Infectious Disease and Microbiology – Dirofilaria Species

Overview

Dirofilaria species are nematode helminths that primarily infect animals. Human infection is uncommon and usually represents an accidental zoonotic infection.

Important species include D. immitis, D. repens, D. tenuis, and D. ursi.

Microbiologic Characteristics

Dirofilaria species are filarial nematodes.

Humans are not the usual definitive hosts, which is why the parasite often fails to complete its normal life cycle in human tissues.

Epidemiology

Different species are associated with different animal reservoirs.

D. immitis is the dog heartworm and only rarely causes human disease.

D. tenuis is primarily a parasite of raccoons in the United States.

D. repens is mainly a parasite of dogs and cats in Europe.

D. ursi is associated with bears in Canada.

Human Infection

Human infection is rare.

Because humans are accidental hosts, disease usually results from localization of immature or adult worms in tissue rather than from widespread filarial infection.

Dirofilaria immitis

D. immitis may cause either pulmonary or cutaneous disease in humans.

Pulmonary infection often results from a worm lodging in a branch of the pulmonary circulation.

Pulmonary Dirofilariasis

Pulmonary infection may produce a localized pulmonary nodule.

Patients are often asymptomatic, and the lesion may be discovered incidentally on chest imaging.

The nodule can sometimes resemble:

• Primary lung cancer

• Metastatic disease

• Granulomatous infection

The diagnosis is frequently made only after surgical excision and histopathologic examination.

Cutaneous Dirofilariasis

D. immitis and several other Dirofilaria species may occasionally produce cutaneous or subcutaneous lesions.

These may appear as:

• Subcutaneous nodules

• Localized swelling

• Inflammatory masses

Some lesions may contain a recognizable worm.

Other Species

Other Dirofilaria species, particularly D. repens and D. tenuis, may also cause cutaneous manifestations in humans.

These infections remain uncommon and are usually localized.

Microfilaremia

Microfilaremia is rare in humans.

This is an important distinction from classic human filarial diseases, in which circulating microfilariae may be a major diagnostic feature.

Diagnosis

Diagnosis is usually made by detecting the worm in tissue.

This typically requires:

• Biopsy of the lesion

• Surgical excision

• Histopathologic identification of the nematode

Blood tests are generally less useful because circulating microfilariae are rarely present.

Treatment

The principal treatment is surgical removal of the affected lesion or tissue.

Once the localized worm is removed, additional antiparasitic therapy is often unnecessary.

High-Yield Clinical Pattern

Incidental pulmonary nodule

• ●

No obvious pulmonary symptoms

• ●

Histology showing a filarial nematode

→ Consider Dirofilaria immitis

Exam Essentials

Genus: Dirofilaria

Type: Nematode helminth

Human role: Accidental host

Classic species: D. immitis

Animal association: Dog heartworm

Human manifestations: Pulmonary or cutaneous disease

Other species: D. repens, D. tenuis, D. ursi

Microfilaremia: Rare in humans

Diagnosis: Worm identified in excised tissue

Treatment: Surgical removal

Key clinical pearl: Human dirofilariasis is usually a localized zoonotic infection, often presenting as a pulmonary or subcutaneous nodule, and diagnosis is typically made by histologic identification of the worm after excision.