Foals and adult horses commonly develop diarrhea, sometimes with devastating consequences. When should you become concerned based on clinical pathologic findings? And what tests should the veterinarian order at what time points? These were questions Laura Javsicas, VMD, Dipl. ACVIM, of Rhinebeck Equine, in New York, addressed during her presentation at the Northeast Association of Equine Practitioners Convention, held Sept. 21-24, 2022, in Saratoga. She described a step-by-step workup of a foal with diarrhea, including tips and tricks to consider when interpreting bloodwork and ordering “diarrhea panels.”

Case Presentation

A 7-month-old Thoroughbred colt was found down in the field, in a state of shock, with blue mucous membranes. He was perfectly fine the day before, according to the owner. When Javsicas and her team first examined the foal, his capillary refill time (the time it takes for the gums to return to pink after being pressed with a finger—normally it should take less than 2 seconds) was greater than 5 seconds, and his extremities were cold. He did not have diarrhea at that time.

“But it is common to not see diarrhea in foals if they are so dehydrated there is nothing to pass,” said Javsicas.

A venous blood gas evaluation revealed acidemia (pH 7.14 and a bicarbonate of 11.9, which should be 24) and metabolic acidosis. Sodium and ionized calcium levels were both low, but glucose was normal. Lactate was markedly elevated.

“Why was lactate elevated? Because it is a product of anaerobic metabolism, which occurs secondary to decreased oxygenation, decreased oxygen delivery to the tissues secondary to poor perfusion, lack of oxygen-carrying capacity, and lack of cellular function,” explained Javsicas. “But the trends in lactate are more important than an individual measurement when managing these cases.”

At that point, her recommended work-up was extensive and included additional laboratory testing, abdominal ultrasound, and a diarrhea panel (even though the foal had not yet passed diarrhea).

“You can treat with tetracycline (an antibiotic) while you’re waiting for your other test results, especially if the horse is located in the Northeast and Potomac horse fever (PHF) is suspected,” said Javsicas.

Remarkably, the foal’s complete blood count (CBC), including white blood cell (WBC) count, was normal with no evidence of band cells (an indication of infection).

“The hematocrit and spun PCV were elevated,” said Javsicas. “I trust a spun packed cell volume more than the hematocrit as in some cases the machine can get an erroneous result.”

The fibrinogen (an acute phase protein in the blood, high levels of which indicate inflammation) was at the upper end of the normal range, and serum amyloid A (SAA, another acute phase protein) was only mildly increased at 568. The chemistry revealed increased aspartate transaminase, which can be seen with severe hypovolemia (low blood volume and dehydration). The alkaline phosphatase (an enzyme produced by bone and liver) was elevated, as would be expected in a growing foal.

By now he had diarrhea, so Javsicas turned her attention to the fecal analysis. If this were an adult horse, she said her main differentials would be Salmonella, Clostridium difficile and perfringens, PHF, coronavirus, rotavirus, antibiotic-induced diarrhea (if history dictates), and cyathostomiasis.

“In a young horse, however, I would focus more on Lawsonia, rotavirus, Salmonella, Clostridia, and Rhodococcus equi,” she said.

Interpreting Diarrhea Panels

The best way to differentiate between the various infectious causes of diarrhea is to run a PCR (DNA-based) diarrhea panel. However, the panel might not be testing for what you want.

“You need to be familiar with what your lab is actually testing in their diarrhea panel,” said Javsicas. “And an adult panel may be different from what you would want in a foal’s diarrhea panel.”

Regarding diarrhea panels, Javsicas offered the following advice:

Salmonella A Salmonella PCR has a very rapid turnaround time and is highly sensitive, perhaps too sensitive in some situations. So if it is positive, you must be cautious of overinterpreting the result, because Salmonella might not be the true culprit. For this reason, Javsicas recommended taking serial cultures.

You need a number of serial samples for diagnosis and to ensure the infection is cleared after treatment because the bacteria shed sporadically, and biosecurity is a concern.

A negative result, on the other hand, is quite helpful, said Javsicas, noting “with a severely hypovolemic patient, they may not be actively shedding Salmonella due to the small or dilute sample.”

Clostridium perfringens The C. perfringens test detects the gene for the causative toxins not the toxin itself. In some cases an ELISA test for the toxin might be more clinically relevant.

Potomac horse fever PCR on whole blood or feces can help diagnose PHF, with blood testing likely yielding positive results earlier in the infection.

“If you’ve already started treating for PHF, then you have a decreased chance of a positive sample,” said Javsicas. “Be sure to save pretreatment blood samples in case you want to send out samples later. But, you may still be able to detect PHF in the feces even if you’ve treated and the blood is negative. The alternative test—an immunofluorescence antibody test—has a high rate of false positives. I rely on the PCR test.”

Coronavirus Coronavirus is an emerging disease among horses, especially in the fall and winter. Fecal PCR is the only test, and fecal shedding starts three to nine days post-infection. If multiple horses in one barn are affected with mild fevers, veterinarians often suspect a coronavirus outbreak. 

L. intracellularis L. intracellularis has low fecal shedding; therefore, serology (bloodwork) or, preferably, a combination of both serology and a diarrhea panel is ideal. If you don’t see hypoproteinemia (low blood protein levels), this differential should be low on your list. Use laboratory testing in conjunction with abdominal ultrasound evidence of marked small intestinal thickening to make a diagnosis.

Rotavirus Most rotavirus ELISA and PCR tests only test for rotavirus A. Few labs currently test for the newly identified rotavirus B. Both forms of the virus can cause severe diarrhea in 1- to 4-day-old foals and is extremely contagious with very high (100%) morbidity. Although mortality is low, this disease has a huge economic impact on breeding farms.

Cryptosporidium Despite being included in many diarrhea panels, this disease caused by a protozoal intestinal parasite has questionable clinical significance. “I usually ignore it,” said Javsicas.

She emphasized the importance of performing a fecal float on all diarrhea cases. That said, a diarrheic sample might not be very accurate as it is not concentrated. Always deworm these horses based on history, particularly if they’re hypoproteinemic.

The Foal’s Outcome

The treatment plan for the foal described in this case was supportive care until the diarrhea panel results returned. With fluid therapy (both crystalloids and colloids), antibiotics, and low-dose flunixin meglumine, his pH improved to 7.37, bicarbonate increased to 24, and base excess improved.

Javsicas was thrilled with the foal’s lab results, but only fleetingly.

“Eighteen hours later the WBC count dropped to 2,500, bands increased to 425, and SAA increased to 1,500, consistent with an acute gastrointestinal infection,” she explained. “We ultimately diagnosed salmonellosis based on fecal PCR and culture. No change in treatment was initiated, and the foal was supported in the hospital until the fecal PCR was negative on three tests performed 12 hours apart. Only then was the foal released from hospital.”