Horses can get coccidiosis, but it is uncommon and rarely causes serious illness. Only one valid species of coccidia, Eimeria leuckarti, infects horses, donkeys, and zebras. Unlike cattle, poultry, and goats, where coccidiosis is a major concern, horses tend to carry the parasite without developing significant clinical disease.
How Common Is It in Horses?
Prevalence varies widely depending on geography, age, and management practices. A survey of Kentucky horse farms found that 41% of foals tested positive for coccidia oocysts (the egg-like stage shed in manure), and 85% of farms had at least one positive foal. Studies from other countries report lower rates: around 9% of horses in Poland, roughly 3% in Turkey, and less than 1% in parts of Iran. The wide range reflects differences in climate, stocking density, and hygiene rather than any real geographic limitation of the parasite. Eimeria leuckarti exists wherever horses do.
Despite these detection rates, confirmed cases of clinical disease are rare. Most positive results come from routine fecal testing and never progress to illness. Horses appear to develop immunity relatively quickly, which is why adult infections are especially uncommon.
Foals Are Most at Risk
Foals are far more likely than adult horses to shed oocysts and, in unusual cases, show signs of disease. Their immune systems are still developing, and they encounter the parasite for the first time in contaminated pastures or stalls. Most foals pick up the infection during their first few months of life, shed oocysts briefly, and move on without any noticeable symptoms. By the time they mature, they typically carry enough immunity to keep the parasite in check.
Adult horses that do test positive usually shed very low numbers of oocysts. Clinical coccidiosis in a healthy adult horse is genuinely rare and would raise questions about underlying immune problems or concurrent disease.
How Horses Get Infected
Transmission follows the fecal-oral route. An infected horse sheds oocysts in its manure. Those oocysts need time in the environment to mature (a process called sporulation) before they become infectious. Once sporulated, they can survive in soil and bedding for extended periods. A horse ingests them while grazing contaminated pasture, eating off the ground, or drinking from fouled water sources.
Crowded conditions, wet environments, and poor manure management increase the concentration of oocysts and raise the odds of exposure. Foals housed in stalls or small paddocks with other young horses face the highest risk simply because oocysts accumulate in a confined space.
Symptoms of Equine Coccidiosis
When coccidiosis does cause illness in a horse, the damage happens in the lining of the intestine. The parasite invades and destroys cells in the gut wall, triggering inflammation. Possible symptoms include:
- Diarrhea, which can range from mild and watery to profuse
- Weight loss, particularly in young or already compromised animals
- Poor coat quality and a generally unthrifty appearance
- Swelling under the belly (ventral edema), caused by protein loss through the damaged gut
- Intestinal bleeding in severe cases, sometimes visible as blood in the stool
In mild infections, the only sign may be slightly loose stool and reduced growth in a foal. Severe cases with hemorrhage and dehydration are the exception, not the rule. Many horses shed oocysts without showing any symptoms at all.
Diagnosis
Coccidiosis is diagnosed through a fecal flotation test, the same basic technique used to detect worm eggs. A small manure sample is mixed with a dense solution that causes parasite eggs and oocysts to float to the surface, where they can be examined under a microscope. Eimeria leuckarti oocysts are distinctively large and dark, making them relatively easy to identify once a veterinarian knows to look for them.
The challenge is that finding oocysts in manure does not automatically mean the horse is sick from coccidiosis. Because most infections are subclinical, the diagnosis of clinical coccidiosis requires matching the fecal findings with actual symptoms, particularly diarrhea, and ruling out other more common causes like bacterial infections, sand ingestion, or parasitic worms.
Treatment and Management
Because clinical coccidiosis in horses is so infrequent, there is no widely standardized treatment protocol specific to equine coccidiosis the way there is for cattle or poultry. When treatment is needed, veterinarians typically turn to antiprotozoal drugs from the triazine family, which work by disrupting the parasite’s ability to reproduce inside gut cells. Supportive care for dehydration and inflammation may also be necessary if the horse has significant diarrhea.
Prevention matters more than treatment for most horse owners. Keeping stalls and paddocks clean, removing manure regularly, avoiding overcrowding, and ensuring foals have dry, well-drained living areas all reduce oocyst buildup in the environment. Rotating pastures when possible gives oocysts less chance to accumulate to dangerous levels. These same practices also help control intestinal worms, so the effort does double duty.
Why It’s Less of a Problem in Horses Than Other Livestock
Coccidiosis is a devastating disease in poultry, calves, lambs, and goat kids, where it can cause outbreaks with high mortality. Horses are different for a few reasons. They host only a single valid coccidia species, while cattle and poultry host many, some highly pathogenic. Eimeria leuckarti also appears to be less aggressive in how it damages the gut compared to species that affect other livestock. And horses seem to develop protective immunity efficiently, limiting the window of vulnerability to early life.
If you’re seeing diarrhea or weight loss in a foal, coccidiosis belongs on the list of possibilities, but it’s far less likely to be the culprit than common bacterial or viral infections. In adult horses with chronic diarrhea, coccidia would be an unusual explanation. A fecal test can quickly clarify whether the parasite is present, and your veterinarian can determine whether it’s actually contributing to the problem or just an incidental finding.