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This disease in humans results from infection with a protozoan blood parasite transmitted by a species of the mosquito genus Anopheles. The human clinical condition known as malaria is caused by infection with one of four species of the genus Plasmodium: Plasmodium falciparum, P. vivax, P. malariae, and P. ovale. The first two are the most common and most important, and P. falciparum infection often can be fatal in the absence of treatment.
The Plasmodium species are blood parasites, although some also invade liver cells where they lie dormant until later release brings a relapse with fevers associated with the destruction of red blood cells. The vector mosquitoes imbibe the parasites with the bloodmeal, and the sexual stages unite in the mosquito gut to create a stage which invades the gut wall and forms a cyst, which in turn releases many infective stages (sporozoites) which invade the salivary glands, and are injected into a new host when the mosquito feeds. The sporozoites invade liver cells and later developmental stages of the parasite invade red blood cells which they disrupt (causing fever), form sexual stages and the cycle is completed.
Malaria is characterised clinically by fever (usually periodic), varying degrees of anaemia and splenic enlargement, and a range of syndromes resulting from the physiological and pathological involvement of certain organs, including the brain, liver and the kidneys.
Mosquito vectors of human malaria vary with global region and sometimes even locality, but all belong to the genus Anopheles. However, not all Anopheles can transmit the parasites; relatively few species are important primary vectors, with others of secondary importance, and many of little or no consequence.
Malaria is one of the most widespread of all human parasitic diseases, and in the early part of the last half century more than two-thirds of the world's population lived in endemic areas. By about 1950 it was estimated that the annual incidence of the disease was approximately 250 million cases, with about 2.5 million deaths per year, and significant interference in the agricultural and industrial development of tropical countries. From the 1950s, the World Health Organisation instigated and co-ordinated efforts towards worldwide eradication of the disease.
By the early 1970's the population freed from the risk of malaria transmission had increased from 400 million to over 1200 million, with the disease being eradicated from the whole of Europe, most of North America, most of the Caribbean and large parts of South America, and Singapore, Japan, Korea, Taiwan and Australia.
However in recent years the position has deteriorated in some countries where control efforts were making an impact, and the disease has returned to others from which it had been regarded as eradicated. In Africa south of the Sahara, where little eradication has been attempted, the distribution of malaria has remained essentially unchanged. In 1988, the global incidence of malaria was estimated to be of the order of 110 million cases annually with some 270 million people infected. At the present time, the momentum for global malaria eradication has declined, and few tropical countries maintain the enthusiastic and often efficient vector control efforts that existed in the 1960's.
Prevention of malaria in many countries has been heavily dependent on anti-malarial drugs and residual insecticides since the 1950s, but this has broken down in a lot of places for various reasons, including development of drug resistance by the parasites, insecticide resistance by the mosquitoes, and failures in administrative and logistical systems at central, regional and local government levels. Thus, the importance of self protection for local inhabitants and visiting travellers in"at-risk" areas has become more and more emphasised in recent years. This involves the use of protective clothing where possible, use of chemical repellents, and the use of bed nets when sleeping in unscreened or otherwise unprotected situations. Drugs for protection against infection are still useful in certain areas, but expert advice should be sought with respect to the regions and localities being visited, the time to be spent there, and other factors likely to relate to exposure to infection.
Malaria in Australia
In Australia, malaria has been endemic, but the malaria was declared eradicated from the country in 1981. Little is known of local vectors because few outbreaks were studied. However, laboratory investigations have revealed that a number of local Anopheles species are susceptible to infection, and An. amictus, An. annulipes, An. bancroftii, An. farauti and An. hilli have been possibly involved in field transmission.
Anopheles farauti is a major vector of malaria in Papua New Guinea, and it is presumed to be the species of greatest concern in the north of Australia. However, this species is recognised to be a complex of closely related species, the members of which cannot be differentiated by eye, and the capability to transmit malaria of the different members is unknown. In southern Australia, An. annulipes has apparently been the vector where occasional cases of malaria have been contracted, but this species is likewise a species complex and the relative capacities of the various members to transmit malaria is also unknown.
Although malaria is no longer endemic in Australia, approx. 700-800 cases occur here each year in travellers infected elsewhere, and the region of northern Australia above 19oS latitude is the receptive zone for malaria transmission. Occasional cases of local transmission occur in the Torres Strait islands and rarely in northern Queensland, and vigilance is required to prevent reestablishment of the infection in some northern localities.
Richard C. Russell
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