Dogs and cats become infected when they ingest the oocysts (coccidia eggs) from fecal-contaminated ground.  This can occur when a pet grooms/licks the dirt off.  In some cases, the oocysts are swallowed by mice and then the pet is infected after eating the mouse. 

After being ingested, the oocyst breaks open and releases eight “sporozoities”.  Each of these “sporozoites”  finds an intestinal cell and begins to reproduce inside it.  Ultimately, the cell is so full of what are called “merozoites” at this stage that the cells bursts, releasing more “merozoites” which seek out intestinal cells of their own.  It is important to note how thousands of intestinal cells can become infected and destroyed as a result of accidentally swallowing a single oocyst.

As the intestinal cells are destroyed in larger and larger numbers, intestinal function is disrupted and a bloody, watery diarrhea results. 

Coccidia infections can only be detected by a microscopic fecal exam.  Unfortunately, small numbers of coccidia can be hard to detect and parasites may not be evident until later in the course of the condition.

Coccidiosis is treated with a medicine that inhibits coccidial reproduction.  Once the numbers stop expanding, it is easier for the patient's immune system to catch up and wipe the infection out. 

While there are species of coccidia that can infect people,(Toxoplasma  and Cryptosporidium) the Isospora species of cats and dogs cannot.  Other pets may become infected from exposure to infected fecal matter but it is important to note that this is usually an infection of  young animals (i.e. the immature immune system tends to let the coccidia infection reach large numbers whereas the mature immune system probably will not.)

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