Differential Diagnosis for Secondary Cellulitis in a Pediatric Patient
Single Most Likely Diagnosis
- Cellulitis caused by Streptococcus or Staphylococcus: This is the most likely diagnosis given the patient's presentation of increasing redness and swelling around the bite area, which is a common manifestation of bacterial cellulitis. The fact that the patient is afebrile and behaving normally does not rule out this diagnosis, as systemic symptoms can be variable.
Other Likely Diagnoses
- Insect bite reaction: An exaggerated immune response to the insect bite could cause the swelling and redness, especially if the patient has a history of allergic reactions to bites.
- Erythema migrans (Lyme disease): Although less common, if the bite was from a tick, Lyme disease could be a consideration, especially if the patient was in an endemic area. However, the absence of a distinctive "bull's-eye" rash and systemic symptoms makes this less likely.
- Spider bite (e.g., brown recluse): Certain spider bites can cause necrotic lesions and significant local reaction, but these are relatively rare and usually have distinctive features.
Do Not Miss Diagnoses
- Necrotizing fasciitis: Although rare, this is a life-threatening condition that requires immediate surgical intervention. The increasing area of swelling and redness could be indicative of this condition, especially if there are signs of severe pain, crepitus, or skin necrosis.
- Methicillin-resistant Staphylococcus aureus (MRSA) infection: MRSA can cause severe skin and soft tissue infections, including cellulitis, and is important to consider due to its resistance to common antibiotics.
- Rabies: If the bite was from an animal suspected of having rabies, this would be a critical diagnosis not to miss, given its almost invariably fatal outcome if not promptly treated.
Rare Diagnoses
- Ehrlichiosis or anaplasmosis: These are tick-borne illnesses that can cause systemic symptoms and rash, but they are less common and typically present with fever and other systemic symptoms.
- Cat-scratch disease: Caused by Bartonella henselae, this infection can lead to regional lymphadenopathy and a papule or pustule at the site of the cat scratch, but it's less likely given the description of a bug bite.
- Leishmaniasis: A parasitic infection transmitted by sandflies, which can cause skin lesions, but it's rare in most regions and usually associated with travel history to endemic areas.