The Best Protocol to Treat Equine Skin Wounds by Second Intention Healing: A Scoping Review of the Literature

Equine skin wound treatment continues to be a challenge for veterinarians. Despite being a frequent practice, it remains difficult to choose an evidence-based treatment protocol. This study aimed to comprehensively explore the literature and provide a scoping review of therapeutic strategies for equine skin wounds and identify knowledge gaps and opportunities for future research. This review was conducted using specific criteria to select literature that described methods to manage second intention wound healing. After removing duplicates and screening papers for suitability, 81 manuscripts were included for data extraction. Of these, 59 articles were experimental studies, 10 were case reports, 9 were case series, and 3 were clinical studies. The most frequent wound location was the distal limbs. Macroscopic assessment was the main tool used to evaluate treatment effectiveness. All of the case reports, case series, and clinical studies reported positive outcomes with regard to the treatment used, while only 36% of the experimental studies found significant healing improvement in treated wounds compared to control groups. It was found that there are many treatments that have exhibited controversial results, and there exists a lack of evidence for the adoption of specific treatment protocols.

Macroscopic, Histologic, and Immunomodulatory Response of Limb Wounds Following Intravenous Allogeneic Cord Blood-Derived Multipotent Mesenchymal Stromal Cell Therapy in Horses

Limb wounds are common in horses and often develop complications. Intravenous multipotent mesenchymal stromal cell (MSC) therapy is promising but has risks associated with intravenous administration and unknown potential to improve cutaneous wound healing. The objectives were to determine the clinical safety of administering large numbers of allogeneic cord blood-derived MSCs intravenously, and if therapy causes clinically adverse reactions, accelerates wound closure, improves histologic healing, and alters mRNA expression of common wound cytokines. Wounds were created on the metacarpus of 12 horses. Treatment horses were administered 1.51-2.46 × 10⁸ cells suspended in 50% HypoThermosol FRS, and control horses were administered 50% HypoThermosol FRS alone. Epithelialization, contraction, and wound closure rates were determined using planimetric analysis. Wounds were biopsied and evaluated for histologic healing characteristics and cytokine mRNA expression. Days until wound closure was also determined. The results indicate that 3/6 of treatment horses and 1/6 of control horses experienced minor transient reactions. Treatment did not accelerate wound closure or improve histologic healing. Treatment decreased wound size and decreased all measured cytokines except transforming growth factor-β3. MSC intravenous therapy has the potential to decrease limb wound size; however, further work is needed to understand the clinical relevance of adverse reactions.

Anti-fibrotic Actions of Equine Interleukin-10 on Transforming Growth Factor-Beta1-Stimulated Dermal Fibroblasts Isolated From Limbs of Horses

Fibroproliferative disorders occur in both humans and horses following skin injury. In horses, wound healing on the limb is often complicated by the formation of fibroproliferative exuberant granulation tissue, characterized by persistent expression of pro-fibrotic transforming growth factor-beta1 (TGF-β1) and deficient expression of anti-inflammatory interleukin-10 (IL-10). IL-10 has been shown to directly modulate fibrotic gene expression in human fibroblasts, so we hypothesized that equine IL-10 (eIL-10) may exert similar anti-fibrotic effects on equine dermal fibroblasts. Cell-lines were created from the limb skin of six individual horses. Recombinant eIL-10 was produced and purified, and its effects on the cells investigated in the presence and absence of equine TGF-β1 (eTGF-β1). Myofibroblast differentiation and collagen production were examined using immunofluorescent cytometry, cell contractility in a collagen gel assay, and fibrotic gene expression using quantitative PCR. In response to eTGF-β1, fibroblasts increased in contractility and expression of alpha-smooth muscle actin, collagen types 1 and 3, and matrix metalloproteinase 1, 2, and 9. Equine IL-10 limited cell contractility and production of alpha-smooth muscle actin and type 3 collagen, and decreased mRNA levels of eCol3a1 and eMMP9, while increasing that of eMMP1. Opposing effects on eTGF-βR3 and eIL-10R1 gene expression were also observed, with mRNA levels decreasing following eTGF-β1 treatment, and increasing with eIL-10 treatment. These findings indicate that eIL-10 limits the pro-fibrotic effects of eTGF-β1, potentially through the modulation of fibrotic and receptor gene expression. Further investigations are warranted to assess the therapeutic utility of eIL-10 in the treatment of exuberant granulation tissue.

Homing and Engraftment of Intravenously Administered Equine Cord Blood-Derived Multipotent Mesenchymal Stromal Cells to Surgically Created Cutaneous Wound in Horses: A Pilot Project

Limb wounds on horses are often slow to heal and are prone to developing exuberant granulation tissue (EGT) and close primarily through epithelialization, which results in a cosmetically inferior and non-durable repair. In contrast, wounds on the body heal rapidly and primarily through contraction and rarely develop EGT. Intravenous (IV) multipotent mesenchymal stromal cells (MSCs) are promising. They home and engraft to cutaneous wounds and promote healing in laboratory animals, but this has not been demonstrated in horses. Furthermore, the clinical safety of administering >1.00 × 10⁸ allogeneic MSCs IV to a horse has not been determined. A proof-of-principle pilot project was performed with two horses that were administered 1.02 × 10⁸ fluorescently labeled allogeneic cord blood-derived MSCs (CB-MSCs) following wound creation on the forelimb and thorax. Wounds and contralateral non-wounded skin were sequentially biopsied on days 0, 1, 2, 7, 14, and 33 and evaluated with confocal microscopy to determine presence of homing and engraftment. Results confirmed preferential homing and engraftment to wounds with persistence of CB-MSCs at 33 days following wound creation, without clinically adverse reactions to the infusion. The absence of overt adverse reactions allows further studies to determine effects of IV CB-MSCs on equine wound healing.

Osteopontin expression in healing wounds of horses and in human keloids

REASONS FOR PERFORMING STUDY

Convincing evidence shows that persistent or excessive expression of osteopontin (OPN) is linked to fibroproliferation of various organs in laboratory animals and in man, such that its downregulation is a logical therapeutic objective.

OBJECTIVES

To investigate OPN expression in an equine model of wound healing and in clinical specimens of equine exuberant granulation tissue and human keloids in an effort to better understand the contribution of this protein to inflammation-associated skin fibrosis.

STUDY DESIGN

Description of gene and protein expression in an experimental equine model of wound healing and clinical specimens in horse and man.

METHODS

Osteopontin gene expression was evaluated by quantitative PCR, while protein expression was investigated by means of immunohistochemical staining.

RESULTS

Quantitative PCR showed that the OPN gene is expressed in normal intact skin of horses and continues to be expressed during the wound-healing process. An increase in gene expression was observed throughout the phases of wound healing, with a final decrease at wound closure. The protein was not detected in normal skin. Keratinocytes in wound-edge samples did not express the protein, whereas dermal immunoreactivity was confined to inflammatory cells. Healed wounds were devoid of staining. Equine exuberant granulation tissue showed immunoreactivity of the surrounding epidermis, infiltrating neutrophils, mononuclear cells, endothelial cells and fibroblasts. Human keloids showed OPN immunoreactivity throughout the epidermis as well as in mononuclear cells and scattered fibroblasts.

CONCLUSIONS

Immunohistochemical data show a different pattern of expression between normally healing and fibrotic wounds (exuberant granulation tissue and keloids), thus suggesting a role in fibroproliferation in horses and man.