Diarrhea can be a common problem in horses, but how do owners know when it is serious?
Diarrhea, defined as loose stools, or excessive and overly-frequent defecation, occurs when the intestine does not complete absorption of electrolytes and water.
Simple changes in feed, exposure to lush grass, or a bite of moldy hay can cause brief irritation of the bowel, giving a horse diarrhea for a day or two, but anything more than that could be from a variety of more serious causes.
Bacteria, viruses, and toxins are all factors that can damage the lining of the bowel and lead to diarrhea and other complications.
The organisms that cause diarrhea are mostly bacteria – Salmonella and Clostridium difficile are among the most common.
Clostridium difficile is associated with antibiotic use in both people and horses.
While antibiotics are useful to kill bad bacteria, they can also kill good bacteria at the same time, upsetting the balance of flora in the body, notes internal medicine specialist Dr Peter Heidmann, from the Palm Beach Equine Clinic in Wellington, Florida.
If a horse goes on antibiotics for any reason, such as a wound or an infection, that can upset the good bacteria in the intestines and cause bad bacteria, such as Clostridium difficile, to grow.
Clostridium difficile can be found naturally in the environment. There are various types of salmonella, most adapted to birds or to cattle or other livestock, so horses that are around livestock have a higher rate of becoming infected with that particular bacteria.
Horses can also carry salmonella and not have any symptoms, so they can pass it to each other. If the healthy flora in the horse’s body is thrown off by even a small change in diet, or something bigger like a colic episode, or antibiotics, then salmonella can grow up in its place.
Another bacterial cause of diarrhea in the US can be a disease called Potomac Horse Fever. A bacteria called Neorickettsia risticii, which is carried by snails and conveyed by flies like caddis flies, causes Potomac Horse Fever.
For this reason, horses that live near rivers or streams can become infected. During warm weather months, caddis flies pick up the bacteria from streams and can transfer the disease to nearby horses who accidentally eat the flies or larvae.
There are hotbeds for Potomac Horse Fever throughout the US, including the Potomac basin where it was first described, as well as many parts of the East Coast, and areas of Oregon, northern California, and Montana.
A viral cause of diarrhea commonly seen is Coronavirus. This gastrointestinal virus shreds the intestinal lining and can cause horses to become very sick. The body has to reline the bowel, and it does so quickly, but it takes three to five days, during which the horse may have severe diarrhea and secondary infections.
“Coronavirus was thought for a long time to just be an opportunistic infection and that the virus would take advantage of the horse already being sick, but now it is more and more believed to be the cause of its own type of disease,” Heidmann says.
“Like all of these diseases, it causes damage to the lining of the bowel and supportive care must be used to help the horse heal.
“Unlike bacterial infections, however, you cannot directly treat the organism, since there aren’t appropriate drugs to directly treat coronavirus in horses.”
Outside of the infectious causes of diarrhea, there are mechanical causes, such as ingestion of sand, which can be a common problem in some locations. Sand is irritating to the lining of the bowel and can cause damage from its weight, as well as its abrasiveness.
In general, sand is irritating enough that the body cannot retain the fluid that it needs in the intestines. As a result, it will cause secretory diarrhea where too much water is being lost. Clearing the sand usually solves the problem and the bowel is then able to re-establish a healthy lining.
A final cause of diarrhea in horses is toxins. Toxic plants, such as Oleander, can be fatal in large doses, but if ingested in small amounts, can be a severe irritant to the bowel. Other toxins that a horse can ingest in the environment, such as phosphate or insecticides, may also cause diarrhea.
The single most important treatment for diarrhea, no matter the cause, is supportive care. Supportive care includes providing intravenous fluids to replace the fluids lost, providing protein in the form of plasma for the protein lost due to lack of absorption, as well as balancing electrolytes.
The next most important step is taking measures to either re-establish good gut flora or to remove the bad bacteria. In the past, a powdered charcoal was used, which is great for absorbing bacteria, but does not absorb the water.
A gastrointestinal health supplement called BioSponge® came on the market in the early 2000s through the company Platinum Performance. The product is a purified clay powder that binds the toxins, and also binds the water, so that the horse loses less fluid in their diarrhea.
While absorbing the bad bacteria and toxins is important, also providing good bacteria in the form of probiotics can also be very helpful.
“Probiotics are very variable in their efficacy, but there are some bacteria that are known to be associated with gut health,” Heidmann notes. “The good bacteria in people, and in horses, that has the most data for being helpful is Saccharomyces Boulardii. Old-fashioned brewers yeast is also Saccharomyces, but it is a different species, Saccharomyces cerevisiae.
“One of the best ways to re-establish healthy flora is transfaunation, which is taking a healthy horse’s manure, filtering it, and then tubing it into the sick horse,” Heidmann explains.
“That is one of the most dramatic treatments out there. It provides the good ‘bugs’ that the horse is losing through the diarrhea. You will often see foals eating their mother’s manure. It is an instinctual habit to get the good bugs into their stomach.
“We only do that in the sickest of cases. Whatever the route, it makes a big difference to provide the good bugs because that creates the environment for the gut to heal.”
While some antibiotics are warranted in the right situation, Heidmann points out that they are not necessary as often as people think.
“With people or dogs, if we get salmonella or some other intestinal infection, we almost always go on antibiotics, but because antibiotics are the cause of many cases of colitis in horses, in general that is not the best strategy, he says.
“There are a couple of exceptions. Clostridium difficile does respond to antibiotics, metronidazole being the most common one. For Potomac Horse Fever, Tetracycline broad spectrum antibiotics are the best.”
Biosecurity measures should also be taken to protect healthy horses from an infectious barn-mate. Heidmann recommends complete isolation of the sick horse while it is ill, and for a minimum of two full weeks after the infection has been clinically resolved.
This includes no horse-to-horse contact, as well as no shared use of wheelbarrows, pitchforks, etc.
Molecular and DNA testing can be done to make sure that the horse is infection-free, but he warns that testing can be problematic.
“There is a very high number of false negatives, meaning there is truly some infection there, but the lab cannot find it,” Heidmann says.
“There can be times when the horse is shedding bugs, but the tests do not pick it up. The state-of-the-art standard of care is a DNA test called PCR, and yet you still have to do multiple tests to get a positive test and get a diagnosis. Still, the best way to be safe is to continue testing until you are sure.”
Heidmann warns of common complications in severe diarrhea cases, laminitis being highest on the list.
With the sickest of horses, it is unfortunately not uncommon for the veterinarian to get the gut fixed over three to five days, and then find that the feet have started to become very inflamed due to toxins in the bloodstream.
If the horse loses the lining of its intestine, then the good and bad bacteria that are supposed to be contained in the intestine can ‘leak’ out into the bloodstream and are free in the abdomen. Those bacteria are then dying either from an attack by the immune system or antibiotics, and they release endotoxins into the bloodstream, which along with other inflammatory products, can cause laminitis.
Another serious complication is blood clotting. The sick horse may become very low on blood protein when the bowel lining is damaged, which can cause clotting abnormalities. The horse may have difficulty clotting or they may become prone to abnormal increases in clotting. The horse might seem better, and then it will develop a clot somewhere in the body. It can be anywhere, but it is most often in the intestine itself, which is usually fatal.
In general, horses like this are treated with supplemental protein in the form of plasma. In some cases, the veterinarian will also provide anticoagulant medications.
Although some cases of diarrhea are brief and easily resolved, Heidmann warns owners that serious cases can go downhill fast, and it is important to refer to an expert.
“The biggest sign of a problem is duration,” Heidmann explains.
“If it is one day, it could be that they had a bite of bad food or something simple. If there are fevers or lethargy, those are instant warning signs.
“If it lasts for days, or if they go off their feed, those are instant warning signs. That is when you should call your veterinarian right away, especially because as they start to go downhill, these complications really amplify. The worst cases are the ones that have been smoldering for a day or two.”