Diagnosing Diarrhea in Foals

Signs and tests that can help you and your veterinarian quickly identify the various causes of foal diarrhea.
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foal with diarrhea
Rotavirus is a common diarrhea culprit found primarily in newborns. | Courtesy Dr. Nathan Slovis
Little intestines might make little droppings of poop, but there’s no reason for those little droppings to be soft or—worse—runny. If you find liquid poop stains on your foal’s hind limbs and/or tail, that baby needs a veterinary exam and lab work right away.

Because foals can dehydrate quickly, and because their immune systems are still relatively fragile, they often go downhill fast when diarrhea sets in. While immediate steps will aim to treat clinical signs and keep the baby hydrated and safe from leaking toxins, the ultimate goal is to determine what disease is plaguing him and stop its progression as soon as possible. For that, breeders need a diagnosis that’s not only quick but also accurate, our sources say.

Fortunately, science has been making good progress over the years with regard to diagnosing the causes of foal diarrhea. In this article, we’ll look at all the ways experts work up foal diarrhea cases to get their tiny equine patients back to health as soon as possible.

To Get it Right, Get What’s Wrong

Diarrhea results from enterocolitis—inflammation of the intestinal tract. Knowing what has set off that inflammation is critical for stopping it, because an accurate diagnosis allows for more targeted treatment, says Nathan Slovis, DVM, Dipl. ACVIM, CHT, director of the McGee Center at Hagyard Equine Medical Institute, in Lexington, Kentucky.

So while you’ll want to start providing supportive care to a sick foal right away, you still need to start working up a diagnosis as fast as possible, Slovis says. Foals—especially newborns—can quickly succumb to the dehydration and weakness caused by infectious diseases.

In addition, certain kinds of pathogens (disease-causing organisms) can wreak havoc at lightening speeds. For example, types A and C of the Clostridium perfringens bacteria can cause the intestines to start necrotizing, leading to rapid death.

Noah Cohen, VMD, MPH, PhD, Dipl. ACVIM-LA, Patsy Link Chair in Equine Research and director of the Equine Infectious Disease Laboratory at Texas A&M University’s College of Veterinary Medicine and Biomedical Sciences, in College Station, agrees.

“Knowing sooner helps with decisions about management,” Cohen says. For that reason, Cohen developed the first polymerase chain reaction (PCR) test for one common foal diarrhea culprit, Salmonella spp, nearly 30 years ago. The test cut the waiting time from several days to less than a day, meaning foals got the right medications and biosecurity in place in record time.

An accurate diagnosis also helps prevent undesirable effects of the wrong medication or blanket treatments, says Slovis. “You don’t want to shotgun antimicrobials if you’re not sure what you’re treating, because you could end up disrupting the microbiota in the bowel and make things worse,” he says. “Moreover, knowing which microbial agent or agents cause diarrhea can help to take appropriate steps for control and prevention in the herd, and inform strategies for monitoring spread or future recurrence.”

Step 1: Assess the Farm

A major clue when making a foal diarrhea diagnosis comes from careful evaluation of his surroundings, says Slovis.

“People don’t always realize that a physical exam includes a sort of physical exam in the facility to see what’s going on,” he says. “Is it clean? What’s the everyday practice of feeding and cleaning stalls? What about movement of horses in and out of facility? What about water point sources? Are there only certain stalls or certain batches of hay or certain fields where there are problems? How old are the other animals? Has the feed been changed recently? How long has the problem been going on? What time of year is it? All these questions are really important for getting to an accurate diagnosis.”

For example, when young foals develop fever and diarrhea in August or September along the East Coast, these combined signs should point toward Potomac horse fever (PHF), he says.

“Knowing the history of the farm is very important,” Slovis says, adding that veterinarians pick up clues best by visiting the farm themselves.

Step 2: Assess the Foal

Diarrhea indicates enterocolitis, but a clinical examination and recent history of the foal can point to other clues that might aid in diagnosis, says Slovis.

The most important clue is the foal’s age, our sources say. In particular, newborn foals—up to about two weeks of age—can have very different reasons for their enterocolitis compared to older foals. Rotavirus, for example, is a common diarrhea culprit found primarily in newborns that veterinarians rarely diagnose in foals over six months of age. By contrast, Lawsonia intracellularis, which causes equine proliferative enteropathy (EPE), generally affects weanlings but not newborns.

The foal’s age will help determine which laboratory tests the veterinarian orders, Slovis says.

The clinical exam itself might provide few helpful clues, says Cohen. That’s because many of the signs associated with foal diarrhea—dehydration, lethargy, lack of interest in nursing, and fever—are similar. But what about other physical or behavioral signs, such as certain postures, suckling habits, coat condition, or facial expressions? Can any other visible characteristics help flag a cause for the diarrhea?

“Other than age, no,” Cohen says.

Even so, such signs can at least help distinguish between foals that are truly sick and those that aren’t, Slovis says. “I’m not (particularly concerned) about the ones that are looking at you and running around having fun and nursing, and they just have a little diarrhea, like from foal heat,” he says. In such cases, owners can continue to monitor the foals closely for worsening of the condition, but they probably don’t need further diagnostics at that point.

Step 3: Assess the Poop

The real answers to what’s causing a foal’s diarrhea are usually in the feces, our sources say. Sampling the manure can provide a wealth of information about the pathogens circulating in the foal’s digestive system.

Composite PCR panels allow technicians to screen for eight to 10 of the most likely culprits—tailor-designed for particular age groups and environments—in one fell swoop. What’s more, they give results in 24 to 36 hours, “which is amazing,” Slovis says. PCR tests can detect the genetic material (e.g., DNA) of these pathogens and even reveal how many the sample contains. “It gives a good idea of who may be the real players in that sample,” he says. “These panels are my go-to (for working up foal diarrhea).”

Cohen appreciates these customized multi-pathogen panels, as well. “Some foals may have co-infections, say, Cryptosporidium parvum (a protozoa) and rotavirus,” he explains. By finding both pathogens—rather than stopping with a single positive test—veterinarians can treat for both diseases right away and plan for appropriate management to prevent additional cases in a herd.

Microbiological culturing is an older and slower—but still useful—technique, they say. Technicians incubate the fecal sample in the laboratory to see what bacteria grow and reproduce, which can be visible under microscope usually within about five days. That’s slow—and foals can get much sicker without treatment during that wait time—but the advantage is culturing can reveal whatever bacteria grow because, unlike PCR, it’s not a specific test. “For some of the bacteria that are out there, we don’t have a molecular (PCR) test yet,” Slovis says. “So with PCR, you may miss some.”

Cohen says cultures can be useful complements following a positive PCR test. “For bacteria such as salmonella, I like to have culture results to be able to test for antimicrobial susceptibility and to know the organisms are truly replicating in the foal,” he says. PCR tests can detect DNA even when the pathogen is dead, he adds.

“I also like being able to have the isolate to compare to any additional isolates from the farm, barn, or hospital in the case of an outbreak or to compare to other isolates from other times and locations,” he explains.

If protozoa are the root of your foal’s problem, fecal cytology can usually pick it up within minutes compared to a 36-hour PCR panel, says Slovis. “We literally take a fresh sample of manure and spread it on a microscope slide, and we can see those live protozoa moving around,” he says.

If the issue is intestinal parasites, fecal egg counts can reveal whether the foal has over-the-threshold parasite loads for roundworms, ascarids, strongyles, or Strongyloides westeri (equine threadworm), which foals can acquire through milk.

While evaluating poop is your best bet for finding the culprit, it can be tricky business, says Slovis. The pathogens might not be in every dropping—especially if the diarrhea is loose and dilute—and they might not be detectable every day. “This means maybe doing a test on Day 1 and then again on Day 3, in case you missed something,” he says.

And protozoa die after about an hour outside the colon or if they’re frozen, he adds. That can “camouflage them in the manure,” because they’re not moving. “It’s tough to see.”

And despite all these techniques, it’s still possible the veterinarian will come up empty-handed, Slovis cautions. “We’re not looking at an 80 to 100% chance of getting a diagnosis, and people need to understand that,” he says.

The Pros and Cons of Blood Tests

Sampling a sick foal’s blood might provide helpful diagnostic information, says Cohen.

Immunoassays—which detect the presence of pathogen-specific antibodies circulating in the bloodstream—can point toward certain kinds of infections, he says.

An advantage of immunoassays is their speed, Slovis adds. They’re based on an enzyme-linked immunosorbent assay (ELISA) test—similar to a home pregnancy test for humans—and can give results within 25 minutes for diseases such as EPE and rotavirus or pathogens like Clostridium difficile.

ELISA tests, however, are prone to false negatives in foals because veterinarians often use tests that haven’t been validated in horses, he says. “They tend to use ELISA tests that were designed for humans,” he explains. “Those tests can be good when they are positive for a diagnosis, but sometimes the pathogen level is too low for these tests to detect.”

A complete blood cell count can be useful for white blood cell counts, which reflect the level of infection “and give you an idea of how sick the animal is,” Slovis says.

Supportive Care and Biosecurity During the Waiting Game

Foal diarrhea test results aren’t immediate. So in the meantime, the goal is to keep the patient as stable as possible while preventing further disease spread throughout the farm, our sources say.

“Treatment for foal diarrhea is largely supportive and nonspecific, and it’s often initiated well in advance of obtaining diagnostic information about the agent or agents causing disease,” says Cohen.

In general, diarrhea causes significant water loss, especially in young animals, leading to rapid dehydration, Slovis explains. “They’re shocked; their blood pressure is down; they’re not absorbing nutrients properly; they might even be hypoglycemic,” he says.

Poor hydration can lead to a dangerous drop in blood pressure, he adds. Meanwhile, the inflammation in the gut can lead to leaky bowel syndrome. Many microorganisms that can live safely within the digestive system can become toxic if they escape the intestines. When the gut is inflamed, those endotoxins can literally “leak” through the intestinal walls and pass into the body and bloodstream, poisoning the animal to death, says Slovis.

To control these issues while waiting for test results, foals should receive intravenous fluids to keep them hydrated and possibly broad-spectrum antibiotics to keep bacterial toxins under control until the veterinarian reaches a definitive diagnosis, he says.

Good veterinary management also means implementing immediate biosecurity measures.

“I generally implement precautions for biosecurity in all foals,” Cohen says. That includes separating sick foals from healthy ones and handling healthy foals before handling sick foals (and not vice versa), as well as disinfecting equipment, changing clothes, and cleaning hands and boots after handling infected animals.

“The biggest cause for the spread of (equine disease) is humans,” Slovis says.

Take-Home Message

A sick foal with diarrhea needs supportive care as soon as possible, our sources say. But just as urgently, that baby needs an accurate diagnosis. While cultures, PCR, and immunoassays can’t always point to a definitive diagnosis, they’re the best science currently offers and can provide you with critical information about what pathogens are affecting your young horse. Armed with such information, you can get targeted and effective treatments to your sick foal, putting him back on a healthy track.


Written by:

Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates. Follow Lesté-Lasserre on Twitter @christalestelas.

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