Dirofilaria repens is a parasitic roundworm that primarily establishes infections in dogs and other carnivores like cats, foxes, and wolves. This parasite belongs to the same family as the more widely known heartworm, Dirofilaria immitis. Adult worms are white and slender, with females capable of reaching lengths of 25-30 cm. They typically reside within the subcutaneous tissues just beneath the skin of their animal hosts.
Transmission and Life Cycle
The cycle begins when a mosquito from a susceptible genus, such as Aedes, Culex, or Anopheles, bites an infected animal and ingests microfilariae, the parasite’s microscopic larvae, from the animal’s bloodstream. These microfilariae undergo development within the mosquito’s body over a period of 10 to 16 days, maturing into infective larvae.
Once the larvae have reached this infective stage, they migrate to the mosquito’s mouthparts. When the mosquito takes its next blood meal, these larvae are transmitted to a new animal host. Inside this new host, the larvae continue their development through two more molts, eventually becoming adult worms that settle in the subcutaneous tissues. The maturation process within the definitive host takes approximately six to seven months, after which adult female worms can begin producing a new generation of microfilariae.
Clinical Signs in Animal Hosts
Many canids and felines infected with Dirofilaria repens show no outward signs of disease and are considered asymptomatic carriers. When symptoms do appear, they are most commonly related to the skin. The presence of adult worms can lead to the formation of distinct subcutaneous nodules or lumps, which are granulomas that form around the parasites.
Other dermatological issues may arise, including generalized itching (pruritus), skin inflammation (dermatitis), papules, and patches of hair loss. These signs are often the result of an allergic-type hypersensitivity reaction to the microfilariae circulating in the body. While the skin-related symptoms can cause discomfort, infections with D. repens are generally less severe than those caused by its relative, Dirofilaria immitis, which migrates to the pulmonary arteries and can lead to serious cardiopulmonary disease.
Human Infection and Symptoms
Humans can become infected with Dirofilaria repens through the bite of a carrier mosquito, but they are considered accidental or “dead-end” hosts. This means the parasite is unable to complete its life cycle and reproduce within the human body. The infection is typically caused by a single larva that fails to reach full maturity and eventually dies within the host’s tissues.
The host’s immune system responds to the trapped and dying worm by forming an inflammatory nodule around it. These nodules most frequently appear in subcutaneous tissue, often on the upper body, face, or arms, areas commonly exposed to mosquito bites. A notable presentation is ocular dirofilariasis, where the worm or the resulting nodule is found in or around the eye, such as under the conjunctiva (the clear membrane covering the white of the eye) or in the eyelid.
While subcutaneous and ocular nodules are the most common manifestations, the migrating larva can occasionally end up in other locations. Less frequent sites for nodules include the lungs, where they can be mistaken for tumors on chest X-rays, as well as the male genitals or female breast tissue. The infection in humans is localized and self-limiting.
Diagnosis and Treatment
In animal hosts like dogs, diagnosis often begins with identifying microfilariae in a blood sample. A modified Knott’s test can be used to concentrate microfilariae from the blood, making them easier to detect. If skin nodules are present, a veterinarian might perform a fine-needle aspirate to look for larvae or recommend an ultrasound.
For humans, a diagnosis is almost always made after a nodule is surgically removed. The excised tissue is sent to a pathologist, who identifies the worm, confirming the cause of the lesion. Because humans do not typically have circulating microfilariae, blood tests are not useful for diagnosis.
Treatment for both animals and humans primarily involves the surgical excision of the nodule containing the adult worm. In humans, this removal is curative, and further treatment is often not needed. Veterinarians may opt for additional therapies for infected dogs, including specific antiparasitic drugs to eliminate any remaining microfilariae from the bloodstream.
Prevention and Geographic Spread
Prevention centers on parasite control for pets and mosquito avoidance for both animals and people. For pets, regular administration of macrocyclic lactone parasiticides, the same class of drugs used to prevent heartworm, is effective. These preventative treatments should be started in young animals in endemic regions and are prescribed by a veterinarian. Using mosquito repellents formulated for pets can provide an additional layer of protection.
This parasite is traditionally found in parts of Europe, Africa, and Asia. However, its geographic range has been expanding northward into areas previously considered non-endemic, such as Central and Northern Europe. This spread is linked to factors like climate change, which creates more favorable conditions for the mosquito vectors, and the increased movement of infected pets across borders. This expansion makes public awareness and preventative measures important for safeguarding health in new regions.