A missionary from Chicago is sent to work in a geographic region of a Cental American country wher onchocerca volvulus is endemic. Infections due to this tissue nematode (onchocerciasis) are a major cause of 'river blindness', since microfilariae migrate through subcutaneous tissues and concentrate in the eyes. Which one of the following durgs can be used prophylactically to prevent onchocerciasis?