Nodular lesions due to infestation by Dirofilaria repens in dogs from Italy

Background: Canine subcutaneous dirofilariosis caused by Dirofilaria repens is an important cause of human dirofilariosis in the Old World.

Objectives: To describe the clinical and pathological features of infestation by D. repens in dogs with cutaneous nodules and to discuss key diagnostic procedures.

Animals: Sixteen privately owned dogs of different breeds, ages and genders with subcutaneous nodules were positive for D. repens on at least one diagnostic test.

Methods: Diagnostic tests included cytological examination of fine-needle aspiration biopsies, detection of circulating larvae by Knott's test, morphological and molecular identification of adult parasites and microfilariae, and histopathological examination of excised nodules.

Results: Twelve animals were positive for D. repens microfilariae, using cytology and at least one other diagnostic test. Four of twelve dogs screened by Knott's test had circulating D. repens microfilariae, and three of them were co-infected with Dirofilaria immitis. In eight dogs, adult nematodes were morphologically identified as D. repens. Nodular panniculitis was observed in all cases, with nodules containing either one or several cross-sections of nematodes and microfilariae. The nematodes extracted were identified as D. repens by an RT-PCR on the second internal transcribed spacer (ITS-2).

Conclusions and clinical importance: Adults and larvae of D. repens can induce skin nodules in dogs, and these lesions should be carefully evaluated in endemic areas. Clinicians should be aware that correct tissue sampling (including the storage of part of a sample in ethanol instead of formalin) is pivotal to achievement of a specific diagnosis. Cytology of fine-needle tissue aspirates was shown to be useful.