What's Really Causing Your Horse's Health Issues? A Deeper Look at Gut Health, Immunity and Disease

It's frustrating watching your horse struggle with the same problems again and again - mud fever that won't clear, thrush that keeps coming back, allergies that seem to appear out of nowhere - especially when you've done everything you've been told to do. This post looks at some of the most common equine skin, hoof and immune conditions, and digs into what's really driving them beneath the surface.

Every disease ultimately comes back to two things: deficiency and toxicity. Rather than covering the basics you already know - hydration, regular feeding, minimising stress - we'll explore the deeper factors that often get missed, but that can make a real difference to your horse's long-term health.

Common Horse Skin and Hoof Conditions

Mud fever appears on the lower limbs and backs of the legs as inflammation, scabbing and cracked, matted skin. Horses with pink skin or white socks are more susceptible, and it's contagious between horses. It's triggered by Dermatophilus congolensis, a bacterium that lives in soil and takes hold when skin repeatedly cycles between wet and dry and immunity is already compromised. Normally, beneficial bacteria and immune cells keep it in check - it's only when that balance breaks down that the bacteria gets the upper hand.

Spray-on commensal microbial (probiotic) formulas work by letting live, beneficial microbes migrate under the scabs to heal the skin directly, so stripping scabs first isn't necessary. Keeping the horse dry during treatment helps too.

Rain scald is caused by the same bacterium and thrives in hot, humid conditions, showing up as matted hair and scabbing across the back and rump. Light-pigmented horses are more prone to it. As with mud fever, restoring beneficial microbes - rather than just stripping scabs - supports faster healing. Seek veterinary advice if you see fever, lethargy or appetite loss.

Seedy toe and white line disease are secondary conditions - they start with a mechanical hoof problem (long toes, flared walls) that creates cracks bacteria can enter. Because there's no oxygen under the hoof wall, it's an ideal environment for anaerobic bacteria to multiply. Seedy toe shows as cracking where the hoof wall meets the sole; white line disease occurs when bacteria break down keratin in the inner hoof wall. Both are more common in horses with laminitis. Good, regular hoof care is the best prevention, and in immune-compromised horses, resolving these often comes down to genuine dietary change and rebuilding a diverse population of beneficial gut microbes - not topical treatment alone.

Thrush affects the frog of the hoof and is caused by anaerobic bacteria, sometimes alongside fungi - you'll usually smell it (a strong odour and black discharge) before you see it. Daily cleaning with a spray-on commensal microbial blend lets "native" beneficial bacteria outcompete the pathogenic ones; twice-daily application is generally recommended for an active infection.

Allergies are becoming more common in horses, showing up as itchy skin, rashes, coughing or wheezing - any mucosal surface can be affected when immunity is compromised. The obvious triggers (insects, pollen, chemicals, certain plants) are really just the surface story. The deeper question is why your horse has suddenly started reacting to things it's been exposed to its whole life.

Why Do These Conditions Keep Happening?

In every condition above, bacteria or fungi have managed to get past the horse's commensal microbes - the beneficial, "native" organisms living on the skin, in the hooves and throughout the body. So the real question isn't "what caused this infection" but "why did my horse's immune and microbial defences fail to stop it?"

Nothing happens in isolation, and every condition relies on predisposing factors already being present. It's common practice to treat only the symptom that shows up, without asking what led to it. Think of it like a headache: paracetamol treats the pain, but not the dehydration or magnesium deficiency that caused it. The same logic applies to your horse - symptoms need treating for comfort, but lasting change comes from also addressing what's happening underneath. Just as you'd take your car in for a warrant of fitness every six months, it's worth applying the same thinking to your horse's health.

Here are three foundational pillars worth understanding.

1.       Good Nutrition

New Zealand has excellent equine nutritionists, so we won't cover feeding basics here - but glyphosate exposure is worth flagging. Where possible, source feed and hay that hasn't been treated with glyphosate-based weedkillers or used as a pre-harvest desiccant. Glyphosate has antimicrobial properties strong enough that it's been patented for that purpose, meaning regular exposure through sprayed hay or grain can have an ongoing, antibiotic-like effect on gut bacteria. It also binds to minerals in plant tissue - iron, manganese, copper, zinc, magnesium and cobalt - making them less available even when feed looks nutritionally complete on paper. Corn, alfalfa, soy and beet pulp are common carriers, since many are grown from glyphosate-tolerant seed.

If you can't fully change your horse's feed, a good vitamin and mineral supplement, a quality probiotic containing commensal microbes, and a periodic detox protocol are sensible next steps. Asking your hay supplier whether their hay is tested for glyphosate is a reasonable question these days.

2.       Microbial Balance

Every horse - like every living creature - carries more microbial cells than host cells, which tells you how important they are. Even small shifts in hindgut microbiome diversity can trigger colic or other health issues. These gut microbes live in the mucosal lining alongside lymphocytes (the immune cells responsible for gut protection), and the two work in close partnership: the immune system helps keep microbial communities balanced, while depending on those same microbes to function properly.

Certain medications disrupt this more than people realise. Antibiotics are the obvious one, but NSAIDs, hormone treatments and proton-pump inhibitors like omeprazole (often used long-term in racehorses for ulcers) all exert an antibiotic-like effect on beneficial gut bacteria. A 2024 University of Georgia study found that phenylbutazone (Bute), one of the most widely used equine NSAIDs, actually integrates into intestinal cells and causes mitochondrial injury and oxidative stress - offering a mechanistic explanation for why long-term NSAID use is so often linked to gut and ulcer problems.

Commensal microbes support digestion, help maintain correct hindgut pH, physically block pathogenic bacteria from attaching to the gut wall, and help metabolise nutrients. Disrupting this balance, even briefly, has consequences that show up later - whether you see them immediately or not.

3.       Leaky Gut

"Leaky gut" describes a gastrointestinal tract that's become too permeable, letting contents that should stay inside - bacteria, fungi, endotoxins - pass into the bloodstream. The immune system, sitting right at that mucosal interface, reacts to anything that shouldn't be there, and this ongoing immune response is what shows up externally as allergy symptoms. Left unaddressed, constant inflammation eventually overwhelms and weakens the immune system as a whole.

Leaky gut in horses has historically been under-researched compared to other species, but that's changing. Researcher Dr Wendy Pearson, presenting at the 2025 American Association of Equine Practitioners Convention, is leading a University of Guelph study exploring whether activated hemp seed powder can support gut barrier function during physical stress. Separate research has shown that combining transport with exercise measurably increases gut permeability and inflammation markers in horses - but a prebiotic fermentation product (derived from Aspergillus oryzae) prevented that stress-induced "leakiness" from occurring at all. This reinforces what holistic practitioners have observed clinically for years: gut barrier integrity is central to immune resilience, and it responds well to targeted nutritional support.

This is also why reaching for a single symptom-relief medication - Bute for pain, say - doesn't resolve the underlying picture. Long-term Bute use has itself been linked to gastric ulceration and kidney or liver stress, which can compound the very gut issues driving the original symptoms.

Supporting the Immune System Naturally

Probiotics give the horse's body - not just the gut - the tools it needs to keep pathogenic bacteria in check. The goal isn't adding "a" probiotic, but increasing microbial diversity and species count, so the immune system has a genuinely broad toolkit to draw on.

It's easy to default to quick-fix, symptom-only treatments. The natural alternative - good diet, restoring healthy bacteria, and addressing gut integrity - is slower, but far more foundational. Gastrointestinal issues are the second-leading cause of death in horses, so keeping the gut nourished and repaired should be routine care, not just a reaction to crisis.

A few natural allies worth knowing:

• Meadowsweet – traditionally used to support gastric comfort and ulcer healing
• Dandelion – rich in nutrients and supportive of liver detoxification
• Slippery elm – helps rebuild and soothe the mucosal gut lining
• Psyllium seed – useful for horses grazing sandy soils, helping move accumulated sand through the gut (roughly 50g per 100kg of bodyweight, fed for four consecutive days each month)

As always, check with your vet before starting any new treatment, particularly around dosing and suitability for your individual horse.

Recent Research in Equine Gut and Immune Health

• A 2024 controlled study at the University of Georgia found that phenylbutazone causes measurable mitochondrial injury and oxidative stress in intestinal cells, and disrupts the gut microbiome and metabolome - adding weight to concerns about long-term NSAID use in horses.
• A 2025 Frontiers in Immunology study examined how commercial oral probiotic supplements affect gut barrier function and mucosal immunity in horses, reinforcing the biological plausibility of probiotic support for gut integrity.
• New research led by Dr Wendy Pearson at the University of Guelph (2025) is investigating leaky gut syndrome directly, including whether activated hemp seed powder can support gut barrier function during transport and exercise stress.
• A study published in Animals (2023) found that combining transport with exercise significantly increased gut permeability and inflammation markers in horses - and that a prebiotic Aspergillus oryzae fermentation product prevented this stress-induced permeability increase.
• A 2024 review in Animals summarised current understanding of equine gut dysbiosis, noting that many commercial probiotics on the market don't reliably contain the organisms or concentrations stated on the label - underscoring the importance of quality and species diversity when choosing a probiotic.
• A 2024/2025 study on equine pastern dermatitis found distinct differences in skin microbial communities between healthy and affected horses, and that topical application of a beneficial bacterial strain (Weissella cibaria) produced favourable shifts in the skin microbiome - supporting the same "restore the beneficial microbes" approach discussed above for mud fever and rain scald.

* Do check with your vet after doing your own research too, that its ok to go ahead with these treatments, based on the specifics of your horse.

Disclaimer: This content is for informational purposes only and is not meant to diagnose, treat, or replace consulting a primary veterinarian for individualized care.