Factors influencing the pattern of imported malaria

Data on imported malaria in industrialized areas are known to be incomplete because of underreporting and lack of homogeneity. These facts and the complexity of factors influencing the transmission of malaria render their interpretation difficult. The relevance of various factors is usually not fully considered, although their impact on recommendations for chemoprophylaxis may be important.

All malaria cases imported from Kenya from 1988 to 1996 that were reported to the Federal Office of Public Health of Switzerland were analyzed. The reciprocal impact on data interpretation with regard to Plasmodium species, chemoprophylaxis, onset of first symptoms after return, male or female sex, seasonal fluctuation, duration of stay, nationality groups, and fatal outcome was analyzed.

Multivariate analysis showed a significant impact of Plasmodium species, regular chemoprophylaxis, and long duration of stay on the latency of malaria attacks. African origin and repeated stays were confounders with regard to adherence to chemoprophylaxis. The local situation of malaria transmission and the development of tourist figures were found to influence the evolution of malaria rates. These factors must be analyzed simultaneously to prevent errors in data interpretation. A higher proportion of tertian malaria cases (caused by Plasmodium vivax or Plasmodium ovale) than in previous reports was recorded owing to the impact of chemoprophylaxis and longer outbreak latencies. Seventy-five percent of tertian malaria cases were diagnosed within 6 months after return.

Factors influencing the pattern of imported malaria must be assessed in relation to each other, especially if data from different countries and various chemoprophylaxis regimens are compared. Furthermore, regular malaria chemoprophylaxis with mefloquine given until 4 weeks after return from an endemic area is not adequate to prevent tertian malaria. Regular chemoprophylaxis was found to cause longer latencies for all malaria species.