Dirofilaria and other zoonotic filariae
- Parasite
- Exam priority: medium
Filarial worms of dogs and wild animals spread by mosquitoes. In people they die young and leave a lung nodule or a lump under the skin.
Also called: Dirofilaria immitis, dog heartworm, Dirofilaria repens, Dirofilaria tenuis, dirofilariasis
Quick facts
- Type
- Parasite · Nematodes
- Gram / shape
- Long thin worms
- Spread
- Bite of a mosquito · Not transmitted between people
- Causes
- Pulmonary dirofilariasis · Subcutaneous dirofilariasis
- Treatment
- Surgical removal of the nodule or worm
At a glance
- Shape
- Long thin worms; in humans usually a single immature or dead worm inside a granuloma
- Family
- Onchocercidae
- Type
- Filarial nematodes of animals; humans are dead-end hosts
- Size
- Adults in dogs are several centimetres; human nodules usually 1–3 cm
Morphology
- In human tissue the worm is identified by its cuticle, muscle layer and lateral cords.
- D. repens and D. tenuis have a ridged cuticle; D. immitis has a smooth one.
- Microfilariae almost never appear in human blood.
- Like other filariae, Dirofilaria carries Wolbachia endosymbiotic bacteria.
Transmission
- Bite of a mosquito (Aedes, Culex, Anopheles, Mansonia) that has fed on an infected dog or wild animal.
- Not transmitted between people.
Reservoir: D. immitis: dogs, also cats and wild canids; D. repens: dogs and cats (Old World only); D. tenuis: raccoons (North America)
Vector: Mosquitoes of the genera Aedes, Culex, Anopheles and Mansonia
Isolation precautions
What the care team adds to standard precautions.
- Standard
Life cycle
- A mosquito injects third-stage larvae into the skin of a dog or other animal host.
- In dogs, D. immitis adults live in the pulmonary arteries and heart; D. repens under the skin.
- Females release microfilariae into the blood, which a mosquito swallows.
- In the mosquito the larvae develop to the infective stage and move to the mouthparts.
- In a person the larvae rarely mature; they die and are walled off in a nodule.
Virulence factors
- A dying D. immitis worm is carried to a small pulmonary artery, causing a small infarct and granuloma.
- Subcutaneous species provoke an inflammatory nodule with eosinophils.
- Ocular worms cause conjunctival irritation and can damage the eye.
Diseases it causes
- Pulmonary dirofilariasis: a solitary coin lesion on chest imaging that mimics lung cancer Solitary pulmonary nodule →
- Subcutaneous dirofilariasis: tender, sometimes migratory nodules on exposed skin, breast or scrotum
- Ocular and periocular dirofilariasis: a worm under the conjunctiva or in the eyelid
Clinical picture
- Pulmonary form: often asymptomatic, found on a chest X-ray; sometimes cough, chest pain, fever.
- Nodules are round and well defined, usually under 3 cm.
- Subcutaneous form: a nodule that may move, on the face, limbs or trunk.
- Mild eosinophilia may be present.
Diagnosis
- Pulmonary: suspected from a coin lesion; confirmed when excision or biopsy shows the worm.
- Subcutaneous or ocular: worm identified in the removed nodule (histology, morphology).
- PCR on tissue identifies the species.
- Serology is supportive only and cross-reacts with other helminths such as Toxocara.
Treatment
Surgical removal of the nodule or worm, which is usually both diagnostic and curative; anthelmintic drugs are generally not needed in humans.
Drug class only; doses and regimens are set by the clinician from the current guideline.
Prevention
- Monthly heartworm preventives for dogs and cats reduce the animal reservoir.
- Mosquito control and personal protection (repellents, screens, clothing).
Test yourself
Think of the answer, then tap the question to see it.
What is the typical finding of human pulmonary dirofilariasis?
A solitary, well-defined coin lesion on chest imaging that mimics lung cancer.
How is Dirofilaria immitis transmitted to humans?
By the bite of a mosquito that has fed on an infected dog or other animal host.
Why are microfilariae not found in the blood of infected people?
Because the larvae rarely mature in humans; they die before producing microfilariae.
What is the usual treatment of human dirofilariasis?
Surgical removal of the nodule or worm; anthelmintics are generally not needed.
Connections
← Back to the group: Nematodes
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: CDC - DPDx - Dirofilariasis (CDC, 2019); Human and Animal Dirofilariasis: the Emergence of a Zoonotic Mosaic (Clinical Microbiology Reviews (PMC), 2012).
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Representative model, for educational purposes only.