18-FDG Uptake in Pulmonary Dirofilariasis

Solitary pulmonary nodules are a common finding on chest radiography and CT. We present the case of an asymptomatic 59-year-old male found to have a 13 mm left upper lobe nodule on CT scan.  The patient was asymptomatic and the CT was performed to follow up mediastinal and hilar lymphadenopathy that had been stable on several previous CT scans.  He had a history of emphysema and reported a 15 pack-year smoking history.  PET-CT was performed which demonstrated mild 18-FDG uptake within the nodule. Given his age and smoking history, malignancy was a consideration and he underwent a wedge resection.  Pathological examination revealed a necrobiotic granulomatous nodule with a central thrombosed artery containing a parasitic worm with internal longitudinal ridges and abundant somatic muscle, consistent with pulmonary dirofilariasis. Dirofilaria immitis, commonly known as the canine heartworm, rarely affects humans.  On occasion it can be transmitted to a human host by a mosquito bite.  There are two major clinical syndromes in humans: pulmonary dirofilariasis and subcutaneous dirofilariasis.  In the pulmonary form, the injected larvae die before becoming fully mature and become lodged in the pulmonary arteries.