What is the next step in treating a 12-year-old girl with Dirofilaria (dirofilariasis) in her cheek that has not subsided after 3 weeks on Diethylcarbamazine (DEC)?

Surgical Excision is the Definitive Treatment for Subcutaneous Dirofilariasis

For a 12-year-old girl with Dirofilaria in the cheek that has not responded to 3 weeks of DEC, proceed immediately to surgical excision of the parasite, as this is the definitive treatment for subcutaneous dirofilariasis. Medical therapy alone is insufficient for established subcutaneous infections.

Why DEC Failed and What to Do Next

Understanding the Treatment Failure

• DEC is not effective for adult subcutaneous Dirofilaria worms - the medication primarily targets microfilariae (larval forms) in the bloodstream, not adult worms encapsulated in subcutaneous tissues 1
• Subcutaneous dirofilariasis (most commonly D. repens) presents as nodular lesions where adult worms are physically encapsulated, making them inaccessible to systemic antiparasitic drugs 2
• The diagnosis of subcutaneous dirofilariasis is typically established by surgical removal and histopathologic examination of the adult worm, not by medical treatment 1, 2

Immediate Management Algorithm

Step 1: Surgical Excision

• Complete surgical removal of the subcutaneous nodule containing the adult worm is the primary and most effective treatment 1, 2
• This provides both definitive diagnosis (through histopathologic examination) and cure 2
• The procedure is straightforward and can be performed under local anesthesia for facial lesions 2

Step 2: Post-Surgical Medical Therapy

• After surgical excision, consider adding ivermectin 150-200 μg/kg as a single dose followed by DEC 2 mg/kg three times daily for 4 weeks to eliminate any potential microfilariae or residual parasites 1
• Alternatively, doxycycline 200 mg daily for 6 weeks can be used as it targets the Wolbachia endosymbiont present in filarial species and has demonstrated efficacy in microfilaremic dirofilariasis 3

Critical Pre-Operative Considerations

Rule Out Co-Infections Before Any Further DEC Use

• Before administering any additional DEC, you must exclude onchocerciasis and loiasis co-infection through skin snips, slit lamp examination, and daytime blood microscopy 4, 5
• DEC can cause severe reactions including blindness, hypotension, and fatal encephalopathy in patients co-infected with these parasites 4, 6
• If the patient has traveled to co-endemic regions (sub-Saharan Africa, parts of Central/South America), this screening is mandatory 7

Expected Clinical Features

• Eosinophilia is typically absent in subcutaneous dirofilariasis - all six patients in one case series had normal eosinophil counts 1
• Serum IgE levels are usually normal 1
• Symptoms are generally mild and nonspecific 1
• The most common locations are the head (including cheek), thoracic wall, and upper limbs 2

Why Medical Therapy Alone is Inadequate

• A comprehensive review of 397 cases worldwide showed that correct diagnosis and cure were achieved through surgical removal and histological identification, not medical treatment alone 2
• The adult worms are physically encapsulated in subcutaneous tissues, creating a barrier to systemic medications 2
• Internal localizations (including 9 pulmonary cases) were consistently misdiagnosed as malignant neoplasms until surgical excision and histopathology were performed 2

Common Pitfalls to Avoid

• Do not continue DEC monotherapy - three weeks without response indicates the need for surgical intervention 1
• Do not assume eosinophilia will be present - its absence does not rule out dirofilariasis 1
• Do not delay surgical excision - prolonged medical therapy without surgical removal will not resolve subcutaneous infections 2
• Do not administer additional DEC without screening for co-infections if the patient has appropriate travel history 4, 5, 6