Delayed attacks of malaria in visitors to the tropics.

Out of 80 Danish college students and their teachers visiting the Middle East, West Pakistan, and India by bus in two four-month journeys during the summer and autumn of 1971, six individuals developed an apparently first attack of vivax malaria within seven to nine months after their return to Denmark. No cases of malaria were seen in the group during the journeys. Prophylaxis with chloroquine (Resochin) was strictly observed when visiting the areas in which malaria is endemic.All cases responded rapidly and completely to a full course of treatment with chloroquine. The most probable cause of the delay of the clinical manifestations is drug prophylaxis, which can prevent a clinical attack but not always eradicate the parasite.