Dirofilaria repens is a parasitic roundworm transmitted by mosquitoes that lives beneath the skin of dogs, its primary host, and increasingly turns up in humans across Europe, Asia, and parts of Africa. Unlike its better-known relative Dirofilaria immitis, which lodges in the heart and lungs, D. repens takes up residence in subcutaneous tissue, forming nodules that can appear almost anywhere on the body. What makes this parasite worth paying attention to is how quickly its range has expanded in recent decades, showing up in countries and latitudes where it had never been recorded before.
How the Parasite Gets Around
D. repens depends on mosquitoes to complete its life cycle. An infected dog (or other canid) carries microscopic larval forms called microfilariae in its bloodstream. When a mosquito feeds on that dog, it picks up microfilariae, which then develop inside the mosquito through several larval stages. Once the larvae reach the infective third stage (L3), they migrate to the mosquito’s mouthparts and get deposited into the next animal the mosquito bites.
The range of mosquito species that can serve as competent vectors is broader than researchers once assumed. Studies have confirmed that Aedes japonicus and Aedes geniculatus, two species common in temperate Europe, can support full development of D. repens to the infective L3 stage. In one laboratory trial, infective larvae appeared as early as nine to ten days after mosquitoes fed on infected blood, and vector efficiency indices for D. repens reached high levels in multiple mosquito populations tested.1PubMed Central. Development of Dirofilaria immitis and Dirofilaria repens in Aedes japonicus and Aedes geniculatus Field surveys in Turkey have identified D. repens DNA in the head-thorax regions of several additional species, including Culex pipiens, Aedes caspius, and Anopheles maculipennis, confirming they can carry infective-stage larvae ready for transmission.2PubMed. Potential Mosquito Vectors of Dirofilaria immitis and Dirofilaria repens in Aras Valley, Turkey The invasive Asian tiger mosquito (Aedes albopictus), which has spread aggressively across southern Europe in recent years, has also been flagged as a vector of growing concern.3PubMed Central. Global Research Trends in Emerging Zoonosis Due to (the Filarial Nematode) Dirofilaria repens (1955-2025)
What It Does to Dogs
Dogs are the primary definitive host, meaning adult worms can mature, mate, and produce microfilariae that circulate in the blood, keeping the transmission cycle going. Infections are often clinically silent. Many dogs carry the parasite for months or years without showing obvious signs, which is precisely what makes them such effective reservoirs: a seemingly healthy dog can be shedding microfilariae into the blood that mosquitoes then pick up and spread.4PubMed Central. Occurrence of Dirofilaria repens in wild carnivores in Poland
When clinical signs do appear, they tend to involve the skin. Dogs may develop subcutaneous nodules, each containing one or several adult worms surrounded by inflammatory tissue.5PubMed. Nodular lesions due to infestation by Dirofilaria repens in dogs from Italy Some cases progress to more widespread skin disease, including diffuse dermatitis with pustules, areas of hair loss, thickened and darkened skin, and scaling.6PubMed. Allergic dermatitis by Dirofilaria repens in a dog: clinical picture and treatment Itching is sometimes present, sometimes absent, which adds to the diagnostic challenge. These dermatological signs can easily be mistaken for allergies, bacterial skin infections, or other parasitic conditions, so veterinarians in endemic areas need to keep D. repens on their differential list. Co-infections with D. immitis also occur, particularly in regions where both parasites circulate.7PubMed. Nodular lesions due to infestation by Dirofilaria repens in dogs from Italy
Cats as Accidental Hosts
Cats can get infected, but they are far less hospitable to the parasite than dogs are. European guidelines describe cats as “imperfect hosts” for dirofilarial worms. After a mosquito deposits infective larvae, only a small fraction develop to adulthood, and only a small percentage of exposed cats end up harboring mature worms.8PubMed Central. Dirofilarioses in cats: European guidelines from the ABCD on prevention and management When adult worms are found, they sometimes turn up in unexpected locations. In one case from Romania, a five-year-old cat undergoing routine neutering was found to have two adult D. repens worms inside the scrotum, one female measuring about 12.5 cm and one male about 6.5 cm. That cat also had circulating microfilariae, confirming a patent infection.9PubMed. First report of Dirofilaria repens infection in a microfilaraemic cat from Romania A similar finding was reported in a stray cat in Spain, where two female worms were removed from the spermatic cord during a trap-neuter-release procedure, and circulating microfilariae were confirmed by multiple tests.10PubMed Central. First report on natural infection by Dirofilaria repens in a cat in Spain
These feline cases tend to be reported as “firsts” for a given country, which suggests either that cat infections are genuinely rare or that nobody was looking hard enough until recently. The practical takeaway for cat owners in endemic regions is that prevention matters for cats too, even though they are not the parasite’s preferred host.
How Humans Get Infected and What Happens
Humans get D. repens the same way dogs do: a mosquito carrying infective larvae bites them. In people, the parasite has traditionally been considered unable to complete its life cycle. A single worm typically migrates through subcutaneous tissue and eventually dies, triggering an immune response that walls it off into a nodule. Most human cases involve exactly that: a firm lump under the skin, sometimes painless, sometimes itchy or tender, that shows up weeks to months after the initial mosquito bite.
Nodules have been reported on limbs, the trunk, the face, the breast, and the genitals. In one case, a 16-year-old boy in India presented with forearm swelling that had been growing for two months; imaging revealed a parasitic cyst, and surgical removal confirmed D. repens.11PubMed Central. Human Subcutaneous Dirofilariasis A young woman living in Rome developed a subcutaneous nodule on her left arm after what she initially took for an insect bite, which evolved from a red wheal into a firm nodule over several weeks. Ultrasound revealed a characteristic ribbon-like structure with parallel hyperechoic lines inside the lesion.12PubMed Central. A case of human Dirofilaria repens in Rome, Italy: A clinical and radiological challenge A case in Austria presented as a completely painless subcutaneous nodule with the diagnosis confirmed only after excision and histological examination.13PubMed. A case of human Dirofilaria repens infection, causing an asymptomatic subcutaneous nodule Scrotal nodules have also been documented, where histology revealed a degenerating roundworm inside an abscess cavity surrounded by fibrotic tissue and inflammatory cells.14PubMed Central. Dirofilariasis in Male Genitalia: A Case Report and Literature Review
Because these nodules can mimic tumors, cysts, or abscesses, they often prompt imaging studies and sometimes unnecessary worry about cancer before the worm is identified. Surgical excision is generally both diagnostic and curative for subcutaneous cases.
When the Worm Reaches the Eye
The eye is the other body site where D. repens frequently turns up in humans, and these cases tend to be the most alarming for patients and clinicians alike. The worm can lodge under the conjunctiva, where it is sometimes visible as a thin, coiled structure beneath the transparent tissue covering the white of the eye. It can also migrate into the deeper orbital tissues. Reported complications include swelling of the eyelid or lacrimal gland, inflammation of the uvea or sclera, glaucoma, and in rare instances, intraocular dirofilariasis.15PubMed Central. Subconjunctival human dirofilariasis by Dirofilaria repens in the Mediterranean Basin
In one Spanish case, a 43-year-old man presented with acute redness at the edge of his right eye. Examination revealed a large nematode beneath the conjunctiva, and the intact worm was surgically removed and identified as a mature but infertile female D. repens. Serum antibody levels were monitored for six months after surgery and confirmed parasitological cure.16JAMA Ophthalmology. Subconjunctival Infection With Dirofilaria repens: Serological Confirmation of Cure Following Surgery As with subcutaneous cases, surgical removal of the worm is the standard treatment for ocular dirofilariasis. There is no approved drug regimen for killing the worm in situ inside the eye, so getting it out physically is the priority.
Are Humans Really a Dead-End Host?
For decades, the standard teaching was that humans are dead-end hosts for D. repens: the parasite can arrive and form a nodule, but it cannot produce circulating microfilariae, so it cannot be picked up by another mosquito and spread further. This assumption has been challenged. Scattered case reports have documented microfilariae in human blood or tissue samples. In one case from Italy, fine-needle aspiration of a deep-seated swelling near the iliac region pulled out numerous microfilariae. The larvae were initially mistaken for a different filarial species before molecular analysis confirmed them as D. repens.17PubMed Central. Dirofilaria repens microfilariae from a human node fine-needle aspirate: a case report
More recently, evidence of persistent microfilaremia in a human patient has been documented, raising the uncomfortable possibility that at least some people could function as more than dead-end hosts and could theoretically contribute to the transmission cycle.18PubMed Central. Not a Dead-End Host: First Confirmed Persistent Microfilaremia in Human Dirofilaria repens Infection This remains a rare finding, and it would be premature to say humans are significant reservoirs. But it does complicate the public-health picture and underscores why surveillance for human cases matters beyond just treating the individual patient.
The Wolbachia Connection
Like many filarial nematodes, D. repens carries an intracellular bacterial symbiont called Wolbachia. These bacteria live inside the worm’s tissues and appear to be essential for the parasite’s survival and reproduction. What makes Wolbachia clinically relevant is that it also drives part of the inflammatory response that the host mounts against the worm. When worms die and break down, Wolbachia are released into surrounding tissue, triggering immune reactions.
Immunohistochemical staining of skin nodules from 14 human patients showed numerous Wolbachia bacteria within intact worms and some positive staining in the surrounding inflammatory tissue. Blood tests from 11 of those patients came back positive for antibodies against the Wolbachia surface protein.19PubMed. Wolbachia in Dirofilaria repens, an agent causing human subcutaneous dirofilariasis This relationship has a practical therapeutic angle: antibiotics that target Wolbachia, particularly doxycycline, can weaken or kill filarial worms by eliminating their bacterial partner. In at least one human case, a patient was treated successfully with doxycycline alone when they were reluctant to use the standard antiparasitic drug ivermectin.20PubMed Central. Case Report: Successful Treatment of a Patient with Microfilaremic Dirofilariasis Using Doxycycline
How D. repens Is Diagnosed
In dogs, the most accessible screening tool is the modified Knott’s test, which concentrates microfilariae from a blood sample so they can be examined under a microscope. Morphometric analysis of the microfilariae allows reliable distinction between D. repens, D. immitis, and other filarial species that may be circulating in the same region.21PubMed Central. Morphometric analyses of canine blood microfilariae isolated by the Knott’s test enables Dirofilaria immitis and D. repens species-specific and Acanthocheilonema genus-specific diagnosis PCR-based molecular assays offer more definitive species identification and can be applied to blood samples, excised tissue, and even formalin-preserved specimens.22Parasitology International. Improved detection of Dirofilaria repens DNA by direct polymerase chain reaction
In humans, the diagnosis is usually made after surgical excision. A lump gets removed, and the histopathologist identifies the characteristic thick cuticle with external longitudinal ridges, prominent lateral cords, and paired uteri packed with microfilariae in female worms.23Acta Cytologica. Dirofilaria repens Diagnosed by the Presence of Microfilariae in Fine Needle Aspirates In some cases, fine-needle aspiration has yielded the diagnosis before excision. One memorable report described a 21-year-old woman whose fine-needle biopsy of a forearm swelling literally pulled a dead adult female D. repens out through the puncture site, an 80.5 mm worm dragged along with the needle.24Sri Lanka Journal of Dermatology. Human dirofilariasis – an unusual mode of presentation at fine needle aspiration Ultrasound can be helpful before surgery: the “filarial dance sign,” where a worm’s movements are visible in real time, and the parallel hyperechoic double-line pattern on static images both suggest a living parasite inside a nodule.25PubMed Central. A case of human Dirofilaria repens in Rome, Italy: A clinical and radiological challenge
One persistent challenge is that commercially available antigen tests designed for D. immitis heartworm screening do not reliably detect D. repens. Improved diagnostic tools have been called for to bring D. repens detection up to the standard that exists for its better-funded cousin.26PubMed Central. Recent advances on Dirofilaria repens in dogs and humans in Europe
Treatment and Prevention in Animals
In dogs with clinical disease, treatment aims to eliminate both the adult worms and the circulating microfilariae. A combined protocol using doxycycline (to target Wolbachia and weaken the adults) along with ivermectin administered repeatedly over several months has shown promise in clearing microfilariae from the blood.27PubMed. Treatment of Dirofilaria repens microfilariaemia with a combination of doxycycline hyclate and ivermectin For skin disease, a topical spot-on formulation combining imidacloprid and moxidectin has been used successfully in at least one reported case of allergic dermatitis caused by D. repens.28PubMed. Allergic dermatitis by Dirofilaria repens in a dog: clinical picture and treatment
Prevention relies on the same class of drugs used against heartworm: macrocyclic lactones such as ivermectin, moxidectin, milbemycin, and selamectin. Monthly administration kills the infective larvae before they can mature into adults. In endemic areas, year-round prevention is recommended, and veterinarians have been urged to promote preventive measures more aggressively among pet owners.29PubMed Central. Recent advances on Dirofilaria repens in dogs and humans in Europe Screening imported dogs is another critical control measure, since moving an infected, untreated dog into a non-endemic area that has competent mosquito vectors is essentially an introduction event for the parasite.
A Parasite on the Move
D. repens has historically been considered a parasite of southern and eastern Europe, with core endemic zones in Italy, the Balkans, and parts of the former Soviet Union. That picture has changed sharply. Over the past two to three decades, cases in dogs and humans have been recorded progressively farther north and west. A systematic review of European distribution data concluded that higher temperatures accelerate larval development inside mosquitoes, prolonged warm periods extend the transmission season, and there has been a significant increase in at-risk areas, particularly in northern countries.30PubMed Central. Vectors, host range, and spatial distribution of Dirofilaria immitis and D. repens in Europe: a systematic review There is evidence that D. repens has spread faster than D. immitis from the endemic zones of southern Europe into northern Europe.
Finland is a good example of how far north the parasite has reached. Research on D. repens transmission in southeastern Finland identified summer temperature as the most obvious factor limiting the parasite’s northern distribution, since larval development inside the mosquito simply cannot proceed below certain thermal thresholds. With ongoing warming, further spread into Fennoscandia is expected.31PubMed Central. Dirofilaria repens transmission in southeastern Finland
Wild Animal Reservoirs
Dogs are not the only animals keeping this parasite circulating. Wild carnivores, particularly red foxes and golden jackals, have tested positive for D. repens in multiple European countries. A survey in the Vojvodina region of Serbia detected both D. immitis and D. repens in fox and jackal populations, while only D. immitis turned up in stray dogs from the same area, suggesting that wild canids may maintain their own semi-independent transmission cycle for D. repens.32PubMed. Molecular survey of Dirofilaria species in stray dogs, red foxes and golden jackals from Vojvodina, Serbia In Poland, D. repens was found in wild carnivores across several regions, with the highest infection rates in areas that also had the highest recorded prevalence in dogs.33PubMed Central. Occurrence of Dirofilaria repens in wild carnivores in Poland
The existence of wildlife reservoirs complicates control. You can put every pet dog in a region on preventive medication, but if foxes and jackals are carrying the parasite and feeding the local mosquito population with microfilariae, transmission to humans and untreated domestic animals can continue. This is one reason some researchers have framed D. repens as a One Health problem that cannot be solved by veterinary medicine alone. It sits at the intersection of wildlife ecology, mosquito control, climate policy, and human medicine, and managing it requires thinking across all four.