In this article republished from Equine Health magazine, equine surgery specialist Patrick J Pollock, BVMS, CertES (Soft Tissue), FHEA, Dip ECVS, MRCVS, explains the different stages in wound healing, why equine wounds sometimes don’t heal, why moist wound healing techniques have superseded dry and offers some wound healing tips

WHETHER created by the surgeon’s scalpel, or a strand of rusty fence wire, wounds elicit a consistent response in all mammals. A biological process of wound healing follows and is made up of an orderly series of steps aimed at restoring continuity to  tissue injured. Veterinary surgeons are faced with many types and causes of wound in daily practice and yet the approach and management of wounds in horses has not changed greatly in 20 years, except that in today’s world it seems there are numerous products sold to vets and horse owners that purport to “accelerate” wound healing. Although many make a very positive contribution to wound healing — improving the local wound environment, donating moisture, removing necrotic debris and providing infection control — to date no product is available that speeds up healing beyond that which occurs when wound healing is optimised.

Sadly some of the products can be used in a way that attempts to circumvent effective wound healing techniques. Indeed no matter where one travels across the globe, veterinary surgeons and horse owners feel compelled to treat wounds with substances of dubious value— these include oil-based ointments, or particulate containing sprays, or even ash, hydrocarbons and animal faeces.

Wounds move predictably down a common pathway of healing, and knowledge of the four overlapping healing phases can determine appropriate steps in wound management:
Inflammation — During the inflammatory phase, neutrophils (1)* predominate in the wound environment, releasing enzymes that break down cellular debris. (image right)

The same wound subjected to primary closure, following lavage and surgical debridement

Debridement — Monocytes invade the wound and become macrophages which begin to remove debris and induce fibroblasts to enter the wound.
Repair  — Also referred to as the fibroblastic or proliferative stage of healing. The fibroblast predominates giving the wound strength and laying down collagen and connective tissue. Fibroplasia or the formation of granulation tissue is the hallmark of the repair phase, which begins, in optimal conditions, around three days after wounding.

In the debridement phase, a large amount of necrotic debris and purulent material is present

Maturation  — Also called the remodelling phase, maturation processes begin around two weeks after wounding and may continue for six to 12 months. Granulation tissue and collagen production decline, the wound becomes stronger, although it should be remembered that the scar is never as strong as the tissue it replaces.

Veterinary surgeons in equine practice are frequently called upon to manage traumatic wounds and the healing of such wounds is often protracted in equines compared to other species. Complications associated with wounds are expensive and can lead to significant wastage, as affected animals may be unable to continue with an athletic career.

The repair phase, where healthy granulation tissue fills the wound

Wound closure, either primary or delayed, is always the preferred method of treatment, however this is not always possible due to large tissue deficits and the high levels of contamination common to equine wounds. For these reasons healing by second intention is often the only option. Second intention healing relies on contraction and epithelialisation* of the wound and is often slow.

Complications such as the development of exuberant granulation tissue (proud flesh) and sequestrum formation (see below right) are common with chronic wounds entering what is known as a “non-healing state”. These problems are particularly apparent in distal limb wounds and can largely be attributed to differences in the inflammatory response and the presence of damaging enzymes that slow, or halt, wound healing. This delay in healing is characterised by large numbers of neutrophils within the wound.

A large sequestrum can be seen above the radio-opaque marker (needle)

Why don’t equine wounds heal?
In horses there are many reasons why wounds may enter a so called non-healing state. This might be due to a number of iatrogenic (2) factors, or the general health status of the animal, however more often than not one of the following factors is implicated.
◊ Infection
◊ Excessive movement (see below)
◊ Presence of contamination, foreign bodies, scab and necrotic tissue
◊ Large tissue deficits
◊ Loss of blood supply (often confounded by the lack of a collateral circulation in the equine distal limb)
◊ Poor tissue oxygenation at the wound surface
◊ Continued and repeated trauma to the wound
◊ Transformation of wound tissue (most commonly this leads to the formation of equine sarcoid) (Figure 8).
◊ Presence of exuberant granulation tissue.

The first step in wound therapy should be to determine which stage of healing the wound is currently in. The clinician can then determine if any of the above factors are present, and which steps are necessary to eliminate the problem in order to move the wound to the next stage.

Movement is a common cause of complications when dealing with equine wounds. In the case shown here, a wound on the palmar aspect of the heel bulbs is immobilised using a foot cast to facilitate healing. Twelve days separate the images to the left and to the right

Copious lavage with an appropriate fluid is an excellent method for removing debris. Image below shows wound after the lavage process

What can we do better?
Since 400 BC when Galen and Hippocrates determined that removal of necrotic debris from wounds aided healing, a great deal of research has contributed to the science and practice of wound management in human beings and horses.

In the last two centuries, the use of antiseptics and introduction of antimicrobials led to great leaps forward, however in the 1960s, the work of Winter and others changed the way wounds were managed forever when it was determined that epithelisation(3) was retarded by the dry scab which normally covers a superficial wound and, if the formation of the scab was prevented, the rate of epithelisation was markedly increased. This single finding more than any other, based on work carried out on the wounds of pigs, inaugurated an era of moist wound healing.

Wound healing tip:
Remove debris from the wound surface with copious lavage and maintain a moist environment at the wound surface with the use of an appropriate moisture donating dressing such as a hydrogel. Saline is the most appropriate lavage solution, however water is a close second. 

Wilmink and others demonstrated that inflammation in the wounds of horses is weak and protracted, and that horse leukocytes produce relatively less reactive oxygen species essential to bacterial killing, than do the leukocytes(4) of native breed ponies. They also produce lower levels of other mediators required to reinforce the inflammatory response and to induce tissue formation and wound contraction. The weak inflammatory response exhibited by horses following wounding handicaps their ability to prevent and/or eliminate wound infection.

Wound healing tip:
Avoid the use of non-steroidal anti-inflammatory drugs in the early stages of wound healing, as these drugs may prolong the inflammatory phase even further and may impede the ability of the horse to eliminate bacteria from the wound surface

Malignant transformation of equine wounds can occur, in this case a sarcoid has developed at the site of a wound on the lateral aspect of the hock

Since the earliest times, the presence of wound infections has been associated with delayed or complicated wound healing. Wound infection results from the attachment and multiplication of microbes, rendered possible by an imbalance between bacteria and host factors. The most significant microbe-related factor is the bacterial inoculation dose; an inoculum(5) of 105 organisms/gram of tissue is a challenge above which wounds may become infected. This number is generally reached in the wounds of human beings after approximately six hours, the so-called golden period. However this value is likely to be lower in patients with an impaired immune response, poor tissue perfusion(6) or the presence of a foreign body, and in horses where environmentally acquired contamination is very common due to the conditions in which horses live.

As a result, techniques to reduce bioburden are key to reducing the risk of wound infection. Brett and others demonstrated that the most effective technique was early wound irrigation. However veterinary surgeons are frequently presented with wounds long after the infection is established. Lavage, and debridement are still effective, however topical antimicrobial treatment is often required. With the emergence of increasing numbers of microbes with resistant sensitivity profiles, the challenge is to identify other techniques effective at eliminating infection.

The use of dressings that donate silver ions to the wound is one such method, as is the use of certain honey solutions. The antimicrobial properties of honey which are attributed to many factors including acidity, hydrogen peroxide content, osmotic effect and phytochemical components have long been known and although Manuka honey from New Zealand has been the focus of research, on-going work at Glasgow University Veterinary School has demonstrated similar results with a number of honeys produced here in the UK, including heather honey. In addition to inhibiting microbial growth, honey may also have a role to play in controlling inflammation and promoting the healing process through the modulation of cytokines(7), fibroblast proliferation and angiogenesis.

Wound healing tip:
A number of medical grade honey dressings, and silver dressings are available and can be effective. Furthermore the use of hydrogels mixed with antimicrobials (following appropriate culture and sensitivity) can be very effective when treating a wound infection.

Work by Christine Theoret and her team suggest that tissue hypoxia(8) is common in the wounds affecting the distal limbs of horses, and that this hypoxic state may contribute to the formation of exuberant granulation tissue (proud flesh). This may help to explain why hyperbaric oxygen therapy has been shown to optimise wound healing. Furthermore in human beings a link between tension at the wound surface and over-expression of pro-fibrotic cytokines has been demonstrated. If this also occurs in horses, it may provide an explanation for the profound effect that movement has on the incidence of complications during wound healing in horses.

Wound healing tip
Appropriate bandaging, removal of debris, and choosing the correct dressing will reduce the risk of wound hypoxia. Splinting, casting and appropriate bandaging can be effective in eliminating movement.

 In human beings, chronic non-healing wounds are commonly encountered and cost the NHS £bn2.3-3.1  annually (conservative estimate from 2008). In many instances these chronic wounds affect older members of society and may never fully heal but are simply “managed” until the natural end of the individual’s life. Identification of patients afflicted by these chronic wounds is a priority. The recent development of “patient side” testing for the presence of biomarkers for non-healing wounds has allowed health professionals to identify patients that require advanced intervention early after initial presentation. Although at an early stage, current work at the University of Glasgow Veterinary School has identified the presence of these markers of the “chronic non-healing state” in the wounds of horses. It is hoped that these markers might finally provide a technique for determining which of our many veterinary interventions truly lead to improved wound healing.

References
Brett, D, Owens, M,D, Joseph, C, Wenke, P (2007). “Early wound irrigation improves the ability to remove bacteria”. Journal of Bone & Joint Surgery.   89:1723-1726
Posnett, J, Franks, P J (2008). “The burden of chronic wounds in the UK”. Nursing Times; 104: 3, 44–45
Theoret, C.,L. (2009). “Tissue engineering in wound repair the three ‘Rs’ — repair, replace, regeneration. Vet Surg. 38:905-913
Wilmink, J M, van Weeren, P R, Stolk, P W, (1999). “Differences in second intention wound healing between horses and ponies: histological aspects”. Equine Vet J. 31:61-67
Winter, G. D. (1962). “Formation of the scab and the rate of epithelisation of superficial wounds in the skin of the young domestic pig”. Nature 193:293 -294

© Equine Health
(Managing wounds in horses first appeared in Equine Health magazine, issue 10, March/April  2013)

Articles republished from Equine Health are kindly supplied by Chris Keate, editor, Equine HealthChristine Keate has been a journalist in the equine industry for 20 years. She was a columnist at Horse and Hound on health and welfare issues and then editor of Horse Health for five years, before launching Equine Health as part of MA Healthcare. She has a deep love and understanding of horses and the issues that affect their health and well-being.

Bridging the gap – Equine Health magazine brings together equine vets, industry professionals and horse owners; improving communication and understanding, leading to better equine health and welfare.

*Out and About Dressage layman’s glossary
(1) Neutrofils, a type of immune cell that is one of the first cells to travel to the site of an infection 
(2) Iatrogenic, induced inadvertently by medical treatment or diagnostic procedures
(3) Epithelialisation, when epithelial cells migrate across the new tissue to form a barrier between the wound and the environment 
(4) Leukocytes, type of blood cell that lacks haemoglobin and is therefore colourless

(5) Inoculum – a substance (a virus or toxin or immune serum) that is introduced into the body to produce or increase immunity to a particular disease
(6) Perfusion, is the process of oxygenated blood being delivered to the tissues of the body
(7) Cytokines, cell signalling molecules that aid cell to cell communication in immune responses and stimulate the movement of cells towards sites of inflammation, infection and trauma 
(8) Hypoxia, diminished availability of oxygen to the body tissues 

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