Cryptosporidium: A Particularly Nasty Type of Coccidia
Coccidia are a common parasite of young puppies and kittens. They are single celled organisms that cause diarrhea as they reproduce inside intestinal cells, ultimately killing the cell inside which they divide. As their numbers increase, so does the number of intestinal cells being killed until so many cells are killed that the host animal dies from its severe bloody diarrhea or the hostís immune system wins and kills off the Coccidia. Drugs used against Coccidia can suppress reproduction of the organism and thus keep the numbers down, but only the hostís 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.
Calf with diarrhea. picture from Highgate Veterinary Clinic, UK
Generally speaking, carnivores like dogs and cats get infected with Isospora species of while livestock (goats, sheep, calves, and even rabbits) get Eimeria species and these species of Coccidia do not cross over.
Cryptosporidium are similar to Coccidia and, until recent advances in molecular biology showed us otherwise, they were believed to be simply another species of Coccidia. Cryptosporidium have some uniquely unpleasant features:
- Cryptosporidium oocysts are so small that they are very 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 very 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 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.
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.
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.
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 one thinks to look for it. PCR testing, a very sensitive type of 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. Clindamycin in combination with Tylosin is currently favored.
THE U.S. PUBLIC HEALTH SERVICE
AND INFECTIONS 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
For more details on pet related guidelines for HIV+ individuals, we recommend these links:
The Centers for Disease Control and Prevention (The CDC) has a page on Preventing Infections from Pets:
Page last updated: 3/1/2011