Author: PathologyMCQ Editorial Team
Category: Parasitology / Infectious Pathology
Last Updated: October 2026
Read Time: 12 minutes
Difficulty: Moderate (MD/DNB, FRCPath, NEET-SS)

At a glance

• What: a mosquito-borne zoonotic filarial nematode; dogs are definitive hosts, humans are accidental, usually dead-end hosts.
• Where in humans: subcutaneous nodules and periocular or subconjunctival lesions, often excised as a cyst or tumour.
• Key histology: a large worm with a thick cuticle bearing fine external longitudinal ridges, polymyarian muscles and an eosinophil-rich granulomatous reaction.
• In this post: a zoomable Dirofilaria virtual slide with an Enterobius mimic, an annotated Enterobius slide, a nematode look-alikes table, and 10 interactive MCQs including 6 slide-based questions.

What is Dirofilaria repens?

Dirofilaria repens is a mosquito-borne filarial nematode of dogs and other carnivores. Humans are accidental hosts in whom the worm rarely reaches sexual maturity, yet it can still cause striking local inflammation, most often in the subcutaneous tissue and around the eye. A closely related species, Candidatus Dirofilaria hongkongensis, has been described in Asia and is separated from D. repens by molecular methods.

Human cases, including rare microfilaraemia, are increasingly reported, which makes D. repens a frequent topic in pathology and microbiology exams.

Why it is exam-relevant

• It is a zoonotic filarial infection with dogs as reservoirs.
• Humans usually lack microfilaraemia, so diagnosis depends on histopathology.
• Questions test cuticular morphology (ridges vs bosses vs alae), muscle type, host roles and the differences from Loa loa and D. immitis.

Life cycle and hosts

Role	Organism	What happens
Definitive host	Dogs, foxes, cats and other carnivores	Adults live in the subcutis; females release microfilariae into the blood
Vector	Mosquitoes (Aedes, Culex, Anopheles)	Microfilariae develop into infective L3 larvae and are passed on during a bite
Accidental host	Humans	Larvae migrate and usually die before maturing, causing a nodule
Endosymbiont	Wolbachia bacteria	Needed for worm survival and reproduction; target of doxycycline

Epidemiology

• Endemic in parts of Europe, Asia and Africa; India (particularly Kerala) and Sri Lanka report many human cases.
• Spreading into new regions with climate change and pet travel; also seen in returning travellers.
• Infected dogs are often asymptomatic, so the reservoir goes unnoticed, making canine infection a public health issue.

Gross and microscopic pathology

Gross pathology

• Subcutaneous nodule often mistaken for a cyst, lipoma or soft tissue tumour.
• Periocular, eyelid or subconjunctival lesions, sometimes with a visible moving worm.
• Occasionally a migratory swelling as the worm moves; rarely breast, scrotum or deeper sites.

Gross appearance is never diagnostic; microscopy is essential.

Microscopic features, step by step

1. Whole worm at low power

• A large worm in cross or oblique section (often several hundred micrometres across) within a fibrous, inflamed nodule.
• Always scan at low power first; fragmented or degenerated worms can mimic cysts or necrotic debris.

2. Thick, multilayered cuticle

• A dense, refractile, laminated cuticle forms the outer boundary.
• Pearl: thick laminated cuticle = nematode, not cestode or trematode.

3. Prominent muscle layer

• Tall, numerous muscle cells in each quadrant (polymyarian-coelomyarian), divided by two broad lateral cords.
• Muscle + cuticle together confirm a true nematode, not debris.

4. Central intestine

• A small central intestine confirms an adult helminth rather than a larva or artefact.
• Spotter rule: intestine present = adult nematode; cestodes have no gut.

5. Female worm with uterine microfilariae

• Paired uterine tubes containing microfilariae prove a mature female.
• This is uncommon in humans, which is why microfilaraemia is rare.

6. The diagnostic clue: longitudinal cuticular ridges

• Fine, evenly spaced external longitudinal ridges separate D. repens from D. immitis (smooth cuticle) and Loa loa (bosses).
• Host reaction: granulomatous inflammation with many eosinophils, histiocytes and giant cells, sometimes abscess and fibrosis.

See it on a real case: open the Dirofilaria repens tab in the virtual slide viewer and zoom onto the worm’s surface.

Differential diagnosis: tissue nematodes and other parasites

Parasite	Usual site	Cuticle and key clue
Dirofilaria repens	Subcutis, eyelid, conjunctiva	Thick cuticle with fine external longitudinal ridges; polymyarian muscles
Dirofilaria immitis	Lung (pulmonary infarct, coin lesion)	Smooth cuticle; worm inside a thrombosed pulmonary artery branch
Loa loa	Subcutis, crossing the conjunctiva	Irregular cuticular bosses; Calabar swellings; microfilaraemia common
Onchocerca volvulus	Subcutaneous nodules over bony points	Many coiled worms in a fibrous nodule; cuticle with annular ridges and striae
Wuchereria / Brugia adults	Lymphatics and lymph nodes	Smaller worms with a smooth cuticle inside dilated lymphatics
Enterobius vermicularis	Appendiceal or colonic lumen	Small; paired triangular lateral alae; meromyarian muscles; little reaction
Cysticercus (Taenia solium larva)	Subcutis, muscle, brain	Cestode: no intestine, folded tegument with microvilli, scolex with hooklets

Annotated slide: Enterobius in the appendix

Use the region selector to jump to the labelled features of this pinworm, then compare its size and lateral alae with the ridged D. repens above.

Clinical features in humans and dogs

Feature	Humans	Dogs
Host role	Accidental, usually dead-end	Definitive host
Presentation	Solitary subcutaneous nodule, periocular or subconjunctival lesion, migratory swelling	Often asymptomatic; sometimes itchy nodules or dermatitis
Microfilaraemia	Rare	Common; source of mosquito infection
Usual diagnosis	Histology of the excised nodule, PCR	Blood tests for microfilariae (modified Knott test), PCR

Diagnosis and pitfalls

• Histopathology of the excised worm is the gold standard in humans.
• PCR and sequencing confirm the species, especially in degenerated worms, and separate D. repens from related species.
• A worm extracted intact from the eye can be identified by its morphology; blood smears are usually negative in humans.

Common pitfalls

• Calling the nodule a cyst, lipoma or soft tissue tumour clinically.
• Missing a degenerated worm hidden in an eosinophilic abscess; deeper levels help.
• Expecting microfilaraemia, or confusing the worm with Loa loa (bosses) or D. immitis (smooth).

Treatment

• Humans: surgical excision of the nodule or worm is both diagnostic and curative; drugs have a limited role.
• Dogs: macrocyclic lactones, doxycycline against Wolbachia, and mosquito control to break transmission.

More practice: Dirofilaria quiz

Key takeaways

• Dogs are definitive hosts and mosquitoes the vectors; humans are accidental, usually dead-end hosts.
• Microfilaraemia in humans is rare, so histopathology (plus PCR) makes the diagnosis.
• Ridges = D. repens; bosses = Loa loa; smooth = D. immitis; lateral alae = Enterobius.
• Expect an eosinophil-rich granulomatous reaction around a dead worm.
• Surgical excision is diagnostic and curative in humans.