Concurrent occurrence of malignant melanoma and amyloidosis in a horse.

In a 16-year-old mare of mixed breed, with gray coat, which was used for 2 years for production of polyvalent anti-snake antiserum, some nodules appeared on the underside of the tail and perineum. The animal gradually became weakened and showed weight loss and eventually, was euthanatized. Based on necropsy findings and histopathologic examination, affection to malignant melanoma was confirmed. By applying Congo red for detection of amyloidosis, deposition of amyloid within the wall of arterioles of spleen and a small amount inside the hepatocytes were observed.