Coccidia Common Parasite of Puppies or Kittens

Cryptosporidium: A Particularly Nasty Type of Coccidia.

Coccidia are a common parasite of young puppies and kittens. They reproduce inside intestinal cells, ultimately killing the cells within which they divide and producing a nasty diarrhea when too many intestinal cells have been killed. As the Coccidia increase in numbers, so does the number of intestinal cells being killed.  Potentially, so many intestinal cells can be killed that the host animal dies from severe bloody diarrhea. Survival is a matter of the host's immune system versus the dividing Coccidia. Drugs used against Coccidia can suppress reproduction of the organism and thus keep the numbers down, but only the host’s immune system can actually rid the body of the infection.

Coccidia infection is the scourge of the dairy industry, as it is one of the most common causes of calf death. Coccidia infection is similarly a nightmare for small puppies and kittens.

Generally speaking, carnivores like dogs and cats get infected with Isospora species while livestock (goats, sheep, calves, and even rabbits) get Eimeria species. These species of Coccidia do not cross over.

The easiest way to understand Cryptosporidium species and how they cause disease is to think of them as another kind of Coccidia. For many years, they were believed to actually be another type of Coccidia although advances in molecular biology have shown them to be different; however, from a practical perspective, they act like Coccidia. Cryptosporidium have some uniquely unpleasant features that are not shared by routine species of Coccidia:

Cryptosporidium oocysts (the "eggs" that can be seen on a routine fecal test) are so small that they are difficult to detect under the microscope with normal testing methods.

  • Cryptosporidium from dogs and cats do not readily infect humans with the exception of immunosuppressed individuals. For these people, infection is life threatening. (Cryptosporidium from livestock more readily infects humans, causing severe diarrhea that sometimes results in hospitalization.)
  • Because they are not truly Coccidia, drugs that would work on Coccidia do not have any effect on Cryptosporidium.
  • Out in the world, Cryptosporidium oocysts (the infective stage) are tough. They resist bleach and most other normal cleansers. Only prolonged exposure to ammonia or extreme temperatures can kill them.
  • In calves, only 10 oocysts (somewhat like an egg) are needed to establish a significant infection (i.e., that is how many the calf has to swallow). This is a particularly small number. We do not know how small the number is for other species.

How Infection Occurs

The life cycle of this organism is rather complicated and it is not necessary to understand all the stages. The short version is that an oocyst (sort of like an egg) is passed in the feces of an infected animal. This oocyst is swallowed by another animal by licking dirt off its fur, drinking contaminated water or some such activity. The oocyst releases sporozoites (sort of like a spore) into the intestinal tract of the new host. The sporozoite infects an intestinal cell and divides. The spores divide into other stages with other names, which in turn infect more cells.

All this cell division occurs asexually for a while until eventually the Cryptosporidium begins a sexual phase: instead of making more copies of itself by simple cell division it produces male and female cells. Fertilization occurs, yielding oocysts like the egg that started it all.

There are actually two types of oocysts: one that is thick-walled and ready to be passed in feces to face the external world, and one that is thin-walled and just infects the host over again from the beginning.

This is an important and bad fact so we will say it again: the thin-walled oocyst infects the host over again from the beginning. No contaminated water is needed. No dirty fur necessary. This is now a self-perpetuating infection.

How Rare is this Parasite?

The prevalence of Cryptosporidium oocyst shedding in dogs has varied from 2% to up to 15% to 20% in stray dog populations. Fecal specimens from 200 stray dogs impounded at the San Bernardino City and County animal shelters were screened for Cryptosporidium oocysts and 2% of dogs were found to be oocyst positive. A similar survey of 206 cats revealed oocyst shedding in 5.4%. Most infections are subclinical, meaning that the host animal is not sick.

Humans tend to get their own species of it (Cryptosporidium hominis) while cats and dogs each have their own Cryptosporidium. Pet ownership has not been found to be a significant risk for humans with cryptosporidiosis (i.e., most infected humans get infected from other humans or from livestock).

This is generally good news except for the immunosuppressed owner who might adopt an infected pet without knowing it. Remember, in the immunosuppressed individual, cryptosporidiosis can be a life-threatening infection.

Screening for Infection

A routine fecal flotation test, as is recommended annually for most pets, is likely to miss Cryptosporidium as it is such a small organism. The good news is that an ELISA test kit is available that can detect all species of Cryptosporidium and can be easily run by most veterinary laboratories. Detecting Cryptosporidium is not a problem if you think to look for it. PCR testing, a sensitive test that looks for a target organism's DNA, is also available.


The bad news is that treatment is difficult. Nothing can really be described as highly effective. A medication called paromomycin has been effective but is highly toxic to the kidneys. A medication called nitazoxanide has been effective but causes nausea and diarrhea. Azithromycin and Tylosin also have activity against Cryptosporidium and are sometimes used.

The U.S. Public Health Service and the Infectious Diseases Society of America both recommend that HIV-infected individuals should not bring into their homes:

  • Animals with diarrhea
  • Stray dogs or cats
  • Dogs or cats under age 6 months.

See more details on pet-related guidelines for HIV-positive individuals.

The Centers for Disease Control and Prevention (CDC) has information on preventing infections from pets.

CDC tip: Protect yourself against getting Crytosporidium from animals. Simply wash your hands with running water and soap after any contact with animals and animal feces (stool).

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