Unusual aspect of cuticular morphology in human pulmonary dirofilariasis

A 79-year-old woman was found to have a slowly enlarging pulmonary nodule in the right upper lobe, and partial lung resection was performed for diagnostic and therapeutic purposes. Histopathological examination revealed fragments of a filarial worm within the nodule. The parasite showed features compatible with Dirofilaria immitis, including a thick multilayered cuticle, abundant somatic musculature, and prominent internal longitudinal ridges projecting into the lateral cords. However, the external cuticular surface showed localized ridge-like elevations, making distinction from Dirofilaria ursi difficult on morphology alone. A partial sequence of the mitochondrial 12S rRNA gene was amplified from formalin-fixed, paraffin-embedded tissue and analyzed phylogenetically. The sequence clustered within the D. immitis clade and was clearly separated from D. ursi and Dirofilaria repens. This case highlights a diagnostic problem in tissue-based identification of pulmonary dirofilariasis and demonstrates the value of molecular analysis when histopathological morphology is ambiguous.