Occult sarcoid represent the earliest form of the disease. Although they are early forms, it does not necessarily mean that they have been a recent occurrence. Some of these lesions remain static for years without any clinical implication. The occult sarcoid can occupy large areas, or be very small and subtle
These sarcoids usually appear as a roughly circular hairless area, or an area that has altered hair quality. Occasionally the only change may be subtle changes in the hair coat colour, thickness and density. The later forms of occult sarcoid usually appear as grey hairless, often circular or roughly circular areas. Careful palpation of the lesion will usually reveal one or several tiny nodules in the skin in the affected area. The skin may feel slightly thickened and lacking in its normal elasticity. As with all sarcoids, they can occur at any body site however are rare on the lower leg region.
They can be confused with rub marks from tack/rugs, as well as many skin conditions such as: Ringworm, Alopecia Areata, Pemphingus Foliaeceus and Dermal/epidermal Naevus.
Verrucous sarcoids usually have a grey, scaly or warty appearance (hence the term verrucous, verrucose or wart-like sarcoid). This is the origin of the original and very misleading term equine warts! Individual lesions can be small and ‘papilloma-like’ extensions from skin (very similar to warts), but frequently they coalesce into larger lesions and sometimes extend over extensive areas. The majority of early verrucous lesions have an occult halo. Usually there is alopecia (loss of hair) but sometimes the hair is sparse and has thinner character when compared to normal hair. Sometimes small shot-like, solid nodules can be felt within them. The skin lacks normal flexibility and may crack easily.
The surface of most verrucous lesions is scaly with a lot of flaking and dense keratin / dandruff like change. Flakes and scale are easily rubbed from the surface. However, some are simply scaly in texture. There may be some localised ulceration of the surface exposing red, fleshy tissue underneath.
As with all sarcoids they can occur anywhere on the body. Lesions on the lower leg, especially around the coronary band, can occur but are rare. The Verrucous sarcoid can be mistaken for many other conditions such as: Ringworm, Alopecia Areata, Pemphingus Foliaceus, Cheloid/Hypertrophic scar at a wound site, or Hypertrophic skin as a result of repeated trauma or rubbing.
Nodular sarcoids are easily recognisable. They have discrete, firm, defined nodules under the skin and are commonly located in the eyelid, axilla (armpit), inner thigh and groin regions. This type of sarcoid can be of variable size and be single or multiple. Some have interlocking nodules giving an appearance like a bunch of grapes. Bleeding exudative masses can form from these sarcoids when they ulcerate.
There are two types of nodular sarcoids; Type A which have no skin involvement, meaning that the skin can be moved freely over the surface; and Type B which are firmly attached to the overlying skin, meaning that the skin cannot be moved over the surface of the nodule.
Type A nodular
These are characterised as firm nodular structures lying under the intact skin – there is no skin involvement. They lie deep beneath the skin and have quite thick skin overlying the sarcoid, however can still be extensive. This may be an explanation for their depth and common failure to erupt through the skin, or influence the skin in a secondary fashion (thinning or verrucous/occult change or hair loss). Occasionally the overlying skin may become hairless/thinned and may break open. The lesion then presents as an ulcerated nodule, or may fall away spontaneously leaving a haemorrhagic cavity. Extensive groups of nesting nodules that may be linked together can develop in one or more locations.
Pathologically the type A nodule has a loose fibro-cellular capsule that appears to confine the lesion to its immediate area. The capsule shields the tumour from the skin. Historically these tumours were referred to as fibroma and fibroma durum. Many pathologists are unhappy to make the diagnosis of sarcoid in these cases because the pathologically distinctive (pathognomonic) feature of palisading of the basal epidermal cells and formation of rete pegs with hyperkeratosis are not present at all. However, trauma to these lesions will quickly result in a more typical sarcoid pathology.
Type A1 nodules can be moved freely and independently over the tissues beneath and around. This means that the nodule (or group of nodules) has no attachment to the skin and no significant attachments to the surrounding and deep tissues. Surgical treatment or ligation of these is relatively easy and safe.
Type A2 nodules have deep attachments to the underlying tissues and therefore, although the skin overlying it can be moved independently the nodule cannot be moved independently of the deep tissues. These cannot be surgically removed or ligated. Recurrence is very common.
Type B nodular
These are characterised by having visible and/or palpable skin involvement i.e. the skin cannot be moved independently of the tumour; when the skin is moved the tumour moves with it. This type of sarcoid can develop from a type A nodule, especially if interfered with, however they can develop spontaneously over time. Sometimes the overlying skin is alopecic (hairless) and scaly (verrucous). Some also ulcerate over larger or smaller areas to expose a fleshy bleeding surface. This progression can be very rapid. Once these lesions become ulcerated they often develop rapidly into fibroblastic or malignant lesions. Pathologically, these nodules have poorly defined margins. This is consistent with their ‘fixed’ nature and lack of independent mobility.
Eyelid nodules of this type are particularly dangerous – it has been shown that they have a major propensity to deep and extensive ‘roots’ that permeate the muscles of the eyelids. This is why eyelid tumours are potentially VERY dangerous and should not be interfered with ill-advisedly. Professional help must be obtained as soon as there is any suspicion of changes.
Type B1 nodules have no deep attachments.
Type B2 nodules are involved with the surrounding tissues and structures. They have a deeper attachment to the underlying structures and then the lesion has a bound-down feel to it. This represents a very serious form of the disease.
The fibroblastic sarcoid is fleshy and aggressive in appearance.
They can conveniently be divided in to two groups. They sometimes have a narrow pedicle (type 1) or they can be broadly based with no pedicle (type 2).
Type 1 Fibroblastic
These can be further classified as Type 1a where there is no root extension beyond the pedicle; or Type 1b where there is narrow pedicle but a root or foot exists in the attached body part. This is vital for treatment options. Removal of the external mass of a type 1a fibroblastic sarcoid would result in a cure, but removal of the bulk external mass from a type 1b would be a potential disaster!
Type 2 Fibroblastic
Also known as sessile or rooted, the type 2 fibroblastic sarcoid is a very serious form and treatment is always problematic. This is because the roots are broadly based, highly penetrating and can extend far beyond what is visible. They are prone to bleeding easily. Because of this, these are particularly attractive to flies who like to surface-feed on the lesion. This can cause considerable surface ulceration and further infection. They are common at all sites and may develop quickly from a milder form following damage, they commonly develop at the site of skin injuries on the limbs.
Treating this form early is critical to get the sarcoid more manageable before the fly season starts and thus trying to prevent it being spread between animals or to other sites on the same horse.
Many lesions are actually mixtures of the other types. Mixed sarcoid is a common diagnosis. Where possible this classification should be avoided – the predominate type should be used instead, but there are circumstances when a predominate type is not readily identifiable.
- These include variable mixtures of two or more of the other types.
- If a diagnosis of mixed sarcoid is made then the corollary should be to state the predominate type present.
- Often common in long standing lesions or where there is repeated minor trauma (e.g. tack or harness rubbing or where inappropriate treatment has been attempted).
Malignant sarcoid is applied to any lesion where there is extensive local or wider spread through the skin and subcutaneous tissues. It is rare. This is the most aggressive type in which the tumours spread extensively through the skin with cords of tumour tissue interspersed with nodules and ulcerating fibroblastic lesions. There are often some overlying verrucous and occult changes in the skin. The skin need not be broken and there may be no superficial ulceration, especially in the elbow region and the flank fold regions. Usually, affected horses have multiple sarcoids at other sites but isolated malignant lesions can develop; this is commonest at the point of the elbow, the medial thigh and the side of the face.
There is no recorded case in which the sarcoid has spread to internal organs and so, until recently, the term malevolent was used to try to differentiate it from the metastatic disseminating tumours also referred to as ‘malignant’. Quite commonly the term is wrongly applied to some large lesions – the size is not the important aspect – it’s the extent of local infiltration and extension that defines the type.