The following is reported by Schaffer et al (2013)
# % Sarcoid 1,543 46 SCC 633 19 SCC in situ 28 1 Melanoma 344 10 Soft tissue sarcoma 238 7 Squamous papilloma 189 6 Benign soft tissue 114 3 Fibroma 80 2 Lipoma 21 1 Hemangioma 7 .2 Leiomyoma 3 0.1 Myxoma 3 0.1 Mast cell tumor 99 3 Lymphoma 57 2 Giant cell tumor 35 1 Hemangiosarcoma 20 1 Epithelial tumors 51 0.4 Basal cell tumor 39 1 Carcinoma 8 0.2 Apocrine tumor 4 0.1
Schaffer PA, Wobeser B, Martin LE, Dennis MM, Duncan CG. Cutaneous neoplastic lesions of equids in the central United States and Canada: 3,351 biopsy specimens from 3,272 equids (2000-2010). J Am Vet Med Assoc. 2013 Jan 1;242(1):99-104. doi: 10.2460/javma.242.1.99. PMID: 23234288.
There are 6 different clinical presentations of sarcoid - they all look similar histologically
- occult
- verrucose
- nodular
- fibroblastic
- mixed
- malignant
Bovine papillomavirus-1 (Deltapapillomavirus) and sometimes BPV-2 are external factors in the development of sarcoid. In the Karalus et al (2024) the majority were BPV-2.
Horses dont have the immunological ability to remove infection.
About half of the horses, especially those in contact with cattle, have BHV-1 DNA in normal skin.
The characteristic changes histologically are an infiltrative stromal tumor with proliferation of fibroblastic cells in contact with an epidermis that has prominent rete ridges that insert into the fibroblastic cells.
Karalus et al (2024) studied 49 cases, and examined reoccurrence. They found that 12 of the 49 recurred after surgical excision. Of the following features evaluated (mitotic count, cellularity, necrosis, nuclear pleomorphism, and inflammation) only mitotic count correlated with recurrence. Four of 5 sarcoids with a mitotic count of greater than 20 in 2.37 mm2 recurred This was significantly higher sarcoids with a count of less than 20 where 8 of 44 cases recurred. No allowance for the histological tumor free distance of surgical margins was done. They divided that cases into complete and incomplete excision, and most were called incomplete. As most clinicians do marginal excisions, perhaps this is relevant to marginal excisions.
Karalus W, Subharat S, Orbell G, Vaatstra B, Munday JS. Equine sarcoids: A clinicopathologic study of 49 cases, with mitotic count and clinical type predictive of recurrence. Vet Pathol. 2024; 61: 357-365.
Ogłuszka M, Starzyński RR, Pierzchała M, Otrocka-Domagała I, Raś A. Equine Sarcoids-Causes, Molecular Changes, and Clinicopathologic Features: A Review. Vet Pathol 2021; 58: 472-482.
The horse has a propensity to develop vascular tumors that are interesting excercises in characterisation. Vascular hamartoma, hemangioma and angiomatosis are all reported, and often the photomicrographs look similar. Wise et al (2021) published a detailed discussion of one such tumor in the horse.
Wise LC, Crosby DE, Walton AN, Neef A, Shearer PL, Woodward DJ, Hilbert BJ. A tumour of vascular origin in a horse. Equine Vet Educ 2021; 33: e353-e356.
Schöniger and Summers reported on 2 horses with nerve sheath tumors - neurofibroma. Their criteria were:
- high and poor cell areas – Antoni A and B
- nuclear pallasading
- Verocay bodies
- Hyalinised microvessels
- Nerve fibres at edge or within mass
- S100 + strong and diffuse in Schwann cells
- Schwannomas have typical fusiform cells
Subtypes are localized or plexiform (multinodular) then classic, collagenous, cellular and pigmented.
Neurofibromas have buckled or wavy nuclei – S100 cells are Schwann cells which are part of the tumour.
Are composed of Schwann cells, neurofibrocytes and perineural cells.
Subtypes are localized, plexiform or diffuse (infiltrative) then then classic, collagenous, cellular and pigmented.Schöniger S; Summers BA(2009) Localized, Plexiform, Diffuse, and Other Variants of Neurofibroma in 12 Dogs, 2 Horses, and a Chicken. Vet Pathol 2009 46: 904-915.
Sullins KE. Melanocytic tumours in horses. Equ Vet Educ 2020; 12:624-630