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Neonatal Diarrhea in Foals

At the 2020 AAEP Convention, Nathan Slovis, DVM, DACVIM, presented a thorough look at what’s new in neonatal diarrhea.
foal nursing gray mare field

Mare and foal pairs parallel each other’s shedding pattern for Salmonella.

During foaling season, equine practitioners expect to treat cases of neonatal diarrhea. At the 2020 AAEP Convention, Nathan Slovis, DVM, DACVIM, of Hagyard Equine Medical Institute in Kentucky, presented a thorough look at what’s new in these cases.

Slovis pointed out that more than 20% of foals develop infectious diarrhea by 6 months of age. Due to hypovolemia and/or septic shock, foal diarrhea can be life-threatening.

Malabsorption occurs when poorly absorbed lactose begins to ferment and leads to osmotic diarrhea that decreases absorption of gastrointestinal water and electrolytes. Secretory diarrhea occurs from the effect of bacterial endotoxins and their impact on the enteric nervous system. Endotoxin (lipopolysaccharide) can set off an inflammatory cascade that increase cytokines.

The causes of diarrhea are many, so it is important to obtain a good history, determine whether other animals in the barn are affected, and if so, in what age groups? What is the consistency of the feces? Is there a peculiar odor? (For example, Clostridium difficile has a unique smell.) Identify whether any medications have recently been administered to the diarrheic foal. Intestinal ultrasound is useful to demonstrate colitis, enterocolitis or some other GI abnormality.

Lab testing with PCR is very sensitive in detecting DNA of pathogens compared to detection of protein or antibodies by ELISA testing, said Slovis.

Rotavirus Diarrhea. One of the most common causes of neonatal diarrhea is rotavirus, which invades the intestinal epithelial cells, and in particular the sides, tips, villi and the brush border responsible for lactase formation to digest milk. Incomplete milk digestion allows disaccharides to reach the hindgut, where they ferment to cause bloat and diarrhea. The enteric nervous system also creates secretory diarrhea.

This virus tends to affect foals younger than 3 months of age, but especially those that are 1-2 days old. Slovis advised that a foal usually sheds rotavirus for 10 days but can shed up to eight months while remaining subclinical. The PCR test for rotavirus is equine specific—be sure the test is validated for horses.

One complication of rotavirus is development of pyloric/duodenal stenosis. Prevention is possible with a killed vaccine for the G3 strain. (The G14 strain also circulates, but there is no vaccine for it yet.) Pregnant mares immunized at 8, 9 and 10 months of every pregnancy are able to confer immunity to the foal via transfer of colostral antibodies. Rotavirus vaccine reduces morbidity from 80% to 30% as well as decreases duration of diarrhea from 7.3 to 1.8 days. Equally important is the need for proper hygiene and disinfection of stalls with phenols or accelerated hydrogen peroxide. Bleach is not effective. Another preventive strategy is to house foals outside.

Clostridial Diarrhea. Clostridium difficile can be present in asymptomatic carriers. For example, Slovis reported that >50% of healthy human neonates carry toxicogenic C. difficile. Toxin A destroys tight junctions, so bacteria leaking into the bloodstream can cause endotoxic shock. Toxin A also stimulates sensory neurons, which release substance P when inflamed or stimulated. Mast cells release serotonin, then myenteric ganglia release acetylcholine with subsequent secretion of chloride. All this leads to severe diarrhea.

C. difficile is everywhere. A stressed postpartum mare can shed bacteria, and an udder-seeking foal ingests spores with subsequent increased shedding in the environment. C. difficile is identified in 16% of mares and 25% of foals.

One new finding of import is that Clostridium difficile can take to the air. Human studies have identified bedside air samples growing C. difficile. Consider what happens when stalls are mucked out and how that activity aerosolizes pathogens. In addition, C. difficile spores are difficult to eradicate from surfaces.

Diagnosis of C. difficile relies on finding toxin in the feces, which is identified via ELISA testing. PCR doesn’t detect toxins, but it detects genes responsible for making toxin. One-quarter of C. difficile species are non-toxicogenic. The toxins are stable in the refrigerator at 4 degrees C for 60 days. However, Slovis cautioned against storing samples in Styrofoam because it can bind the toxins.

Another protocol tests for glutamate dehydrogenase (GDH), an enzyme produced at high levels with C. difficile.

Salmonella. As many as 7-15% of normal horses shed Salmonella. An asymptomatic horse with Salmonella spp. might shed 100-200 colonies per gram of feces, whereas a symptomatic carrier sheds 10 million colonies per gram of feces along with exhibiting diarrhea and fever. Postpartum mares typically amplify shedding. One thousand colonies can grow to more than one billion colonies within six to eight hours at 100 degrees F. Slovis said that one fly can carry 6,000 Salmonella bacteria, and maggots in Salmonella-contaminated manure remain Salmonella positive during their four weeks of life. Similarly, mice eat manure and maintain Salmonella positivity in the area.

Studies demonstrated that resistant strains of Salmonella don’t decrease long-term survival or increase colic or abnormal feces in hospitalized horses; nor do they increase these risks in herdmates.

Previous recommendations to monitor for Salmonella shedding suggest five negative cultures, but current thinking is that three consecutive negative cultures within a month should be sufficient. Antimicrobial drugs can increase the duration of bacterial shedding. Mare and foal pairs parallel each other’s shedding pattern.

Vaccination of a pregnant mare with an oral S. typhimurium/Newport-resistant strain done at 9 and 10 months of pregnancy has shown excellent efficacy. This vaccine has a conditional license only in Kentucky at this time. In other states, an equine practitioner can request conditional licensing if anticipating a local problem.

Enterococcus durans. Enterococcus durans, previously named Streptococcus durans, colonizes the small intestinal mucosal surfaces between 2-10 days of age to cause diarrhea in affected foals. The organism is found in blood, joints and the umbilicus. Slovis suggested easy sample acquisition for a PCR test by using a Swiffer on catheter and foaling carts.

Biosecurity practices are essential in all situations dealing with foals. It is important to disinfect foaling kits, and Slovis recommended getting away from leather halters. Dip pans should be used regularly along with new or disposable boot covers for entry into stalls. Hand hygiene is critical using soap and water and/or alcohol sanitizer. 

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