Summary

Equine sarcoids are likely caused by bovine papillomavirus. Sarcoid‐affected horses are capable of transmitting the virus, often by flies, to other horses to cause development of sarcoids. Early signs that a sarcoid is developing within a wound are dehiscence, unhealthy appearance of granulation tissue, lack of wound contraction, and inability to control exuberant granulation tissue (EGT). Sarcoids in a wound are difficult to diagnose histologically because sarcoid tissue is often admixed with granulation tissue. Pathologists unfamiliar with the histologic appearance of sarcoids often misdiagnose the lesion. Recurrence of a sarcoid after simple surgical excision is common. Other treatments that may offer better success than excision include: cryotherapy; laser surgical excision; immunotherapy in the form of cell‐wall extracts of mycobacteria or subcutaneous implantation of sarcoid tissue; repeated topical application of a cytotoxic drug, such as cisplatin, administered alone or in combination with electric‐impulse therapy, the anti‐mitotic drug, 5‐fluorouracil, or an antiviral ointment, such as acyclovir or imiquimod; and radiation therapy. Sarcoid contamination of wounds can be prevented by covering wounds, if possible, and by fly control.

Introduction

Horses have been affected by sarcoids for centuries,¹ but the condition was first described by Jackson in 1936.² Sarcoids occur in all equine populations, with a prevalence varying between 0.6% and 12%.³ This unique, benign fibroblastic tumor is, by far, the most prevalent skin neoplasm of horses. Analyses of records from veterinary diagnostic pathology laboratories showed that 24% of all tumor biopsy specimens and 46% of biopsy specimens obtained from cutaneous neoplasms contained tissue determined to be a sarcoid when examined histologically.^4,5 Other equidae, including donkeys, mules, and zebras, are also affected.³ Conversely, the role of the sarcoid in failure of wound healing has only recently come to light.⁶

The equine sarcoid has the following six different clinical manifestations:

1. Occult (flat) sarcoid: the mildest and most superficial form is called the occult form. It is characterized by a vague (often circular) change in the color or density of the hair coat. The epidermis is thickened, and there is invariably at least one small nodule within the affected area.
   
2. Nodular sarcoid: the nodular form is solid and often spherical. Lesions of this type can vary widely in size and number; some involve the dermis, whereas others are entirely subcutaneous.
   
3. Verrucose sarcoid: the verrucose form is “warty” in appearance and usually ill defined. The size can vary widely from limited zones to extensive plaques.
   
4. Fibroblastic sarcoid: the fibroblastic form is fleshy and exophytic in appearance and is usually exudative and infected. Lesions can be pedunculated or sessile with a broad base. The latter type is that most commonly involved in problems with healing and is often similar in appearance to EGT.
   
5. Mixed sarcoid: mixed sarcoids show characteristics of several of the other types and are relatively common, occurring spontaneously or in association with certain types of wounds.
   
6. Malignant (malevolent) sarcoid: the malignant form is a rare and very dangerous expanding invasive tumor. It can arise spontaneously and may also develop when a milder form (most often the fibroblastic type) is subjected to repeated trauma (including biopsy). Although this form is rare, it can develop in sarcoid‐contaminated wounds, especially if repeated surgeries are performed (usually in response to excision of tissue presumed to be EGT).

Occult and nodular forms seldom complicate wound healing, except when these forms are subjected to incomplete surgical removal or are traumatized.

Etiopathogenesis

There is extensive evidence to indicate that equine sarcoids are caused by bovine papillomavirus 1 (BPV‐1) and BPV‐2, which are associated with transient, benign warts in their natural bovine host. Some investigators have speculated, however, that cattle‐to‐horse viral transmission does not result in development of sarcoids, because the majority of sarcoids evaluated harbored an equine‐specific version of the BPV not found in cattle.⁷ Conversely, stabling of sarcoid‐affected horses with unaffected horses has been shown to result in the transmission of BPV and development of sarcoids.⁸ Using polymerase chain reaction (PCR) techniques, viral DNA has been found in sarcoid tissue and in intact, normal skin of sarcoid‐affected horses, but not in the skin of unaffected horses.⁹ In one report, however, DNA of BPV was found in normal skin of horses unaffected with sarcoid but exposed to sarcoid‐affected horses (i.e., latent infections).¹⁰ Similarly, the proportion of horses having BPV DNA (73%) or BPV RNA (64%) identified in their skin samples was found to be the same in horses affected with sarcoids and in healthy horses living in contact with cattle having recently suffered BPV infection.¹¹ Furthermore, BPV DNA (30%) and BPV RNA (20%) were also detected in healthy, control horses. In a recent study, severity of disease, characterized by the number and growth rate of sarcoids, nevertheless, showed a highly significant correlation with intralesional viral load.¹² Altogether, these findings have implications concerning the diagnosis of sarcoids using PCR.

Some horses appear to be genetically predisposed to develop sarcoid. The incidence of sarcoids in Quarter Horses is nearly twice that of Thoroughbreds, and the incidence in Standardbreds is half that of Thoroughbreds.¹⁰ Predisposition to sarcoids appears to be strongly associated with particular major histocompatibility complex (equine leukocyte antigen) haplotypes¹³ and may be related to suppressed local immunity.¹⁴

Because sarcoids tend to develop in wounds, flies attracted to wounds may act as vectors of either viral particles or viral‐infected fibroblasts. The infection can be transmitted from horse to horse or from one site to another on the same horse. Indirect transmission of BPV may also occur via tack or grooming equipment.¹⁰

Horses have a reputation for developing non‐healing wounds, particularly those involving the distal aspect of the limb (carpus/tarsus and distally to these joints). The vast majority of equine wounds, however, heal if inhibitory factors, such as infection, movement, necrotic tissue, foreign body, etc., are removed. When a wound is affected by a sarcoid, however, the healing inevitably becomes suspended, often indefinitely. Sarcoid transformation of wounds is, therefore, a serious complication associated with cutaneous wounds in horses.⁶

Fibroblastic sarcoids and verrucose sarcoids are the usual types of sarcoid encountered as a complication of wound healing.^6,15–17 Sarcoids developing in wounds on the trunk tend to be verrucose and to remain superficial, whereas those on the limbs are likely to be the invasive, fibroblastic type.³

Transformation of a wound into a sarcoid often goes unrecognized, mainly because of the similar appearance of the sarcoid tissue to granulation tissue or a keloid/hypertrophic scar. Treatment of a wound affected by sarcoid transformation is difficult because consistently effective treatments are lacking. This chapter focuses primarily on the clinical features of sarcoid transformation and the challenges of treating those wounds in which sarcoid is identified.

Any type of wound is susceptible to transforming into a sarcoid, and even a small wound, such as a skin puncture at an injection site or caused by an insect bite, can be transformed into a non‐healing, sarcoid‐contaminated wound. Surgical wounds can easily be infected with BPV, either naturally or by iatrogenic transfer of BPV‐infected tissue into the wound during surgery (e.g., a castration site).

The time required for a sarcoid to develop at a wound after infection is extremely variable. In some cases, the transformation is obvious early in healing, whereas for other wounds, transformation occurs much later, even long after the wound appears to have healed (e.g., months or years).

Sarcoid transformation within a wound can occur in the following ways:

• Injury (accidental or surgical) to sarcoid‐affected skin. In this case, a sarcoid is wounded, and if the wound extends into normal skin adjacent to the tumor, the entire wound is susceptible to sarcoid tranformation (Figure 21.1).
  
• Development of a sarcoid in a wound in normal skin when there are sarcoids at other sites (Figure 21.2). In this case, the virus or cells from the sarcoid are transmitted from the tumor into the wound. Cellular transfer from a sarcoid to a wound at another site can easily take place, and the “autograft” of cells results in a high probability of cellular survival in the wound. Vector transmission provides the only plausible explanation for this transfer because there is no evidence to support hematogenous dissemination of either sarcoid‐transformed cells or virus or genomic particles of the virus.¹⁸
  
• Sarcoid transformation occurring at a wound when there are no sarcoids at other sites, but the host has contact with horses affected with sarcoids (Figure 21.3). In this situation, cells or particles of virus from the sarcoid of one horse are transferred into the wound of another horse, probably by a vector. Alternatively, the horse may have a latent infection of BPV, and epithelial damage activates the virus to cause formation of a sarcoid.
  
• A wounded horse is in contact with a fomite, such as tack or grooming equipment, containing the BPV.¹⁰
  
• Sarcoid can be transferred iatrogenically into a surgical site when surgical instruments (or even hypodermic needles) are used at a sarcoid site and then used for a surgical procedure at another site (Figure 21.4).
  
• Failure to remove all sarcoid cells during surgical excision of a sarcoid. In this case, residual sarcoid is responsible for recurrence. Rapid growth or transformation of the sarcoid is common under these circumstances, even after harvesting a sample from a sarcoid (Figure 21.5). The time required for recurrence is unpredictable. Wound dehiscence, failure of the wound to epithelialize, a lack of wound contraction, or any combination of these complications may indicate that the sarcoid has recurred.

Larger wounds may be more susceptible to the development of sarcoids than smaller wounds in the following situations: (1) when normal skin is wounded and sarcoids are present at other sites; (2) when normal skin is wounded, and the horse is in contact with horses that have sarcoids; or (3) when normal skin is wounded and the horse is in contact with a source of BPV (a vector or fomite). Small wounds, such as a wound created by a needle puncture (Figure 21.4), heal more quickly than large wounds, thereby decreasing the risk of prolonged exposure to vectors for transmission of virus or sarcoid cells. The proximity of a sarcoid to a wound can influence the likelihood of the wound undergoing sarcoid transformation (Figure 21.6), but this may depend on the feeding behavior of insects.

Flies have been implicated in the epidemiology of sarcoids; they may act as vectors for the BPV particles, but it seems far more likely that the sarcoid‐transformed cell itself is transferred from a sarcoid to a wound (Figure 21.7). In this event, the location of the wound has an important bearing on the type of sarcoid that develops. Transfer of sarcoid cells to a wound on a limb (e.g., elbow/stifle and distal regions) tends to induce formation of a fibroblastic sarcoid (Figure 21.8), whereas a wound on the body and neck more commonly develops into a verrucose sarcoid (Figure 21.9).^6,17

Warm, still weather results in a large population of flies and far more fly activity on the horse. Flies frequently feed on sarcoids, especially if the sarcoids are ulcerated. Sarcoid cells seem to have a better chance of survival if they are transferred, by a fly, from a sarcoid near a wound than from a sarcoid remote from it (Figure 21.7).

When a sarcoid is diagnosed on the limb of a horse, a wound (ranging from a minor abrasion to a full‐thickness laceration) very likely preceded the sarcoid; sarcoids rarely develop spontaneously on the limb distal to the femorotibial joint (stifle)/cubital joint (elbow) in the absence of cutaneous trauma.

Clinical aspects

A general medical history of the horse and a specific history of the wound should be obtained at the outset. Establishing if the horse has other sarcoids or has had sarcoids is important. Even if previous treatment for a sarcoid was successful, the horse is likely genetically prone to develop other sarcoids. The presence or history of sarcoids affecting in‐contact horses is also important. Given the familial tendencies to develop sarcoids,¹⁹ information concerning sarcoid history of related horses may be particularly important. Horses with sarcoids at other sites are far more likely to develop a sarcoid in a wound than are those without a sarcoid. Close contact of a wounded horse with a sarcoid‐affected horse may be an important factor in contamination of a wound with sarcoid cells. Wounds occurring during summer (i.e., fly season) are far more likely to develop a sarcoid, and the longer the wound remains unhealed or uncovered, the greater is the opportunity for sarcoid transformation to take place (Figure 21.10).

A physical examination of the wound is important because factors that inhibit wound healing, such as infection, movement, impaired local blood supply, necrotic tissue, or foreign bodies, may also facilitate transformation of the wound into a sarcoid. When sarcoids are detected at other sites on the wounded horse, transformation of a wound into a sarcoid becomes likely.

Identification

Early stages of sarcoid transformation of a wound are often impossible to recognize. Granulation tissue and bacterial pyogranuloma can appear remarkably similar to a wound that is partially or exclusively sarcoid (Figure 21.11a‐c). Likewise, a keloid or hypertrophic scar can easily be mistaken for a verrucose sarcoid and vice versa (Figure 21.11d,e).

Establishing whether or not a sarcoid is involved in a non‐healing wound is important because the management of EGT and sarcoids is very different. Martens et al.²³ described a PCR method to identify sarcoid that is based on the presence of BPV DNA, collected by swabbing the suspected sarcoid lesion; this test may be particularly valuable for detecting sarcoid in non‐healing wounds.³ The test is unsuitable for detecting occult‐type sarcoids,²³ however, and due to high prevalence of BPV DNA in normal equine skin, the test may result in a false‐positive diagnosis.³ Presence of any type of sarcoid lesion at other sites on the body should immediately alert the clinician to the possibility of sarcoid transformation in a non‐healing wound.²⁴

Differential diagnosis of sarcoid‐contaminated wounds

The differential diagnosis of sarcoid transformation is complicated because there may be admixtures of granulation tissue, scar tissue, and infection within the wound. In some cases, sarcoid transformation is quite obvious to an experienced clinician. The majority of sarcoids in a wound, however, are difficult to diagnose. The differential diagnoses for equine sarcoids in wounds include the following:

• (exuberant) granulation tissue (Figure 21.11b);
  
• staphylococcal pyogranuloma (pseudomycetoma) (Figure 21.11c);
  
• oomycotic or fungal granuloma (e.g., Pythium spp., Conidiobolus coronata, etc.);
  
• foreign body granuloma;
  
• keloid scars, which can closely resemble verrucose sarcoids (Figure 21.11e).

Various features help to differentiate these clinical conditions (Table 21.1).

Pathology

Histologic examination of the tissues excised from a normal healing wound characteristically reveals organizing granulation tissue with a well‐recognized parallel arrangement of new blood vessels (see Figure 1.7). The healing process whereby normal granulation tissue is formed has been outlined in Chapter 1, and certainly it is well recognized by pathologists. Although samples obtained from a chronic, granulating, non‐healing wound may contain some possible causative factors (e.g., bacterial or fungal infection, foreign matter, necrotic tissue, etc.), the normal underlying structure of the granulating wound remains invariably recognizable.

The histologic appearance of sarcoids, both within a wound and at other sites, is well documented.²⁵ There is little difference in the histologic features of sarcoids in wounds and those arising de novo (i.e., without the history of a wound at the site), except where admixtures of sarcoid and granulation tissue are present within the wound. Histologic characteristics of a sarcoid include fibroblastic proliferation, with fibroblasts arranged in a whorling pattern (Figure 21.12). Random arrangement of fibroblasts is also common. The fibroblasts themselves can show anisocytosis (abnormal variations in cell size) and anisokaryosis (abnormal variation in nuclear size in the cells). The number of mitotic figures varies but usually is low or rare. The abundance and features of the extracellular matrix also vary.

The absence of overlying epithelium in tissues taken from an ulcerated sarcoid can complicate diagnosis. The first descriptions of the equine sarcoid were made by Jackson, who, in 1936, identified a variable epithelial component to the typical sarcoid.² Skilled pathologists usually have little difficulty in recognizing the full range of sarcoid types, even in the absence of epidermis. The diagnosis, however, depends heavily on the harvest of a truly representative sample of tissue, and this is often a major problem when sarcoid and granulation tissue are admixed.

Samples obtained by shaving off the surface of a granulating wound bed may show infected, ulcerated granulation tissue only. A far better representation of the true nature of the non‐healing tissues is obtained when a core biopsy sample is taken from the depth of the tissue. Deep core biopsy samples may reveal sequential “layers” of sarcoid and granulation tissue or nests of sarcoid tumor cells. Sampling of only one tissue type or inappropriate histologic preparation of the sections may, therefore, be deceptive; a single small sample may show only sarcoid, only pyogranuloma, or only granulation tissue. Because treatment of a non‐healing wound based on results of a single, small sample may be inappropriate, several deep biopsy samples using a wide biopsy punch (6–8 mm) should be harvested to maximize the chances of an accurate diagnosis.

Biopsy technique is important. The site should not be scrubbed or otherwise molested. Desensitizing the wound with local anesthetic solution is usually not necessary because both sarcoid and granulation tissue are free of nerve endings. If the wound must be desensitized, a regional nerve block, if possible to perform, is preferred to local infiltration of the site with an anesthetic solution, which may cause inflammation. A suspicious area of granulation tissue, such as a nodular region denser than surrounding tissue, should be sampled. When a preferred site for harvesting cannot be identified, several 6‐mm to 8‐mm biopsy punch samples should be taken from the central portion of the tissue, and each sample placed in an individual container of 10% formalin. Photographing the biopsy sites is useful so that the pathologist can see the appearance of tissue sampled. Samples taken from the skin margin are not usually diagnostic; moreover, taking samples from the margin is ill advised, because doing so encourages the tumor to extend further into normal skin. Small lesions may be subjected to excisional biopsy with the precautions outlined later in Treatment/management. Verrucose changes in wound sites are harvested in the same way, although for many of these samples, the skin edge can be included.

The value of histologic examination cannot be overstated because findings influence the therapy and the prognosis. The concurrent presence of a sarcoid and granulation tissue within a wound occurs with some frequency. Sarcoids inhibit healing, regardless of the amount of sarcoid tissue found in the wound. When “granulation tissue” is removed as a part of wound management, it should be submitted for histopathologic examination. Because of the variations in matrix type and staining characteristics of sarcoid tissue and inexperience of the pathologist, sarcoid tissue may be erroneously identified as fibroma, fibropapilloma, neurofibroma, neurofibrosarcoma, Schwannoma, low‐grade fibrosarcoma, or spindle‐cell tumors of various types.