[Pulmonary dirofilariasis found unexpectedly during thymectomy].

A 56-year-old woman in Kitakyushu City was operated on after diagnosis of mediastinal tumor with myasthenia gravis. Besides the thymoma, a small nodule was palpated in the lower lobe of the left lung and biopsied. A segmented worm was found in sections of this granulomatous nodule. A cross section of the degenerated worm (400 X 310 micron) showed a 3-layered tegument and internal cuticular ridges with high lateral chords. The muscle layer consisted of high and polymyarian muscles. Based on these characteristics, this worm was identified as an immature dog heartworm, Dirofilaria immitis. The recent sharp increase of dirofilariasis cases reported in Japan is discussed.