Causes, Biofilm, and Modern Treatment
Wound healing in horses, particularly on the limbs, is one of the greatest challenges in veterinary practice. The fact that wounds in horses so quickly become chronic or fail to heal is due to a combination of anatomical and physiological characteristics specific to the species.
Why do wounds become chronic?
Hypergranulation: Formation of wild flesh
Horses have a physiological tendency toward excessive formation of granulation tissue. Since the inflammatory phase of wounds below the carpal or tarsal joint is often mild but extremely prolonged, the body produces new tissue uncontrollably. This “excess granulation tissue” overgrows the wound margins and mechanically prevents new skin cells (epithelial cells) from covering the area.
Poor Blood Circulation and Tissue Tension in the Legs
Most chronic wounds develop distally (below) the knee and elbow. In this area, there is hardly any protective muscle tissue; instead, it consists almost entirely of tendons, bones, and skin. The skin here is under enormous mechanical tension, which makes wound healing more difficult. In addition, capillary blood flow in this area is poorer than in the trunk, meaning that less oxygen and nutrients reach the wound bed.
Constant Movement and Mechanical Irritation
It is difficult to keep a horse still. Standing up, lying down, and constant movement in the joint area cause the newly formed, highly sensitive tissue to be repeatedly torn at the microscopic level.
High Infection Pressure and Biofilms
The barn environment is never sterile. Wounds on the legs are constantly exposed to bedding, dust, feces, and urine. If a wound remains open for an extended period, these bacteria aggregate to form a biofilm. This protective slime film makes the pathogens resistant to the body’s own immune cells as well as to many topical antibiotics, and severely impedes healing.
Severe Contamination at the Time of Injury
Many horse wounds occur in the paddock or in the stable due to trauma such as fence injuries (paddock wire), being kicked, or getting caught on objects. These wounds are usually dirty; tissue is crushed, pockets form, and dirt is forced deep into the wound. Such traumatized and poorly perfused tissue provides the perfect breeding ground for chronic infections.
Biofilm Formation
As soon as bacteria colonize the wound, they attach themselves to the tissue and quickly form a protective slime layer composed of proteins and sugars. Within this barrier, the pathogens are resistant to the body’s immune defenses and conventional antibiotics. Without targeted mechanical and chemical destruction of this mucus layer, the biofilm keeps the wound in a persistent inflammatory state and blocks any progress toward healing.
Hypochlorous Acid (HOCl): Effective against biofilm without causing resistance
Hypochlorous acid (HOCl) is a substance produced naturally by the body, including by a horse’s white blood cells as part of its natural immune defense.
Due to its small molecular size and electrical neutrality, hypochlorous acid can easily penetrate the protective mucus layer of the biofilm rather than bouncing off it. HOCl destroys the cell walls of bacteria through oxidation and simultaneously breaks down the structural matrix of the biofilm mechanically.
Since HOCl is based on a purely physical mechanism of action, it highly effectively eliminates even multidrug-resistant pathogens without allowing them to develop new resistances.
Sources:
Jørgensen E, Bjarnsholt T, Jacobsen S. Biofilm and Equine Limb Wounds. Animals (Basel). 2021 Sep 27;11(10):2825. doi: 10.3390/ani11102825. PMID: 34679846; PMCID: PMC8532864.
Nadia Ayurini Anantama, Charis Du Cheyne, Ann Martens, Susanne Pauline Roth, Janina Burk, Ward De Spiegelaere, Jule Kristin Michler, The granulation (t)issue: A narrative and scoping review of basic and clinical research of the equine distal limb exuberant wound healing disorder, The Veterinary Journal, Volume 280, 2022, 105790, ISSN 1090-0233, https://doi.org/10.1016/j.tvjl.2022.105790.
Marchant, K., Hendrickson, D.A. & Pezzanite, L.M. (2024) Review of the role of biofilms in equine wounds: Clinical indications and treatment strategies. Equine Veterinary Education, 36, 152–168. Available from: https://doi.org/10.1111/eve.13919