Differential Diagnosis for Rash on Arms and Legs

Single Most Likely Diagnosis

• Cercarial dermatitis (Swimmer's itch): This condition is caused by an allergic reaction to the cercariae of certain species of schistosomes, which are found in freshwater lakes and rivers. The patient's history of river swimming in Alex Bay and the description of the rash as small, bumpy, and itchy are consistent with this diagnosis.

Other Likely Diagnoses

• Insect bites: The patient's outdoor activities during vacation increase the likelihood of insect bites, which can cause itchy, bumpy rashes.
• Contact dermatitis: Exposure to certain plants, such as poison ivy or oak, or other allergens during outdoor activities can cause a contact dermatitis rash.
• Heat rash: The warm weather during vacation can contribute to the development of heat rash, which is characterized by small, itchy bumps.

Do Not Miss Diagnoses

• Lyme disease: Although less likely, Lyme disease is a serious condition that can cause a rash, and the patient's outdoor activities in an area where Lyme disease is endemic increase the risk.
• Erythema migrans: This is a rash caused by a tick bite and can be a sign of early Lyme disease or other tick-borne illnesses.

Rare Diagnoses

• Schistosomiasis: While the patient's symptoms are more consistent with cercarial dermatitis, schistosomiasis is a rare but serious condition that can be acquired through contact with contaminated freshwater.
• Leptospirosis: This bacterial infection can cause a rash, among other symptoms, and is typically acquired through contact with contaminated water.