Cryptosporidium: A Particularly Challenging Type of Coccidia
WHAT ARE COCCIDIA AND WHY ARE THEY 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 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 its 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.
HOW INFECTION OCCURS
The life cycle of this organism is rather complicated and it is probably 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 via 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 a very 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 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 the 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% 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 Cryptosporidium (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 once or twice a year for most pets, is likely to miss Cryptosporidium. There are two main reasons for this: first, Cryptosporidium is extremely small and second, oocysts are only intermittently shed. At the present 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 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. 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.
Page last updated: 8/19/2021