Retrospective Review With Genetic Analysis of Canine Microfilarial Dermatitis in the Western United States.

Background: Microfilarial dermatitis was described once in dogs of the western United States. The organisms were not identified.

Hypothesis/objective: To identify nematodes as a cause of dermatitis and describe clinical features, treatments and therapeutic responses.

Animals: Eight client-owned dogs with suspected or definitively diagnosed microfilarial dermatitis based on appropriate clinical history and therapeutic response had skin biopsy samples collected.

Materials and methods: Retrospective review of electronic medical records taken between January 2010 and December 2022 from a multicentre dermatology speciality group in the United States for dogs, followed by PCR and genome sequencing of microfilariae from formalin-fixed tissue.

Results: Pruritus (eight of eight), plaques (five of eight) and lesions of the head (six of eight) were commonly reported. Four had microfilariae on histopathological evaluation. One sample was genetically consistent with Cercopithifilaria bainae; another was consistent with Onchocerca lupi. Two formalin-fixed paraffin-embedded samples were not available for sequencing. Dogs received oral pulse dosing of fenbendazole (Panacur; Merck Animal Health) (mean 41 mg/kg) at 10-day intervals for 4 weeks and injectable ivermectin (Agri-Mectin; AgriLabs) (mean 0.42 mg/kg) given orally once weekly for 6 weeks. Three dogs had adverse drug events. One dog had recurrent clinical signs after experiencing full resolution. Complete resolution without recurrence was reported in two of eight dogs. The times to complete resolution were 63 days (Dog 5) and 65 days (Dog 3).

Conclusions and clinical relevance: This is the first confirmed report of clinical dermatitis secondary to O. lupi microfilariae in a dog, and the second clinical description of canine C. bainae microfilarial dermatitis in the United States. Paired ivermectin and fenbendazole may be an appropriate treatment for suspected or definitively diagnosed canine microfilarial dermatitis.