In a laboratory at Utrecht University, researchers asked a quiet but pressing question: could an old remedy, manuka honey, help where modern drugs are faltering? Infected corneal ulcers in horses have grown harder to treat as bacteria learn to resist the antibiotics we depend on, and the study finds that a concentrated honey solution can halt the bacteria that cause these ulcers, even the resistant ones. Yet the finding remains a promise made in glass dishes rather than in living eyes, and the honey's effect depends on exact dosing, since too little can quietly help the very germs it is meant
Manuka Honey Shows In Vitro Activity Against Bacteria Behind Equine Corneal Ulcers
At 4%, bacterial growth seemed to be enhanced
Mark
So what did the Utrecht team actually find?
Mimi
That a 12% manuka honey solution stopped growth of every bacterium they tested from equine ulcers, resistant strains included.
Mark
Why honey, of all things?
Mimi
Resistance. No resistance to manuka honey has been described, even at sub-inhibitory levels, and it has reported antioxidant and anti-inflammatory effects too.
Luke
Let's be clear on scope, though. This was bacterial isolates in a lab. No horse eyes were treated.
Mimi
Right, and the authors say as much. They want clinical studies.
Mark
What about combining it with antibiotics?
Mimi
Mixed. Tetracycline, chloramphenicol and fusidic acid were boosted against Staphylococcus. Ofloxacin and cloxacillin looked slightly weaker for some bacteria. Gentamicin and tobramycin were unchanged.
Luke
And the 4% result is the one to remember. Growth seemed enhanced, especially Streptococcus.
Mimi
Which matters because tears dilute anything applied to the eye.
Luke
So the right concentration on the cornea is unknown. Whether horses tolerate it is unknown too; stinging is only noted in humans.
Mark
So it's promising but early.
Mimi
Yes, a lab signal, not a treatment recommendation.
Der Puls
- Infected corneal ulcers in horses are stubborn, and the bacteria behind them increasingly shrug off standard antibiotics.
- A 12% manuka honey solution stopped every tested bacterium common to these ulcers, whatever their resistance profile.
- Mixing honey with some antibiotics sharpened their effect against staphylococci, while a weak 4% dose let streptococci grow.
- The findings remain laboratory-bound, and researchers are now pressing toward dosing studies and clinical trials in horses before any eye is treated.
In a laboratory at Utrecht University, researchers asked a quiet but pressing question: could an old remedy, manuka honey, help where modern drugs are faltering? Infected corneal ulcers in horses have grown harder to treat as bacteria learn to resist the antibiotics we depend on, and the study finds that a concentrated honey solution can halt the bacteria that cause these ulcers, even the resistant ones. Yet the finding remains a promise made in glass dishes rather than in living eyes, and the honey's effect depends on exact dosing, since too little can quietly help the very germs it is meant to stop. What comes next is the slower work of testing whether a horse's eye can bear it.
A team at Utrecht University has been testing whether manuka honey can help with one of the more stubborn problems in equine eye care: infected corneal ulcers. The work was done on bacteria grown in the laboratory, not in horses, and it should be read that way.
The motivation is resistance. Bacteria that cause these ulcers, including Streptococcus, Staphylococcus and Pseudomonas, have often grown resistant to antibiotics, sometimes after earlier topical treatment. Manuka honey, by contrast, has not been shown to provoke resistance, and it carries reported antioxidant and anti-inflammatory properties.
The researchers first found the concentrations at which honey stops bacterial growth and at which it kills bacteria. They then paired a lower honey dose with seven common eye antibiotics. The headline result was clear: a 12% solution inhibited every tested bacterium, regardless of resistance. The combinations were more mixed. Honey strengthened several antibiotics against staphylococci, slightly weakened others, and left a few unchanged.
There was also a warning. At 4%, honey appeared to help Streptococcus grow. Because tears dilute whatever is applied to the eye, dosing will matter a great deal.
The authors see potential but call for further laboratory work and a clinical study to learn whether the equine eye tolerates honey well. Until then, the finding is a promising step rather than a treatment.
A team at Utrecht University in the Netherlands has tested whether manuka honey could help treat one of the more stubborn problems in equine eye care: infected corneal ulcers, known clinically as ulcerative keratitis. The work was done on bacterial isolates in the laboratory, not in horses, and the findings should be read in that light.
The motivation is resistance. Antimicrobial resistance is not only a systemic issue in human and veterinary medicine; it also shows up in ulcerative corneal disease. The main ophthalmic pathogens known to resist antimicrobial drugs include Streptococcus species, Staphylococcus species and Pseudomonas aeruginosa. Earlier studies have linked acquired resistance to prior topical antibacterial treatment. By contrast, no resistance to manuka honey has been described so far, even at sub-inhibitory concentrations.
That history led the researchers to look at sterile, preservative-free manuka honey as an addition to corneal treatment. Besides its reported antibacterial activity, the honey has reported antioxidant and anti-inflammatory effects.
The work proceeded in two stages. First, the team determined two thresholds for the honey: the minimal inhibitory concentration, at which bacterial growth is stopped, and the minimal bactericidal concentration, at which bacteria are killed. Second, they combined a sub-inhibitory 5% manuka honey concentration with seven topical antimicrobials: chloramphenicol, tetracycline, gentamicin, ofloxacin, tobramycin, cloxacillin and fusidic acid.
The headline result is that a 12% solution inhibited growth of the bacteria commonly found in equine ulcerative keratitis, regardless of their resistance profile. The minimal inhibitory concentration was 12% for every bacterium tested. Bactericidal activity appeared at concentrations from 8% to 28% for one methicillin-resistant coagulase-negative Staphylococcus, S. sciuri.
The combination results were mixed. Adding 5% honey to tetracycline, chloramphenicol or fusidic acid strengthened the effect against Staphylococcus species, including methicillin-resistant isolates. The effect of ofloxacin and cloxacillin seemed slightly reduced with honey for several, though not all, bacteria. Gentamicin and tobramycin showed no change from the addition.
There was also a warning. At a very low honey concentration of 4%, bacterial growth appeared to increase, particularly for Streptococcus species. This matters because the tear film dilutes whatever is applied to the eye, so a formulation that starts strong could drift into a range that is counterproductive.
The authors concluded that manuka honey has inhibitory and bactericidal activity against multi-resistant bacteria common in equine corneal disease, making it a potential topical drug for infectious keratitis. They call for more work on the best concentration, and for a clinical study to see whether the equine eye tolerates it; in humans, first application can cause brief ocular stinging. The study, by Barvelink, Brok, Djajadiningrat-Laanen and colleagues, appeared in Veterinary Ophthalmology in November 2025. Next steps are further laboratory work and trials in horses.
Bemerkenswerte Zitate
A solution of 12% manuka honey effectively inhibited the growth of bacteria commonly found in equine ulcerative keratitis, regardless of their antimicrobial resistance profile.— Study authors, Utrecht University
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