Circulating melanin-containing cells and neutrophils with phagocytized melanin granules in a horse with disseminated melanoma.

This research deals with the diagnosis and exploration of a rare incident of widespread malignant melanoma in an 18-year-old Thoroughbred Cross gelding horse. In this case, unique features were detected, such as circulating neutrophils containing melanin granules.

Presenting Symptoms and Clinical Examination

• The horse under consideration was referred to the Cummings School of Veterinary Medicine at Tufts University due to a low-grade fever of unknown cause, limb swelling, and weight loss over three weeks.
• The initial physical examination led to finding black, smooth nodules on the lower side of the horse’s tail.

Ultrasound and Blood Count Observations

• An ultrasound was performed revealing an enlarged spleen with different textured content, nodules in the liver, as well as signs of peritonitis and fibrin deposition.
• The blood count showed mature neutrophilia, a high concentration of mature white blood cells. Some of the neutrophils contained granules that didn’t react to Perl’s Prussian blue staining but responded positively to Fontana-Mason staining, which is indicative of melanin presence.

Necropsy Results

• Post-mortem examination of the body showed that the spleen filled approximately one-third of the abdominal cavity and it was hard with copious black pigmentation.
• Blackness was noticed in the marrow space of the 18th thoracic vertebra.
• The tracheobronchial, mediastinal, and splenic lymph nodes were approximately five times larger than normal size and were heavily pigmented.
• As a result of these findings, the ultimate anatomical diagnosis was widespread malignant melanoma with extensive spleen involvement and vascular and hemolymphatic dissemination of the neoplasia.

Significance of the Findings

• This study contributes to veterinary medicine by being the first to report neutrophils in circulation with melanin-containing granules in the context of widespread melanoma.
• This finding helped confirm the diagnosis of melanoma before the horse’s death, establishing a new diagnostic marker for the disease.
• The observations made in this study may be useful for future cases and could potentially deepen our understanding of how melanoma proliferates within the body, particularly in horses.