Diarrhea in Foals: Infectious Diarrhea
By Siobhan McAuliffe, MVB, DACVIM, Foal Medicine Specialist
Part 2 in a 3-part series on diarrhea in foals
There are many causes of infectious diarrhea in foals. Among those are viral and bacterial infections. Many different bacteria can cause diarrhea either by direct effects in the intestine or as a result of septicemia (infection in the bloodstream). Foals can get a blood infection from navel ill or bacteria that go through the gut lining soon after birth, and the resulting systemic infection creates diarrhea along with all the other signs.
During the first 24 hours of life, the foal has a very open gut that can absorb large molecules (to allow for absorption of antibodies from colostrum). If the foal ingests bacteria, they can also be transferred across to the bloodstream. Ability to absorb antibodies through the gut wall wanes during the first 24 hours; the first hours are most crucial for getting an adequate amount.
Foals infected during the first week of life with salmonella tend to get it from the mare. Up to 10% of adult horses have salmonella in the GI tract and may pass organisms in manure. The stress of foaling often induces shedding of the bacteria with salmonella organisms then being present around the udder and tail. In these situations wrapping the tail and cleaning the udder of the mare while making sure the foal gets good quality colostrum early can dramatically reduce the incidence of disease.
To date two strains of Clostridia have been shown to affect foals: C. perfringens and C. difficile. New research indicates that there may indeed be additional strains but this is an area of active research. Infection with C. perfringens bacteria, which live as spores in the environment, can be very severe with the foal becoming infected in the first week of life. All foals get clostridial organisms colonizing the gut, but some C. perfringens produce toxins that can cause gut damage, which allows the toxins (and even the bacteria) to get into the bloodstream.
Toxin producing C. difficile can cause diarrhea in foals of any age. Healthy foals that get good colostrum can still get C. perfringens infections in the first week of life. Affected foals may get severe bloody diarrhea, become septic, and may go into shock. As this is a bacterium that may be present in the environment it is an infection that is difficult to prevent. Foals younger than seven days might die if they develop this type of infection. Affected foals can be bright, then suddenly become ill and die (becoming dull or showing signs of abdominal pain, then collapsing and going into shock), sometimes before showing signs of diarrhea. C. perfringens can produce several types of toxins, including beta-toxin that causes necrosis (tissue death) of the intestinal wall and hemorrhaging. Salmonella and clostridia are the major bacterial problems in young foals in the first week of life.
Toxin producing E. coli can also cause diarrhea in foals but it is not as common a cause of diarrhea in foals as it is in calves.
Rotavirus is a common cause of diarrhea in foals. The source of infection is other foals or mares that are shedding the virus and passing it in their manure. If a foal ingests the virus, it can cause severe diarrhea, but it’s more a fluid-losing diarrhea than the bloody diarrhea you see with salmonella or clostridia. Coronavirus is being increasingly recognized as a cause of diarrhea in foals and commonly occurs in combination with other viral or bacterial infections. Foals are less likely to die with Rotavirus infection than with Salmonella or Clostridial infections but the fluid loss can be extreme.