11/08/2026
A 54-year-old woman from Croatia was evaluated for persistent peripheral eosinophilia, intermittent migratory skin rashes, fatigue, and myalgia. Her clinical history included the surgical removal of a subcutaneous nodule from her left upper arm, which was later found to contain parasitic structures on histopathological examination.
Despite several investigations, the underlying infection was not immediately identified. It was only after further parasitological evaluation that microfilariae were detected in her peripheral blood.
The subsequent investigation revealed several important findings:
• Microfilariae were repeatedly detected over a four-month period (August–December 2024).
• Modified Knott's testing demonstrated microfilarial concentrations of 1.0 and 0.33 microfilariae/mL in October and December, respectively.
• Molecular testing through cytochrome c oxidase subunit I (COI) gene sequencing confirmed the parasite as Dirofilaria repens.
• Wolbachia endosymbionts, which are associated with Dirofilaria species, were also detected in the tissue samples and microfilariae.
The patient received doxycycline therapy (100 mg twice daily for 14 days). Ivermectin was considered but was ultimately not administered. During subsequent follow-ups, microfilariae were no longer detected, and her eosinophil count returned to within the reference range.
𝗪𝗵𝗮𝘁 𝗺𝗮𝗸𝗲𝘀 𝘁𝗵𝗶𝘀 𝗰𝗮𝘀𝗲 𝘂𝗻𝗶𝗾𝘂𝗲?
Dirofilaria repens is a zoonotic, mosquito-transmitted filarial nematode primarily associated with dogs, which serve as its principal reservoir hosts. Humans are traditionally considered accidental or dead-end hosts because the parasite generally does not establish sustained microfilarial production in humans.
Human infections most commonly manifest as subcutaneous nodules or ocular dirofilariasis, although other anatomical sites, including the lungs, lymph nodes, skeletal muscles, and reproductive tract, have also been reported.
Microfilaremia in humans is particularly uncommon. Previous reports have documented occasional detection of microfilariae, but these were generally transient or identified at a single time point.
What distinguishes this report is the repeated detection of circulating microfilariae over several months, providing the first confirmed documentation of persistent microfilaremia in a human infected with D. repens.
𝗜𝗺𝗽𝗹𝗶𝗰𝗮𝘁𝗶𝗼𝗻𝘀 𝗳𝗼𝗿 𝘃𝗲𝘁𝗲𝗿𝗶𝗻𝗮𝗿𝘆 𝗺𝗲𝗱𝗶𝗰𝗶𝗻𝗲
This finding raises important questions about the host-parasite relationship and challenges the traditional assumption that humans are exclusively dead-end hosts. However, it does not establish that humans can routinely serve as reservoir hosts capable of sustaining onward transmission.
The authors emphasize the importance of a One Health approach, integrating human medicine, veterinary medicine, and vector surveillance to better understand the epidemiology of D. repens.
For veterinarians, this is another reminder of the public health implications of canine vector-borne parasites. Dogs remain important reservoir hosts, making appropriate prevention, diagnosis, and surveillance essential not only for animal health but also for reducing zoonotic transmission.
Follow VetEd on Facebook and Instagram for more veterinary updates.
𝗥𝗲𝗳𝗲𝗿𝗲𝗻𝗰𝗲
Vranjković, M.P., et al. (2025). Not a Dead-End Host: First Confirmed Persistent Microfilaremia in Human Dirofilaria repens Infection. Microorganisms, 13, 2263.
