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Coccidia are intracellular parasites of the lining of the small intestine that occur with good frequency.  They are microscopic parasites detectable on routine fecal tests in the same way that worms are, but coccidia are not worms and are not susceptible to deworming medications. They are also not visible to the naked eye. Coccidia infection causes a watery diarrhea that is sometimes bloody and can be a life-threatening problem to an especially young or small pet. The fluid loss can be dangerously dehydrating.  Othertimes, the infection can be assymptomatic.  Puppies are most commonly affected. Coccidia infection is especially common in young animals housed in groups (in shelters, rescue areas, kennels, etc.) This is a common parasite and is not necessarily a sign of poor husbandry.  Most infections are not apparent and resolve on their own by self immunization. Occasionally, signs will occur and can be quite severe in heavily compromised pups. Adult dogs usually display significant immunity.

Life Cycle

Oocysts (pronounced o'o-sists) are passed in stool. In the outside world, the oocysts begin to mature or sporulate. After they have adequately matured, they become infective to any host (dog or cat) that accidentally swallows them. To be more precise, coccidia come from fecal-contaminated ground. Infection occurs upon ingestion of contaminated feces or food. 

Detection of Coccidia

A routine fecal test is a good idea for any new puppy or kitten whether there are signs of diarrhea or not. This sort of test is also a good idea for any patient with diarrhea and is recommended at least once a year for healthy dogs and cats as a screening test. Coccidia are microscopic and a test such as this is necessary to rule them out. 


The most common medicines used against coccidia are called coccidiostats.  The time it takes to clear the infection depends on how many coccidia organisms there are to start with and how strong the patient’s immune system is.  A typical treatment course lasts about a week or two, but it is important to realize that the medication should be given until the diarrhea resolves plus an extra couple of days.  Sometimes courses as long as a month are needed.