[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Nematodes](https://durgical.com/en/microbiology/groups/nematodes/) › Dirofilaria and other zoonotic filariae

# 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.

Updated Nov 13, 2026

Also called: Dirofilaria immitis, dog heartworm, Dirofilaria repens, Dirofilaria tenuis, dirofilariasis

## Quick facts

**Type**
Parasite · [Nematodes](https://durgical.com/en/microbiology/groups/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

On this page

- The basics Key points
- At a glance
- Morphology
- Transmission
- Life cycle
- In Colombia
- Memory aid
- What the colours mean

- Clinical Virulence factors
- Diseases it causes
- Clinical picture
- Diagnosis

- Treatment Treatment
- Prevention

- Study Test yourself

- Connections Connections

## Key points

- Dog heartworm in a person = coin lesion in the lung that mimics cancer.
- Humans are dead-end hosts: no microfilaraemia.
- Spread by mosquitoes from dogs.
- D. repens/D. tenuis: subcutaneous or eyelid nodule.
- Excision is the diagnosis and the cure.

## 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 →](https://durgical.com/en/conditions/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).

## In Colombia

D. immitis is present in Colombian dogs, and human pulmonary dirofilariasis has been reported sporadically in Colombia and elsewhere in South America, mainly southern Brazil. D. repens does not occur in the Americas.

## Memory aid

The dog's heart worm gets lost in a human lung: it dies, leaves a coin, and the surgeon finds a worm where a tumour was feared.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/dirofilaria-and-other-zoonotic-filariae/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

## Connections

## Related 4

- [Wuchereria bancrofti / Brugia malayi](https://durgical.com/en/microbiology/wuchereria-bancrofti-brugia-malayi/)
- [Onchocerca volvulus](https://durgical.com/en/microbiology/onchocerca-volvulus/)
- [Mansonella ozzardi / Mansonella perstans](https://durgical.com/en/microbiology/mansonella-ozzardi-mansonella-perstans/)
- [Toxocara canis / Toxocara cati](https://durgical.com/en/microbiology/toxocara-canis-toxocara-cati/)

## Related conditions 1

- [Solitary pulmonary nodule](https://durgical.com/en/conditions/solitary-pulmonary-nodule/)

[← Back to the group: Nematodes](https://durgical.com/en/microbiology/groups/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](https://www.cdc.gov/dpdx/dirofilariasis/index.html) (CDC, 2019); [Human and Animal Dirofilariasis: the Emergence of a Zoonotic Mosaic](https://pmc.ncbi.nlm.nih.gov/articles/PMC3416488/) (Clinical Microbiology Reviews (PMC), 2012).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/microbiology/dirofilaria-and-other-zoonotic-filariae/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/dirofilaria-and-other-zoonotic-filariae/