Differential Diagnosis for 35yo Male with Skin Ulceration after Spider Bite
- Single most likely diagnosis:
- Necrotic arachnidism (Brown recluse spider bite): The patient's history of a spider bite followed by skin ulceration, worsening pain, and lack of systemic symptoms such as fever or elevated WBC count is consistent with a brown recluse spider bite. The fact that the ulceration has not improved and has worsened despite antibiotic treatment (Augmentin) also points towards this diagnosis, as necrotic arachnidism is not typically responsive to antibiotics alone.
- Other Likely diagnoses:
- Soft tissue infection (e.g., cellulitis or abscess): Although the patient has completed a course of Augmentin, the presence of worsening pain could suggest a persistent or resistant bacterial infection. The absence of fever or elevated WBC count does not rule out a localized infection.
- Pyoderma gangrenosum: This is a rare skin condition that can cause ulceration, but it can be considered in the differential due to the presence of a painful ulcer that is not responding to typical treatments.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed):
- Gas gangrene (Clostridial myonecrosis): Although rare and less likely given the lack of systemic symptoms and the specific history provided, gas gangrene is a medical emergency that requires prompt recognition and treatment. It can present with severe pain out of proportion to the physical findings, which aligns with the patient's worsening pain.
- Methicillin-resistant Staphylococcus aureus (MRSA) infection: MRSA can cause severe skin and soft tissue infections that may not respond to standard antibiotic treatments like Augmentin. The presence of worsening pain despite antibiotic treatment raises the concern for a resistant organism.
- Rare diagnoses:
- Lyme disease or other tick-borne illnesses: If the "spider bite" was actually a tick bite, conditions like Lyme disease could be considered, especially if the patient has been in areas where ticks are common. However, the primary presentation of Lyme disease is usually a distinctive rash (erythema migrans) accompanied by systemic symptoms.
- Cutaneous leishmaniasis: This parasitic infection can cause skin ulcers and is typically associated with travel to endemic areas. It's a rare consideration in this context without specific travel history or exposure risks.