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Adult stages of Dirofilaria repens (Nematoda, Filarioidea) reside in the subcutaneous tissues of the definitive or occasional host as dogs, other animals, and humans, and it is transmitted by mosquitoes. Canine infections with adults and circulating larvae of D. repens are often considered asymptomatic, although in some cases, the parasite causes subcutaneous nodules, diffused dermatitis, skin lesions, and itching. This report provides a complete clinical description of an unusual case of allergic diffused dermatitis caused by D. repens in a naturally infected dog and its successful treatment with the use of a spot-on solution containing imidacloprid 10%/moxidectin 2.5%. The dog presented multiple pustules and alopecic areas with lichenification, hyperpigmentation, and erythematous scaling margins without pruritus. Histological examination was compatible with allergic dermatitis. After being unsuccessfully managed for suspected food hypersensitivity, with a significantly worsening of the lesions, a Knott’s analysis detected nematode larvae in the blood. Morphological and molecular identification showed them to be D. repens. The dog was then treated with a single administration of a spot-on formulation containing imidacloprid 10%/moxidectin 2.5%, and the dermatological signs completely resolved within 2 months after treatment. The dog showed no recurrence of the lesions, and no circulating microfilariae were found upon microscopic and molecular examination for six consecutive months after treatment. This report indicates the apparent primary role of D. repens in causing hypersensitivity-like skin disease without pruritus in a dog. It also confirms, as recently shown elsewhere, the efficacy of imidacloprid 10%/moxidectin 2.5% in the treatment of dermatitis caused by D. repens.