The risk of malaria infection in travelers is seldom known in detail and neither is the efficacy of different prophylactic regimens, due to a lack of controlled trials. Surveillance of malaria diagnosed after return can provide data on risk and efficacy.
An open case-control study was initiated. Imported cases were notified to our department and were studied in 320 permanent residents in Denmark, returning from abroad with malaria from 1997 to 1999. These were compared with a group of 600 travelers who were not infected with malaria and matched by age, sex, and destination. Information on the use of chemoprophylaxis and the length of stay in malarious areas were obtained by questionnaire.
Two hundred cases of Plasmodium falciparum malaria were notified of which 103 had used chloroquine and proguanil, 16 mefloquine, and 3 atovaquone and proguanil as prophylaxis, whereas the rest had taken other drugs or no prophylaxis. This study showed that the risk increased with increasing exposure and that compliance was lower especially for mefloquine users in malaria cases compared with controls. The study provided the first comprehensive data on the use of atovaquone/proguanil to travelers. The estimated efficacy of chloroquine and proguanil, mefloquine, and atovaquone and proguanil in fully compliant users was 1:599, 1:2,232, and 1:1,943, respectively, P. falciparum cases per prescription. The country specific risk data showed that the risk of getting malaria varied from 1 per 140 travelers to Ghana to almost 1 per 40,000 to Thailand, providing data that allow the use of prophylaxis to be restricted to high-risk areas.
There was a considerable variation in risk between the countries with the highest risk in tropical Africa. Chloroquine and proguanil was less efficient compared with mefloquine. Atovaquone/proguanil (Malarone) was at least as efficient as mefloquine, but breakthroughs were observed.