How Can Red Flat Dots On The Back And Arms Be Managed?

My son is in the Peace Corps in Panama and has had a rash since November 2016. He describes it as red flat dots that migrate but are primarily on his back and arms. Heat makes it worsen . When it s bad, he says it feels like a jelly fish sting. They ve checked him for the tropical diseases that can involve rashes and all are negative, although he still has intestinal parasites at last clinic visit. The clinic gave him some oral medication and topical lotion which improves it, but when he runs out, it comes back with a vengeance. He s been told it s an allergic reaction but hasn t been told what exactly he s reacting to, but language barrier might be an issue. He s sleeping in a hammock now, thinking his mattress might be the trigger, but not sure. Any thoughts or suggestions??