What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Equine Sarcoid: Types, Causes, and Treatment Options

Name: Equine Sarcoid: Types, Causes, and Treatment Options
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

Equine sarcoid is the most common skin tumor in horses worldwide, a locally aggressive fibroblastic growth caused by bovine papillomavirus types 1 and 2 (BPV1 and BPV2) [1][2][3]. It is not sarcoidosis. Sarcoidosis is a separate, systemic granulomatous disease of humans and occasionally other species, and equine sarcoid has no relationship to it beyond a confusingly similar name.

Sarcoids behave unpredictably. Some remain as small, stable patches for years. Others grow rapidly, invade surrounding tissue, and recur after nearly every treatment attempted. The clinical form of the tumor, its location, how many lesions are present, and how aggressively it is growing all shape what a veterinarian will recommend. This article explains the six recognized clinical types, the viral and genetic causes, the treatment options with their reported success and recurrence rates, and when a case should be referred to a specialist.

What Is an Equine Sarcoid?

An equine sarcoid is a non-metastasizing but locally invasive skin tumor composed of transformed fibroblasts [3][4]. It is the most frequent neoplasm in equids, affecting horses, donkeys, and mules [3][5]. The Merck Veterinary Manual recognizes equine sarcoids as a distinct clinical entity of the integumentary system [6].

Sarcoids do not spread to distant organs the way malignant cancers do. They spread sideways, infiltrating the dermis, subcutaneous tissue, and sometimes lymphatics in the local area [5][7]. This local invasion is what makes them difficult. A sarcoid removed from the skin surface can regrow from microscopic extensions left behind, which is why recurrence rates after simple excision are high [5][8].

The name "sarcoid" comes from the Greek root for flesh, reflecting the fibrous, fleshy appearance of many lesions. The term "sarcoidosis equine" is sometimes used in search queries, but it is a misnomer. Equine sarcoid and human sarcoidosis are entirely different diseases with different causes, different tissues involved, and different treatments.

The Six Clinical Forms of Equine Sarcoid

Veterinarians classify sarcoids into six clinical types based on appearance and behavior: occult, verrucous, nodular, fibroblastic, malevolent, and mixed [1]. Each type has a characteristic appearance and a typical clinical course. The type matters because treatment response and recurrence risk differ between them.

Occult Sarcoid

Occult sarcoids are flat, hairless, slightly scaly patches on the skin. They are often mistaken for ringworm, rub marks, or a patch of dry skin. They can remain stable for years or slowly progress into other forms. Occult lesions are common on the face, neck, and around the eyes.

Molecular studies show that occult sarcoids have higher expression of the BPV E2 and E6 viral genes compared to fibroblastic or nodular lesions [1]. Despite this, occult lesions tend to be the least aggressive clinically. The proliferative fraction, a measure of how fast cells are dividing, is lower in occult sarcoids than in verrucous or fibroblastic types [9].

Verrucous Sarcoid

Verrucous sarcoids have a warty, cauliflower-like surface. They are often gray, brown, or the color of the surrounding skin. They commonly appear on the face, neck, and torso. Verrucous lesions can be slow-growing or moderately aggressive.

The proliferative fraction in the superficial layers of verrucous sarcoids is significantly higher than in occult or nodular types, which means the surface cells are dividing more actively [9]. Viral gene expression in verrucous sarcoids is second only to nodular sarcoids [9].

Nodular Sarcoid

Nodular sarcoids present as firm, rounded masses under the skin. They are usually hairless and may be single or multiple. Common sites include the groin, sheath, eyelid, and neck. Nodular sarcoids can be mistaken for other skin tumors, which is one reason why a veterinarian may want to avoid a simple biopsy (discussed below).

Nodular sarcoids have the highest viral load and the highest expression of all four BPV genes (E2, E5, E6, and E7) among the four main clinical types [9]. This high viral activity does not always translate into the fastest growth, but it does suggest that nodular lesions are biologically active and should be monitored closely.

Fibroblastic Sarcoid

Fibroblastic sarcoids are the most aggressive of the common types. They appear as fleshy, ulcerated masses that often bleed. They can grow rapidly and invade surrounding tissue. Fibroblastic lesions are common at sites of previous trauma or surgery, and they frequently develop at the edges of incompletely removed sarcoids.

Fibroblastic sarcoids show higher expression of the inflammatory genes IL6 and IL1B compared to other types [1]. The proliferative fraction in the superficial layers is high, similar to verrucous lesions [9]. These tumors are the ones most likely to cause clinical problems and the ones most likely to recur after treatment.

Malevolent Sarcoid

Malevolent sarcoid is the most severe form. These lesions invade locally into surrounding tissues, including lymphatics, and can become life-threatening [1]. Malevolent sarcoids may spread along lymphatic vessels, creating cord-like swellings under the skin. They can ulcerate, become infected, and cause significant pain or functional impairment. In some cases, malevolent sarcoids can be fatal, particularly when they involve the head, neck, or genital region and cannot be completely controlled.

A case report described a 23-year-old Falabella gelding with a recurrent sarcoid of the glans penis. The mass was removed by partial phallectomy, but local regrowth occurred. The horse was euthanized three months later due to urinary retention from suspected regrowth [10]. This case illustrates how malevolent or recurrent sarcoids in sensitive locations can become life-ending.

Mixed Sarcoid

Mixed sarcoids contain more than one clinical type in the same lesion or in adjacent lesions. For example, a nodular sarcoid may have a verrucous surface, or an occult patch may develop a fibroblastic component. Mixed sarcoids are common and should be treated according to the most aggressive component present.

Table: Sarcoid Types, Typical Sites, Treatment, and Recurrence Risk

Type	Typical Site	Typical Treatment	Recurrence Risk
Occult	Face, neck, around eyes	Topical imiquimod, cryotherapy, observation	Low to moderate
Verrucous	Face, neck, torso	Cryotherapy, laser, topical cisplatin	Moderate
Nodular	Groin, sheath, eyelid, neck	Surgical excision with adjunct therapy, laser, BCG	Moderate to high
Fibroblastic	Sites of trauma, previous surgery	Surgical excision with cisplatin, laser, BCG	High (30 to 50% after simple excision)
Malevolent	Head, neck, genital region, lymphatics	Aggressive multimodal therapy, referral	Very high
Mixed	Variable	Treat most aggressive component	Depends on components

What Causes Equine Sarcoid?

The accepted cause of equine sarcoid is infection with bovine papillomavirus types 1 and 2 (BPV1 and BPV2) [2][3][5]. BPV13 has been proposed as a possible additional type, but a European screening study of 135 sarcoid DNA samples from horses and donkeys found no BPV13 DNA, suggesting it is rare or absent in that population [11]. More recent research has detected ovine papillomavirus (OaPV) types 1, 3, and 4 DNA in a subset of equine sarcoids, and OaPV2 has been found in coinfection with BPV1 in at least one case [12][13]. The role of these ovine papillomaviruses in sarcoid development is still being investigated.

BPV infects horses across species barriers. In cattle, BPV1 and BPV2 cause benign fibropapillomas that usually regress on their own. In horses, the same viruses cause sarcoids that persist and recur [4]. The virus does not complete its full life cycle in horses. It is an abortive infection, meaning the virus cannot produce new virions efficiently in equine tissue. Instead, it inserts its DNA into equine fibroblasts and drives them to proliferate [4][14].

How BPV Transforms Equine Cells

BPV1 and BPV2 carry several oncogenes, including E5, E6, and E7. These viral proteins are necessary for maintaining the transformed phenotype of sarcoid fibroblasts [15][7]. When researchers knocked down BPV1 oncogene expression using small interfering RNA, the transformed phenotype of sarcoid fibroblasts reversed [15]. This confirms that the virus is not just a bystander. It is actively driving the tumor.

BPV1 E5 and E6 proteins activate the p38 mitogen-activated protein kinase (MAPK) pathway in equine fibroblasts. Inhibiting p38 reduces proliferation and invasiveness of sarcoid cells [5]. The E6 and E7 proteins increase expression of matrix metalloproteinase 1 (MMP-1), an enzyme that breaks down connective tissue and helps the tumor invade [7]. The E5 protein also contributes to MMP-1 upregulation through an indirect mechanism [7].

A near-natural laboratory model showed that primary equine fibroblasts infected with BPV1 or BPV2 virions become hyperproliferative, lose contact inhibition, and behave like sarcoid cells. The infected cells stably maintained viral DNA at around 150 copies per cell after four passages, and the infection appeared to immortalize them [2]. This model helps explain how a single exposure to BPV can lead to a persistent tumor.

Genetic Susceptibility

Not every horse exposed to BPV develops sarcoids. Some families and bloodlines appear to be more susceptible, suggesting a genetic component. The exact genes involved have not been identified, but the pattern is recognized clinically. A horse with one sarcoid is more likely to develop more, and close relatives of affected horses may be at higher risk.

Viral load studies support the idea that individual immune response matters. Horses with quiescent, slowly growing single tumors or mild multiple lesions had viral loads up to 1.4 copies per cell. In contrast, all equids with rapidly growing or multiple aggressive sarcoids had viral loads between 3 and 569 copies per cell [3]. The virus is present in both groups, but the amount and activity of the virus correlate with disease severity.

How BPV Spreads Between Horses

The mode of BPV transmission between horses is not fully understood. Peripheral blood mononuclear cells (PBMCs) can harbor BPV1 and BPV2 DNA, suggesting that the virus can be present in the bloodstream and may serve as a reservoir for latent infection [14]. In one study, BPV1 DNA was detected in PBMCs of young horses before any skin lesions appeared, and the concentration correlated with tumor growth kinetics [16]. This suggests that the virus can spread systemically before causing visible tumors.

Direct horse-to-horse transmission is considered possible but is not well documented. Flies and other biting insects may play a role in mechanical transmission, though this has not been definitively proven. Shared grooming equipment, tack, and close contact between horses are often mentioned as potential risk factors, but the evidence is circumstantial.

Why Biopsy of a Suspected Sarcoid Can Worsen It

Veterinarians generally avoid taking a biopsy from a suspected sarcoid unless the diagnosis is genuinely uncertain. The reason is that trauma to a sarcoid can trigger explosive growth. Needle aspiration and incisional biopsy can both cause a previously quiet lesion to become aggressive.

This phenomenon is not fully understood, but it is consistent with the observation that fibroblastic sarcoids often develop at sites of previous trauma or surgery. The wound healing response, which involves fibroblast proliferation and growth factor release, may create an environment that favors sarcoid expansion.

If a diagnosis is needed, a veterinarian may use clinical appearance alone or take a sample from the edge of the lesion with minimal disruption. In many cases, the characteristic appearance of a sarcoid is enough to make a diagnosis without biopsy. When in doubt, referral to a veterinary oncologist or dermatologist is preferable to repeated sampling.

Treatment Options for Equine Sarcoid

No single treatment works for every sarcoid. The choice depends on the type, location, size, number of lesions, and the horse's overall health. Treatment is often multimodal, combining surgery with an adjunct therapy to reduce recurrence risk.

Surgical Excision

Surgical removal is a common first-line approach for nodular and fibroblastic sarcoids that can be completely excised. The problem is recurrence. Simple excision alone has a reported recurrence rate of 30 to 50 percent [5][8]. This means that one in three to one in two sarcoids regrows after surgery, often more aggressively than before.

The high recurrence rate is due to microscopic extensions of the tumor that are left behind. Sarcoids do not have a clean capsule, and their tendrils can extend well beyond the visible edge. Surgeons often remove a wide margin of normal tissue around the tumor to reduce this risk, but this is not always possible on the face, eyelids, or genitalia.

Surgery is most effective when combined with an adjunct treatment such as cisplatin, cryotherapy, or laser ablation. These adjuncts target residual tumor cells and reduce the chance of regrowth.

Cisplatin

Cisplatin is a chemotherapy drug that is injected directly into the sarcoid or into the surgical site after excision. It kills dividing cells and is particularly useful for reducing recurrence after surgery. Cisplatin is often combined with sesame oil or another carrier to slow its release and reduce systemic toxicity.

Cisplatin-based protocols achieve higher efficacy and lower recurrence rates than BCG in many clinical settings [17]. It is commonly used for nodular and fibroblastic sarcoids, especially those on the face, eyelids, and genitalia where wide surgical margins are difficult to obtain.

Cisplatin is not a standalone cure. It works best as an adjunct to surgery or debulking. It can cause local tissue reaction, and repeated treatments may be needed. The number of treatments depends on the size and response of the lesion.

Bacillus Calmette-Guérin (BCG)

BCG is an immunotherapy that has been used for equine sarcoid treatment for decades. It activates the immune system through TLR2 and TLR4 signaling, macrophage polarization, and enhanced CD8+ T-cell responses, leading to partial or complete regression in some cases [17].

BCG is injected directly into the sarcoid or given after surgical excision. It is most effective for nodular and fibroblastic sarcoids. However, outcomes are highly variable due to differences in dose, strain, and protocol. Adverse inflammatory reactions are common, and some horses develop severe swelling or abscessation at the injection site [17].

BCG is now rarely considered a first-line therapy because modern alternatives such as cryotherapy, cisplatin, and topical imiquimod achieve higher efficacy and lower recurrence rates in many clinical settings [17]. It remains relevant in resource-limited situations or when other treatments have failed.

Cryotherapy

Cryotherapy uses liquid nitrogen to freeze and destroy sarcoid tissue. It is most effective for small, superficial lesions such as occult and verrucous sarcoids. Cryotherapy is often combined with surgical debulking or other treatments for larger lesions.

The success of cryotherapy depends on how deeply the freeze penetrates. Sarcoids that extend into the subcutaneous tissue may not be completely destroyed by surface freezing alone. Multiple freeze-thaw cycles improve efficacy. Cryotherapy is less useful for large or invasive lesions.

Laser Ablation

Laser therapy, typically with a carbon dioxide (CO2) laser, is used to vaporize sarcoid tissue layer by layer. It provides precise control and can be used on sensitive areas such as the eyelids and genitalia. Laser ablation is often combined with surgical excision or cisplatin.

Laser treatment can be effective for a range of sarcoid types, but recurrence is still possible if the tumor extends beyond the treated area. The laser seals blood vessels and nerve endings as it cuts, which reduces bleeding and pain compared to traditional surgery. It requires specialized equipment and training.

Topical Treatments

Topical imiquimod is an immune response modifier that is applied directly to the sarcoid. It is most effective for small, superficial lesions such as occult and verrucous sarcoids. Imiquimod stimulates the local immune response and can cause the tumor to regress over weeks to months.

Topical cisplatin in a carrier cream or ointment is another option for superficial lesions. It is applied directly to the tumor and surrounding skin. Topical treatments are less invasive than surgery and can be used on multiple lesions, but they require consistent application and may cause local irritation.

Immunotherapy and Vaccines

Several experimental immunotherapies are under investigation. Chimeric virus-like particles (CVLPs) of BPV1 L1-E7 have been tested in a phase I clinical trial. In that trial, 12 horses with an average of more than 22 tumors each were vaccinated. Two horses showed clear improvement, and one horse had regression of five sarcoids, though three relapsed during the study [8].

A more recent approach uses influenza virus vectors expressing BPV1 antigens. In a patient trial involving 29 horses, at least one lesion per horse was injected with the vaccine and then boosted. Complete tumor regression was achieved in 10 of 29 horses, and regression was ongoing in another 10 of 29 horses at the time of the report [18]. This treatment induced a systemic antitumor response, with regression of both injected and non-injected lesions [18]. These therapies are not yet widely available but represent promising future options.

Mermaid Diagram: Decision Path for a Suspected Sarcoid

The following flowchart shows the general decision path a veterinarian follows when evaluating a suspected sarcoid.

flowchart TD
    A[Suspect sarcoid lesion] --> B{Avoid biopsy}
    B --> C[Assess clinical type]
    C --> D{Occult or verrucous}
    C --> E{Nodular or fibroblastic}
    C --> F{Malevolent or mixed}
    D --> G[Topical or cryotherapy]
    E --> H[Surgery with cisplatin or laser]
    F --> I[Refer to specialist]
    G --> J[Monitor for recurrence]
    H --> J
    I --> J
    J --> K{Recurrence}
    K --> L[Reassess and consider multimodal therapy]
    K --> M[Continue monitoring]

Recurrence Rates and Treatment Success

Recurrence is the central challenge in sarcoid treatment. The reported recurrence rate after simple excision is 30 to 50 percent [5][8]. This figure is for surgery alone. When surgery is combined with cisplatin, laser, or cryotherapy, recurrence rates are lower, though exact percentages vary by study and lesion type.

BCG immunotherapy produces partial or complete regression in some cases, but outcomes are highly variable [17]. Modern alternatives such as cryotherapy, cisplatin, and topical imiquimod achieve higher efficacy and lower recurrence rates in many clinical settings [17].

The influenza vector vaccine trial reported complete regression in 10 of 29 horses, with ongoing regression in another 10 of 29 at the time of the report [18]. These results are encouraging but the treatment is still experimental.

Viral load may predict how a sarcoid will behave. Horses with quiescent, slowly growing single tumors had viral loads up to 1.4 copies per cell, while all equids with rapidly growing or multiple aggressive sarcoids had viral loads between 3 and 569 copies per cell [3]. This suggests that measuring viral load could one day help guide treatment decisions, though it is not yet a routine clinical test.

When to Refer to a Specialist

Referral to a veterinary oncologist, dermatologist, or surgeon is appropriate in several situations:

The sarcoid is on the eyelid, cornea, or genitalia, where surgery is technically difficult and recurrence can cause serious complications.
The sarcoid is malevolent or has spread along lymphatics.
The horse has multiple sarcoids (more than five to ten lesions) or lesions that are rapidly growing.
Previous treatments have failed or the sarcoid has recurred more than once.
The sarcoid is causing pain, bleeding, difficulty eating, urinating, or breathing.
The diagnosis is uncertain and a biopsy might be needed.

A case of a recurrent penile sarcoid in a Falabella gelding ended in euthanasia three months after partial phallectomy due to urinary retention from suspected regrowth [10]. This outcome underscores the importance of early referral for sarcoids in sensitive locations.

Practical Implications for Owners

If you find a lump, patch, or warty growth on your horse, do not try to remove it yourself. Do not apply over-the-counter creams or ointments without veterinary guidance. Do not allow anyone to take a biopsy or needle sample without a veterinarian's assessment, because trauma can worsen the lesion.

Take photographs of the lesion with a ruler or coin for scale. Note when you first noticed it and whether it has changed in size, shape, or color. Share this information with your veterinarian.

If your horse has a history of sarcoids, monitor the skin regularly, especially around previous treatment sites, the face, neck, groin, and genitalia. Early detection of recurrence makes treatment easier.

Fly control and good hygiene may reduce the risk of spread, though the evidence for this is not definitive. Do not share grooming equipment, tack, or blankets between horses with sarcoids and horses without them.

What Is Still Uncertain

The exact mechanism of BPV transmission between horses remains unclear. The role of ovine papillomaviruses in sarcoid development is an active area of research [12][13]. The genetic factors that make some horses more susceptible are not fully identified. The best treatment protocol for each sarcoid type has not been established through large randomized trials. Viral load testing is not yet a standard clinical tool, though it shows promise as a prognostic indicator [3].

The relationship between viral gene expression and clinical behavior is complex. One study found that viral DNA copy number did not differ between subtypes but was significantly higher in animals with fewer tumors [1]. Another study found that nodular sarcoids had the highest viral load and gene expression [9]. These findings suggest that the interaction between the virus and the host immune response, rather than viral load alone, determines how a sarcoid behaves.

Limitations and When to Contact a Veterinarian

This article is educational and is not a substitute for veterinary diagnosis or treatment. Every sarcoid case is different, and treatment decisions should be made by a licensed veterinarian who has examined the horse.

Contact a veterinarian promptly if:

A skin lesion is growing rapidly, bleeding, or ulcerating.
A sarcoid is located near the eye, mouth, nostril, anus, or genitalia.
Your horse has difficulty eating, breathing, urinating, or defecating.
A previously treated sarcoid regrows.
New lesions appear after treatment.
Your horse seems painful or distressed.
You are unsure whether a lesion is a sarcoid or something else.

Do not wait for a sarcoid to become large or invasive before seeking help. Early intervention is almost always easier and more successful than treating an advanced lesion.

Frequently Asked Questions

What is the difference between equine sarcoid and sarcoidosis?

Equine sarcoid is a skin tumor caused by bovine papillomavirus. Sarcoidosis is a separate systemic disease that affects humans and some animals, with no relationship to equine sarcoid.

What causes equine sarcoid?

Equine sarcoid is caused by infection with bovine papillomavirus types 1 and 2. Genetic susceptibility in some horse families also plays a role.

Can equine sarcoid spread to other horses?

BPV DNA has been found in blood cells of infected horses, and transmission between horses is considered possible but is not fully understood.

Why should a sarcoid not be biopsied?

Trauma from a biopsy can trigger explosive growth of a sarcoid. Veterinarians usually diagnose sarcoids by appearance alone to avoid this risk.

What is the recurrence rate after sarcoid surgery?

Simple excision alone has a reported recurrence rate of 30 to 50 percent. Combining surgery with cisplatin, laser, or cryotherapy reduces this risk.

What is the most aggressive type of equine sarcoid?

Malevolent sarcoid is the most aggressive form. It invades local tissues and lymphatics and can be fatal.

Can equine sarcoid be cured?

Many sarcoids can be successfully treated, but recurrence is common. Some sarcoids, especially malevolent ones, are difficult to cure completely.

When should a horse with a sarcoid be referred to a specialist?

Referral is appropriate for sarcoids on the eye or genitalia, malevolent or multiple lesions, recurrent sarcoids, or any sarcoid causing pain or functional problems.