Cats get coccidia by swallowing microscopic egg-like structures called oocysts, either directly from contaminated environments or by eating infected prey animals. The parasite spreads through a fecal-oral route: an infected cat sheds oocysts in its stool, those oocysts mature in the environment, and another cat ingests them. Kittens and cats in crowded or stressful settings are the most likely to become infected and show symptoms.
The Fecal-Oral Route
Coccidia (specifically a genus called Cystoisospora in cats) completes its reproductive cycle inside a cat’s small intestine. The final product of that cycle is oocysts, tough-shelled structures that pass out in the cat’s feces. Once in the environment, oocysts need time and the right conditions to become infectious, a process called sporulation. In warm, moist conditions (between about 86°F and 99°F), sporulation can happen in under 16 hours. In cooler or drier conditions, it typically takes 7 to 10 days.
A cat becomes infected when it swallows sporulated oocysts. This can happen in several ways: grooming paws that touched contaminated soil, sniffing or licking a surface where infected feces were present, or sharing a litter box with a cat that’s shedding oocysts. Kittens are especially prone to picking up the parasite because they explore the world with their mouths and are often housed in close quarters with other cats.
Eating Prey Animals
Hunting is another major route of infection. Most vertebrates, including mice, birds, and other small animals, can serve as intermediate or “paratenic” hosts. In these animals, the coccidia parasite encysts in tissue but doesn’t multiply. When a cat catches and eats an infected mouse or bird, the encysted parasites are released during digestion and begin their reproductive cycle in the cat’s intestine. Invertebrates like insects can also physically carry oocysts on their bodies, acting as transport hosts. An indoor cat that catches a mouse or a kitten that eats a beetle could pick up coccidia this way.
Why Oocysts Are So Hard to Avoid
Coccidia oocysts have a thick, resistant shell that makes them remarkably durable. In moist, protected environments, sporulated oocysts can survive for a full year or longer, including through winter. They don’t sporulate well below about 68°F or above 104°F, but once they’ve already sporulated, they can persist through a wide range of conditions. Direct sunlight and thorough drying are among the few natural forces that reduce their numbers.
Most common household and kennel disinfectants don’t kill coccidia oocysts. Quaternary ammonium compounds (the active ingredient in many pet-safe cleaners) are ineffective against them. Steam cleaning can destroy oocysts but is difficult to apply thoroughly in real-world settings. Even high-pressure washing is not recommended because it can scatter oocysts to previously clean surfaces. The most effective strategy, according to the University of Wisconsin Shelter Medicine Program, is mechanical removal: promptly scooping feces, scrubbing surfaces with detergent, and keeping areas dry and exposed to sunlight.
Why Kittens Are Most at Risk
Adult cats with healthy immune systems often carry coccidia without ever showing symptoms. They can shed low numbers of oocysts in their stool, quietly contaminating shared spaces. This is a particular problem for kittens, whose immune systems are still developing. A kitten born to or housed with an asymptomatic adult carrier can be exposed to oocysts from its first days of life.
Stress and overcrowding dramatically increase the risk and severity of infection. Shelters, catteries, and pet stores create ideal conditions for coccidia outbreaks: many animals in close proximity, shared litter areas, high stress levels, and immature immune systems. Coccidia is considered an opportunistic pathogen, meaning it causes the most damage when a cat’s defenses are already compromised. Foster care, by reducing stress and separating kittens from large populations, gives them a better chance of fighting off the infection on their own.
Signs of Infection
Many cats, especially healthy adults, never show any signs despite carrying the parasite. When symptoms do appear, they’re most common in kittens under six months old. The hallmark sign is watery diarrhea, which may contain mucus or blood. Kittens can also become lethargic, lose their appetite, and become dehydrated quickly, which is the main danger with young or small animals. In severe cases, the fluid loss from persistent diarrhea can be life-threatening.
Veterinarians diagnose coccidia by examining a stool sample under a microscope, using a technique called fecal flotation that concentrates the oocysts so they’re easier to spot. Because oocysts aren’t shed continuously, a single negative test doesn’t always rule out infection, and repeat testing is sometimes needed.
Reducing Your Cat’s Exposure
You can’t make the environment completely coccidia-free, but you can significantly reduce the risk. Scoop litter boxes at least once daily, since oocysts need time to sporulate before they become infectious. Clean litter boxes with hot water and detergent rather than relying on chemical disinfectants alone. If your cat goes outdoors, keep the yard free of standing water and fecal waste, and ensure resting areas get direct sunlight.
For multi-cat households or foster setups, keep new animals separated until they’ve been checked for parasites. Kittens coming from shelters or stray situations should have a fecal test as part of their initial veterinary visit. If you have outdoor cats that hunt, understand that every mouse or bird they eat is a potential source of reinfection, and routine fecal screening is worth building into their care schedule.