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Cryptosporidium is a Particularly Challenging Type of Coccidia for Pets
Revised: August 04, 2023
Published: February 28, 2005

Watusi Calf
Photo Courtesy Dr. Becky Lundgren

What are Coccidia and Why they are Bad

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 nasty diarrhea when too many intestinal cells have been killed. As the coccidia increase in number, so does the number of intestinal cells being killed. Potentially (for very small or immune-compromised pets), so many intestinal cells can be killed that the host animal dies from its severe bloody diarrhea. Survival is a matter of the host's immune system versus the dividing coccidia. There are drugs that can kill certain types of coccidia, but for Cryptosporidium, drugs just reduce the number of organisms and may not be able to clear the infection fully. (More on this later).

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

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

Cryptosporidium vs. "Garden Variety" Coccidia

The easiest way to understand Cryptosporidium species and how they cause disease is to think of them as another kind of coccidiaCryptosporidium have some uniquely unpleasant features, however, 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. More routine coccidia oocysts are about ten times larger and thus less likely to go undetected.

  • Routine intestinal coccidia is not a human threat. Cryptosporidium is another story, and human threat depends on the species of Cryptosporidium involved and the immune status of the person in question. Cryptosporidium from dogs and cats do not readily infect humans, with the exception of immunosuppressed individuals. For these people, the infection can actually be life-threatening. (Cryptosporidium from livestock more readily infects humans, causing severe diarrhea that sometimes results in hospitalization even in humans who are not immune-suppressed).

  • Drugs that would work on routine species of coccidia do not have any effect on Cryptosporidium.

  • In calves, only ten oocysts 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 but the point is that this is a virulent organism and a small exposure can lead to a very pronounced disease in an individual under stress or with immune compromise due to immaturity, concurrent disease, or small size.

  • Possibly, the most ominous difference between routine coccidia and Cryptosporidium is that Cryptosporidium can yield a self-perpetuating infection, whereas routine coccidia will run its course.

How Infection Occurs

Out in the world, Cryptosporidium oocysts (the infective stage shown below) are very tough. They resist bleach and most other normal cleansers. Only prolonged exposure to ammonia or extreme temperatures can kill them. This photograph depicts the oocyst releasing sporozoites after it has been swallowed by its new host (as described below). The sporozoites are the stage that kills intestinal cells.

Microscopic Cryptosporidium
Cryptosporidium. Photo courtesy of CDC.

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 the 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.

 Arrows show Cryptosporidium organisms attacking intestinal cells
Arrows show Cryptosporidium organisms attacking intestinal cells. Photo by CDC.

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 thing 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 is necessary. This is now a self-perpetuating infection.

If there is any good news in this, it is that most hosts have healthy immune systems and are able to coexist with low numbers of Cryptosporidium without diarrhea. As will be seen, it is unlikely that medications will eradicate Cryptosporidium; the goal in therapy is to eliminate diarrhea rather than clear the organism completely.

How Rare is this Parasite?

The prevalence of Cryptosporidium oocyst shedding in dogs has varied from 2% to up to 15 -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 serious, so a screening test for a new pet is a good idea in this situation.

Screening for Infection

A routine fecal flotation test, as is recommended once or twice a year for most pets, is likely to miss Cryptosporidium. There are two main reasons: first, Cryptosporidium is extremely small, and second, oocysts are only intermittently shed. At this time more specialized testing such as PCR testing for Cryptosporidium DNA or ELISA tests for Cryptosporidium proteins are the best way to go, but these tests were developed to detect the Cryptosporidium species that infect humans so there may be some issues with test sensitivity. Screening routinely for Cryptosporidium is probably not warranted for healthy animals unless they are going to be spending time with immune-compromised individuals.


The bad news is that treatment is difficult. Nothing can really be described as highly effective. A medication called paromomycin has 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. Fortunately, most infected animals have healthy immune systems and, even though medication may not be fully effective, the patient's own immune system is usually able to control, if not fully curtail, the infection. Many animals appear to carry this organism without symptoms.

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 the age of six 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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