, bacteria, diet changes, stress, or noninfectious intestinal disease. That means a positive fecal test for Eimeria does not always prove it is the only problem.
For pet parents, the practical takeaway is this: coccidiosis is real, but it is not the most common explanation for every loose stool in a foal. Your vet will look at the whole picture, including age, hydration, fever, appetite, farm history, and whether other foals are affected.
Symptoms of Coccidiosis in Horses
- Loose stool or diarrhea
- Intermittent diarrhea that comes and goes
- Poor weight gain or weight loss
- Dull or rough hair coat
- Reduced nursing or decreased appetite
- Lethargy or weakness
- Dehydration, tacky gums, or sunken eyes
- Fever
Some horses with Eimeria leuckarti shed oocysts and look completely normal. When illness does happen, it is more likely to affect foals or young horses, and diarrhea may be mild, intermittent, or part of a more complicated intestinal problem.
Call your vet promptly if your foal has repeated diarrhea, stops nursing well, seems weak, develops a fever, or shows any signs of dehydration. Diarrhea in horses can lead to major fluid loss quickly, and even a short period of worsening stool can become serious.
What Causes Coccidiosis in Horses?
Horses become infected by swallowing infective, sporulated oocysts from contaminated manure, water, feed, bedding, or surfaces in the environment. After the parasite is ingested, it invades intestinal cells and reproduces there. The oocysts passed in fresh manure are not immediately infective. They must first sporulate in the environment, which for Eimeria leuckarti can take about three weeks under favorable conditions.
Because of that life cycle, manure buildup and damp, contaminated housing can increase exposure over time. Young horses are more likely to be affected because their immune systems are still developing and they may have heavier environmental exposure.
It is also important to know that finding Eimeria does not always mean it is the main cause of diarrhea. Some infected horses have no clinical signs, and foals may have more than one problem at once. Your vet may also consider rotavirus, salmonellosis, clostridial disease, sand, dietary upset, or other intestinal disorders depending on the history and exam.
How Is Coccidiosis in Horses Diagnosed?
Diagnosis starts with a full exam and a careful history. Your vet will ask about the foal's age, nursing, manure consistency, fever, recent stress, deworming history, and whether other horses on the property have diarrhea. Bloodwork may be used to look for dehydration, inflammation, protein loss, or electrolyte changes.
Fecal testing is the main way to look for Eimeria leuckarti oocysts, but this parasite can be easy to miss. The oocysts are unusually large and heavy, so routine flotation methods may not recover them well. Your vet may request a concentrated flotation method, higher-specific-gravity solution, sedimentation, repeat fecal exams, or submission to a diagnostic laboratory if suspicion remains high.
Because foal diarrhea has many possible causes, diagnosis often includes ruling out other diseases rather than relying on one test alone. In severe or unclear cases, your vet may recommend fecal PCR panels for infectious diarrhea, ultrasound, or additional testing to guide isolation, treatment, and prognosis.
Treatment Options for Coccidiosis in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Farm call or clinic exam
- Basic fecal testing, often including flotation or repeat fecal checks
- Hydration assessment and temperature monitoring
- Oral fluids if appropriate and directed by your vet
- Short-term nursing care, manure cleanup, and isolation from vulnerable foals
Recommended Standard Treatment
- Exam plus CBC and chemistry panel
- Targeted fecal testing through a diagnostic lab
- Supportive care such as oral or IV fluids depending on hydration
- Electrolyte support and close monitoring
- Discussion of anti-protozoal treatment if your vet believes coccidia are clinically important
- Biosecurity steps and recheck plan
Advanced / Critical Care
- Hospitalization or intensive ambulatory care
- IV fluids and repeated electrolyte monitoring
- Expanded infectious diarrhea testing and imaging
- Plasma or additional supportive care if protein loss or systemic illness is present
- Frequent reassessment for sepsis, endotoxemia, or another primary intestinal disease
- Strict isolation and nursing support
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Coccidiosis in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Does my foal's fecal test strongly support *Eimeria leuckarti*, or could this be an incidental finding?
- What other causes of foal diarrhea are most likely in this case?
- Is my foal dehydrated enough to need IV fluids, or is home monitoring reasonable?
- Which fecal method was used, and do we need repeat testing or a diagnostic lab submission?
- Should we isolate this foal from others on the property right now?
- Are anti-protozoal medications appropriate here, or is supportive care the main treatment?
- What signs would mean this has become an emergency today?
- What cleaning and manure-management steps will lower exposure for other foals?
How to Prevent Coccidiosis in Horses
Prevention focuses on reducing manure contamination and limiting a foal's exposure to infective oocysts. Pick up manure regularly in stalls, paddocks, and high-traffic turnout areas. Keep feed off the ground when possible, clean water sources often, and avoid overcrowding young horses in wet or heavily contaminated spaces.
Because Eimeria leuckarti oocysts need time in the environment before they become infective, sanitation matters. Frequent manure removal can interrupt that cycle. Good drainage, dry bedding, and routine cleaning of foaling and nursery areas also help reduce overall infectious pressure.
There is no routine vaccine for equine coccidiosis. Strategic parasite control, strong colostrum management, and prompt veterinary evaluation of any foal with diarrhea are more useful than trying to treat every loose stool the same way. If one foal on the farm develops diarrhea, ask your vet whether isolation, fecal testing of herd mates, or broader biosecurity steps make sense for your setup.