Caribbean itch

A 25-year-old man came to the office in July after awakening with a pruritic rash on his chest, back, and abdomen. He reported feeling feverish with chills and malaise, but had no symptoms of respiratory distress and his vital signs were normal. He mentioned that he had gone snorkeling in the Caribbean the previous day, but did not remember being stung by any marine life. He said he was wearing swimming trunks and a t-shirt while in the ocean.

The patient had a diffuse erythematous maculopapular rash on his neck, chest, back, abdomen, shoulders, and axillae (Figure 1 and Figure 2).

FIGURE 1
Rash on abdomen

FIGURE 2
Macules

What is the most likely diagnosis?