What inhibits equine wound healing
Infection
Body-foreign particles may lodge in the wound or lacertion and trigger infection which is the most common reason for wounds not to heal. Your horse will try to expel damaged-beyond-repair cells and body-foreign particles and as a result, you will see angry-looking red tissue that exudes pus. This yellow or green* thick opaque liquid consists of dead white blood cells, bacteria with dead tissue debris, serum and, if body-foreign particles can be decomposed, these also. Myeloperoxidase (MPO) colours pus green. MPO is an intensely green antibacterial protein that is synthesized in neutrophils and monocytes, packaged into primary granules and released either into the phagosome or the extracellular space where it catalyses the conversion of H2O2 to hypochlorous acid (HOCl). MPO doesn’t just kill pathogens, it also changes the wound pH. Nature is very clever… HOCl is the most effective disinfectant in the chlorine family available in dilute solution! Green pus denotes a more serious wound infection than one with yellow pus.
Pus is ‘process’ that should not be supressed. Instead of reaching for an antibiotic to combat bacterial infection (it suppress the expel mechanism), do remember that bacteria have a greater capacity to survive and grow at alkaline pH values. I refer to what is known as Biological Terrain and bacteria actively changing the tissue and interstitium pH so as to improve their own chances of survival. As the horse owner, you can and should make that Biological Terrain inhospitable to bacteria. For instance by making the wound slightly more acidic, no bacteria can survive that. Systemic antibiotics are not very effective against wound infections, not even as a preventive. In an infected wound, treatment needs to be very localized. Yet putting antibiotic cream on the surface of wounds can be damaging to tissue, destroying the very cells you are trying to protect. A better approach is a thorough cleaning of an infected wound to remove debris, dead tissue and reduce the number of bacteria.
Movement
Tissue that doesn’t remain still cannot repair itself, so wounds on a high-motion area of the body may be slow to heal or not heal at all. If the wound is over a fetlock, it is easy to see how it will move with each step, but the same thing can occur with a deep wound on the back from the movement of the muscles underneath it. You may not see it on the surface, but it is just as damaging. The solution is to immobilise the area, which can be done through a variety of methods. Keeping your horse stabled may be enough, but often you need to do more like for instance casts for wounds on the feet. Stable rest and splints can solve the problem for many locations.
Foreign bodies
Anything inside a wound that should not be there counts as a foreign body e.g. wood splinters, dirt, grass, metal and bone fragments. The horse’s body will try to expel the material over time, and may ultimately be successful, but wound healing can’t and mustn’t continue until that happens. That is why you don’t want to ‘close a wound’ by applying topical creams or potions. Safe healing happens from the inside out. Some foreign bodies in wounds cause more problems than others. Not all materials appear on x-ray, so a visual inspection or surgical exploration of a wound is sometimes necessary to rule out foreign body contamination. Some materials are more likely to wreak havoc within the tissue. A bit of glass isn’t going to harbour infection as much as, say, a piece of soil or cloth that’s been pushed into the wound. Some foreign bodies simply delay healing until the body can eject them, others can lead to a cascade of infection and greater complications. In severely contaminated wounds, nonselective chemical debridement with hydrogen peroxide or hypotonic or hypertonic saline may be used. These use the osmotic gradient to lyse and destroy bacterial cells (good) but also destroy fibroblasts which slows wound healing (not ideal). Mechanical debridement followed by autolytic debridement (facilitated natural process) is what suffices in most cases. Regardless of what the body-foreign particle is, once the material is removed or expelled from the wound, the healing starts right back up again.
Necrotic tissue
Dead tissue within a wound will bring healing to a standstill, typically by inducing infection. A vet dealing with a fresh wound will debride it during the initial treatment, removing tissue that is already dead or likely to die quickly. In smaller wounds, you can do the same thing with copious flushing with water or saline solution. But that initial cleanup, mechanical debridement (sterile scalpel blade, scissors or carbon dioxide lasers), is not always enough. Tissue can die within a wound in days, weeks or even months after the initial injury. This does not mean that the wound was treated poorly. The death of tissues is sometimes a natural part of the healing process. The only solution at that point is to visually inspect the wound, identify the dead tissue and flush or cut it away. Vets typically cut tissue back until they see bleeding, tissue with a blood supply is alive, so vets assume that’s where the necrotic tissue ends [image 4 in the above slider shows a laceration cut back to blooded tissue and properly clean for suturing]. Autolytic debridement is what happens if you maintain wounds in a moist environment. This allows wound exudate —which is rich in endogenous enzymes such as proteolytics, fibrinolytics and collagenases; growth factors and cytokines— to act on the wound and provide an optimal environment for healing. The collective components of wound exudate stimulate the migration of fibroblasts, epithelial cells, vascular tissues, neutrophils, and macrophages. In turn, neutrophils and macrophages control bacterial contamination of the site. Nowadays there are commercial products for exogenous enzymatic debridement , these use streptokinase, trypsin, fibrinolysin, protease, or best of all, collagenase. Knottenbelt makes a strong case for simply enabling Nature because 9 out of 10 wounds will heal well that way.
Continued trauma
A wound that is continually aggravated by for instance tack rubbing, or chewing by your horse or by tight bandages, will not heal. This is very common with foot wounds, particularly wounds to the heel bulb, where your horse may step on it with another foot, reopening the entire area. Even thick, strong grass can cause enough trauma to wounds on the lower limb to keep them from healing. The solution is simple enough: identify and stop the repeated trauma and the wound will heal. This means that your horse is not in work (no under-saddle work), uses a neck cradle to keep him from fussing with the wound or using a different bandaging technique. “Once you stop the trauma, wounds tend to heal very quickly” says Knottenbelt.
Reduced blood supply
Blood brings to the wound site the constituents essential to healing (optimise what nutrients are in the bloodstream). Without adequate circulation, tissue repair fails. If an artery is occluded then everything downstream may be affected. Bleeding is an indicator of an adequate blood supply and a vet, making a close visual inspection, can usually identify poor circulation. In respect of overgrowth of granulation tissue that prevents epithelialization of a wound, it is a fallacy that proud flesh develops from a limited blood supply. It is wrong to think that lower limbs have reduced blood flow and that therefore proud flesh is likely to develop there. Legs have plenty of blood flow, otherwise they would fall off, and you can see the blood supply right in the proud flesh if you cut it. Proud flesh develops because healing has stalled due to some other factor on this list.
Poor oxygenation
Wounds require oxygen to heal and they receive that through two routes: the air surrounding the wound and hemoglobin in the blood. A wound lacking oxygen, from either source, will look purple. It won’t look like a good, healthy wound. Leaving a wound uncovered can expose it to more oxygen, of course, but covering it may actually be better. If you put a semi-occlusive dressing over a wound then you cut off the oxygen it receives from the air, but the bloodstream will begin to bring in more oxygen to compensate for that. Thus oxygen gets delivered to the deepest healing tissues, not just those on the surface and in a higher concentration. Yet again favouring the inside-out approach.
Poor overall health & nutrition
The general health status of a horse can influence how his wounds heal. Cushing’s Disease is the classic example. These horses have lowered immune function, so wounds may be slow to heal or prone to infection. A horse on steroid medications or one with a disease like Lymphoma could have similar issues. Poor nutrition can also lead to delayed wound healing. Knottenbelt: “Be sure that you are feeding a horse properly when he has a healing wound. Make sure he’s getting enough protein in his diet. That is what his body will be using to create new tissue.” That is where Aloeride aloe vera comes into it, with (instead of protein that needs to be broken down into amino acids) readily absorbable 7 out of the 8 dietary essential amino acids (Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Valine, there’s no consensus on Tryptophan yet). Aloeride also gives your horse 12 dietary non-essential amino acids (Alanine, Arginine, Asparagine, Cysteine, Glutamic Acid, Glycine, Histidine, Proline, Serine, Tyrosine, Glutamine, Aspartic Acid). Then there are all the other nutrients, and β (1,4) linked polysaccharides to conduct all into function.
Tissue deficit
The natural healing process has distinct phases. Without adequate healthy tissue within the wound to serve as a foundation, one or more phases may be delayed or never occur. If you’ve got a tissue deficit, there may not be enough of a biological foundation for new tissue to form over. If you’ve got a huge wound left to heal by second intention (without stitches) it may stay open forever. The solution, he says, it to provide what the body cannot. There are new high-tech wound dressings that create a biologic scaffold of sorts, using collagen or amnion or intestines from other animals, that provide the missing foundation and allow tissue to begin to fill in and eventually develop a very normal appearance. These can be incredibly helpful with larger wounds that might not ever close on their own.
Local factors
This category of healing interrupters includes conditions immediately in and around the wound. A wound may not heal in a very cold or a very hot environment, cells do not function well in either of these. Very wet or a very dry conditions can affect healing. A wound may not heal if there is pocketing in the tissues. To identify these conditions, a veterinarian will have to look at the bigger picture and be a bit creative with solutions. You may need to protect the wound with bandaging (most do well with moist, warm dressings) or you may need to move the horse to a less severe environment. Or, the vet may have to alter the wound itself, removing tissue to eliminate pockets or uneven tissue beds.
Iatrogenic idiocy
Knottenbelt’s more than qualified opinion is, that what people put on their horse’s wounds ranges from unnecessary commercial products to harmful homemade preparations. The huge majority of equine wounds do just fine with a thorough flushing with warm water or saline and then being covered with a simple hydrogel bandage. For first aid treatments, Knottenbelt recommends hydrogel dressings. Hydrogels provide and maintain a moist wound environment. They donate fluid to the wound and can be used as a filler for a desiccated cavity where they can restore a physiologically sound moist wound healing environment. Most hydrogels will aid debridement of necrotic wounds and may also have a bacteriostatic effect due to the presence of propylene glycol which acts as a humectant which prevents the gel drying out. “Horses have a reputation for wounds that don’t heal and I’m certain it’s because of the ridiculous salves owners and even some veterinarians will put in the wounds. Wounds do not need physical help healing do not scrub wounds that are healing just fine and do not pull scabs off. Scabs are there for a reason and keep scarring to a minimum.”
Tumour transformation
When the tissue inside a wound develops tumours, healing stops and managing the case can be very challenging. Sarcoids can develop deep inside wounds. If the horse has a sarcoid somewhere else on his body, it is easy for those cells to be transferred to the wound by flies. Then you have a sarcoid in the wound and this is a very serious situation that has to be recognized and dealt with quickly. Often the forming sarcoid is mistaken for proud flesh and owners delay calling the veterinarian or attempt to treat it themselves, making the situation worse. If you compare two wounds, one with proud flesh and one with a sarcoid, they can look identical, but the treatments are diametrically opposite. You need to treat the tumour with cancer drugs and if you try to cut it out -like proud flesh- then you are going to make it worse. Vets should do pathology on every suspected case of proud flesh to ensure it’s not a tumour.
Professor Derek Knottenbelt qualified from Edinburgh University in 1970. His expertise in equine internal medicine is recognised internationally. He is a Diplomate of the European College of Equine Internal Medicine and of the American College of Veterinary Internal Medicine and held a personal chair at the University of Liverpool until his retirement in 2010. He is now a consultant professor at the University of Glasgow and has published a plethora of journal articles and something crazy like 12 textbooks. He received the Animal Health Trust Scientific Award in 2003, the BEVA Blue Cross Welfare Award in 2004, and received an OBE in the Queen’s Birthday Honours of 2005 for his services to Equine Medicine.
