Coccidia are single-celled intestinal parasites that invade the lining of a cat’s gut, multiply inside the cells, and cause watery or bloody diarrhea, especially in kittens. The species that infect cats belong to the genus Cystoisospora (formerly called Isospora), and while the infection, called coccidiosis, can look alarming, it’s treatable and typically resolves within a week or two of starting medication.
How Cats Get Infected
Coccidia spread through a straightforward cycle. An infected cat sheds microscopic egg-like structures called oocysts in its feces. Once outside the body, these oocysts need warmth, moisture, and oxygen to become infectious, a process called sporulation. In ideal conditions this can happen in as little as six hours, though it more commonly takes seven to ten days. When another cat swallows sporulated oocysts, whether by sniffing contaminated litter, grooming dirty paws, or exploring a soiled environment, the parasite takes hold in the intestinal lining and begins reproducing.
Cats can also pick up coccidia by eating infected prey. Mice and other small animals can act as transport hosts, carrying the parasite without getting sick themselves. When a cat catches and eats one of these animals, the coccidia are released in the gut and begin their lifecycle. Research has even documented transmission from wild bobcats to domestic cats through shared prey animals, so outdoor and hunting cats face higher exposure.
Why Kittens Are Most at Risk
Healthy adult cats often carry coccidia without showing any symptoms at all. Their mature immune systems keep the parasite in check, limiting how much damage it can do to the intestinal lining. The real trouble shows up in kittens, particularly those under six months old, and in cats whose immune systems are compromised by stress, overcrowding, or other illness.
Kittens are vulnerable for two reasons: their immune defenses are still developing, and they tend to encounter the parasite in high doses in environments like shelters, catteries, or breeding facilities where many animals share space. In experimental settings, newborn cats exposed to high doses of the parasite developed diarrhea within three to four days of infection. That rapid onset explains why coccidiosis can escalate quickly in young kittens, especially when dehydration sets in.
Symptoms to Watch For
The hallmark sign is diarrhea, which can range from soft and mucousy to watery and streaked with blood. In mild cases, a kitten may simply have loose stools for a few days. In more severe infections, you’ll see:
- Watery or bloody diarrhea that persists for several days
- Dehydration, noticeable as sunken eyes, dry gums, or skin that doesn’t snap back when gently pinched
- Loss of appetite and weight loss
- Lethargy or a kitten that seems unusually weak
- A dull, rough coat in kittens that have been infected for a while
Adult cats shedding coccidia often look completely normal. You may never know they’re infected unless a routine fecal test picks it up. This silent shedding is one reason the parasite circulates so effectively in multi-cat households.
How Coccidia Is Diagnosed
Veterinarians diagnose coccidiosis by examining a stool sample under a microscope. The standard method is a fecal flotation test, where a small amount of feces is mixed with a special solution that causes oocysts to float to the surface, where they can be collected on a glass slide and identified. The oocysts are oval, clear, and distinctive enough for a trained eye to spot.
One catch: cats don’t start shedding oocysts the moment they’re infected. There’s a gap, called the prepatent period, between when a cat swallows the parasite and when oocysts appear in the stool. If a kitten has diarrhea but the first fecal test comes back clean, your vet may recheck a few days later or start treatment based on clinical suspicion, particularly if the kitten came from a shelter or breeder environment where coccidia is common.
Treatment and Recovery
Coccidiosis is treated with antiparasitic medications that either stop the parasite from reproducing or kill it outright. Drugs that directly kill the organism tend to work faster and require a shorter course of treatment. In one widely cited protocol, treated cats showed a rapid drop in oocyst shedding along with noticeable improvement in stool consistency within just a few days.
Most kittens start feeling better within three to five days of beginning medication. The full course of treatment typically lasts five to fourteen days depending on the drug used and the severity of infection. Kittens with significant dehydration may need fluid support during the first day or two to stabilize while the medication takes effect.
Clearing the environment matters just as much as treating the cat. Oocysts are tough. They resist most standard household disinfectants. Steam cleaning, boiling water, and ammonia-based cleaners are the most effective options for contaminated surfaces. Litter boxes should be scooped at least daily (ideally twice daily) to remove feces before oocysts have a chance to sporulate and become infectious. In shelters or multi-cat homes, separating symptomatic animals and thoroughly sanitizing shared spaces helps break the cycle of reinfection.
Can Humans Catch It?
The species of coccidia that infect cats are host-specific, meaning they complete their lifecycle only in felines. The Cystoisospora organisms your cat sheds cannot establish an infection in humans. A different, unrelated species of coccidia can cause intestinal illness in people, but it’s not the same organism and is not transmitted by cats. So while basic hygiene around litter boxes is always smart, you don’t need to worry about catching coccidiosis from your cat.
Preventing Reinfection
Because oocysts can survive in the environment for months under the right conditions, prevention is an ongoing effort rather than a one-time fix. Keep litter boxes clean, since removing feces before oocysts sporulate (within the first day or so) is the single most effective step. If your cat hunts, be aware that prey animals are a consistent source of reexposure. Indoor cats in single-cat homes rarely encounter the parasite after their initial treatment, but cats in shelters, foster homes, or multi-cat households may need periodic fecal checks, especially when new animals are introduced.
Kittens adopted from shelters or rescue groups are worth testing even if they seem healthy. A quick fecal flotation during their first vet visit can catch an asymptomatic infection before it becomes a problem, and early treatment prevents the diarrhea and dehydration that make coccidiosis dangerous in young cats.