Coccidia (sometimes misspelled “coxidia”) are microscopic single-celled parasites that infect the intestinal lining of animals, causing a disease called coccidiosis. They belong to the phylum Apicomplexa and are classified as obligate intracellular parasites, meaning they can only survive and reproduce inside a host’s cells. The two most common genera are Eimeria and Cystoisospora (formerly called Isospora), and nearly all species are host-specific, meaning the coccidia that infect a dog cannot infect a cat or a chicken.
How Coccidia Infect and Spread
The infection starts when an animal swallows tiny egg-like structures called oocysts, typically by sniffing or eating contaminated feces, soil, or prey animals. Fresh oocysts shed in feces aren’t immediately dangerous. They first need to go through a maturation process called sporulation, which requires air, moisture, and warmth. For most species this takes 24 to 48 hours, though under ideal warm and humid conditions it can happen in as little as 6 hours. In cooler environments, sporulation may take 7 to 10 days.
Once a sporulated oocyst is swallowed, digestive enzymes break it open and release the parasites inside. These organisms migrate to specific sections of the intestine, burrow into the cells lining the gut wall, and begin reproducing. The parasite goes through two to three rounds of asexual replication inside those cells, producing waves of new parasites that burst out and invade neighboring cells. After several rounds, the parasites shift to sexual reproduction, forming new oocysts that pass out in the animal’s feces to start the cycle over again. Each round of replication destroys intestinal cells, which is what causes the symptoms of disease.
Which Animals Are Affected
Coccidia infect a wide range of birds, mammals, and reptiles. Poultry, cattle, and swine are heavily impacted in agricultural settings, while dogs and cats are the most commonly affected companion animals. Young animals are far more vulnerable than adults because their immune systems haven’t yet developed resistance. In adult animals, coccidia often cause no visible illness at all, though the animal may still shed oocysts and spread infection to others.
Coccidiosis in poultry is one of the most economically significant parasitic diseases in agriculture. Infected broiler chickens show reduced growth rates and poor feed efficiency. Depending on the species involved, the infection can range from mild fluid loss and nutrient malabsorption to severe inflammation with hemorrhaging and sloughing of the intestinal lining.
Symptoms of Coccidiosis
The hallmark symptom is diarrhea, which can range from soft and watery to bloody depending on the severity of the infection and which part of the intestine is affected. In dogs and cats, the parasites can invade the lower small intestine, cecum, and colon, destroying the cells that line the intestinal villi. This leads to inflamed intestinal crypts, flattening of the absorptive surface, and villous atrophy, which in practical terms means the gut loses its ability to absorb water and nutrients properly.
Young puppies, kittens, calves, and chicks may develop dehydration, weight loss, poor appetite, and lethargy. In severe cases, especially in crowded or unsanitary conditions, coccidiosis can be fatal. Adult animals with healthy immune systems typically carry the infection without showing symptoms, but they act as silent spreaders in kennels, shelters, and farms.
How Coccidia Are Diagnosed
Veterinarians diagnose coccidiosis through a fecal flotation test. A small stool sample is mixed with water, filtered to remove debris, and then mixed with a special flotation solution (usually sugar water or salt water). Because oocysts are lighter than this solution, they float to the surface, where they’re collected on a glass coverslip and examined under a microscope. The number of oocysts per gram of feces helps gauge the severity of infection.
One limitation of this method is that fecal samples containing mucus can reduce detection accuracy by roughly half, according to laboratory comparisons of flotation techniques. A single negative test doesn’t always rule out infection, particularly early in the course of disease before the parasite has completed its first reproductive cycle and begun shedding oocysts.
Treatment Options
Treatment focuses on stopping the parasite’s reproduction and giving the animal’s gut time to heal. Sulfa-based drugs have been the traditional go-to for decades. Newer antiparasitic medications, particularly ponazuril (a derivative of toltrazuril), have become increasingly popular because they directly target the parasite’s reproductive stages rather than just slowing its growth.
A study of shelter dogs and cats with confirmed coccidiosis found that ponazuril given at 50 mg/kg daily for three days was the most effective dosing protocol, eliminating detectable oocyst shedding in about 93% of dogs and 88% of cats by day 3 to 4. A single dose at the same concentration was somewhat less effective (77% of dogs, 80% of cats), and a lower single dose cleared infection in only 69% of dogs and 48% of cats. Animals with very high oocyst counts at the start of treatment were significantly more likely to still be shedding parasites at the first recheck.
It’s worth noting that clearing the parasite from the stool doesn’t always mean diarrhea resolves immediately. In that same study, fecal consistency at the day 3 to 4 recheck wasn’t closely tied to whether the animal was still actively infected, suggesting that intestinal healing takes additional time even after the parasites are gone.
Why Coccidia Are Hard to Eliminate From the Environment
One of the most frustrating aspects of coccidia is how tough the oocysts are. Sporulated oocysts can survive for up to a year in moist, protected environments as long as they aren’t exposed to freezing temperatures or extreme heat. They resist most common household and kennel disinfectants. High-concentration ammonia solutions can destroy them, but the fumes are harsh enough that animals need to be removed from the area during treatment, making it impractical for many settings.
This environmental resilience is why reinfection is so common, especially in shelters, breeding facilities, and farms. Simply treating the animal without addressing the environment often leads to a revolving cycle of infection. Thorough physical cleaning to remove fecal material, proper ventilation to keep surfaces dry, and using clean water sources are the most effective strategies for reducing oocyst buildup.
Can Humans Get Coccidia?
The coccidia species that infect dogs, cats, and livestock (Eimeria, Cystoisospora) are host-specific and do not infect humans. You cannot catch coccidiosis from your puppy’s diarrhea or from handling infected animals.
However, the broader group of coccidia-related parasites does include a few species that infect people. Cryptosporidium parvum is a zoonotic species that can spread from animals to humans, and Cystoisospora belli (formerly Isospora belli) is a human-only parasite that causes intestinal illness, particularly in immunocompromised individuals. Cyclospora cayetanensis, another human coccidia, spreads exclusively between people and has no known animal host. These human-infecting species are distinct organisms from the ones your veterinarian treats in pets.
Preventing Coccidiosis
In kennels, shelters, and multi-pet households, prevention centers on breaking the fecal-oral cycle. Prompt removal of feces before oocysts have time to sporulate is the single most effective measure. Since sporulation can happen in as little as 6 hours under warm, humid conditions, daily cleaning is the minimum, and twice-daily cleaning is better in warm climates or indoor facilities.
Keeping living areas dry and well-ventilated slows sporulation considerably. Food and water bowls should be elevated or positioned away from areas where animals defecate. New animals entering a facility should ideally have a fecal test before being mixed with the general population, since healthy-looking adults can shed large numbers of oocysts without showing any signs of illness. For species like Cystoisospora in dogs and cats, there’s also an indirect transmission route: the parasite can form dormant cysts in the tissues of prey animals or rodents, which then infect a dog or cat that eats them.