A 4-year-old female thoroughbred race horse died of acute peritonitis cause
d by necrotizing granulomatous duodenitis. Yersinia enterocolitica was immi
mohistochemically demonstrated in macrophages in granulomas developed in th
e duodenum, lung, liver and abdominal lymph nodes. The yeast-like fungi wer
e found in the cytoplasmic vacuoles of macrophages in the lung that infiltr
ated into the granulomas and surrounding alveoli with congestive edema. The
yeast-like fungi were positively stained by Gomori-Grocott chromic acid me
thenamine silver stain and immuno-histochemically stained with anti-histopl
asma antibody. In this case, it was considered that granulomas formed in th
e duodenum, lung, liver and abdominal lymph nodes were primarily caused by
Yersinia enterocolitica due to idiopathic weakening of the immune system. Y
east-like fungi immunohistochemically identified as histoplasmas secondaril
y infected the lung. This is the first case regarded as equine histoplasmos
is capsulati in Japan.