History and Geography

Since at least the mid-Pleistocene, many thousands of generations of humans have been parasitized by the plasmodia (Garnham 1966). Species very similar to the human plasmodia have also been described from gorillas and chimpanzees; indeed the relationships of the malaria parasites of humankind and the African apes are extremely close (Dunn 1966; Garnham 1966). It is therefore certain that human malaria is an Old World disease in its origins. There has, however, been some debate about the timing of malaria's appearance in the Western Hemisphere (McNeill 1976). Some have suggested that malaria was present in the New World long before the time of European contact, but a strong case can be made that the hemisphere was malaria-free until the end of the fifteenth century (Dunn 1965; McNeill 1976).

Malaria could have reached the New World before 1492 only as an infection of migrants from northeast Asia or by pre-Columbian sea-borne introductions. The possibility that humans brought malaria overland into North America from Siberia can almost certainly be discounted; conditions for malaria transmission were unsuitable in the far north during and after the Pleistocene, as they are today. It is equally unlikely that the Vikings could have introduced malaria in the centuries before Columbus. These voyagers came from regions of Europe and the North Atlantic (Greenland, Iceland) presumably free of malaria at that time, and they seem to have visited northeastern North American coasts that were north of any possible receptive anopheline mosquito populations. Similarly, any voyagers landing on American shores from the central or eastern Pacific could not have carried the parasites with them because islands in that region are free of anopheline vectors and thus of locally transmitted malaria. Voyagers reaching American coasts from eastern Asia (e.g., fishermen adrift from Japan) could conceivably have introduced malaria, but this possibility too is remote.

Moreover, and more to the point, colonial records indicate that malaria was almost certainly unknown to the indigenous peoples of the Americas. It is also evident that some areas that supported large pre-Columbian populations soon became dangerously malarious after European contact (McNeill 1976). The absence in aboriginal American populations of any of the blood genetic polymorphisms associated with malaria elsewhere in the world is another kind of evidence consistent with the conclusion that the

Western Hemisphere remained free of the disease until contact (Dunn 1965).

In the first two centuries after 1492, malaria parasites must have been introduced many times from Europe and Africa. Anopheline vectors were at hand, species native to the Americas. Together with smallpox, measles, and other infectious diseases brought across the Atlantic from the Old World, malaria soon began to contribute to the depopulation of the indigenous peoples, especially those of the lowland American tropics (Borah and Cook 1963; Crosby 1973; McNeill 1976).

From its early, usually coastal, sites of introduction, malaria spread widely in North, Central, and South America, limited principally by altitude and latitude - that is, by factors controlling the distribution of vector mosquitoes. By the nineteenth century in North America, the disease was prevalent in much of the Mississippi Valley; seasonal transmission occurred even in the northernmost areas of the Mississippi basin (Ackerknecht 1945). Malaria transmission, unstable and seasonal, also extended into the northeastern United States, well north in California, and far to the south in South America. By the eighteenth and nineteenth centuries, malaria had also become established as a stable endemic disease in the American subtropics and tropics, including most of the islands of the Caribbean.

The Old World gave malaria to the New; the New World, however, provided the first effective remedy for the disease. It is recorded that a sample of cinchona bark, taken from a Peruvian tree as a medicinal, was carried to Europe in 1632 by a Spanish priest (Russell et al. 1963). The bark was soon discovered to provide relief from certain intermittent fevers. This therapeutic action allowed Richard Morton and Thomas Sydenham in England in 1666, and Francesco Torti in Italy in 1712, to begin the process of defining malaria as a clinical entity separable from other fevers, which failed to respond to cinchona (Russell et al. 1963). By the end of the seventeenth century, cinchona bark was an important export product from Peru, well established in the treatment of intermittent fevers.

The human malaria parasites have evolved in association with their evolving hosts and have followed the human species through most, but not all, of its dispersal in Africa, Asia, and Europe. Transmission, dependent on suitable anopheline mosquitoes, has always been limited by all those environmental conditions, influenced by latitude and altitude, that control vector breeding and survival.

The antiquity of the human—Plasmodium associa tion in the Old World is symbolized, biologically, by the existence in modern populations of some of the blood genetic polymorphisms. Early texts - Chinese, Hindu, Chaldean, Greek - give us other evidence of this ancient association (Russell et al. 1963; McNeill 1976; Bruce-Chwatt and Zulueta 1980). Malaria was probably endemic in Greece by the fourth century, B.C.; Hippocrates described the types of periodicity of intermittent fevers - quotidian, tertian, quartan -and took note of the enlargement of spleens in those who lived in low, marshy districts. In Italy, too, the intermittent fevers were well known, for example, to Cicero, and well described by Celsus, Pliny the Elder, and Galen (Russell et al. 1963).

Malaria was certainly a disease of some importance in the centuries of Roman domination of Europe and the Mediterranean basin. However, in their review of malaria in early classical times, especially in relation to military operations, L. J. Bruce-Chwatt and J. de Zulueta (1980) reached the conclusion that malaria was much less destructive than it has been in recent centuries, primarily because P. falciparum was absent or rare and the other species were less intensely transmitted. The conclude that P. falciparum failed to spread in those centuries because Anopheles atroparvus was refractory as a vector for the falciparum strains then introduced from time to time by travelers. A. atroparvus, basically a zoophilic species, was also a poor vector for P. ma-lariae and P. vivax. By late classical times, however, it appears that two other anopheline species, Anopheles labranchiae and Anopheles sacharovi, had been introduced and dispersed along the coasts of southern Europe from their North African and Asian origins. These anthropophilic species were much more effective as vectors for all three of the species of plasmodia. By the final centuries of the Roman Empire, malaria was a more lethal force and may have contributed to the social, political, and cultural decline that had set in.

After the fall of the Roman Empire, the history of malaria in Europe and the Mediterranean region is obscure for many centuries (Bruce-Chwatt and Zulueta 1980). With few exceptions medieval medical writers provide only sketchy and confusing accounts of cases and outbreaks that may have been due to malaria. During the Renaissance, the historical record is clearer and richer, but malaria appears not to have represented a major problem to society in those centuries. It is not until the seventeenth and eighteenth centuries that malaria became resurgent in Europe, not only in the south but, in periodic outbreaks, as far north as the Netherlands, Ger many, southern Scandinavia, Poland, and Russia (Bruce-Chwatt and Zulueta 1980).

Through all of these centuries the record of malaria's human impact in Asia and Africa is fragmentary. With the onset of European colonization, however, it soon became obvious that endemic malaria was a threat almost everywhere in the Old World tropics, especially to the colonizers (who were, of course, generally nonimmune upon arrival in the tropics). The beginnings of tropical medicine and modern malariology are entangled in this recognition (Dunn 1984).

The modern era in malariology began in the last two decades of the nineteenth century with the identification of the causal parasites and the recognition of the role of anopheline mosquitoes as vectors. These discoveries provided the rationale for new strategies in malaria control, developed during the first third of the twentieth century. Malaria control itself was not a new concept; nor was the control of mosquitoes a new idea at this time. Humankind had sought from ancient times to control biting mosquitoes as pests (Russell et al. 1963). The new rationale, however, provided for much more specific vector control, directed principally at those anophelines that proved to be important in transmission. Malaria control was further strengthened in the 1930s with the introduction of synthetic antimalarials, useful not only in treatment but also in prophylaxis. The 1940s brought further advances in chemotherapy together with the first of the residual insecticides, DDT.

By the late 1940s and early 1950s, the resources for treatment and control of malaria appeared to be sufficiently formidable to justify attempts at national eradication in a few countries (e.g., Venezuela, Italy, the United States). Early success in local or aerial eradication, based on residual spraying, together with concerns about the emergence of anopheline resistance to DDT and other insecticides, led to a decision in 1955 to commit the World Health Organization to malaria eradication. This commitment pushed many countries into supposedly time-limited eradication campaigns, often with considerable WHO financial and adivsory support. Some of these campaigns were successful, especially in countries with unstable malaria, but others faltered after dramatic initial reductions in incidence. By the end of the eradication era, in the early 1970s, some hundreds of millions of people were living in areas where campaigns had eliminated endemic malaria (Clyde 1987), but in many other areas malaria continued to prevail, forcing a return to long-term control strategies.

In 1980 the World Health Organization began to recommend that malaria control be coordinated with primary health care. The experiences, especially the difficulties, of the eradication years also made it imperative that modern malaria control be strongly supported by epidemiological field work with recognition of local variability in transmission patterns and of sociocultural, economic, and human behavioral influences on transmission and control (Bruce-Chwatt 1987). Malaria control and therapy continues to be complicated by mosquito resistance to insecticides and by parasite resistance to drugs. Vaccine development proceeds, although slowly. The problems posed by malaria persist, but they are not insoluble. Malaria will remain endemic in many countries in the twenty-first century, but the prospects are good for steady improvement in control.

Frederick L. Dunn

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