Whipple's disease

A 52-year-old man comes to the office because of pain in his wrists and knees. He is a farmer and initially thought that these symptoms were related to the strain of his work. However, NSAID treatment had led to minimal improvement and he is worried because he has also developed watery diarrhea with cramping for the past three weeks. His pants have felt loose, he says. Biopsy from his small intestine shows PAS-positive macrophages in the lamina propria. Which of the following is the most appropriate initial treatment?