Horses are notorious for getting injured, with leg wounds a common occurrence. Prompt attention and veterinary care usually resolve an injury with minimal problem. However, some wounds simply won’t heal. In those cases, it is prudent to consider the presence of bacterial biofilms that help bacteria evade the horse’s immune response and delay healing.
A Danish study evaluated a possible link between impaired healing and biofilms [Jorgensen, E.; Bjarnsholt, T.; and Jacobsen, S. Biofilm and Equine Limb Wounds. Animals 2021, 11, 2825; doi.org/10.3390/ani11102825]. The authors identified biofilm infection in nearly all equine limb wounds. They found that delayed healing due to biofilms tended to occur primarily on distal limb wounds rather than on body wounds.
The researchers noted that biofilms made bacteria less susceptible to anti-microbial drugs (AMDs) due to tolerance, which is different from genetically inherited AMD resistance. Tolerance needs time to develop, so biofilms only a couple of days old are more successfully treated with AMDs than biofilms of longer duration. Biofilms exert their adverse actions in multiple ways:
- Neutrophils release enzymes and oxidative radicals to significantly injure surrounding tissue.
- More collateral damage is incurred by antibodies that cannot bind to bacteria—instead, they form immune complexes that activate opsonization and the complement system.
- Oxygen consumed by biofilms and leukocytes lowers oxygen tension in the tissues.
All these instances amplify hypoxia and inflammation with increasing biofilm formation and collateral injury to the tissues. The result: a non-healing wound.
The incidence of occurrence also depends in part on the wound bed and its microenvironment.
It is difficult to identify biofilms with routine diagnostic tests or visual observation of typical signs of inflammation. Tissue biopsy and electron microscopy, PCR and culture might yield some information. Suspicion of a biofilm issue is likely for a non-healing limb wound on a horse. The authors noted that “systemic antibiotics have no effect on the bacterial burden in granulating wounds and should therefore mainly be used for acute wound infections.”
Ideally, the objective is to prevent biofilm formation in the first place, and that can be accomplished by local and/or systemic treatment with AMDs in a wounded horse within the initial 24 hours.
Once a biofilm has become established, treatment relies on debridement—sharp dissection with a scalpel is one effective technique—as many times as necessary. Each debridement effort physically removes biofilm and bacteria, providing an additional 24-hour window until biofilms reform and reestablish. During that period, topical and/or systemic treatments are key to limiting bacterial growth and biofilm production.
Debridement at bandage change further helps with resolution, particularly if crevices exist in the wound. The authors had no preference for topical treatment, but they suggested favorable results using polyhexamethylene biguanide (PHMB), nanocrystalline silver or silver sulfadiazine, and with antibiotics.
Another strategy is to apply hypertonic saline, sugar or honey for a few days to reduce the burden or organisms and help remove necrosis and exudates. For Pseudomonas aeruginosa infections, they recommended topical use of gauze-soaked 2% acetic acid for 20-30 minutes along with appropriate AMDs.