Moderate severity
Coccidiosis
A single-celled intestinal parasite that causes watery, sometimes bloody diarrhoea in kittens. Adults often carry it silently.
⚠️ Common symptoms
Coccidiosis is a disease of circumstances more than of bad luck. The parasite is common, adult cats frequently carry it without incident, and the difference between an invisible passenger and a kitten on a drip is usually density, hygiene and stress. That is why it turns up so predictably in the places where cats are gathered — rescue intakes, breeding catteries, a barn litter, a household that has just taken in more cats than its litter trays can serve.
Inside the kitten, the parasite invades the cells lining the small intestine and multiplies within them until they rupture. Repeated over an enormous number of cells, this strips away the absorptive surface of the gut, and the result is watery diarrhoea, poor nutrient absorption and, in a young animal with little reserve, rapid dehydration. Blood appears when the damage is severe enough to breach the surface. A kitten that fails to thrive, stays small, and has a dull coat and a persistently loose stool is a familiar picture in shelter medicine.
Diagnosis needs a microscope and, quite often, patience. The oocysts are shed intermittently, so a single negative faecal flotation in a suspicious case means little; a pooled sample collected over three consecutive days is far more informative. It is also worth remembering that coccidia frequently travel in company. Kittens with coccidiosis often have roundworms, Giardia or Tritrichomonas as well, and clearing one while missing another explains a lot of apparently failed treatments.
The single most common misconception is that routine worming covers it. It does not. Standard anthelmintics target roundworms and tapeworms and have no effect whatsoever on protozoa, so a kitten dutifully wormed every fortnight can still be riddled with coccidia. Treatment requires a specific drug, and increasingly that means ponazuril or toltrazuril, which have made the older two-week sulfonamide courses feel cumbersome by comparison.
Owners often ask whether they can catch it. Cystoisospora species of cats are host-specific and are not considered a human health risk, which is a genuine relief in a household with children. That is emphatically not true of every intestinal parasite of cats, so the reassurance should follow a diagnosis rather than precede one — which is another argument for getting the faecal sample looked at rather than guessing.
🔍 Causes
The species that matter in cats are Cystoisospora felis and Cystoisospora rivolta, protozoal parasites that invade the cells lining the small intestine and multiply there, destroying the cells as they go. Cats become infected by swallowing sporulated oocysts from a contaminated environment, or by eating an infected rodent that carries dormant stages in its tissues. Oocysts are shed in faeces and become infective in the environment after a day or two, which is why prompt litter tray hygiene is disproportionately effective. Overcrowding, poor sanitation and stress are the classic amplifiers, which is why outbreaks cluster in shelters, catteries and rescue situations. Adults commonly develop immunity and shed without any signs, acting as a reservoir for kittens.
🛡 Prevention
Hygiene does most of the work. Scoop litter trays at least daily and preferably twice, since oocysts need a day or more in the environment to become infective and removing faeces promptly interrupts that. Provide enough trays for the number of cats, clean them with hot water and detergent, and remember that most household disinfectants do not kill coccidial oocysts — steam or a dedicated product is needed for a serious contamination. Keep kittens off ground contaminated by adults where possible, control rodents and discourage hunting, quarantine new arrivals, and reduce stocking density and stress in multi-cat environments. Routine worming products do not treat coccidia, a point that trips up a great many owners.
💊 Treatment
Sulfadimethoxine has long been the standard treatment, given over a course of one to three weeks, and works by halting the parasite's reproduction while the cat's own immune system clears the infection. Ponazuril and toltrazuril are increasingly used and have the practical advantage of a much shorter course, often a single dose repeated once. Supportive care matters as much as the drug in small kittens, where fluid losses are dangerous: subcutaneous or intravenous fluids, a highly digestible diet, and careful attention to warmth and weight. Because the infective stage survives in the environment, treatment must be paired with hygiene — litter trays scrubbed and changed daily, bedding washed hot, and faeces removed promptly. Every kitten in an affected litter is treated together, whether or not it has signs.
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⚕️ Medical disclaimer
- This information is educational only and is not a substitute for veterinary advice.
- Symptoms overlap between conditions — only a vet can diagnose accurately.
- Never withhold treatment or self-medicate based on information found online.
- When in doubt, call your vet. Early intervention almost always leads to better outcomes.