Tumor. Now there's a word guaranteed to strike fear into anyone's heart. Loosely defined, a tumor is an uncontrolled or incorrect growth of cells, which can invade normal tissue and disrupt functions. It can be benign (meaning it's slow-growing and doesn't tend to spread) or malignant (a fast-growing, aggressive tumor that easily metastasizes, or spreads to other tissues). We're not immune to tumors, and neither are our horses.
The good news, if you can call it that, is that the vast majority of tumors that affect horses are of the benign variety. Cancer isn't nearly as common a killer in equines as it is in humans. In fact, the two most common types of tumors in horses—melanomas and sarcoids— usually are benign and often cause little disruption in a horse's daily life. Some horses have been known to live with these skin growths for upward of 20 years. Under certain circumstances, however, both of these types of tumors can be a nuisance and a health risk, not to mention unsightly. Let's take a look at each type of tumor, then examine some of the newest research ideas and treatment options.
The Sarcoid Scenario
You've probably seen a sarcoid tumor on a horse at some point in your travels; they're the most common type of skin tumor in equines, occurring in an estimated one out of every 100 horses. The word sarcoid is a contraction of the word sarcoma (a tumor originating from the connective tissue) and the suffix "-oid,” which means "like.” So, a sarcoid is a tumor "like a sarcoma.” The difference is that sarcomas generally are malignant, and sarcoids usually are not.
The average equine sarcoid is found on the skin surface and resembles a wart. Some are small and flat, with a crusty surface or a normal skin covering. This type (sometimes called a verrucous sarcoid) grows very slowly and might remain static for years at a time, or might even spontaneously disappear. Others, sometimes called fibroblastic sarcoids, are more aggressive and invasive. They have a raised, bumpy surface, might bleed or ooze serum if they're bumped or rubbed, and can rapidly proliferate into large, angry-looking masses.
Sarcoids can appear anywhere on the skin surface, and horses of any age, breed, or color can develop them, although they tend to show up in horses which are middle-aged or better. Some researchers have noted that Quarter Horses seem to show a predisposition. A tendency to develop sarcoids might run in families, although at this point there's no strong proof of that. The areas most frequently affected include the skin of the head (especially the mouth, eyelids, and ears), legs, tailhead, underside of the barrel, and any area where there exists a previous wound site or scar tissue.
Sarcoids can appear singly, but often they'll show up in several locations on a horse, a characteristic that leads many researchers to believe that there might be an infectious or viral cause for these tumors. There are precedents in human medicine, most notably a virus called HPV (for human papilloma virus), which has been strongly linked to certain types of cervical cancer in women.
Interestingly, most equine sarcoids are infected with a closely related micro-organism, the bovine papilloma virus (BPV). BPV causes ordinary warts in cattle—non-malignant and pretty harmless. In horses, which are not the virus' natural hosts, BPV seems to stimulate something far nastier. Whether the virus causes the sarcoids, or is just somehow associated with their growth, is unclear at this point. BPV has been demonstrated in lab situations to cause equine cells to develop tumor-like characteristics, but the viral infection is not enough to cause the actual development of a sarcoid (at least, not so far).
Researchers including Elizabeth Carr, DVM, Diplomate ACVIM, who currently is exploring treatment options for sarcoid tumors as part of a PhD project at the Department of Veterinary Surgery and Radiology, University of California, Davis, suspect that horses are a "dead-end host” for BPV. She thinks that the virus goes through an altered life cycle in equine cells, never managing to replicate and shed virus particles as it does in cattle. As a result, sarcoids do not seem to be contagious from horse to horse. It's more likely that the horses somehow contract the virus from the environment (not even necessarily from being in contact with a cow with warts). Regardless of how it gets there, however, it's very possible that BPV could act as the most significant initiating event in the development of a sarcoid.
"People have suspected for a long time that there's some relationship between this virus and sarcoid tumors in horses,” says Carr. "It's been demonstrated that more than 90% of sarcoids do have the DNA for the bovine papilloma virus, which is a recognized oncovirus (a virus capable of expediting malignancy in cells). We know we can create a tumor in a horse by injecting it (subcutaneously or intra-dermally) with BPV, but they tend to spontaneously disappear within a few weeks or months. That tells us there may be other factors at work which influence the viral expression.
"Histologically, sarcoid cells don't vary a lot,” adds Carr, "but clinically, there's a great deal of variation in the degree of aggression in sarcoid tumors seen in horses. We suspect that in some tumors, more of the oncogene is being expressed. As with any type of virus, the name of the game (from the virus' point of view) is to avoid recognition by the host's immune system. For the most part, BPV does a good job of going undetected. Our goal is to come up with a vaccination which would serve as a ‘wake-up call' for the horse's immune system, forcing it to recognize the presence of the virus and attack it.”
Work on a sarcoid vaccine is still in the very early stages at UC/Davis, Carr emphasizes, but she expects that a working version probably will become available within five years.
"Potentially, you could easily develop a vaccine, which could simply and effectively treat existing sarcoids.”
She suggests it would be just as easy to develop one which could prevent sarcoids—and such a vaccine wouldn't even be particularly expensive to produce. But because people don't tend to view sarcoids as a life-threatening condition, the economic viability of such a vaccine might be questionable. And without the participation of pharmaceutical companies, products like this never become commercially available. Interest in a vaccine to treat existing tumors, therefore, is a better bet from a financial standpoint.
Although slow-growing sarcoids often cause minimal trouble to a horse, if they appear in an area where tack or equipment might rub against them (for example, near the mouth where a bit would rest, or on the girth-line), they can crack and bleed frequently and can cause significant discomfort. Larger masses are even more troublesome, sometimes splitting and becoming infected by flies and maggots. There's always the possibility that a sarcoid tumor can interfere with normal functions, for instance, when one forms on an eyelid or ear.
For all of these reasons, you might want to consider treatment for your horse's sarcoids.
"It's best to at least get them evaluated, as soon as possible after you notice they've appeared,” recommends Carr. "Whether you pursue treatment depends on the size of the tumor and its location, but once they get past a certain size, or the tumor changes to malignancy, you may not have the option of treating them anymore. For most sarcoids, it's a good idea to explore treatment early on.”
The trickiest part of all this might be deciding which type of treatment is best. There are a bewildering number of options, all with pros and cons and variable success rates. Part of the difficulty is that no two sarcoids seem to be alike in how they respond to treatment. Tumors have been known to disappear spontaneously when nearby tumors are removed or treated. At the other end of the spectrum, an incomplete treatment might stimulate sarcoids to become wildly aggressive and invasive. There is no one single treatment that is uniformly successful in curing sarcoid tumors; in some cases, control is the best you can hope for.
Options for treatment, all of which have a moderate record of success, include the following:
- surgical excision
- cryonecrosis (freezing with liquid nitrogen)
- radiation therapy
- destruction by surgical laser
- chemotherapy, usually in the form of drugs injected locally into the tumor
- injection of immune stimulants, either systemically or locally into the tumor.
Frequently, veterinarians will use more than one of these methods in combination, depending on the size and location of the sarcoid and whether it is benign or rapidly growing. Surgical excision probably is the most common approach, although it works best with flat, wart-like sarcoids. Vigorously active tumors are more difficult to remove surgically, as they have a tendency to recur at the same site. Carr notes that sarcoid tumors tend to develop finger-like extensions into the normal tissue, so it can be next to impossible to ensure that all the tumor cells have been eradicated.
Often, surgery is used in combination with other treatment methods. Large tumors might need to be "de-bulked” (surgically reduced in size) before other methods can be applied successfully; for example, the visible parts of the tumor might be surgically trimmed as much as possible, then cryosurgery could be used to freeze the margins of the tumor site (the aim being to kill any remaining tumor cells).
Injection with immune-stimulant drugs is a useful approach with sarcoids that are in locations difficult to approach surgically (for example, on the eyelid) or when multiple tumors are involved. Cosmetically, this approach has the advantage of far less visible scarring than surgery, but its success rate varies from horse to horse. One approach that has been used over the past couple of decades is injection of a vaccine called BCG (bacillus Calmette-Gurein), also used to combat tuberculosis in humans. BCG serves as an immunostimulant, enhancing the body's response to tumor-specific antigens. Its usefulness seems to be limited to small tumors, and occasionally the treatment seems to stimulate sarcoids to become aggressive. As a result, BCG has fallen out of favor as a sarcoid treatment in recent years.
More promising is a chemotherapy drug called cisplatin, which Carr says has "about an 80%-90% success rate over two years, if owners are committed to the treatment regimen.”
Cisplatin can be injected intra-lesionally (directly into the tumor) if the sarcoid is small (one to two centimeters in diameter). If the tumor is larger, surgical "de-bulking” might be necessary first. Otherwise, a prohibitive quantity of the drug would have to be used, since the tissue's ability to diffuse cisplatin is limited. Although the research team at UC Davis uses cisplatin as its treatment of choice for many sarcoids, Carr notes that at the moment, sourcing the drug in an appropriate concentration is difficult.
"There are diluted forms readily available, but they're not nearly as effective, we think. The results are definitely connected to the concentration of the drug, and to it being administered regularly, every two to three weeks.”
Radiation therapy is another modality with which the UC Davis team has had good success, particularly with sarcoids that are very large or difficult to excise because of their location. By an owner's standards, radiation might seem a bit daunting. Because the horse must remain absolutely still for the treatments (which only last a couple of minutes each), he must be placed under a general anesthesia, and the usual regime is six treatments over a three-week period. That's quite a lot of anesthesia, and of course there's an extended hospital stay involved as well. However, if your horse has large or invasive tumors, radiation might be the most effective way to go. Not every university veterinary hospital is equipped to provide this therapy, so you might want to do some investigating to find the facility nearest you.
Regardless of the approach you use to tackle your horse's sarcoids, it's important to be realistic about the results. Sarcoids are tenacious, and frequently recur, sometimes within weeks of their apparent eradication. The treatment might only succeed in reducing the size of the tumor, or reducing the number or severity of the tumors. Cures are rare; control is a more practical goal.
Every owner of a gray horse lives in fear of melanomas, those characteristic black or brown nodules that so often appear on the skin around and under the tail. Rightly so, for estimates suggest that more than 80% of gray horses over the age of 15 will develop at least one melanoma tumor during its lifetime. The "why” hasn't been determined. We know that melanomas are tumors of the melanocytes, the cells that produce skin pigment. We know that in aging gray horses there appears to be a disturbance in the metabolism of melanin, which stimulates local over-production of dermal pigment. But why gray horses are particularly susceptible isn't yet clear. (One thing is reasonably certain—unlike melanomas in humans, which might be triggered by overexposure to ultraviolet radiation, gray-horse melanomas don't appear to be linked to an overdose of sun. The shady locations in which they tend to develop, and the fact that the skin of gray horses is black, and thus well-protected from U/V radiation, attest to this.)
Melanomas can be hard or soft, solitary or appearing in clusters. Often, they're subcutaneous when they first appear, covered by normal skin, but as they develop over time, they become more obvious, and their surfaces can become ulcerated and/or infected. Although they're generally dark brown, gray, or black in color, it's also possible for these tumors to be unpigmented (amelanotic), a situation which makes diagnosis considerably more challenging (microscopic examination of biopsy samples being the only way to identify these "invisible” tumors).
The underside of the tail, the perineal and peri-anal regions, and the penis and sheath in males are the most common locations for melanomas to sprout. They also can be found on the ear margins, elsewhere on the head, in the jugular region, and near or on the parotid salivary gland. It's quite possible for melanomas to spread internally as well, most commonly gravitating to the serosal surfaces of the liver, spleen, and lungs.
Much of the time, melanomas are fairly benign, slow-growing tumors, and are more unsightly than dangerous. They might remain that way for years or even decades. Within each tumor, however, lurks the potential for an overnight change to malignant growth, which swiftly can change the situation from a cosmetic nuisance into something life-threatening.
They might appear as rapidly spreading series of lumps or nodules, or even as vast, rippling sheets of black tumor masses across the tissue. Malignant melanomas can interfere with a horse's excretory functions, with breeding and foaling, or, if found in other locations on the body, such as the back or neck, with working under saddle or in harness.
Because malignant melanomas frequently appear in multiple sites, and because they have a high rate of metastasis, they can be very difficult to cure. Gray horses aren't the only ones affected; horses of other coat colors also can develop melanomas. The incidence in non-grays is much lower, but when melanomas do appear in these horses, the tumors are more likely to be aggressive.
All breeds of horses are susceptible to melanomas, but their incidence is probably over-represented in Percherons, Arabians, and Andalusians, all breeds in which the color gray is very common.
Melanomas in dogs and humans have a complex system of classification, but equine melanomas usually are characterized as benign or malignant. They can develop in one of three distinct ways:
a) The melanomas develop slowly, over a number of years, without metastasis, and can remain benign for 10 to 20 years (this is the most common scenario).
b) Benign melanomas that have existed for months or years suddenly assume malignant characteristics and begin to spread rapidly externally and/or internally.
c) Melanomas are malignant from their first appearance and readily metastasize. In rare cases, they might even be congenital (present at birth). Fortunately, this is the least common possibility.
As with sarcoids, sometimes melanomas require no treatment at all, if they're slow-growing and not interfering with any of the horse's daily functions. Many gray horses exist happily enough for years with benign melanomas that cause them little or no discomfort.
However, just because a tumor appears to be inactive doesn't mean you should stop monitoring it. There always is the possibility that something will trigger a change to malignancy, so it's important to examine tumors regularly for changes in size, number, or appearance.
"If you're the owner of a gray horse,” says Carr, "the best thing you can do is closely monitor him, from the age of about six on up. Melanomas will develop eventually. When they do, talk to your veterinarian early on and discuss treatment—don't leave it till they get very large.”
Treatments For Melanomas
A bewildering array of treatment options exists for melanomas, and again, as with sarcoids, no one method has emerged as having a uniformly rewarding success rate. Your veterinarian might want to try one or more of the following:
Surgical Excision—The simplest approach for slow-growing, small to moderate-sized tumors (less than three centimeters in diameter) in surgically accessible locations. Large "sheets” of melanoma tissue are not good candidates for surgery, as it would prove very invasive and the chances are slim that you'd be able to remove 100% of the tumor. Even with solitary tumors, rapid recurrence often is a problem, as it's difficult to tell whether you've been able to remove all the abnormal cells. Surgical excision might be used to "de-bulk” a large tumor mass in order to improve the chances of success of other treatment methods, such as one of the following:
Cryonecrosis—As with sarcoids, freezing with liquid nitrogen might help kill the remaining cells in the tumor "bed” after the majority of a large melanoma has been removed by surgery or with surgical lasers. The surface tissue is frozen to -20° Celsius, allowed to thaw, then frozen a second time. This procedure usually can be done with the horse standing and sedated. Combining surgical removal with "cryo” treatment rarely cures melanomas, but it very often can keep the tumors of a manageable size when the freezing is repeated once or twice a year.
Chemotherapy—Cispla-tin, the chemotherapy drug, also has been found to be useful for single, small-to-moderate-sized melanomas, injected intra-lesionally (into the tumor itself) every two weeks for a total of four treatments. (Systemic chemotherapy—in which the drug is injected into the muscle or a vein rather than into the tumor itself—has proven to have little or no effect on equine melanomas.) The results vary depending on how successfully the drug diffuses through the tissues (the injection technique usually involves using a fine-gauge needle and many applications of the drug, five to eight millimeters apart). Encouragingly, though, cisplatin has been used on pregnant broodmares and breeding stallions with no signs of toxicity. Tumors do tend to recur a few months after cisplatin treatment ceases, but horses develop no resistance to the drug, so the treatment can be repeated if necessary.
Radiation—Radiation therapy, similar to that used on sarcoid tumors, also is a possibility if you are lucky enough to have an appropriately equipped veterinary hospital nearby. Says Carr, "We've been using radiation therapy on some melanomas in awkward locations with pretty good success.”
Latest Therapy: Cimetidine
One of the most promising advances in the treatment of melanoma is the drug cimetidine (trade name Tagamet). Borrowed from human medicine, where it has been shown to treat malignant melanomas and some other types of tumors, cimetidine has been used with good success in horses since 1985. The drug often is able to reduce the size and number of a horse's tumors, although veterinarians caution there is no way to tell whether an individual horse will respond to the treatment until it is tried. It's estimated that 30%-50% of horses treated do respond to some degree. A good response is a 50% reduction in the size and/or number of melanomas, and no further progression of the disease for several years. (In a few cases, cimetidine has even been documented to cause tumors to regress completely, apparently curing the horse.)
Researchers aren't completely sure how cimetidine works on melanomas, but they do know that in an active melanoma, T suppressor cells are present to dampen the immune response to the abnormal cells. Cimetidine appears to block the activation of these T suppressor cells, thus helping stimulate the body's anti-tumor defense system. Not surprisingly, cimetidine works best on tumors that are actively growing. Results with tumors that aren't changing in size or appearance generally have been disappointing.
Although cimetidine is a ray of hope for many owners, it's an expensive and inconvenient drug, best administered orally at a level of 2.5 milograms/kilograms of body weight, at eight hour intervals, over a course of weeks or months. Giving the drug only once or twice a day doesn't appear to give nearly the same results. If your horse is going to respond to cimetidine, you'll start to see a change in the size and/or number of the tumors within two to seven weeks. If that occurs, your veterinarian will continue the therapy until he/she sees no further change for a period of two to three weeks. If your horse's tumors show no changes after three months of treatment, the therapy should be discontinued, as it will be evident by then that you'll get no response.
One of the positive things about cimetidine treatment is that, while it rarely cures a horse of melanomas, it can slow or even halt the progress of the disease for months or years after the treatment ceases, adding valuable time to your horse's life. If the tumors do become active again, they often respond well to a second course of cimetidine. The drug also can be used in combination with surgery, cryonecrosis, and chemotherapy (cisplatin), and no toxic effects have been recorded to date. But, says Carr, diligence and dedication are key to the therapy.
"The bottom line is that if it works (for your horse), you've got to keep using the drug throughout his lifetime.”
A Melanoma Vaccine?
The research team of Alain Theon, DVM, MS, at UC Davis currently is investigating the possibility of a vaccine for melanoma tumors, according to Carr. She notes, "Unless it's a virally mediated tumor (which melanomas are not), the best use of a vaccine is for picking up microscopic metastases, not for eliminating the tumor completely.”
Meanwhile, at the Veterinary Oncology Services and Research Center in West Chester, Penn., K. Ann Jeglum, VMD, Diplomate ACVIM (Oncology), offers a service currently unique in North America—she can produce a "custom-made” vaccine (technically known as an autochthonous vaccine) for your horse's melanoma tumors. When a tumor is excised surgically, the tissue can be packed on ice and sent to Jeglum's lab by overnight courier (the fresher the material, the better). On arrival, it is minced, placed in culture media, and the live tumor cells extracted. From this sample, for a base price of about $500 (plus the cost of shipping), a vaccine is created that is designed to help the horse's immune system recognize and attack his own tumor cells.
The recommended protocol for the vaccine is to administer it once every two weeks for a total of 12 weeks, then once every four to six weeks until the supply of vaccine derived from the tumor runs out (this depends on how many live cells were able to be extracted from the sample). Jeglum recommends that each dose of the vaccine be delivered along with an immunostimulant product such as Nomagen or EquiStim, and that the vaccine be injected intradermally, in front of a lymph node that drains to the tumor site. As for the success rate, certified veterinary technician Kim Wilkinson-Kahn, who does much of the vaccine production at Jeglum's clinic, says statistics have not yet been generated, but that "most clients report some response,” and that several horses have experienced regression of their tumors.
Magic treatment? No, but, when it comes to dealing with sarcoids and melanomas, there isn't one, simple answer that will suit every horse. The best recommendation is to keep your expectations realistic and explore all the treatment options with an open mind and the advice of as many veterinary experts as you can rally around you. In most cases, these tumors are anything but a death sentence—but that's no reason to stick your head in the sand.
About the Author
Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.
POLL: It's Derby Week!