Melanoma in horses: distribution by sex, age, nationality, and slaughterhouse findings in 28 horses from Northern Italy

Abstract

Melanomas are dermal-epidermal and subcutaneous tumors commonly found in horses, especially those with grey coats. This study aimed to characterize melanomas in slaughtered horses by analyzing their distribution according to sex, age, and nationality, and to describe the veterinary inspection interventions required at slaughterhouses based on lymph node involvement and distant metastases in a province of northern Italy. Between January and December 2024, 182 grey horses were examined, and 28 cases of melanoma were identified (15% of grey horses) with an average age of 14 years: 8 males (mean age 13 years) and 20 females (mean age 15 years). The nationalities represented were 21 French, 2 Hungarian, 2 Italian, 2 Slovenian, and 1 Polish. The most commonly affected sites were the base of the tail and perineal area (26 cases), followed by the cardiac muscle, and trachea. Nineteen cases were classified as dermal melanomas, 5 as melanocytomas, 2 as anaplastic melanomas, and 2 as noncutaneous melanomas; 2 had lymph node metastases, 14 localized lesions, and 12 distant metastases. Eighteen horses underwent tumor removal with carcass trimming, while 10 required complete carcass condemnation. This study provides information on the clinical behavior of melanomas in slaughtered horses and the interventions required for meat processing during postmortem inspection in slaughterhouses.

Introduction

Melanomas are the most common dermal–epidermal and subcutaneous tumors in horses, especially in grey-coated horses (Andersson 2024; MacKay 2019). They account for approximately 6–15% of all diagnosed equine skin tumors (Pimenta 2023). Although the incidence of these neoplasms increases with age (Hollis 2024; Phillips 2013), cases have also been reported in young horses (Pimenta 2023).

In grey horses, melanoma typically presents as pigmented skin nodules that vary in size and shape (Pimenta 2023). These nodules are often located in heavily pigmented areas, such as the head, neck, anus, and genitals [https://ceh.vetmed.ucdavis.edu/health-topics/melanoma-horses]. Some horses may develop visceral melanomas, which can impair the function of internal organs such as the intestines and kidneys [https://madbarn.com/melanomas-in-horses]. In horses examined at slaughterhouses, melanomas typically present as firm, black, discrete, and raised nodules on the skin, particularly in the perianal area (beneath the tail) and on the lips and eyelids. These lesions vary in size and number, with early melanomas appearing as small bumps that may coalesce into larger, more aggressive masses over time. In some cases, ulcerated tumors may bleed and exude black, tarry material (van der Weyden 2020).

Common clinical signs include behavioral changes, weight loss, lethargy, and difficulty swallowing, particularly when melanomas develop near the digestive tract (Foreman 2024). Advanced or large perianal and perirectal tumors can impede defecation, leading to conditions such as constipation and straining, due either to extra-luminal obstruction of the rectum or to severe pain and discomfort (Haegeman 2025; Diakakis 2021). In mares, melanomas located around the vulva may also cause reproductive problems [https://equinesarcoid.co.uk/melanoma-in-horses]. In some cases, melanomas affect internal organs such as the liver, spleen, and lungs, producing signs like respiratory distress or digestive disturbances, although relatively few horses present with such symptoms [https://ceh.vetmed.ucdavis.edu/health-topics/melanoma-horses].

Melanoma in horses is diagnosed through clinical examination of characteristic black, firm lumps or nodules, particularly in grey horses, followed by fine-needle aspiration or biopsy of suspected lesions (Metcalfe 2013). A veterinarian then analyzes the tissue sample to confirm the diagnosis, differentiate melanoma from other conditions, and determine whether the tumor is benign or malignant (McNeil 2023; Campagne 2012). Imaging techniques such as ultrasound and CT scans may also be used to evaluate the size and extent of internal masses.

Treatment options for equine melanoma, in addition to surgical excision, include immunotherapy with the Oncept melanoma vaccine, cryotherapy with liquid nitrogen for superficial lesions, various chemotherapy approaches (intralesional, topical, or systemic), laser therapy using carbon dioxide or diode lasers, and newer modalities such as nanosecond-pulse irreversible electroporation. These treatments are often combined, with the optimal approach depending on factors such as the tumor type, size, and location, as well as the horse’s age and overall health (MacKay 2019).

This study aimed to characterize melanomas in slaughtered horses by analyzing their distribution according to sex, age, and nationality, and by describing the slaughterhouse veterinary inspection interventions required in relation to lymph node involvement and distant metastases.

Materials and methods

The study was based on the analysis of 4,212 horses slaughtered in 2024 at a certified slaughterhouse in the Province of Reggio Emilia, Northern Italy, under the supervision of the Veterinary Service of the Department of Public Health. Of these, 182 (4.3%) were grey horses directly intended for slaughter, originating from EU countries, including France, Hungary, Slovenia, Italy, and Poland. The nationality of the animals was recorded from the passport (release document), which coincided with the nationality of breeding, and from the traceability documents of entry into Italian territory.

Approximately 100 animals arrived at the slaughterhouse per week. They were inspected while alive and, after slaughter, a second inspection was conducted to confirm lesions observed antemortem. During postmortem inspection at the equine slaughterhouse, specific symbols indicated whether visual inspection was sufficient or if further evaluation by palpation and incision was required for each pathology. For each pathology confirmed by the Istituto Zooprofilattico, the regulatory reference justifying exclusion from human consumption was provided, along with the classification of the meat as Animal By-Products (ABP) according to Regulation (EC) No 1069/2009.

Carcasses contaminated with feces, ingesta, hair, or dirt were identified, and the affected parts were removed as Category 2 ABPs. Equine liver and kidneys were always excluded from human consumption because of cadmium risk. European rules require inspection of muscles and lymph nodes in the shoulders of grey horses for melanosis and melanomas, including incision of the kidneys. If health risks were identified during inspections, the veterinarian performed additional checks, including kidney palpation and possible laboratory tests.

Tissue samples from melanocytic lesions were collected at the slaughterhouse, refrigerated, and sent to the Zooprophylactic Institute (ZI) of Reggio Emilia for gross evaluation and fixation in formalin. After 24 h, the samples were forwarded to the ZI of Parma for paraffin embedding and standard histological processing. For each sample, two 4-µm-thick tissue sections were prepared, stained in parallel with hematoxylin and eosin, and subjected to potassium permanganate bleaching to facilitate light microscopic examination. The histological findings reported in the study were derived from the literature (Pimenta 2023; Mauldin 2016) and supplemented with the principal observations made during the histological examination of the cases under study, with references provided to support their classification.

The veterinary pathologist classified and diagnosed the lesions histologically as follows:

• Melanocytoma: Non-encapsulated, well-demarcated dermal neoplasms composed of multifocal aggregates of polygonal or spindle cells supported by fine fibrovascular and/or myxoid stroma. Neoplastic cells occasionally contained granular black cytoplasmic pigment and exhibited mild anisocytosis, anisokaryosis, and cellular pleomorphism;

• Dermal melanoma: Non-encapsulated dermal or subcutaneous neoplasms of variable size, well-demarcated and occasionally infiltrative, composed of round to polygonal neoplastic cells with prominent nucleoli and heavily pigmented cytoplasm, accompanied by numerous associated melanomacrophages;

• Anaplastic melanoma: Infiltrative neoplasms with poorly defined margins and potential vascular invasion. These tumors were characterized by nests and cords of epithelioid or spindle cells supported by delicate fibrovascular stroma. Neoplastic cells displayed variable amounts of black cytoplasmic pigment, marked nuclear pleomorphism, multiple nucleoli, severe anisocytosis and anisokaryosis, and a high mitotic rate, and are characterized by a greater propensity to give internal metastases.

Results

In 2024, 28 horses with melanoma were identified, representing 15% of 182 grey horses examined (Table 1). The mean age was 14.2 years (range 1–27 years): 8 males (mean age 13 years) and 20 females (mean age 15 years). By nationality, 21 were French, 2 Hungarian, 2 Italian, 2 Slovenian, and 1 Polish. Lesions were located at the base of the tail and perineal area in 26 cases, in the cardiac muscle in 1 case, and in the trachea in 1 case. Morphologically, 19 were diagnosed as dermal melanomas, 5 as melanocytomas, 2 as anaplastic melanomas, and the remaining 2 as melanomas with noncutaneous localization.

Table 1.

Veterinary service of the Province of Reggio Emilia, 2024. Characteristics of the 28 horses diagnosed with melanocytic tumors

ID	Sex	Age (years)	Nationality	Topography	Morphology
1	F	13	French	skin of the tail	dermal melanoma
2	F	17	French	skin of the tail	melanocytoma
3	M	10	Hungarian	skin of the tail	dermal melanoma
4	F	13	French	skin of the tail	dermal melanoma
5	M	21	Italian	skin of the tail	melanocytoma
6	F	20	French	skin of the tail	dermal melanoma
7	F	3	Polish	skin of the tail	melanocytoma
8	F	15	French	skin of the tail	dermal melanoma
9	M	2	French	skin of the tail	dermal melanoma
10	F	20	French	skin of the tail	anaplastic melanoma
11	M	4	Slovenian	skin of the tail	anaplastic melanoma
12	F	1	Slovenian	skin of the tail	melanocytoma
13	M	18	French	skin of the tail	dermal melanoma
14	M	12	French	skin of the tail	dermal melanoma
15	M	21	French	skin of the tail	dermal melanoma
16	F	1	Hungarian	skin of the tail	melanocytoma
17	F	13	French	skin of the tail	dermal melanoma
18	F	18	French	heart	melanoma
19	F	19	French	skin of the tail	dermal melanoma
20	M	16	French	skin of the tail	dermal melanoma
21	F	18	French	trachea	melanoma
22	F	12	French	skin of the tail	dermal melanoma
23	F	20	French	skin of the tail	dermal melanoma
24	F	27	French	skin of the tail	dermal melanoma
25	F	12	French	skin of the tail	dermal melanoma
26	F	17	French	skin of the tail	dermal melanoma
27	F	21	French	skin of the tail	dermal melanoma
28	F	15	Italian	skin of the tail	dermal melanoma

According to EU Regulation 2019/627 Article 22, postmortem inspection in horses was performed visually, with palpation and incision used when necessary to establish a definitive diagnosis. During carcass inspection, the kidneys were examined visually: in all 28 cases, no kidney was submitted for further investigation since the lesions were located only around the serosa and the organ showed no pathognomonic signs.

Regarding the postmortem procedure (Table 2), neoplastic removal by trimming was performed in 18 horses, while complete carcass condemnation was required in 10 horses. Of the 28 cases, 2 had lymph node metastases, 14 had localized lesions, and 12 had distant metastases.

Table 2.

Veterinary service of the Province of Reggio Emilia, 2024. Presence of lymph node metastasis and type of procedure in 28 horses diagnosed with melanocytic tumors

ID	Sex	Age (years)	Nationality	Lymph node	Metastasis	Procedure
1	F	13	French	–	–	tumor removal
2	F	17	French	–	–	tumor removal
3	M	10	Hungarian	–	–	tumor removal
4	F	13	French	–	yes	carcass condemnation
5	M	21	Italian	–	yes	carcass condemnation
6	F	20	French	–	yes	carcass condemnation
7	F	3	Polish	–	–	tumor removal
8	F	15	French	–	yes	carcass condemnation
9	M	2	French	–	yes	carcass condemnation
10	F	20	French	–	yes	carcass condemnation
11	M	4	Slovenian	yes	–	carcass condemnation
12	F	1	Slovenian	–	–	tumor removal
13	M	18	French	–	yes	carcass condemnation
14	M	12	French	–	yes	tumor removal
15	M	21	French	–	yes	tumor removal
16	F	1	Hungarian	–	–	tumor removal
17	F	13	French	–	–	tumor removal
18	F	18	French	–	yes	carcass condemnation
19	F	19	French	–	yes	tumor removal
20	M	16	French	yes	–	carcass condemnation
21	F	18	French	–	–	tumor removal
22	F	12	French	–	–	tumor removal
23	F	20	French	–	–	tumor removal
24	F	27	French	–	–	tumor removal
25	F	12	French	–	–	tumor removal
26	F	17	French	–	–	tumor removal
27	F	21	French	–	yes	tumor removal
28	F	15	Italian	–	–	tumor removal

Distant metastases were observed in the mandible (Fig. 1A), with infiltration of cellular elements forming thin ulcerated bundles, as well as in the myocardium, left lung, left rib cage, and subscapular vertebra (Fig. 1C). The mandibular lesion originated from the lower lip commissure and involved the trachea, myocardium, scapular bones, and lumbosacral region (ilium and sacrum), and the carcass was therefore condemned. In the live animal, a black, globular growth approximately 7 cm in diameter, with a firm, lobated consistency, was identified in the right abdominal region (Fig. 1B). Local metastases appeared as black nodules ranging in diameter from 5 mm to approximately 2 cm. These lesions were visible on the cardiac surface, infiltrating the myocardium and protruding into the ventricular wall (Fig. 1D); in the myocardium, trachea, aortic arch, and right lung (Fig. 1E); and in the gluteal muscle, biceps femoris, and coccygeal vertebrae (Fig. 1F). Additional lesions were also observed in the vertebrae and the epiglottis.

Fig. 1.

Locations of distant, locoregional, and tail base metastases. A: Mandible B: Right abdomen C: Myocardium, left lung, left rib cage, and subscapular vertebra D: Myocardium E: Myocardium, trachea, aortic arch, and right lung F: Gluteal muscle, biceps femoris, and coccygeal vertebrae G: Anal orifice H: Ventral glabrous portion of the tail I: Gluteal muscle, anal orifice, and rectum

For Figs. 1G-I, the lesions originated from the anal orifice and the ventral glabrous area, involving both the superficial and deep muscles of the tail.

Discussion

Melanomas in horses, particularly grey ones, commonly occur in the perineal area, tail, and genitalia. They may result from age-related melanin accumulation rather than true malignancy, affecting up to 80% of grey horses by 15 years of age (Metcalfe 2013).

This study investigated the prevalence, anatomical distribution, and pathological characteristics of melanomas in grey horses slaughtered in Northern Italy, as well as their implications for veterinary inspection and meat safety regulations. Among the 182 grey horses examined, 28 (15%) were diagnosed with melanoma—a prevalence consistent with previous studies reporting rates between 6% and 15% (Pimenta 2023). The affected horses had a mean age of 14.2 years, with a slight sex-related variation (13 years in males and 15 years in females), consistent with earlier research showing a higher frequency of melanoma in older grey horses (Hollis 2024; Phillips 2013; Robert 2025).

The most commonly affected anatomical site was the perineal region, specifically the base of the tail (93%), followed by less frequent occurrences in visceral organs such as the heart and trachea. Morphologically, dermal melanoma was the predominant lesion type (75%), with melanocytomas (18%) and anaplastic melanomas (7%) occurring less often. Notably, three cases were identified in horses under five years of age, all originating from non-French countries, suggesting that although melanoma is primarily age-related, early-onset cases do occur, as previously reported in the literature (MacKay 2019). Metastases were observed in 13 cases (46% of melanoma-affected horses), including two with lymph node involvement and 11 with distant metastases. These findings reinforce existing evidence that equine melanoma, although often slow growing, can progress to malignancy, particularly in older horses (Phillips 2013; Seltenhammer 2014). The metastatic sites observed in this study included the lungs, heart, vertebrae, and epiglottis, highlighting the systemic potential of the disease. The presence of lymph node or distant metastases significantly influenced veterinary inspection outcomes: localized tumors were managed by trimming and local excision, whereas all cases with widespread metastases resulted in carcass condemnation, in accordance with EU Regulations 2019/627, Article 45(f) and (o) and Article 22(m) and (j).

From a veterinary pathology and public health perspective, distinguishing between benign and malignant melanocytic lesions is crucial. Progression from melanocytoma to malignant melanoma has been reported in the literature (Moore 2013), underscoring the need for careful inspection even of lesions not overtly malignant.

This study highlights the role of veterinary inspectors in applying EU Regulation 2019/627, which allows localized excision of neoplastic lesions but requires full carcass condemnation in cases of metastasis. It also underscores the need for targeted postmortem inspections in grey horses, with particular attention to high-risk areas such as the perineum, tail base, and lymph nodes to ensure food safety. Furthermore, the research emphasizes the importance of early surveillance of equine melanomas. Despite their slow growth, approximately 75% of benign lesions eventually progress to malignancy. Raising awareness among veterinarians, owners, and regulators can support early detection and intervention before metastasis occurs. (Phillips 2013).

Although melanomas in grey-horses are often considered “benign” because of their slow growth and encapsulation, many gray horses—particularly those with the G/G genotype—develop multiple tumors early in life that later metastasize. In fact, two-thirds of affected horses show metastases at necropsy, regardless of clinical signs (Brodesser 2025).

Conclusion

This study strengthens the argument for a standardized protocol for equine melanoma assessment in slaughterhouses. Implementing a systematic approach—including histopathological analysis, where appropriate—could enhance food safety and public health while also advancing our understanding of equine melanoma progression. Further studies evaluating potential biomarkers or genetic screening methods may contribute to early detection strategies, with benefits for both veterinary oncology and equine welfare.

Author contributions

Conceptualization, investigation, writing—original draft, visualization, supervision, GM.MI.; investigation and supervision A.P.; writing—review and editing, and visualization, I.B.; investigation and supervision G.DL.; investigation, supervision G.P.; investigation, supervision G.DA.; investigation, supervision A.P.; conceptualization, writing—original draft, investigation, and supervision, L.M. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partially supported by the Italian Ministry of Health - Ricerca Corrente Annual Program 2026.

Data availability

The data supporting the findings of this study are available on request from the corresponding author.

Declarations

Ethics statement

Not applicable.

Informed consent statement

Not applicable.

Conflicts of interest

The authors declare no conflict of interest.

Competing interests

The authors declare no competing interests.