Skin Tumors in Horses: Sarcoids, Melanoma and Squamous Cell Carcinoma

By Dr Duncan Houston

A new lump on a horse is easy to dismiss, especially if it is small, slow-growing, or not causing pain.

That is where owners can get caught out. Many equine skin tumors start quietly. Some remain fairly stable for years, while others grow, ulcerate, bleed, interfere with tack, damage the eye, affect the sheath or anus, or come back aggressively after incomplete treatment.

The most common skin tumors in horses are sarcoids, melanomas, and squamous cell carcinomas. They can look very different, but they all have one thing in common: early recognition gives you more options.

Quick Answer

The most common skin tumors in horses are sarcoids, melanomas, and squamous cell carcinomas. Sarcoids are the most common equine skin tumor and are usually locally invasive rather than metastatic, but they can be difficult to treat and often recur. Any new, growing, bleeding, ulcerated, painful, eye-associated, genital, or non-healing lump should be assessed by a veterinarian rather than treated at home. (MSD Veterinary Manual)

Why Skin Tumors Matter in Horses

Skin tumors are common in horses of all breeds and ages. The American College of Veterinary Surgeons notes that the three most common skin tumors in horses are sarcoids, melanomas, and squamous cell carcinomas, and that early recognition, accurate diagnosis, and early treatment are important for better outcomes. (American College of Veterinary Surgeons)

The mistake I see most often is waiting because the lump “doesn’t seem to bother the horse.”

That can be risky because location matters as much as size. A small tumor near the eye, sheath, penis, vulva, anus, girth, eyelid, pastern, or tack area can become a much bigger problem than a larger lump in a less sensitive location.

What Are Sarcoids?

Sarcoids are the most commonly diagnosed tumors of horses and other equids. MSD Veterinary Manual describes equine sarcoids as representing around 20 percent of all equine neoplasms and 36 percent of skin tumors in horses. (MSD Veterinary Manual)

Sarcoids are not “benign” in the comforting sense. They do not usually spread to internal organs, but they can be locally aggressive. That means they may invade nearby skin and tissue, recur after treatment, become ulcerated, bleed, interfere with tack, and create ongoing welfare problems.

Sarcoids are associated with bovine papillomavirus, especially BPV types 1 and 2, and there may also be genetic susceptibility in some breeds or bloodlines. They are often seen at previous wound or scar sites. (MSD Veterinary Manual)

What Do Sarcoids Look Like?

Sarcoids can look very different from one horse to another.

Common forms include:

MSD Veterinary Manual recognises six clinical forms: occult, verrucose, nodular, fibroblastic, mixed, and malevolent. Sarcoids can occur anywhere, but they are common on the ventral thorax and abdomen, paragenital region, and head. (MSD Veterinary Manual)

In practice, sarcoids are especially suspicious when a lump looks warty, keeps returning, grows at a wound site, bleeds easily, looks like proud flesh but is not healing, or appears as multiple different-looking lesions on the same horse.

Are Sarcoids Cancer?

Sarcoids are tumors, but they behave differently from many cancers.

They are usually considered locally invasive and non-metastatic. That means they can be destructive where they are, but they do not usually spread to distant organs. Morris Animal Foundation describes sarcoids as the most common skin cancer diagnosed in horses and ponies, noting that they do not spread to other parts of the body but can grow large, spread locally, multiply, and affect quality of life. (Morris Animal Foundation)

The practical message is simple: sarcoids are not usually life-threatening, but they should still be taken seriously.

Should a Sarcoid Be Biopsied?

This is where careful veterinary judgement matters.

A definitive diagnosis can be made by biopsy, but suspected sarcoids can react badly to trauma. MSD Veterinary Manual states that biopsy can confirm the diagnosis, but acquiring the sample carries a risk of triggering considerable and uncontrollable lesion expansion. (MSD Veterinary Manual)

That does not mean biopsy is never done. It means the decision should be deliberate.

A vet may avoid biopsy when the lesion is clinically typical and treatment planning does not require tissue confirmation. A vet may recommend biopsy when the diagnosis is uncertain, the lesion could be squamous cell carcinoma or another tumor, or treatment choice depends on histopathology.

The key is this: do not cut, scrape, pick, burn, freeze, or biopsy a suspected sarcoid without a veterinary plan.

Why Sarcoids Are Difficult To Treat

Sarcoids are frustrating because they can extend beyond what you can see.

A small visible mass may have microscopic tumor cells in nearby tissue. If only the obvious lump is removed, recurrence is common. MSD Veterinary Manual reports recurrence rates of 15 percent to 82 percent after surgical excision alone, and notes that excised sarcoids often regrow more aggressively within six months. (MSD Veterinary Manual)

This is why “just cut it off” is often a bad plan.

The best approach depends on tumor type, size, location, whether it has been treated before, whether it is ulcerated, how close it is to sensitive structures, and whether referral-level treatment is available.

Sarcoid Treatment Options

There is no single best treatment for every sarcoid.

Options may include:

Monitoring

Some small, quiet sarcoids may be monitored if they are not growing, ulcerated, irritated, or in a risky location. This should still be a veterinary decision, because trauma or inappropriate treatment can make some sarcoids worse.

Surgical removal

Surgery may work best for small, well-defined lesions where adequate margins can be achieved. It is less reliable when used alone for invasive, recurrent, or poorly defined tumors. MSD recommends surgical excision with margins of at least 0.5 to 1 cm where surgery is chosen. (MSD Veterinary Manual)

Laser surgery

Laser excision can reduce bleeding and may be useful in selected cases, especially when combined with other treatments.

Cryotherapy

Cryotherapy freezes tumor tissue, often after debulking. A systematic review of equine sarcoid treatment found that cryotherapy is among the more effective treatment regimens in selected cases, although outcomes vary with case selection, site, technique, and whether it is combined with surgery. (ABVP)

Local chemotherapy

Intralesional or local chemotherapy may include cisplatin or 5-fluorouracil in selected cases. A systematic review found that radiotherapy, cryotherapy, intralesional cisplatin, and electrochemotherapy were among the most effective regimens reported, but it also stressed that treatment decisions must still be made case by case because the evidence is variable. (ABVP)

Radiotherapy

Radiotherapy can be very effective, especially for difficult sites such as around the eye, but it is expensive, specialised, and not widely available. MSD Veterinary Manual describes local radiotherapy as highly effective for tumors less amenable to traditional therapy, including limb or periocular lesions. (MSD Veterinary Manual)

Immunotherapy or topical prescription treatment

Some cases may use BCG, prescription topical treatments, or other specialist protocols. These should not be attempted without veterinary guidance.

What About Toothpaste or Home Remedies?

You may hear stable-yard stories about toothpaste, bloodroot, caustic creams, herbal pastes, or “sarcoid salves.”

This is dangerous territory.

Some irritant products may appear to shrink or inflame a lesion, but they can also cause pain, tissue damage, ulceration, scarring, eye injury, and aggressive regrowth. A systematic review reported serious adverse outcomes with some sarcoid treatments in the literature, including severe complications around the eye. (ABVP)

The clean rule is: do not apply home remedies or over-the-counter tumor creams to a suspected sarcoid.

Other Common Skin Tumors in Horses

Sarcoids are common, but they are not the only important skin tumor.

Melanoma

Melanomas are especially common in grey horses. UC Davis notes that melanomas are the third most diagnosed skin tumors in horses after sarcoids and squamous cell carcinoma, and that they are commonly observed in grey horses but can occur in any coat colour. (Center for Equine Health)

Typical melanoma locations include:

• Under the tail

• Around the anus

• Genital region

• Lips

• Eyelids

• Throatlatch

• Parotid region

Melanomas often appear as small, black, dome-shaped nodules. They may be slow-growing at first, but they can enlarge, merge into plaques, affect defecation or sheath function, and sometimes spread internally. UC Davis notes that melanomas are usually initially benign but can become malignant and metastasize. (Center for Equine Health)

The old advice was often “leave grey horse melanomas alone.” That is too simplistic. Some can be monitored, but growing, ulcerated, painful, obstructive, or non-grey-horse melanomas deserve a proper veterinary plan.

Squamous cell carcinoma

Squamous cell carcinoma, or SCC, is a malignant skin tumor. MSD Veterinary Manual states that squamous cell carcinomas are the most common malignant skin tumors in horses and are often found in adult or aged horses with white or part-white coats. Common sites include the eyes, lips, nose, anus, and external genitalia, especially the sheath around the penis. (MSD Veterinary Manual)

SCC may look like:

• A non-healing sore

• A crusted or ulcerated patch

• A raised irregular mass

• A bleeding or infected-looking lesion

• Persistent eye discharge

• A mass on the third eyelid

• A foul-smelling penile or sheath lesion

• A lesion around the anus or vulva

UC Davis notes that ocular SCC is the most common cancer of the equine eye and can invade adjacent tissues, causing visual impairment and destruction of the eye. Early detection is associated with better outcomes. (Horse Report)

This is one tumor where delay can be very costly.

Papillomas

Papillomas are warts caused by papillomaviruses. They are often seen in young horses, especially around the muzzle or lips. MSD notes that warts are caused by papillomaviruses and may spread by direct contact or contaminated items such as bedding, blankets, saddles, tack, and environmental surfaces. (MSD Veterinary Manual)

Many young horse papillomas resolve without major treatment, but not every wart-like lesion is a harmless papilloma. In older horses, solitary warty masses need more caution.

Mast cell tumors and other masses

Horses can also develop mast cell tumors, lymphosarcoma, vascular tumors, basal cell tumors, collagenous nevi, cysts, granulomas, abscesses, proud flesh, and other skin masses. MSD describes mast cell tumors in horses as uncommon and usually benign, often appearing as solitary masses most commonly on the head and legs. (MSD Veterinary Manual)

The important point is not to guess from appearance alone when the lesion is changing, painful, ulcerated, or in a sensitive location.

Severity Guide: How Worried Should You Be?

The main checkpoint is not whether the lump is big. It is whether it is growing, ulcerated, bleeding, painful, in a high-risk location, or interfering with normal function.

What Else Can Look Like a Tumor?

Not every lump is a tumor.

Important rule-outs include:

• Abscess

• Cyst

• Insect bite reaction

• Granuloma

• Proud flesh

• Ringworm

• Habronemiasis

• Eosinophilic granuloma

• Wart

• Scar tissue

• Foreign body reaction

• Chronic wound

• Lymph node enlargement

• Hematoma or seroma

• Bacterial or fungal skin infection

Sarcoids can resemble proud flesh, fibropapillomas, and other skin conditions. MSD specifically notes that sarcoids can look like other skin tumors and exuberant granulation tissue. (MSD Veterinary Manual)

This is why a non-healing wound deserves attention. Sometimes it is not a wound anymore. Sometimes it is tumor tissue hiding in plain sight.

When Is This an Emergency?

Most skin tumors are not same-hour emergencies, but some situations need urgent veterinary care.

Call your vet promptly if your horse has:

• A lump involving the eyelid, third eyelid, cornea, or eye surface

• Persistent eye discharge with a visible mass or abnormal tissue

• A tumor on the penis, sheath, vulva, or anus

• Difficulty urinating

• Difficulty defecating

• Bleeding that does not stop

• Rapid swelling or rapid tumor growth

• Severe pain

• Foul smell, discharge, or infection

• A mass that interferes with tack, walking, or eating

• A wound that does not heal as expected

• A sarcoid-like lesion that has been cut, rubbed, or traumatised

• A tumor that changes significantly over days to weeks

UC Davis notes that ocular SCC is easier to treat when lesions are small, while larger or invasive tumors may require advanced imaging or even removal of the affected eye. (Center for Equine Health)

If the eye is involved, do not wait.

How Vets Diagnose Skin Tumors

Diagnosis depends on the lesion.

Your vet may use:

• Physical examination

• History and growth pattern

• Photographs and measurements

• Fine needle aspirate in selected masses

• Biopsy

• Excisional biopsy for small masses

• Histopathology

• Cytology

• Culture if infection is possible

• Ultrasound for deeper masses

• Endoscopy for sheath, penile, or internal concerns

• Eye examination for periocular lesions

• Imaging such as CT for large or invasive ocular tumors

ACVS notes that veterinarians may perform biopsy or complete excisional biopsy depending on mass size, and that referral may be needed depending on location, temperament, need for specialist therapy, or anaesthetic requirements. (American College of Veterinary Surgeons)

The safest plan is to diagnose first, then treat. Guessing can make sarcoids worse, miss SCC, or delay treatment for a tumor that still had a good early outcome.

What Should You Do Right Now?

1. Photograph and measure it

Take clear photos from the same distance every 2 to 4 weeks.

Include:

• One close-up

• One wider shot showing location

• A ruler or coin for scale

• Notes on bleeding, ulceration, pain, or growth

This turns vague impressions into useful evidence.

2. Do not pick, cut, burn, freeze, or scrape it

This matters most with suspected sarcoids. Trauma can inflame the lesion, invite infection, and sometimes make treatment harder.

3. Stop friction

If the lump is under a saddle, girth, bridle, rug, boot, or breastplate, reduce rubbing until your vet has assessed it.

Repeated trauma can ulcerate tumors and complicate treatment.

4. Check high-risk sites

Look carefully at:

• Around both eyes

• Eyelids and third eyelid area

• Muzzle and lips

• Under the tail

• Around the anus

• Vulva

• Sheath and penis

• Girth and saddle area

• Inside thighs

• Previous wound sites

• Lower limbs

Many important tumors hide in places owners do not check routinely.

5. Call your vet if it changes

Growth, bleeding, ulceration, discharge, pain, or location near sensitive tissue changes the urgency.

Do not wait months for a changing mass to “declare itself.” It already has.

Common Mistakes Owners Make

Assuming a sarcoid is harmless because it does not metastasize

Sarcoids can still be locally aggressive, recurrent, painful, ulcerated, and functionally serious.

Cutting or treating before diagnosis

Home removal attempts can worsen sarcoids and delay treatment of SCC or other tumors.

Waiting on eye lesions

Eye tumors are much easier to manage early. Delay can mean more invasive treatment and a higher risk to vision.

Ignoring grey horse melanomas

Some melanomas remain slow, but others enlarge, obstruct, ulcerate, or spread. They should still be monitored and discussed with a vet.

Treating proud flesh that is actually a tumor

A wound that keeps producing abnormal tissue after appropriate care may need biopsy or further investigation.

Using caustic tumor creams

These can cause pain, burns, scarring, eye damage, and uncontrolled inflammation.

How To Prevent or Reduce Risk

You cannot prevent every skin tumor, but you can reduce risk and catch problems earlier.

Useful steps include:

• Check your horse’s skin weekly

• Photograph any new lump and track changes

• Pay special attention to previous wound sites

• Use fly control around wounds and irritated skin

• Avoid sharing contaminated tack or equipment where papillomavirus-related lesions are suspected

• Use UV-protective fly masks for horses at risk of ocular SCC

• Provide shade, especially for horses with white or pink skin

• Inspect grey horses under the tail and around the anus regularly

• Check the sheath and penis in geldings and stallions

• Ask your vet to examine any non-healing wound

• Include a skin check in annual wellness exams

UV protection is especially important for SCC risk. MSD notes that limiting ultraviolet exposure may help prevent squamous cell carcinomas, particularly in white or partially white horses. UC Davis also recommends UV-protective fly masks and peak-sunlight stabling for horses at high genetic risk of ocular SCC. (MSD Veterinary Manual)

Will My Horse Be Okay?

Many horses with skin tumors do well, especially when the lesion is recognised early and treated appropriately.

The outcome depends on:

• Tumor type

• Size

• Location

• Whether it is ulcerated

• Whether it has been treated before

• Whether clear margins are possible

• Whether adjunctive therapy is available

• Whether vital structures are involved

• Whether recurrence is monitored

Sarcoids can be frustrating but are often manageable with a tailored plan. Melanomas may be slow, but grey horses often develop more over time. Squamous cell carcinoma can be highly treatable when caught early, but can become locally destructive and much harder to manage if left too long.

The strongest message is this: a lump is easier to investigate early than to rescue late.

FAQs

Are sarcoids in horses dangerous?

Sarcoids usually do not spread to internal organs, but they can be locally invasive, recurrent, ulcerated, painful, and difficult to treat. Their seriousness depends on size, location, growth rate, and whether they have been traumatised or treated before.

Should I remove a sarcoid myself?

No. Do not cut, burn, freeze, scrape, tie off, or apply home remedies to a suspected sarcoid. Trauma or incomplete treatment can make sarcoids more difficult to manage.

What does squamous cell carcinoma look like in horses?

It may appear as a non-healing sore, crusted area, raised irregular mass, ulcerated lesion, bleeding tissue, or foul-smelling genital lesion. It commonly affects the eye region, lips, nose, anus, sheath, penis, vulva, and other non-pigmented or poorly haired areas.

Do all grey horses get melanomas?

Not all, but grey horses are at much higher risk. UC Davis notes that melanomas are commonly observed in grey horses and tend to increase in size and number as grey horses age. (Center for Equine Health)

When should I call a vet about a lump?

Call a vet if the lump is new, growing, bleeding, ulcerated, painful, infected-looking, near the eye or genitals, interfering with tack, or not healing like a normal wound.

Final Thoughts

Skin tumors in horses are common, but they should not be approached casually.

Sarcoids are the classic trap: often non-metastatic, but locally aggressive and highly frustrating if treated poorly. Melanomas in grey horses may seem routine, but they still need monitoring. Squamous cell carcinoma can look like a sore or crusty patch at first, but delay can cost vision, function, and sometimes the horse’s life.

The safest approach is to photograph, monitor, avoid trauma, and get a diagnosis early. Do not treat a lump based on stable-yard advice or internet pictures. The same “warty” lesion could be a sarcoid, papilloma, SCC, proud flesh, infection, or something else entirely.

Early clarity gives you options. Guesswork takes them away.

If you are unsure whether your horse’s lump is a sarcoid, melanoma, squamous cell carcinoma, wart, proud flesh, or infection, ASK A VET™ can help you work through the signs and decide how urgently your horse needs veterinary care.