Differential Diagnosis for Small Bumps on the Outside of the Lips After Swimming in a River

• Single Most Likely Diagnosis

  • Cercarial dermatitis (Swimmer's itch): This is the most likely diagnosis given the context of swimming in a river and developing small bumps on the skin. Cercarial dermatitis is caused by an allergic reaction to the penetration of the skin by cercariae, which are the larval stage of certain species of schistosomes (blood flukes) that infect birds and other animals. The reaction typically occurs within hours of exposure and can cause intense itching, redness, and small bumps.

• Other Likely Diagnoses

  • Insect bites: Bites from insects such as mosquitoes, flies, or water striders could cause small bumps on the skin after being near or in a river. The bites can lead to allergic reactions, which manifest as itchy bumps.
  • Contact dermatitis: Exposure to certain plants, such as poison ivy, oak, or sumac, or irritants in the water could cause contact dermatitis, leading to skin bumps, redness, and itching.
  • Seabather's eruption (Sea lice): Although more commonly associated with saltwater, similar reactions can occur in freshwater environments due to other aquatic organisms. This condition causes an itchy, bumpy rash.

• Do Not Miss Diagnoses

  • Leptospirosis: While less likely to cause skin bumps directly, leptospirosis is a serious bacterial infection that can be contracted through contact with contaminated freshwater. Early symptoms can be non-specific and may include a rash, but the disease can progress to severe forms if not treated promptly.
  • Schistosomiasis (Swimmer's fever): In rare cases, the penetration of the skin by schistosome cercariae can lead to a systemic infection known as schistosomiasis. This condition can cause a range of symptoms, including fever, and is more serious than cercarial dermatitis.

• Rare Diagnoses

  • Aquagenic pruritus: A rare condition characterized by intense itching after exposure to water, without any visible skin lesions. It's less likely given the presence of bumps but could be considered if the primary symptom is itching.
  • Waterborne allergic reactions to specific organisms: There are various other aquatic organisms that could potentially cause allergic reactions, including certain types of algae or plankton. These would be rare causes of skin bumps after swimming in a river.