Differential Diagnosis for Unusual Skin Sores

The patient's presentation of unexplained skin sores, particularly with a response to Ivermectin and a positive Filariasis IGG4 AB test, suggests several potential diagnoses. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Onchocerciasis (River Blindness): The patient's response to Ivermectin, both topical and oral, along with the positive Filariasis IGG4 AB test, strongly suggests Onchocerciasis. The disease is characterized by skin lesions, including nodules and papular dermatitis, which can leave atrophic scarring. Although the patient has never left the United States, cases of Onchocerciasis have been reported in the U.S. in individuals who have traveled to or immigrated from endemic areas, but there's also a possibility of local transmission in certain regions.

• Other Likely Diagnoses

  • Cutaneous Leishmaniasis: This condition can cause skin sores that are unresponsive to standard antibiotics and may respond to antiparasitic treatments like Ivermectin. The sores often have a firm border and can leave scarring.
  • Lyme Disease: Although less likely given the response to Ivermectin, Lyme disease can cause skin lesions (erythema migrans) and might be considered if the patient has been exposed to ticks.
  • Mycobacterial Infections (e.g., Leprosy): These infections can cause chronic skin lesions that are unresponsive to standard antibiotics. However, the response to Ivermectin is not typical for mycobacterial infections.

• Do Not Miss Diagnoses

  • Cutaneous T-Cell Lymphoma (Mycosis Fungoides): This rare form of skin cancer can present with skin lesions that are resistant to treatment. Although the patient's response to Ivermectin makes this less likely, it's a diagnosis that should not be missed due to its potential severity.
  • Sporotrichosis: A fungal infection that can cause skin lesions, particularly after exposure to contaminated soil or plants. It's less likely given the patient's response to Ivermectin but should be considered due to its potential for serious disease.

• Rare Diagnoses

  • Loiasis: Caused by the Loa loa parasite, this disease can lead to skin and eye lesions. It's rare in the United States and typically associated with travel to endemic areas in Africa.
  • Mansonelliasis: Infection with Mansonella parasites can cause skin lesions among other symptoms. It's rare and usually seen in individuals who have traveled to or are from specific endemic areas.