One can have Malarial relapses thru-out one's life even with-out new misquito bites....his recurrences should become less frequent with good nitrition, good health practises, and medicine.
You are right about the relapses. That's why I am automatically denied the priviledge of giving blood. However, in this case, the DDT would go a long way to reducing the occurances of Denny's malaria. There are a lot of mosquitoes in his office. I think he's taking quinnine again and that can be debilitating, too. I plan on quizzing him about his iron count when I see him next. The parasites eat the red blood cells and so anemia often develops which makes you more susceptible.
Your suggestions are spot-on. Yet, since we live in a malaria area, I do believe the DDT could be a definite plus.