Understanding Sarcoids on Horses: Types and Treatments

What Are Sarcoids on Horses?

• Equine sarcoids are skin cancers that can become invasive without spreading to other organs. Fibroblastic cells in the skin serve as connective tissue, and these tumors form when these cells become cancerous.

• You may see a singular lesion or a cluster of growths. They may range in size, location, and even appearance. Often they are mistaken for melanomas, another form of skin cancer.

• Despite their inability to spread to other organs, they can cause significant discomfort for your horse.

Definition and Overview

• Sarcoids on horses are the most frequently diagnosed tumor in horses, affecting about 12% of horses. They can also be found in other equids, like donkeys and zebras.

• They typically appear as hairless patches with tougher skin or nodules. They can also open into sores. A typical lesion is a few centimeters, although they can grow much larger.

• A comprehensive review of equine sarcoid literature reveals that these growths are composed of tumor tissue that invades the skin locally. They can appear as rough, hairless patches, firm nodules, or even fleshy, ulcerated masses. While they can grow quite large—up to 10 cm in diameter—most remain smaller.

• The primary complication with tumors is their location, and if they interfere with a horse’s comfort, movement, tack, or anything else that impacts how they live. They may not have any impact on some horses.

How Sarcoids Affect Equine Health

• A sarcoid’s size, location, and type determine how it affects a horse.

• They could be cosmetic in nature, without causing any pain.

• These tumors may interfere with tack, creating itching, pain, or even open sores if scratched or torn. There is a normal skin covering over the growth, which can tear and bleed. This wound opens the door for secondary infections and pain.

• Sarcoids also impact their immune system, which affects their overall health.

• These growths are rarely life-threatening, but they can create a lesser quality of life for some horses.

Malignant sarcoids

• Some variations of sarcoids are malignant, meaning they spread throughout a horse’s body. In this case, it won’t spread to other organs, but there may be tumors all over a horse’s skin.

• These lesions send out tendrils under the skin to new locations for more skin lesions to pop up. As you might expect, these are very difficult to treat.

Causes of Sarcoids in Horses

• The primary trigger of sarcoids on horses is the bovine papillomavirus (BPV). There are other contributing factors, otherwise every horse exposed to BPV would develop them.

• There is a genetic component to these growths as well, making some horses more susceptible than others. There may also be an element of skin trauma, suggesting that existing sores may put out the red carpet for BPV to infect the area.

• Incidentally, papillomaviruses are related to aural plaques in horse ears, too.

Role of Bovine Papillomavirus

• Types 1 and 2 of the bovine papilloma virus travel via biting flies from their typical cow targets to horses.

• When BPV infects the skin, it will interfere with the normal DNA replication process. The virus’s DNA embeds itself into the horse’s DNA, and when those cells replicate as they usually do, the virus DNA turns creates a tumor.

• Some sarcoid-free horses have BPV DNA in their cells, which hints at other extenuating circumstances beyond BPV being the sole component.

Around the eye is a common place for sarcoids.

Risk Factors for Developing Sarcoids

Genetic and Environmental Factors

• There are likely genetic factors at play, too. This accounts for the prevalence of BPV in horses without these tumors, and it helps explain why sarcoids are likely to recur, despite successful treatment. Genetic factors may create some level of susceptibility.

• The manner in which BPV transmits may also impact the likelihood of sarcoids on horses. If biting flies carrying this virus have access to a sore or weakened skin, the virus may have a head start.

• If existing lesions are not entirely removed, either because the treatment itself or the application of the therapy wasn’t 100% successful, the sarcoid may recur.

Breed Susceptibility

• There are studies and reviews published that examine the relationship between sarcoids on horses and breed. One review shows that Quarter Horses are twice as likely to develop these tumors as Thoroughbreds, and Thoroughbreds are twice as likely to develop them as Standardbreds.

Read this paper here.

Location

• The specific type of BPV also impacts horses in different parts of the world. A scientific review conducted in Western Canada discovered that most cases were related to BPV type 2. In other parts of the world, like Europe, the BPV is usually type 1.

Read the paper here.

Age and Gender Considerations

• Sarcoids on horses are more commonly diagnosed in younger equines, ages 3 to 6, although there are exceptions, of course. One study looked at over 500 horses and found some interesting things regarding age and gender.

Geldings were at a higher risk of developing sarcoids in comparison with stallions. There was no significant difference in the risk of developing sarcoids between stallions and mares. The risk of developing sarcoid increased with age up to 15 years and then declined.

Read the abstract here.

Environmental Influences

• Two significant things in your horse’s life can influence the likelihood of developing sarcoids.

• One is the prevalence of biting flies that may carry the papilloma virus.

• The other is your horse’s skin health. Many tumors appear at the site of previous scars and wounds. The theory is that open tissues provide a tempting place for biting flies to deposit a virus. Considering that many sarcoids recur or expand when disturbed, confirming this line of thought.

Are Sarcoids in Horses Contagious?

• Sarcoids on horses are not directly contagious, although they can be spread around the barn by flies, grooming tools, saddle pads, and tack. And because sarcoid tissue contains the virus, it’s prudent to keep them from opening.

Transmission Pathways

• Researchers have a general understanding that BPV uses flies as vectors. A fly can feed on an open lesion on one horse and possibly transmit it to another horse.

• Horses with sarcoids may accidentally contaminate themselves and create another tumor.

Preventing BPV Spread Among Horses

• There is no way to eliminate the risks of your horse developing these tumors, but you can mitigate the risks in a few ways.

• Fly control. Limiting exposure to biting flies can reduce rates of transmission. Attack the fly life cycle at all stages, along with proper fly control measures such as fly predators, swift manure and urine mucking, fly sprays, and fly sheets, boots, and masks.

• Wound management. Clean, treat, and monitor wounds. It’s not always ideal or possible to bandage a wound, but you can cover it with fly protection like sheets and boots if necessary.

• Don’t share grooming tools and tack. This guideline is ideal anyway, as many things can hop from horse to horse by way of brushes and tack, like ringworm.

Identifying Sarcoids in Horses

• Sarcoids on horses take time to grow, and the early stages are subtle. You might notice a spot or hairless patch, and it might look like ringworm, a scar, or an area of rough skin.

• They may also create white hairs in the area, resembling a scar, or they may look like proud flesh. This excessive granulation tissue is typically found on the lower legs, so you may wonder what it’s doing elsewhere on their body. Could be a tumor instead!

• It’s a great idea to track progress by taking photos. You can then see the progression of hair loss, size, and texture over time.

Recognizing Early Signs and Symptoms

Look out for these common early signs:

• A small, flat, hairless area with a rough or scaly surface.
• A firm, round nodule or lump under otherwise normal-looking skin.
• Any unusual skin growth at the site of a previous injury or scar.
• Sudden or rapid growth of a pre-existing lump or lesion.

Common Locations on the Body

• Your horse’s whole body is susceptible to developing these tumors, but sarcoids prefer thin skin. Faces and necks often harbor smaller growths, but those on legs and abdomens can grow quite large.

• Other locations include around the sheath, udders, or lower legs.

• Sites of previous wounds are common, too, and may look like a part of the healing process.

• Sarcoids can appear anywhere on a horse’s body, but they have a preference for specific areas. They are often found in regions with thin skin or relatively hairless areas. Knowing these common spots can help you focus your daily skin checks.

• Lesions on the head tend to be smaller, while those on the limbs and abdomen can grow larger. They can also develop at the site of a previous skin wound, where they might be mistaken for excessive healing tissue. The surrounding normal skin may appear completely unaffected, making the growth stand out.

Diagnosing Equine Sarcoids

• Your vet can diagnose your horse’s growths. Early diagnosis will save you time and money in the long run, as you’ll have appropriate treatments early on. Your vet will start with a physical exam and go from there.

Clinical Examination Techniques

• The first step for your vet is to inspect and feel the lesion, noting size, shape, location, texture, and any other identifying characteristics. They will also feel for attachments to underlying tissues.

• Ruling out other conditions like ringworm and summer sores is part of the process, too.

Biopsy and Laboratory Testing

• It may seem logical for a biopsy of the area, but sarcoids are famous for responding to the removal of tissue by triggering excessive and fast growth. Unless it’s necessary, you’ll want to avoid this.

• It’s certainly possible to test any tissues after surgical removal, as this hopefully takes all of the tumor out.

• For questionable tumors, it’s a fine line between a definitive diagnosis and risking an adverse reaction to the biopsy.

Small wounds open doors for infection with BPV.

Types of Sarcoids in Horses

• There are six types of equine sarcoids, which are classified by their appearance and behaviors. The type of growth will influence the treatment plan.

• Horses with multiple lesions can have more than one type of tumor.

Occult Sarcoids

• Occult sarcoids on horses are the least aggressive version, and often signal the start of more to come. Occult means hidden, and in this case, the tumors are small and easily overlooked.

• These often look like an inconsequential scar and are common around eyes, mouths, necks, and elbows. They often hibernate for years, remaining unchanged, but are susceptible to growth if irritated. Occult tumors can often morph into more aggressive types.

Verrucous Sarcoids

• Is it a wart or a verrucous sarcoid? They often look the same, showing grayish skin and a crusty, irregular surface prone to cracking.

• Slow growing, the verrucous variety is also not as aggressive as other lesions, and is often found on faces or near sheaths and udders. Sometimes, white hairs appear in the area, making them look more like scars. And like other forms, irritations can transform verrucous sarcoids into more aggressive types.

Nodular Sarcoids

• Then there are the spherical, firm lumps under the skin. These nodules can be tiny or a few inches across. You’ll typically see these around the sheath, udders, and eyelids.

• Type A nodules are under the skin, and the skin above the bump is freely movable. Type B nodules poke out of the skin and may rupture. A stem or trunk often attaches them.

• As a surprise to nobody, they can morph into more aggressive types if bothered.

Fibroblastic Sarcoids

• Rapidly growing and alarming to see, fibroblastic sarcoids are quite aggressive. They will invade local tissues, often beneath the skin, too. They feel firm, raised, and usually irregular. They also appear wet.

• Because they are easily ruptured and leak blood, they will attract flies. They resemble proud flesh and are common on the face, lower legs, and groin area.

• The fibroblastic variety is the most visually alarming type. They are aggressive, fleshy masses known for rapid growth. These tumors are locally invasive and can sometimes reach the tissues beneath the skin. They often appear as irregular, raised lumps that are firm to the touch.

• Fortunately, they do not metastasize to other organs, but can spread invasively in the area and are difficult to treat.

Mixed-Type Sarcoids

• It’s common to find mixed-type sarcoids on horses, comprised of more than one type. This typically occurs when one type of lesion transforms into another, and you are catching it in the transition stage.

• For example, a horse might have a lesion with both verrucous (warty) and fibroblastic (fleshy) characteristics. You might also see nodular lumps developing within a flat, occult sarcoid. These mixed presentations are common and can make diagnosis and treatment more complex.

Malignant Sarcoids

• Terminology gets confusing here. Malignant (or malevolent) sarcoids don’t metastasize to other organs, but instead aggressively attack the skin.

• Lymph vessels under the skin serve as tumor highways, to the extent that you can feel the cords of tumor peppered with nodules under the skin. These are usually seen on the face, jaw, elbow area, and inner, upper legs.

• While rare, malevolent sarcoids can grow after less aggressive types are traumatized or not fully treated.

This article has many photos of the various types of tumors.

Treatment Options for Sarcoids

• After treatment, there is always a chance for recurrence, and there are no guarantees when addressing these lesions. There are many sarcoid treatment options, and the best results often include a mixture of different types of therapies.

Surgical Options Including Laser Surgery

• Smaller lesions fare well with surgical excision. Ideally, your vet removes the entire sarcoid plus a border of healthy tissue to prevent regrowth. However, this is not always effective, with a 50% recurrence rate.

• It’s best to pair traditional surgery with another treatment.

• Your vet may also use a CO2 laser for surgery. This method minimizes bleeding and is more precise, but it does require special equipment.

Cryotherapy and Topical Treatments

• Sarcoids on horses can also be treated with cryotherapy, which uses extreme cold to freeze the lesion with liquid nitrogen. Freezing may be the best treatment for small areas and has a success rate of around 70%. The treated hair often grows back white.

• Topical chemotherapy agents are another option. Several prescription creams are available that work by attacking the sarcoid’s skin cells. One such cream is imiquimod, which stimulates a local immune response. Another is a cytotoxic cream known as AW4-LUDES (Liverpool cream).

• These treatments can be painful, create significant swelling, and take weeks or months of diligent treatment application. Topicals are best for flat sarcoids.

Chemotherapy and Immunotherapy

• Other chemotherapy drugs, like cisplatin, are injected directly into tumors. They interfere with the DNA of cancerous cells to halt their replication and growth. A course of injections over several weeks is best.

• Immunotherapy aims to stimulate the horse’s own immune system to attack and destroy the sarcoid tissue. The most common form is an injection of a vaccine called Bacillus Calmette-Guérin (BCG), which was initially developed for tuberculosis.

• This treatment is particularly successful for sarcoids around the eye, where surgery is risky. The BCG vaccine triggers an intense inflammatory reaction as the immune system targets the tumor. Multiple injections are usually required for the treatment to be effective.

Radiation Therapy and Other Advanced Methods

• It’s also possible to use radioactive seeds or needles directly in the tumor. For lesions around sensitive areas like the eyes, this is ideal.

• Electrochemotherapy is another advanced method that combines chemotherapy with high-voltage electric pulses. The electric shock makes the tumor cells more permeable, allowing the chemotherapy drug to enter in higher concentrations. This procedure requires general anesthesia.

• A newer option is photodynamic treatment, where a special chemical is applied to the tumor and then activated by a specific type of light. This activation triggers a reaction that kills the tumor cells. These advanced treatments are typically only available at specialized veterinary hospitals.

Benign Neglect

• It’s not always a good idea to ignore a medical issue, but for some horses, benign neglect may be the best treatment for smaller, stable sarcoids. If it’s not growing or interfering with your horse’s health or comfort, it may be best to leave it alone. This is particularly true for occult or small verrucous sarcoids that have remained unchanged for a long time.

• The goal is to avoid the risks of removal that can potentially trigger growth and morphing of the tumor, and leaving it alone may be the best therapy.

• Diligent documentation, including photos, is warranted to track changes and switch to another treatment plan.

Memorizing your horse’s skin, along with their behaviors and reactions to things, is the best way to notice smaller changes that can become big problems! This includes using your fingertips to observe areas of your horse that you can’t see well to feel for changes in the skin.  

There are many treatment options!

Key Highlights

• An equine sarcoid is the most common skin tumor in horses, linked to the bovine papilloma virus.

• There are six main types of sarcoids, ranging from mild, flat lesions to aggressive, fleshy growths.

• While they don’t spread to internal organs, sarcoids can be locally invasive and persistent.

• A wide range of treatment options exists, but successful treatment can be challenging due to high recurrence rates.

• Early diagnosis and a tailored treatment plan from your vet are crucial for management.