Malaria - the disease

The increasing threat of infection

Estimated incidence of clinical malaria episodes—caused by any species - resulting from local transmission, country level averages, 2004 (World Malaria Report 2005)

There are very many millions of people infected with the malaria parasite, and the problem may still be growing, despite the implementation of the Roll Back Malaria Partnership in many countries in 2000.⁵ Older drugs which are no longer effective because of resistance problems are still being used. The world's population is expanding rapidly, particularly in developing countries where malaria is endemic. At the same time, tourism and business travel to the tropics and subtropics are steadily increasing. In some countries, economic problems and political disturbance have interrupted the routine control measures used to reduce exposure to mosquitoes and to treat people already infected with malaria. Factors which may precipitate a malaria epidemic fall into two categories: natural (climatic variations, natural disasters), and man-made (conflict and war, agricultural projects, dams, mining, logging). Most of these factors modify the physical environment, and increase the capacity of mosquitoes to transmit malaria. Some factors also result in massive population movements that expose non-immune populations to malaria infection.⁶

Changes in climatic conditions also increase the threat of malaria. Both mosquitoes and malaria parasites need warm temperatures to multiply and tropical areas of the world have the best combination of adequate rainfall, temperature and humidity allowing for breeding and survival of anophelines.¹

Resistance

A key factor contributing to the increasing malaria mortality rate is the widespread resistance of Plasmodium falciparum to conventional antimalarial monotherapy drugs, such as chloroquine, sulphadoxine-pyrimethamine (SP) and amodiaquine. Multidrug-resistant P. falciparum malaria is widely prevalent in Southeast Asia and some Amazonian regions of South America. Now Africa, the continent with the highest burden of malaria, is increasingly developing resistance to monotherapies such as chloroquine and SP, with increased mortality as a result.

The inappropriate use of antimalarial drugs during the past century has contributed to the current situation. Antimalarial drugs were used as monotherapies and were generally poorly managed in that their use was continued despite unacceptably high levels of resistance. In addition, there has been over-reliance on both quinoline compounds and antifolate drugs, with consequent encouragement of cross-resistance among these compounds.⁷

Over the past decade, a new group of antimalarials – the artemisinin compounds, especially artesunate, artemether and dihydroartemisinin – have been deployed on an increasingly large scale. These produce a rapid therapeutic response, are active against multidrug-resistant P.falciparum malaria, are well tolerated by patients and reduce gametocyte carriage. To date, no parasite resistance to these compounds has been detected. Studies in Southeast Asia have shown that combinations of artemisinin compounds with certain other antimalarials produce high cure rates in just 3 days of treatment.