Today scientists consider the microbiota a metabolic organ of its own, and it’s a growing area of interest because of the role it plays in general health and well-being. New research is showing that microorganisms naturally present in the gut aren’t just designed to break down nutrients. They have a direct impact on the overall well-being of humans and horses alike.
The gut microbiome is a constantly changing combination of fungi, bacteria, and viruses. In humans, the intestinal microorganisms begin changing as early as birth and continue to shift throughout the first year of life as food sources shift from liquids to solids. Researchers at the University of California, Davis, have found the same is true for foals. Starting at birth, foals begin developing a microbiome by ingesting amniotic fluid, the mare’s feces, colostrum (the antibody-rich first milk), and milk, explains Meriel Moore-Colyer, BSc, BHSII, PhD, RAnimSci, professor at Royal Agricultural University, in Gloucestershire, England.
“The immunoglobulins (antibodies) in the mare’s first milk are critical in early defense against disease,” she says. “Foals will get some microbes for gut development from this.”
Colostrum contains antibodies that typically recognize disease-causing bacteria and viruses and target them for destruction. Besides antibodies, colostrum contains many antimicrobial enzymes and proteins, including lysozyme, lactoferrin, and lactoperoxidase.
The specific microorganisms and population levels present directly relate to the food source available. For example, milk contains oligosaccharide carbohydrates, on which certain bacterial species thrive. When milk is the main food source, a large population of bacteria that might metabolize milk components like oligosaccharides is present in the gut.
“Some bacterial populations colonize opportunistically, and they seem to flourish given the lack of other populations,” says Michael Mienaltowski, DVM, PhD, an associate professor at UC Davis.
Nibbling and sucking on hay, straw, or the mare’s feed can also contribute to a foal’s early microbiome development, says Moore-Colyer. As the foal begins ingesting new food sources, the microbiome changes and plays a critical role in developing lifelong immunity and disease resistance. Therefore, supporting gut health from as early as Day 1 can have a lasting impact on an animal’s overall health.
Transitions in Foal Gut Health
Most foals follow a natural transition from milk diets to mouthing things in their environment to eating soil, feces, concentrate, grass, and hay. They watch the other horses on the property chewing and become curious about trying it, too.
“Usually, it is our intention for our foals to pick up those microbial populations that will serve their diets and, thus, help them to ultimately metabolize food,” Mienaltowski says. “However, sometimes events lead to disturbances in the GI microbiota.”
Pathogens (disease-causing organisms) are simply part of everyday life. A foal could encounter another sick foal or pick up bacteria from a fomite (objects or materials likely to carry pathogens) or even a person’s hands. Bacteria live in feces and the soil, both sources foals interact with regularly. A foal could be in an environment where antimicrobials were used, even on other horses, and resistant strains might arise and find their ways into a herd or the soil, or onto the surfaces around him.
“The cleanliness of a facility can contribute to the prevention of microbial disturbances,” Mienaltowski says. “There are even opportunistic pathogens that exist in the foal’s GI (tract) that might flourish if the opportune moments arise for that bacteria to increase in numbers.”
He recommends three steps for supporting foal gut health:
- Keep your facility as clean as possible. Manure buildup in stalls and fields or dirty environments could lead to illnesses in foals, especially if they suffer from GI issues secondary to another illness.
- Know who and what is coming onto your property. Get a good medical history for the horses on your premises, and check to see if any of them have been at veterinary hospitals recently. Be aware of other species, such as cattle, goats, and sheep, on the property. These or wild animals might carry a pathogen that could infect your horse, such as Salmonella or Leptospira interogans (TheHorse.com/17078).
“Be aware of antimicrobial use on the property and antimicrobial use for horses coming onto your property,” Mienaltowski says. “Ask about previous exposures to infectious agents.”
- Recognizing clinical signs. Have your veterinarian immediately examine any foal that appears unwell. Closely monitor foals that are “off” to prevent a situation from taking a turn for the worse. Safeguarding against preventable disease can help you avoid the expense of treating a sick foal and can save his life.
Signs of Trouble and Their Prognoses
Disruptions in a foal’s gut can occur for a multitude of reasons. Pathogen exposure and dietary transitions (from milk to forage) alter the microbiome populations. Antibiotics can wipe out healthy populations, potentially altering the numbers of helpful microbes and creating “space” for other microbes to cause an imbalance known as dysbiosis.
Diarrhea—called scours in foals—is the telltale sign of a gut issue, be it minor or from a serious illness, though it’s normal to see this with the transition from milk to forage and feed. Even if scours isn’t pathogen-related, the associated dehydration can become a serious and even fatal issue in a foal.
“It is important to examine foals that aren’t nursing, are looking lethargic, have issues with mobility or weakness,” says Mienaltowski. “Other symptoms can include elevated temperature, pulse, and respiratory rate. If a foal doesn’t seem to be growing at a rate typical for its breed, this could be a sign too.”
Like human babies, foals can quickly become sick. Therefore, it’s critical to stay in frequent contact with the veterinarian and report issues with abnormal-appearing or -acting foals. The prognosis for a foal suffering from a microbiome imbalance ranges from excellent to poor. The outcome largely depends upon whether the foal is ill from an infectious pathogen, has suffered from failure of passive transfer of immunity, or has experienced another illness or injury.
“If any newborn shows signs of illness, it is important to pay attention and tackle the problem right away,” he says.
In 2019 Mienaltowski and a team of researchers collected fecal samples from foals with and without diarrhea. They noted differences between farms, which he says should make sense: The foals at each farm lived in different environments and were subject to different management styles, and horse populations on each farm had different histories of illnesses and antimicrobial exposure.
“We are still working on understanding the differences in diarrhea by age,” he says. “Some of the differences are related to whether or not the foal was more reliant upon milk in its first couple weeks.”
Pros and Cons of Pre-/Probiotics
Few to no double-blinded studies have shown prebiotics and probiotics to be effective in supporting foal gut health (though they have been shown effective in some adult populations). In fact, some studies have shown that certain probiotics, relative to placebos, led to more days early in life with diarrhea, says Mienaltowski.
“Most researchers in the field agree that the best thing we can do is to make sure that any preventive or therapeutic measure maintains the richness of the GI microbiota for the foal,” he says. “There isn’t any one kind of probiotic that I could recommend because gold-standard studies aren’t published for the horse.”
Mienaltowski says that there is a push for researchers to perform studies on prebiotic and probiotic efficacy. He notes that most clinicians and academics would like to see clinical trials detailing their effectiveness in the horse rather than transposing findings from humans and mice to horses, given the vast difference between each species’ digestive tracts.
Instead of providing supplements, Mienaltowski suggests taking a big-picture approach to determining how your foal’s hindgut (everything beyond the small intestine) transitions to life outside the womb, comparing any outliers or sick horses to the rest of your foals.
“If your veterinarian is recommending a specific prebiotic/probiotic for your foal, I would ask them why they are recommending it and what sort of efficacy has been determined scientifically, if only to know what to expect for what you’re spending,” Mienaltowski says. “Product costs range from $10 to $150 per dose, and I haven’t seen a study proving either price point product as actually being effective in the horse.”
Moore-Colyer agrees that pre-or probiotics shouldn’t be necessary if the foal is healthy and the mare is fit and well. However, she says if the foal has had a bout of scours, they might be helpful.
Looking to the Future
Research focused on the horse’s microbiome, specifically in foals, is a relatively new area of study. Findings from human research are the starting point for launching investigations into horses and other animals.
Peter Huntington, DVM, MANZCVSc, director of nutrition for Kentucky Equine Research (Australasia), says one of the most interesting areas of equine nutrition research, in his opinion, involves long-chain omega-3 fatty acids—eicosapentaenoic acid (EPA) and docosaheaenoic acid (DHA)—from fish oil. He points to research showing numerous health benefits from supplementation, including boosting immunity.
“It would be interesting to explore its value prior to foaling in more detail,” he says. “The addition of long-chain omega-3 fatty acids to the diets of pregnant and lactating mares has shown considerable benefit to both the mare and foal.”
Furthermore, researchers have found that mares who consume diets high in omega-3 fatty acids produce ‘richer’ colostrum, which assists in the passive transfer of colostral antibodies and could jump-start the foal’s health.
“Supplementation of pregnant mares with DHA and EPA confers benefits to the developing fetus and certainly provides benefits to the foal as soon as it is on the ground,” he says. “Researchers have discovered that the placenta may be responsible for transferring DHA and EPA to the fetal nervous system; therefore, supplementing the mare during pregnancy may assist with foal development in utero.”
Mienaltowski sees two other significant trends in research related to foal gut health. The first is fecal transfer of microbes from healthy horse feces into the GI tract of a sick horse. The second is the concern clinicians and private practitioners have about antimicrobial use in mares and foals, and the resulting diarrhea, antibiotic-resistant infections, etc. that might arise.
“With all of these trends, there is much discussion about more sophisticated genomic analyses of microbial populations to get a good idea of the microbes present in sickness and in health for foals and adult horses alike,” he says.