Malaria: Geographical distribution

Many lay people regard malaria as a purely tropical disease. However, the distribution of malaria used to be world-wide. Today, it still occurs in some 100 countries. The situation varies from region to region. Until 1938 there was still P. vivax malaria ("polderkoorts") in Belgium, and in the Netherlands as late as 1958, although there was an unexplained (possibly autochtonous) case of P. malariae infection in a child in Zealand in 1969. The WHO declared the Netherlands officially malaria-free only in 1970. It is chiefly the pollution of surface waters which makes reproduction of Anopheles mosquitoes difficult. Yet Anopheles plumbeus, a vector which preferably breeds in water in hollow trees (and car tyres), Anopheles atroparvus, a brackish water mosquito, and its close relative A. messeae, still occur in the Low Countries. Anopheles atroparvus is able to transmit Plasmodium vivax malaria, but cannot transmit Plasmodium falciparum. Anopheles plumbeus was previously an exclusively zoophilic vector, which in recent years has fed more and more on humans. This mosquito can transmit tropical falciparum malaria. In the last century there were important changes in the lifestyle of humans, resulting in less human/mosquito contact. Effective therapy is also available. All these factors mean that malaria has disappeared in Northwest Europe. Cases in Western countries are generally dealt with swiftly and satisfactorily, and one person with malaria very rarely leads to the infection of others. Chronic reintroduction of the disease in Europe is thus very improbable. To maintain an infectious disease it is necessary for one infectious case to lead to one other infectious case, otherwise the disease will die out in the area. In Europe there are at present insufficient gametocyte carriers and vectors to ensure the continuation of the disease.

Malaria is a very important public health problem. The number of clinical cases is estimated at 150 to 300 million per year. Of these, approximately 1 million, mainly young children die every year in Africa alone. Most lethal infections are due to Plasmodium falciparum. The disease causes symptoms such as fever, shivers, headache and muscular pain, anaemia and splenomegaly. Involvement of the brain often leads to death. For some years P. falciparum has been developing increasing resistance to chloroquine and other anti-malaria products. This, of course, makes treatment difficult. Many mosquitoes which are responsible for transmission of the disease are becoming resistant to a number of insecticides and this makes vector control difficult.

• P. falciparum is the most common form in sub-Saharan Africa. It occurs chiefly in Africa, tropical South America and Southeast Asia. The parasite occurred previously in the Mediterranean basin. It does not occur outside the tropics and subtropics.

• P. vivax has the widest distribution area (previously as far as London, Norway, Denmark, New York, southern Canada and even Siberia). In 1922 the number of cases in Texas was estimated at 500,000. It is the most common form in certain regions (e.g. Maghreb countries). P. vivax preferentially penetrates young red blood cells (reticulocytes). To this end the merozoites have two proteins at their apical pole (PvRBP-1 and PvRBP-2). In 1976 Miller discovered that Plasmodium vivax uses the Duffy blood group antigens (Fy^a and Fy^b) as receptors to penetrate red blood cells. These antigens do not occur in the majority of humans in West Africa [phenotype Fy (a-b-)]. As a result P. vivax does not occur in West Africa. Duffy blood group negative erythrocytes are, in vitro, also resistant to infection with P. knowlesi (monkey malaria).

• P. ovale: chiefly West Africa, less elsewhere in Africa and sporadically in the Far East.
  

• P. malariae is not very common anywhere.

Category: Medicine Notes