Dirofilaria repens: Transmission, Symptoms, and Prevention

Dirofilaria repens is a parasitic roundworm transmitted by mosquitoes that lives in the subcutaneous tissue of dogs and other animals, and occasionally infects humans. While its better-known relative, Dirofilaria immitis, causes heartworm disease, D. repens tends to settle beneath the skin rather than in the heart and lungs, producing nodules that in human patients are frequently mistaken for tumors. The parasite has been spreading rapidly across Europe and parts of Asia in recent decades, and the combination of climate change, expanding mosquito populations, and undiagnosed dogs maintaining the infection cycle makes it a growing public health concern.

How Dirofilaria repens Gets Around

The life cycle of D. repens depends entirely on mosquitoes. An infected dog (or other host) carries microscopic larval stages called microfilariae in its bloodstream. When a mosquito feeds on that animal, it picks up these larvae, which then develop inside the mosquito over a period of roughly two weeks until they reach an infective stage. The next time that mosquito bites a dog, cat, or person, it deposits the larvae into the new host’s skin. From there, the larvae migrate through tissue and mature into adult worms that can live for years under the skin.

A wide range of mosquito species can serve as vectors. Research in Turkey identified several common species carrying D. repens DNA in their head and thorax regions, which indicates the parasites had developed to the infective stage. These included Anopheles maculipennis, Aedes caspius, Aedes vexans, Culex theileri, and Culex pipiens.¹ In Europe, two invasive mosquito species, Aedes japonicus and Aedes geniculatus, have proven to be efficient vectors for both D. repens and D. immitis in laboratory and field studies.² The Asian tiger mosquito, Aedes albopictus, which has spread aggressively into southern and central Europe, has also been flagged as a new potential vector contributing to the parasite’s geographic expansion.³

Not every mosquito species is equally capable of supporting the parasite’s development. Aedes aegypti, despite being one of the world’s most medically important mosquitoes, appears to be resistant to both D. immitis and D. repens.² This means the risk map for D. repens does not perfectly overlap with the risk map for dengue or Zika, even though all are mosquito-borne.

What Happens in Dogs

Dogs are the primary definitive host for D. repens, meaning the parasite can complete its full life cycle in them, producing adult worms that mate and release microfilariae into the bloodstream. Many infected dogs show no obvious signs of illness, which is a major part of the problem. A dog can silently harbor the parasite for years, serving as a reservoir that keeps feeding the transmission cycle through local mosquito populations.⁴

When symptoms do appear, they tend to involve the skin. Dogs may develop subcutaneous nodules, localized or diffuse dermatitis, skin lesions, and itching.⁴ These signs can look like allergic reactions, leading veterinarians down the wrong diagnostic path if they are not thinking about filarial parasites. Treatment is warranted when clinical signs are present, and options have included adulticide drugs and antiparasitic agents.⁵

Dogs can also be co-infected with both D. repens and D. immitis simultaneously, which complicates diagnosis and treatment. A case report from Romania documented a dog harboring both species at once, confirmed by both morphological examination and genetic analysis.⁶ In areas where both parasites circulate, telling them apart through traditional microscopy can be unreliable, particularly since the microfilariae of the two species look similar under a standard blood smear. Molecular methods such as multiplex PCR have been developed to distinguish them accurately in a single test.⁷

Cats and Wild Carnivores as Hosts

Cats can become infected with D. repens, but they are imperfect hosts. After a mosquito introduces the larvae, only a small number develop to the adult stage, and this happens in only a fraction of exposed cats.⁸ When infection does take hold, it tends to present as subcutaneous nodules or skin inflammation, much as it does in dogs. One notable case involved a cat with multiple subcutaneous lumps that looked like a soft tissue tumor on imaging. The masses turned out to be D. repens adults, identified by genetic testing after surgical removal.⁹ This kind of diagnostic confusion is a recurring theme with D. repens across species.

Wild carnivores also play a role. A survey across Poland tested blood and spleen samples from wolves, red foxes, Eurasian badgers, raccoons, raccoon dogs, and martens, and found D. repens DNA in about 3% of samples overall, with positive results in badgers, foxes, and wolves.¹⁰ A separate molecular survey in Serbia detected D. repens in red foxes and golden jackals, and interestingly found that D. repens appeared to circulate more among wild canids than among the stray dog population in that same region.¹¹ This wildlife reservoir complicates control efforts. Even if every pet dog in a region were treated and protected, wild animals could sustain the parasite in the environment.

Human Infection and Why It Gets Misdiagnosed

Humans are accidental, dead-end hosts. D. repens larvae occasionally make it past the skin and develop into an adult worm, but the worm typically cannot reproduce in the human body and does not release microfilariae into the bloodstream. That means an infected person cannot pass the parasite along to another mosquito. The problem for humans is not systemic disease but rather what the single worm does wherever it ends up lodging.

The most common presentation in people is a subcutaneous nodule, a firm lump under the skin that can appear almost anywhere on the body.¹² Nodules on the trunk, limbs, face, and chest wall have all been documented. A case from Vietnam described a painful nodule on the right eyelid that turned out to be subcutaneous dirofilariasis, confirmed by both tissue examination and molecular testing.¹³ The key clinical challenge is that these lumps look like tumors. An epidemiological review emphasized that the medical significance of D. repens in humans is largely tied to this frequent misdiagnosis as malignancy, with many patients undergoing invasive surgical procedures before the true cause is identified.¹⁴

Genital involvement, though rare, has been documented. D. repens can produce pseudotumor nodules affecting the scrotum, epididymis, or spermatic cord, adding urological cancers to the list of conditions it mimics.¹⁵

When the Worm Reaches the Eye

Ocular dirofilariasis is one of the more unsettling manifestations. The worm can migrate to the subconjunctival space, the area just beneath the thin membrane covering the white of the eye, where it is sometimes visible as a moving, pale yellow thread during a slit-lamp examination.¹⁶ Beyond the subconjunctiva, the worm can also migrate into the orbit, settling in the peri-ocular or retro-ocular space. Complications linked to ocular D. repens include eyelid swelling, lacrimal gland inflammation, uveitis, glaucoma, and scleritis.¹⁷

A case from Russia illustrates how diagnostic delays can escalate the situation. A parasitic cyst in the orbit was eventually removed through a surgical approach to the eye socket, yielding a live adult worm measuring 87 mm long. The authors noted that the nodule’s location near critical structures required aggressive surgery, even though the mass turned out to be benign. Ultrasound could have flagged the parasitic origin earlier and potentially avoided the delay.¹⁸

The Expanding Geographic Range

D. repens was historically concentrated in southern Europe, parts of the Middle East, and central and southern Asia. Over the past two decades, it has pushed northward at a rate that has surprised parasitologists. Evidence suggests it has spread faster than its cousin D. immitis into previously non-endemic areas of northern and central Europe.¹⁹

Three factors get the most attention when explaining this expansion. Climate change is warming temperatures enough to extend the mosquito season and push vector-competent species further north. Pet travel within the European Union has become easier, meaning an infected dog from an endemic area can introduce the parasite to a new region. But researchers have argued that the most important factor may be the high rate of undiagnosed dogs silently maintaining the transmission cycle.¹⁹ Because many infections in dogs are asymptomatic, owners have no reason to seek veterinary care, and the dog continues to supply microfilariae to local mosquitoes year after year.

A 2025 systematic review confirmed that both D. immitis and D. repens represent a growing health concern for animals and humans in Europe, and concluded that continued globalization and warming temperatures will probably lead to further spread.²⁰ The arrival of new invasive mosquito species like Aedes albopictus compounds the problem by adding more competent vectors into regions that previously had few.³

Diagnosis in Dogs and Humans

In dogs, detection starts with looking for microfilariae in the blood, traditionally through a modified Knott’s test, which concentrates the larvae from a blood sample. Molecular diagnostics using PCR have become increasingly important, especially in areas where D. repens and D. immitis overlap and visual identification of microfilariae is unreliable. In an experimental infection study, about 80% of infected dogs had detectable microfilariae by the Knott’s test, while 85% tested positive by PCR on blood samples. Agreement between the two methods improved over time, reaching strong concordance by about nine months after infection.²¹ Antibody detection added another layer of sensitivity, with 90% of experimentally infected dogs developing measurable immune responses.

Duplex PCR (testing for both Dirofilaria species simultaneously) can sometimes fail to amplify D. repens DNA in co-infected dogs, even when single-target PCR picks it up. This was documented in naturally co-infected dogs where D. repens DNA was missed in many samples when both species were targeted in the same reaction.²² Veterinarians in endemic areas need to be aware that a negative duplex result does not necessarily rule out D. repens.

In humans, diagnosis usually happens after a surgeon removes a suspicious nodule and a pathologist identifies the worm in the tissue. Since most clinicians in newly endemic areas have never encountered the parasite, it rarely makes the initial differential diagnosis. Ultrasound can sometimes reveal the characteristic appearance of a worm inside a cyst, potentially sparing the patient a more invasive workup. The Russian orbital case is a clear example of how ultrasound, had it been performed earlier, might have shortened the diagnostic timeline and reduced the complexity of surgery.¹⁸

Treatment in Humans

For most human cases, treatment is straightforward in concept if not always in execution. The standard approach for localized subcutaneous or genital dirofilariasis is complete surgical removal of the nodule. Excision is both diagnostic and curative, and in the vast majority of cases, patients recover fully with no recurrence.²³ Some clinicians have used antiparasitic drugs like albendazole or ivermectin, sometimes combined with doxycycline, though surgical excision remains the primary recommendation.

Doxycycline deserves a special mention. D. repens, like most filarial worms, harbors an intracellular bacterial symbiont called Wolbachia. The bacteria are essential for the worm’s survival and reproduction. Doxycycline, a common antibiotic, kills Wolbachia, which in turn weakens and eventually kills the worm. At least one case report documented successful treatment of a microfilaremic human patient using doxycycline alone, chosen because the patient was unwilling to take ivermectin.²⁴ This anti-Wolbachia strategy has become an important concept across filarial medicine, not just for D. repens.

Preventing Infection in Pets and People

Prevention operates on two levels: protecting individual animals and breaking the transmission cycle at the population level.

For dogs, monthly macrocyclic lactone preventives (the same class of drugs used for heartworm prevention) are effective against D. repens when administered consistently. Dogs in endemic or expanding-range areas benefit from year-round chemoprophylaxis, especially since the mosquito season is lengthening with warmer climates. The real gap is not in available drugs but in uptake. Dogs that are not tested, not treated, and not on preventive medication continue to circulate microfilariae to mosquitoes. Research from Corsica underscored this point, finding both Dirofilaria species in local mosquitoes and calling for implementation of preventive chemoprophylaxis alongside vector control to reduce the risk in both dog and human populations.²⁵

For cats, prevention follows the same principle. Although cats are less efficient hosts, they can still develop clinical disease, and the European Advisory Board on Cat Diseases recommends preventive measures in areas where dirofilariasis is endemic.⁸

For humans, there is no prophylactic drug. Protection depends on reducing mosquito bites through personal measures like repellents, long sleeves during peak biting hours, and window screens, and on broader community-level mosquito control. But the single most effective thing a community can do to protect people is to reduce the parasite load in the dog population. If fewer dogs carry circulating microfilariae, fewer mosquitoes pick them up, and the risk to everyone drops.

The Wild Card of Wildlife Reservoirs

Even a perfectly protected domestic dog population would not completely eliminate the risk. The presence of D. repens in wild carnivores across Europe creates a reservoir that is essentially uncontrollable. Wolves, foxes, badgers, and jackals live outside the reach of veterinary chemoprophylaxis, and they interact with the same mosquito populations that bite pets and people. The finding that D. repens circulated more among golden jackals and red foxes than among stray dogs in parts of Serbia suggests that wildlife may sustain the parasite independently of the domestic dog population in some areas.¹¹

The detection of D. repens in wild carnivores in Poland, a country that was not historically considered endemic, reinforces the idea that the parasite’s northward march involves wildlife as well as pets.¹⁰ Researchers have pointed out that this wildlife circulation may help explain why the parasite becomes endemic in newly invaded regions so quickly. Once it establishes in wild animal populations, eradicating it through veterinary interventions alone becomes effectively impossible. The practical takeaway for public health is that D. repens control needs to be thought of as a long-term management challenge rather than an eradication target, combining sustained pet prophylaxis with mosquito surveillance and increased clinical awareness in both veterinary and human medicine.

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