For most of us, it’s a familiar scenario: Your horse sustained a wound nasty enough for your veterinarian to come out and tend. It’s been cleaned and debrided, and now all that remains is the daily routine of bandage changes. But every time you go about the task of removing the old dressing and putting on the new, you scrutinize the wound. Is the reddish color around the edges a sign of trouble? Where did that fluid oozing out of the corner come from? Is this wound healing normally?
Take heart. The odds---and biology--are on your side. The vast majority of equine wounds, both major and minor, heal well and relatively quickly.
“There’s no evolutionary advantage to a wound that doesn’t heal,” says Derek Knottenbelt, BVM&S, PhD, DipECEIM. “They want to heal if they can. And most of them do, with very little intervention from us.” Knottenbelt would know: He is one of the foremost veterinary authorities on equine wound healing, having literally written the book on the topic---Handbook of Equine Wound Management---and consulted on hundreds of cases around the globe.
And all that experience, he says, has taught him an important lesson: “When a wound doesn’t heal promptly there’s always a reason, and until you figure out and address that reason, it never will heal,” says Knottenbelt. “Figure it out and correct it, and then the horse’s body takes over and does a remarkable job and you can take all the credit.”
In fact, Knottenbelt has seen so many wound-healing problems that he’s streamlined the investigative process, ultimately coming up with a checklist. “It’s a pretty famous list, actually, among those of us who deal in these things,” he says. “It’s the 12 inhibitors of wound healing. If I’m faced with a wound that isn’t healing, I go down this list and tick them off, looking for each. Any owner or veterinarian can do the same. It may be just one that’s the problem or it may be three or four, but if you identify and correct them, I can assure you the wound will heal.”
Here is Knottenbelt’s list of factors that stop wounds from healing.
Often signaled by red, angry-looking tissue that exudes pus or has a foul odor, infection is the most common reason that wounds don’t heal, says Knottenbelt, but it’s one of the easiest to remedy if you go about it in the right way.
“The inclination among owners and some veterinarians is to reach for an antibiotic to combat infection, but that’s not the correct approach,” Knottenbelt says. “Wounds become infected for a reason, and you have to figure out why as you begin treatment or you aren’t going to be successful with medications alone.”
Some causes of wound infections, such as contamination or reduced immune function, are found elsewhere on this list. Others are more subtle: “A good example is the pH of the wound. Infection loves a very high pH wound,” says Knottenbelt. “If you can make the wound slightly more acidic, no bacteria can survive that. What you’ve got to do is find out why the wound is alkaline. A veterinarian experienced in wound healing will know to check for this and how to correct it.”
Systemic antibiotics aren’t very effective against wound infections, he says: “We all give horses antibiotic injections as we are treating wounds as a preventive, but if infection sets into a wound, the treatment needs to be very localized. Antibiotics circulating in the blood aren’t likely to make it to the wound site in sufficient levels to be effective at that stage.”
Grabbing a tube of antibiotic cream isn’t always the answer either, he adds: “I’m no fan of putting antibiotics on the surface of wounds. For starters, they can be damaging to tissues, destroying the very cells you are trying to protect. If I’m going to use an ointment to kill an infection in a wound, I do it knowing that I’m also going to harm tissues, but I’m at the point when I have no choice.” A better approach, he says, is a thorough cleaning of an infected wound to remove debris, dead tissue and reduce the number of bacteria.
2. FOREIGN BODIES
Anything inside a wound that shouldn’t be there counts as a foreign body, including wood splinters, dirt, grass 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 continue until that happens.
“Nearly every veterinarian has dealt with a wound that just sits there for a long time and discharges pus without healing,” says Knottenbelt. “It can be baffling until we take an x-ray and see a bit of wire embedded in the tissue. Remove that and it heals up quickly and completely.”
Knottenbelt says that some foreign bodies in wounds cause more problems than others. For starters, not all materials appear on an x-ray, so a visual inspection or surgical exploration of a wound is sometimes necessary to rule out foreign body contamination. And some materials are more likely to wreak havoc within the tissue. “A bit of glass isn’t going to harbor infection as much as, say, a piece of soil or cloth that’s been pushed into the wound,” he says. Some foreign bodies simply delay healing until the body can eject them; others can lead to a cascade of infection and greater complications. But regardless of what it is, once the material is removed or expelled from the wound, the healing starts right back up again.
3. TISSUE DEFICITS
The natural healing process has distinct phases (see “How Wounds Heal,” page 28). Without adequate healthy tissue within the wound to serve as a foundation, however, 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,” says Knottenbelt. “And, 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.
“For instance,” says Knottenbelt, “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.” Your veterinarian will be familiar with these options.
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’s easy to see how it will move with each step,” says Knottenbelt, “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’s just as damaging.”
The solution is to immobilize the area, which can be done through a variety of methods. “Keeping the horse in a stall might be enough, but oftentimes you need to do more,” says Knottenbelt. “We do a lot of casts for wounds on the feet. If a horse cuts his heel bulb, tissue in that area is going to move with every step. So we put a cast on the limb to address that movement and when we take the cast off 10 days later, it’s staggering how well it has done. Some areas of the body are very difficult or impossible to keep still---“you can’t immobilize a tongue,” says Knottenbelt---but stall rests and splints can solve the problem for many locations.
5. NECROTIC TISSUE
Dead tissue within a wound will bring healing to a standstill, typically by inducing infection. A veterinarian 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 is not always enough.
“Tissue can die within a wound days, weeks or even months after the initial injury,” says Knottenbelt. “It doesn’t mean 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 he wound, identify the dead tissue and flush or cut it away.
“We typically cut it back until we see bleeding,” says Knottenbelt. “Tissue with a blood supply is alive, so we assume that’s where the necrotic tissue ends. We might have to do this a few times before the wound heals.”
6. REDUCED BLOOD SUPPLY
Blood brings to the wound site the constituents essential to healing. Without adequate circulation, tissue repair fails. “If an artery is occluded [blocked], everything down the line to that can be affected,” says Knottenbelt, adding that bleeding is an indicator of an adequate blood supply and a veterinarian making a close visual inspection can usually identify poor circulation.
You might think that poor blood supply would cause the development of proud flesh---the overgrowth of granulation tissue that prevents epithelialization of a wound---but that’s not so, says Knottenbelt. “It is a fallacy that proud flesh develops from a limited blood supply,” he says. “There is this idea that lower limbs have reduced blood flow, so proud flesh is likely to develop there. The legs have plenty of blood flow, otherwise they’d fall off. And you can see the blood supply right in the proud flesh if you cut it.” Knottenbelt says proud flesh develops because healing has stalled due to some other factor on his list.
7. POOR OXYGENATION
Wounds require oxygen to heal, and they receive that through two routes: The air surrounding the wound and hemoglobin in the blood supply. “A wound lacking oxygen, from either source, will look purple,” says Knottenbelt. “A veterinarian will be able to see that. It doesn’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, you cut off the oxygen it receives from the air,” explains Knottenbelt, “but the bloodstream will begin to bring in more oxygen to compensate for that and it gets delivered to the deepest healing tissues, not just those on the surface, and in a higher concentration.”
Knottenbelt says that anemic horses or those who are otherwise ill may not have the red blood cell volume to deliver enough oxygen, even with this technique, and a veterinarian might try something a bit more dramatic. “Hyperbaric chambers have been used as a means of delivering concentrated levels of oxygen to slow-healing wounds,” he says. “The horse is placed in a chamber full of concentrated oxygen for a particular length of time each day. These chambers do seem to increase the rate of healing, but it’s unclear by just how much.”
8. CONTINUED TRAUMA
A wound that is continually aggravated--by rubbing tack, chewing by the horse or tight bandages, for instance---will not heal. “This is very common with foot wounds,” says Knottenbelt, “particularly wounds to the heel bulb, where the 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 may mean giving the horse some time off from under-saddle work, using a neck cradle to keep him from fussing with the wound or using a different bandaging technique. “Once you stop the trauma, these wounds tend to heal very quickly,” says Knottenbelt.
9. LOCAL FACTORS
This category of healing interrupters includes conditions immediately in and around the wound. “For instance,” says Knottenbelt, “a wound may not heal in a very cold or a very hot environment--cells don’t 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,” says Knottenbelt. Or, the veterinarian may have to alter the wound itself, removing tissue to eliminate pockets or uneven tissue beds.
10. POOR OVERALL HEALTH
The general health status of a horse can influence how his wounds heal. “Cushing’s disease is the classic example,” says Knottenbelt. “Those 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.”
In less-developed areas of the world, poor nutrition can also lead to delayed wound healing, says Knottenbelt. “This isn’t as much of an issue except in Third World countries, but 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’s what his body will be using to create new tissue.”
11. IATROGENIC IDIOCY
This category, Knottenbelt says, boils down to “stupid things people put on their horse’s wounds” and 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,” he says. For first aid treatments, he says hydrogel dressings, which contain a water-based gel that keeps wounds at an optimal moisture level, can be helpful, as can hydrogel preparations that protect wounds without bandages. “Wounds need nothing else,” he says. “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.”
Likewise, says Knottenbelt, wounds don’t need physical help healing. “Don’t scrub wounds that are healing just fine,” he says, “and don’t pull scabs off. They are there for a reason and keep scarring to a minimum.”
12. TUMOR TRANSFORMATION
When the tissue inside a wound develops tumors, healing stops and managing the case can be very challenging.
“Sarcoids can develop deep inside wounds,” says Knottenbelt. “If the horse has a sarcoid somewhere else on his body, it’s easy for those cells to be transferred to the wound by flies. Then you have a sarcoid in the wound and it’s a very serious situation that has to be recognized and dealt with quickly.” Oftentimes, he says, 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,” he says. “You need to treat the tumor with cancer drugs; if you try to cut it out like proud flesh, you’re going to make it worse. I believe veterinarians should do pathology on every suspected case of proud flesh to ensure it’s not a tumor.”
When your horse has a wound, even a large one, don’t panic. With your prompt attention, and, if needed, help from your veterinarian, chances are it will heal quickly. In the rare event it doesn’t, it’s still not time to worry too much. With a strategic approach to considering and addressing the 12 inhibitors to wound healing, chances are the cause can be quickly identified and eliminated, and your horse will be healing again soon.
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Derek Knottenbelt, BVM&S, PhD, DipECEIM, received his veterinary degree and doctorate from Edinburgh University in Scotland. After 12 years in private practice, he became a professor in equine internal medicine at the University of Liverpool in 1985. In addition to conducting research in equine oncology, dermatology, ophthalmology and wound management, Knottenbelt has written 10 textbooks. He recently retired from academia but now works as a consultant.
This article first appeared in EQUUS issue #471, December 2016.