Intestinal Parasites (Cryptosporidium) in Horses

Key takeaways

Cryptosporidiosis in horses is an infection of the intestine by the protozoa known as Cryptosporidium.

  • This protozoa is commonly found in the feces of animals that show no symptoms of infection
  • Cryptosporidiosis develops most often in foals and immunocompromised horses
  • The protozoa are transmitted by the ingestion of contaminated food, water, or other material
  • The characteristic symptom is yellow diarrhea
  • Foals with diarrhea require immediate veterinary attention to avoid potentially fatal dehydration and electrolyte imbalances
  • Diagnostic tools include fecal smears and cultures, and tests to identify parasitic DNA
  • Treatment involves supportive care to address dehydration and electrolyte imbalances
  • Prognosis is good in most cases, unless severe dehydration develops or the animal is immunocompromised
  • Prevention is challenging since Cryptosporidium is hardy and highly contagious
  • This infection is transmissible between animal species, including humans
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A closer look: Intestinal Parasites (Cryptosporidium) in Horses

The protozoa known as Cryptosporidium are commonly found among healthy horses without symptoms developing. This is because the immune system usually keeps the number of protozoa at a manageable level. Developing symptoms of infection is uncommon in horses.

In horses who are immunocompromised or foals in the first 2 months of life, the risk of developing symptoms of cryptosporidiosis is higher as the immune system is less capable of defending against infection.

Any condition that leads to severe diarrhea, particularly in foals or immunocompromised horses, requires immediate veterinary attention before dehydration and electrolyte imbalances set in. Without care, severe diarrhea is potentially fatal.

Cryptosporidium is also transmissible to humans. In most cases, no illness or only mild illness develops, unless the person is immunocompromised. People with underlying health conditions should not interact with infected horses.

Risk factors

The severity of cryptosporidiosis depends on the severity of the dehydration that results from ongoing diarrhea.

Symptoms of dehydration include:

  • Sticky or tacky mucous membranes
  • Sunken eyes
  • Skin tenting (skin does not return to its normal position quickly when pinched)

In some cases, especially in foals with a failure of passive transfer, concurrent infections occur. Other pathogens such as rotavirus and coronavirus accumulate in the intestine alongside Cryptosporidium, causing severe diarrhea.

Possible causes

Cryptosporidiosis is a symptomatic infection by the protozoa known as Cryptosporidium.

In most cases, these protozoa infect healthy animals, including humans, without causing illness. In certain circumstances, the immune system is not functioning well enough to keep the infection at a manageable level. In these cases, cryptosporidiosis, the symptomatic infection by Cryptosporidium, develops.

Transmission of Cryptosporidium occurs via the fecal-oral route. Horses ingest food, water, or other material that is infected with the feces of an animal that is a carrier. This protozoa is hardy, highly contagious, and transmissible from one species of animal to another, therefore the environment is potentially full of vectors for infection.

Foals are particularly susceptible to cryptosporidiosis. Foals are born without antibodies to parasites and other infectious agents. The foal usually receives antibodies by drinking colostrum, the first milk its mother produces. In some cases, the foal does not get sufficient colostrum or the colostrum is not of sufficient quality for the foal to be protected. This is known as “failure of passive transfer”. Foals that experience failure of passive transfer are at greater risk of developing cryptosporidiosis. Arabian foals with inherited combined immunodeficiency also have a greatly increased risk of developing disease.

Main symptoms

The main symptom of cryptosporidiosis is yellow, watery diarrhea.

Testing and diagnosis

Diagnosis involves identifying the underlying cause of diarrhea, so an appropriate treatment protocol can be made.

Diagnostic tools include:

  • Physical examination
  • Bloodwork
  • Fecal cultures
  • Fecal examination
  • Testing to detect the DNA of infectious agents

Steps to Recovery

There is no definitive treatment for cryptosporidiosis. The aim of treatment is to manage dehydration and electrolyte loss until the immune system can manage the infection. Oral, intravenous, or enteral (through a tube directly into the stomach) fluid therapy is required. In some cases, oral or IV feeding support is required to maintain adequate nutrition.

The prognosis for cryptosporidiosis is good with appropriate supportive care. In cases where severe dehydration occurs, the prognosis is more guarded. Immunocompromised foals also have a guarded prognosis, due to their weak immune system.


Prevention of infection is challenging. Cryptosporidium is hardy, highly contagious, and spreads from species to species through feces. Maintaining a clean environment, especially in birthing areas and enclosures housing newborn foals, is critical.

Areas that are contaminated with infected diarrhea must be disinfected. Many household cleaners are not effective in killing Cryptosporidium. Strategies that are effective in eliminating infection include:

  • Cleaning with ammonia, formalin, hydrogen peroxide, or chlorine dioxide
  • Freeze drying or freezing
  • Heating to 65 degrees celsius (149 degrees fahrenheit)

Horses that are infected with Cryptosporidium must be isolated from all other animals.

Are Intestinal Parasites (Cryptosporidium) in Horses common?

Asymptomatic infection with Cryptosporidium is common. Cases of cryptosporidiosis are uncommon.

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