Malaria control, until recently the sole aim of campaigns in most countries, implies the reduction of the disease until it is no longer a major public-health problem. Control must be maintained by continuous active work. The program is therefore unending. "Vector eradication" means the total elimination of all members of the mosquito species carrying the disease, so that they do not breed when the program is ended. It is therefore a project limited in time. It is feasible, and has been achieved, in some places, but is not practicable everywhere. "Malaria eradication" is the application of the same principle, not to the mosquito but to the malaria parasite, and has been shown already to be applicable in many countries. As generally used, the term does not imply that vector eradication is necessarily to be achieved too. Malaria dies out in infected individuals within 3 years; the aim of eradication is to break the cycle of transmission for 3 years, so that thereafter antimosquito measures can be discontinued, leaving the mosquitos in existence but without the possibility of becoming infected. 6. INSECT RESISTANCE A second development was ominous-resistance to insecticides. As early as 1947, it was found in Greece that the housefly and a kind of house mosquito (Culex molestus) had become resistant to DDT. In 1951, our troops in Korea were threatened by the development of resistance to DDT in body lice. Thereafter resistance developed rapidly all over the world, and disease-carrying insects in many areas became resistant not only to DDT but also to newer and more potent insecticides. By 1955, the number of new species of insects reported as resistant was 37. This phenomenon is the result of genetic selection ("the survival of the fittest"). Insects thus capable of withstanding the insecticides, created a worldwide problem. Not only the malaria program, but the effectiveness of insecticides in agriculture, were threatened. 7. RESPONSE ERADICATION The response of WHO to the favorable and the unfavorable developments was twofold. First, a new and more extensive malaria eradication program was mapped out. Eradication might have remained an exceptional aim if the development of resistance had not made it probable that mere control by spraying over extended periods would be futile. The experience in Greece demonstrated that an eradication campaign would call for the provision of special finance for a limited time only. This represented a necessity more easily understood by the local people and their legislative representatives than indefinitely continued expenditure on malaria control. Eradication, previously regarded as an ideal in favorable circumstances, had therefore now become a matter of urgency. The cycle of transmission had to be broken before insecticide resistance developed more widely. Aware of the increasing danger of resistance in the transmitting mosquitoes, the Eighth World Health Assembly in 1955 decided that the World Health Organization should take the initiative, provide technical advice, and encourage research and coordination of resources in the implementation of a program having as its ultimate objective the worldwide eradication of malaria. The same resolution authorized the Director General to obtain financial contributions and to establish a malaria eradication special account. New requirements New techniques had to be developed for the new program. A malaria eradication program differs radically from mere control in a number of ways. The standard of execution must be perfect, and must be checked by constant surveillance of the population for fresh cases. The source of any case must be investigated, and any spread from that source prevented. As the campaign approaches its goal, the detection of residual foci of transmission becomes more important. This may be difficult as many foci are likely to display themselves only through the movement of residents, to appear as malaria cases in areas supposedly cleared of the disease. The source of the infection must be sought. It may turn out to be overlooked hamlets or possibly a group of regular migrants. Whatever the mechanism of an eradication campaign, its task is accomplished when the evidence of surveillance and a methodical search for cases indicate that transmission is interrupted and that the number of potentially infective carriers is reduced, if not to zero at least to an insignificant level. The active campaign is then discontinued. But it is the essence of a successful eradication campaign that it must include a maintenance organization efficient enough to recognize reintroduction at the earliest possible moment. It is to be assumed that the apparent elimination may not be perfect, and that immigrants or infected mosquitoes from outside the country may reintroduce the disease. The most likely source is an immigrant or a person with longstanding symptomless infection, who will therefore not be detected as a case but will infect a certain number of anophelines. The appearance of even one such case must be the signal for an immediate intensive search for other cases, and also for a temporary and local antimosquito campaign. The number of secondary cases arising from this source will be fairly small, but being nonimmunes they will become highly infective to mosquitoes. Malaria is then transmitted with increasing frequency until an obvious epidemic results. 8. THE RESEARCH RESPONSE The conversion of malaria control programs into the aim of global malaria eradication with its much more demanding criteria showed gaps in knowledge. It necessitated the study of a series of important problems not by routine surveys but by special field and laboratory investigations carried out by experienced research workers. The new problems are typical of the manner in which a disease control program generates problems requiring research for solution. It comprised the following subjects: (a) Behavior characteristics (including biting activity and host preferences) of several anopheline vectors. 38516 59 (b) Phenomena of resistance of anopheline vectors to residual insecticides. Biochemistry and genetics. (c) Drug resistance of malaria parasites. (d) Host-parasite relationship in highly endemic areas. (e) Improvement of insecticide formulations and of spraying equipment. A vast amount of research work on these subjects was carried out by the WHO staff and by national personnel stimulated, assisted, and coordinated by the Organization. Finances for research were not immediately available. Our own Public Health Service was fortunately in a position to be of particular service. The results showed once again that every malaria-eradication program must be carefully adjusted to the local conditions prevailing and that hard-and-fast rules are not applicable in dealing with a community disease which has so many facets. The knowledge thus obtained permitted adaptation of resources to specific needs, saved many disappointments, and corrected many mistakes. Thus, for instance, in areas where the local anopheline was found sheltering in cattle sheds the extension of residual spraying to these outhouses became necessary. Where mud walls had a highly sorptive action on insecticide emulsions a different formulation consisting of a water-dispersible powder was used. Where the local population led a nomadic life antimalarial drugs had to supplement and often to replace the residual insecticides. Finally, where the resistance of the local vector was confined to one insecticide, another insecticide could solve the problem. As is the case in all the research work coordinated by WHO, the Organization actually pays or can pay out of its own meager budget for very little of the malaria research work done at its request or on its behalf. It receives without much cost the invaluable assistance and advice of institutions, laboratories, and individuals. The full extent of these cooperative relationships is outlined in part V. In malaria research, there were ample dividends of benefits to our Public Health Service, both directly and indirectly, in terms of possible applicability of findings to other research problems, for example. Generally, research problems have been approached through both laboratory and WHO field research. (a) Laboratory research WHO has established close relations with some 40 institutions and laboratories throughout the world. They collaborate with the Organization and undertake, on its behalf, a considerable volume of basic or laboratory research in malaria. Although small subventions. or grants-in-aid are paid to several of these institutions, most of them undertake research in the field of malaria at the request of WHO without cost to the Organization. In addition there are some 120 laboratories and individuals in various parts of the world working in close cooperation with the Organization on the resistance of insects to insecticides, including the problem of resistance in anophelines. Only six of these institutions receive any financial aid from WHO. The levels of financial assistance given in recent years and planned for 1959 to institutions and laboratories collaborating with WHO in this field are as follows: Every plan of malaria eradication must be based on a detailed survey of the conditions of the area involved, to determine the strategy of future action. This is necessary because the local features of malaria vary considerably in relation to the parasite species, its mosquito vector and the physical, biological and also socioeconomic environ ment. These preliminary surveys of every malaria campaign and the followup assessments of its results have comprised most of the malaria field research. The studies have required teams with specialists trained in such disciplines as systematic entomology, parasitology, geographical pathology and vital statistics. The need of field research will be recognized when we take into account that the difference in the habits of mosquitos affects the spread of malaria. With a mosquito which prefers human blood (like A. gambiae in Africa) malaria saturates the community, because few mosquitos are required to infect an entire population, and is stable from year to year and difficult to eliminate. With a mosquito which prefers animal blood (like A. culicifacies in India), the maintenance of malaria depends on the existence of a large mosquito population. In the latter case any antimosquito measure which is applied may be sufficient to reduce the number of mosquitos to the point where malaria will die out. 9. IMPRESSIVE RESULTS Has the eradication campaign been a success to date? The answer is that there has never been such a successful attack upon a disease of worldwide importance through a planned international effort (chart 3-C). One of the most remarkable improvements in the world health situation has been due to the widespread control of malaria. Out of the world's total 1958 population of 2,737 million, about 1,163 millions live in areas where malaria is still, or has recently been, endemic. In 1957 the protected population amounted to 454 million. Today's figures of prevalence and mortality are approximately half what they were 12 years ago: the present number of deaths stands at 11⁄2 million and the total cases about 150 million. In practically all the countries where malaria still constitutes a public health problem, programs of control are now being carried out; indeed, in regions other than Africa most countries have introduced nationwide programs. In six countries of Asia-Afghanistan, Burma, Ceylon, India, Indonesia, and Thailand-recent estimates indicate that control programs have already protected 121 million people out of 250 million living in malarious areas. Pilot projects |