Coccidian Infection in Dogs and Cats

Pets with weak immune systems, such as the very young and old, are less able to stop coccidian infection from overtaking their gut.

Puppies and kittens with coccidian infections generally appear sickly with limited energy. By: sarahakabmg
Puppies and kittens with coccidian infections generally appear sickly with limited energy. By: sarahakabmg

Coccidia are single-celled organisms that belong to a family of parasites called protozoa.

Once in the gut of cats or dogs, coccidia have the potential to cause ill thrift and diarrhea. The infectious parts of its life cycle are called oocysts, which are passed in the feces of infected animals. Animals become infected when they sniff infected feces or get it on their paws, groom their coat and swallow the oocysts.

Healthy animals with strong immune systems can often keep the parasite in check and do not become ill. Animals with a weak immune system, such as the very young, old or those with certain illnesses, are less able to stop coccidia from multiplying in their gut and are more likely to develop clinical signs.


Coccidia irritate the lining of the bowel. When present in sufficient numbers, they can cause sufficient inflammation to prompt diarrhea. This diarrhea often contains mucus and sometimes even blood, the result of inflammation of the bowel wall.

Puppies and kittens with coccidian infections tend to be “poor doers” who are skinny and underweight with big potbellies. These young animals often have more than 1 parasitic infection, such as with roundworms and tapeworms, and they generally appear sickly with dull coats and limited energy.


The most common coccidian species to infect dogs is Isospora canis, and in cats it is Isospora felis and Isospora rivolta. The parasites like to colonize mammalian gut and then shed their eggs, or oocysts, out in the host’s feces.

Cats and dogs kept in crowded or unsanitary conditions are more likely to come into contact with feces. If one animal has a coccidian infection, it is likely to be shared. When the pet then grooms herself, she swallows the oocysts and in turn becomes infected.


Diagnosis is made by examining cat or dog feces for the presence of coccidian eggs. These have a transparent shell that contains either 1 or 2 sporoblasts inside.

This and the history of mucus in a young animal’s diarrhea is sufficient evidence to initiate treatment.

Pets in crowded conditions are more likely to share a coccidian infection. By: johnshortland
Pets in crowded conditions are more likely to share a coccidian infection. By: johnshortland


Treatment is slightly problematic because there is no drug licensed for the purpose of killing coccidia in either dogs or cats.

This is more of a legal technicality than a practical problem because treatments effective at killing coccidia exist — drugs belonging to the sulphur-containing antibiotics — but it means using them “off label.” Typically, a 6-day course of a common antibiotic, sulfonamide-trimethoprim, is prescribed for the patient and is highly likely to be effective.

In addition, rigorous hygiene standards should be applied. This includes washing all feed and water bowls in diluted bleach and disinfecting the floors and surroundings. It is also a good idea to bathe the animal at the start and end of treatment to remove any oocysts stuck to her fur that could act as a reservoir of infection.


Good hygiene is key to prevention.

Feces should be removed promptly from kennels and runs so animals do not walk through them. In a cat household, there should be plenty of litter boxes provided that are cleaned thoroughly with diluted bleach at least twice a week.


  • “Coccidial parasites of dogs and cats.” Lindsay & Blagburn. Compendium of Continuing Education for the Practicing Veterinarian, 13: 759–765.
  • The Veterinary Formulary. Yolande Bishop. Publisher: Pharmaceutical Press.


This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Jan. 1, 2016.

Dr. Pippa Elliott, BVMS, MRCVS

View posts by Dr. Pippa Elliott, BVMS, MRCVS
Dr. Pippa Elliott, BVMS, MRCVS, is a veterinarian with nearly 30 years of experience in companion animal practice. Dr. Elliott earned her Bachelor of Veterinary Medicine and Surgery from the University of Glasgow. She was also designated a Member of the Royal College of Veterinary Surgeons. Married with 2 grown-up kids, Dr. Elliott has a naughty puggle called Poggle, 3 cats and a bearded dragon.

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