Malaria: still a threat to travellers

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Abstract

Each year, some 25–30 million international travellers from non-tropical regions visit malaria endemic countries. Up to 10 000 cases of malaria are imported into industrialised countries with an average case fatality rate of around 1%. Malaria still poses a real threat to travellers, particularly in areas with high transmission such as sub-Saharan Africa, Papua New Guinea and the South Pacific islands. The spread of malaria resistant to an increasing number of drugs forces us to adjust chemoprophylaxis regimens. Although new drugs are available and can protect travellers effectively, the number of imported cases to Europe and North America remains substantial. An increasing proportion of imported cases is seen in migrants and foreign-born residents visiting friends and relatives (VFR) in endemic countries. New easy-to-use drugs, better understanding of risk perception of travellers, finding new and innovative ways to reach at risk travellers such as VFRs are among the new challenges to improve malaria prevention for international travellers.

Introduction

Malaria remains one of the most serious diseases worldwide with an estimated 500 million cases occurring each year. It is endemic in 92 countries, with 41% of the world population being at risk of contracting the disease. More than one million deaths are attributed to malaria, the vast majority occurring in African children with an overall mortality rate of 5 per 1000 per year [1]. Malaria in travellers represents only a very small proportion of the global burden of the disease.

After the World Health Organisation launched the Global Malaria Eradication Campaign in 1955, malaria was controlled or eradicated in large areas of the world. Malaria disappeared from previously endemic countries such as Taiwan, Korea, a large proportion of the Caribbean, of the South Pacific, of the Indian sub-continent, North Africa, Southern Europe and the US. In the mid-1960s malaria control deteriorated and strains of Plasmodium falciparum resistant to chloroquine emerged. At the same time Anopheles mosquitoes became increasingly resistant to DDT. In the 1970s malaria increased dramatically. Since then, malaria has returned to areas where it had been controlled or eradicated previously, resistance has spread geographically and reduced the efficacy of many antimalarial compounds.

Transmission levels vary enormously according to the climate, environmental conditions influencing breeding sites and the efficiency of local vectors. In most areas, there are also marked seasonal variations in transmission, influencing the vector population density. The highest level of plasmodium transmission is observed in sub-Saharan Africa, Papua New Guinea (PNG) and the South Pacific islands. While P. falciparum is the predominant species in sub-Saharan Africa and the South Pacific, P. vivax and other species are more frequent in the Americas, the Middle East and most of Asia (Table 1) [2]. In most south-east Asian cities malaria transmission does not occur, but it is still present in cities in India and Africa (with the exception of Addis Ababa, Nairobi and Harare) [3].

Section snippets

Drug-resistant malaria

Chloroquine, a 4-aminoquinoline introduced in 1945, gave us a very efficient tool to combat malaria. With its long elimination half-life, chloroquine could be used as a prophylactic drug. It was cheap, well tolerated and effective against all strains of plasmodia but 12 years after its introduction the first cases of chloroquine resistant falciparum malaria were reported [4]. Since then resistance has spread to almost every part of the world with the exception of Central America and the Middle

Malaria exposure in travellers

During the last three decades tourism has increased dramatically. In 1995, the World Tourism Organisation estimated that there were 565.4 million international tourist arrivals. It is anticipated to reach more than a billion by 2010. Over the last 20 years, international travel to tropical countries has increased enormously. Today, some 25–30 million travellers from non-tropical countries visit malaria-endemic countries each year and up to 10 000 cases of malaria are reported in industrialised

Malaria prevention for travellers

Strategies for malaria prevention in travellers combine three types of interventions: information/education, anti-mosquito measures and use of antimalarial drugs as chemoprophylaxis or standby emergency treatment.

Informing and educating travellers

The vast majority of travellers travel for leisure and vacation. They want to enjoy a 1 or 2 weeks visit in an exotic country and to not have constraints of any kind. Many of the recommendations that doctors provide before travel are not particularly exciting: vaccination, taking daily anti-malaria pills, applying anti-mosquito lotions, avoiding eating salads and cold buffets, no ice cream, no ice cube in soda or whisky. No wonder people do not listen to the doctor's advice and compliance is

Personal protective anti-mosquito measures

Measures against mosquito bites are the first line of protection against malaria. Anopheles spp. mosquitoes bite at night from dusk to dawn. Various means are available to prevent mosquito bites that are best combined because all are not 100% effective alone. A large survey showed recently that air-conditioned rooms were the most effective protective device [11]. Sleeping in room protected by wire mesh is usually safe and mosquito nets are efficacious, impregnation with an insecticide

Chemoprophylaxis vs. standby emergency treatment

Choosing between these two approaches for a specific destination is based on the estimated risk of acquiring malaria (transmission), the predominance of Plasmodium species, the degree of resistance of the local P. falciparum strains to the respective drugs, the tolerability of the drugs to be used and the length of stay in the area at risk. Table 2 indicates incidence rates of malaria in travellers (see Table 2). The dotted line gives a possible cut-off point of incidence above which

Improving compliance and reaching at risk travellers

Do travellers take prophylaxis? In the largest study conducted in this field, the majority (99%) of 44 472 travellers were aware of malaria risk, but only 55.4% were compliant with chemoprophylaxis [18]. Other studies have shown similar results, but with some increase in the proportion of compliant travellers over the years. Compliance with preventive measures taken by North American and European travellers in East Africa increased from 42.4% in 1987 to 61.7% using both regular chemoprophylaxis

Conclusion

Although considerable efforts are being made in the development of malaria vaccines, none will be available for travellers in the near future. Thus chemoprophylaxis will remain the main protective measure available for travellers in areas of high transmission. Research and development should concentrate on well tolerated drugs which can be used for shorter periods of time, or agents active against the liver stage with a long half-life which could be given as one dose prior to departure.

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