Review of research on malaria*

Bull. Org. mond. Sante' 1974, 50, 151-157Bull. Wid Hlth Org. Review of research on malaria* T. LEPES 1 This review ofprogress in malaria research over the periods 1951-1970 and 1970-1973 indicates the results so far achieved in research on the parasite, on the immune response of the host, and on the vector; refers to the means of controlling or eradicating malaria that have been developed in recent years; and outlines the present status of the malaria control and eradication programme. Although impressive results have already been achieved in malaria research, more systematization and concentration of efforts are required if real breakthroughs are to be made. The experience gained in this respect is discussed. One way to assess the present position in malaria research is to review the results published so far. In order to give participants in the El Salvador sympo- sium some idea of the malaria research carried out during the period 1951-71, nearly 10 000 recorded abstracts were consulted, together with many origi- nal texts in toto. Many aspects of this research were dealt with in one phrase or even less and no firm recommendations were made, although general indications were given with the object of further stimulating research. The present symposium was convened with the purpose of defining more precisely the direction that malaria research should take. During the last 10-15 years a great "leap for- ward" was made in malaria research, particularly in the fields of immune response and chemotherapy inasmuch as a great many compounds were tested. An analysis of the literature indicated that, during the period 1971-73, at least 800 papers on malaria research were published. The total number of such papers is no doubt considerably greater, as a certain number, particularly those published in languages less widely understood, were not included in the analysis. Furthermore, the number indicated does not include the many-over 150-papers presented at the Ninth Congress of Tropical Medicine and Malaria, held in Athens in October 1973, or the * Presented at the Symposium on Malaria Research, Rabat, Morocco, 1-5 April 1974. 1 Director, Division of Malaria and Other Parasitic Diseases, World Health Organization, Geneva. papers presented at the Walter Reed Army Medical Center's Panel Workshop on Malaria Research pub- lished later under the title " Basic research on malaria ". The impressive number of publications shows the many aspects of malaria research that are now preoccupying research workers and field malari- ologists. Between 1950 and 1960, the literature provided valuable information on subjects relevant to the planning and manning of malaria control or eradication programmes, and less attention was paid to the host-parasite relationship. By 1960, the development of research techniques and laboratory appliances facilitating the manipulation of cells- and more particularly those dealing with the parasite at the molecular level-led to a new trend of research into the immune response of the host and into the immune phenomena involved in malaria infection. The numerous scientific group meetings, symposia, and panel workshops have also contributed to the systematization of malaria research and, in fact, the recommendations made at those meetings facilitated the selection of subjects for research. Whereas, in the 1950s, 30-50% of the publications related to malaria were devoted to case reports, surveys, the suscepti- bility or resistance of various species of anopheline, and general aspects of control and eradication, in recent years 75% of them have concerned system- atization. It is realized that any classification of the literature published may be considered as arbitrary. However, if the criterion of the single predominant aspect of research is applied, the following distribu- tion is obtained for the last three years: Epidemiology Parasitology Chemotherapy Pathology Immunology MiscellaneousYear & general Prstog & resistance (%/) (0/) (%/) (%o) (%o) (%o) 1971 12 17 17 9 24 21 1972 9 22 18 8 18 25 1973 8 20 20 9 17 26 3173 - 151- T. LEPES Among the miscellaneous publications, the re- search carried out on anophelines with modern genetic techniques is worthy of mention. Clinical observations, on the other hand, mostly refer to basic pathology-more particularly involvement of the brain and kidney and its clinical management- and the nephrotic syndrome observed in Plasmodium malariae infection as the consequence of an auto- immune complex. At the last two Congresses on Tropical Medicine and Malaria great attention was paid to entomology. Thus, at the Ninth Congress, in 1973, 25% of all malaria papers dealt with some aspect of anophe- lines. Other subjects were represented as follows: epidemiology, 20 %; chemotherapy, 15 %; immuno- logy, 10%; parasitology, 8%; and drug resistance, 3 %. Miscellaneous papers of general or local importance accounted for 19 %. This superficial analysis does not provide per se the elements for far-reaching conclusions and recom- mendations. Nevertheless it may facilitate considera- tion of further research needs. RESULTS ACHIEVED SO FAR Immune response of the human host Undoubtedly this subject, more than others, has attracted the attention of research workers and con- sequently the results obtained are more impressive than those on other aspects of malarial infection. There are probably three reasons for the intensive development in this field: refined laboratory research techniques have been elaborated; great advances have been made in immunology in general; and malarial infection per se offers an excellent model for the study of the immune response of the host. As regards the immune response, three major aspects should be considered: serological method- ology for the measurement of that response; the mechanisms involved in the immune response and the possibility of achieving different levels of pro- tective immunity; and, finally, the prospects of active or passive immunization against malaria. Much has been done in the field of malaria sero- logy in recent years, but there is still no technique for measuring protective immunity. Nevertheless, by simultaneously applying two or three techniques of longitudinal follow-up, the recorded changes in the intensity of the immune response may allow con- clusions to be drawn on protective immunity. Among the serological techniques so far developed, immunofluorescence has had the widest application. In recent years certain refinements have been introduced in connexion with the measurement of the intensity of fluorescence and the preparation of the conjugate. Attempts have also been made to explain the role of different antibodies bound to immunoglobulin classes, more specifically IgG and IgM, as conjugates in the indirect fluorescent anti- body test. Whereas, in the early years of application of this technique, cross-sectional surveys were pre- dominant, in recent years more longitudinal studies have been undertaken in different epidemiological situations. As a result, valuable experience has been gained in the interpretation of the data obtained. At the same time, the correct place of this technique in the epidemiological evaluation of malaria control or eradication programmes has been indicated. The sampling of sera in the population of a malarious area is of the utmost importance and the sampling technique used for this purpose has been improved. It is hoped that further refinements will facilitate standardization in all respects. Until recently, the indirect haemagglutination test was the privilege of one laboratory that utilized P. knowlesi as antigen. The test has now been made suitable for use in the field with P. falciparum- sensitized tanned erythrocytes in lyophilized form. Similarly, further experience has been gained with the gel-diffusion technique, which appears to be very sensitive in detecting changes in the immune response following the administration of antimalarial drugs, as shown in field studies in Senegal and in Nigeria. This technique has also proved valuable in studying the antigenic structure of plasmodia. Studies carried out during the last four years on the mechanism of the immune response of the human host, applying one cycle of schizogonic development in vitro, demonstrated the mechanism of humoral immunity. Experiments on monkeys and other laboratory animals, on the other hand, provided an insight into the role of T and B lymphocytes in the initiation of the immune response. At the same time, research on P. knowlesi in monkeys clearly demon- strated the survival of immunogenically variant populations in this species. These findings were of the greatest importance, not only for our understanding of the peculiarity of the immune response of the host following malarial infection, but also for the attempts made by a number of research workers to develop an immunizing agent. The efforts made towards developing a success- ful immunizing agent gave encouraging results, to be taken however with some reserve, though much 152 REVIEW OF RESEARCH ON MALARIA remains to be done before immunization becomes a reality. For understandable reasons, attempts were made to prepare the immunizing agent from blood forms of plasmodia as well as from sporozoites. Blood trophozoites and schizonts were used either after X-irradiation for different periods of time and intensities of rad or after fractionation of the antigen by different procedures, with or without adjuvant. Some methods have proved to be better than others, but in all instances considerable delay in incubation was observed. This indicates that an immunizing agent utilizing blood forms, while not providing total protection, could-if eventually developed for human use-alleviate the disease and possibly reduce to zero the mortality caused by malaria, even though it may not protect from the infection itself. On the other hand, " vaccine " pre- pared from sporozoites irradiated for a well-defined period of time and intensity of rad has proved very promising inasmuch as it provides total protection in a certain proportion of animals. In the rodent model this protection is either total or nonexistent, but the protected animals have resisted challenge with nonirradiated sporozoites of both homologous and heterologous species. Recent experiments on human volunteers have indicated that, whereas the use of irradiated sporozoites may prove to be the right approach towards developing a malaria " vaccine ", further studies are required before any attempts are made for their application on a larger scale. Research on immunization against malaria has already led to improved technical procedures for the fractionation, purification, and gradient separation of sporozoites. Through so-called " circumsporozoite precipitation ", antisporozoite antibody formation was also demonstrated. There are, however, a num- ber of technical problems requiring solution, such as in vitro cultivation of blood forms, cultivation of sporozoites on insect tissue, and mass production of sporozoites in mosquitos- to mention only a few. Nevertheless, these are grounds for hoping that, in the foreseeable future, further important advances will be made towards the development of a successful immunizing agent against human malaria. The parasite Significant developments have taken place in the past few years through research on the parasite itself-e.g., the discovery and description of the life cycle of P. silvaticum as a new species. Further work on the characterization of strains of plasmodia (e.g., the North Korean strain of P. vivax) has indicated that the population of sporozoites of one infective inoculum may consist of genetically different individuals some of which are apt to develop EE forms within a " normal " limit of time whereas others develop very slowly, thus necessitating a very long incubation period. However, this technique is cumbersome and purely empirical. On the other hand, a real progress in strain identification was made recently by introducing the identification of specific enzymes in the blood of individuals infected with given species of Plasmodium. The availability of this technique will certainly further enhance know- ledge of the existence of different strains of the same species-a possibility recognized for many years, although the means of proper identification were still lacking. Other valuable research work has been carried out on the ultrastructure of plasmodia, as a result of which it is now understood how plasmodia invade the host cells and by what mechanism their meta- bolism takes place. Other studies were carried out on the metabolism of plasmodia, particularly as regards proteins, amino acids, purines, folates, lipids, and carbohydrates. All this research should eventu- ally lead to improved in vitro cultivation of plas- modia, although, as already mentioned, little pro- gress has been made in this field so far. The vector The modern genetic techniques introduced into research on malaria vectors have brought to light many important features relating to the sibling spe- cies as well as to genetically distinguishable strain characteristics coded by specific patterns to chromo- some inversion polymorphism. Further research is required for the full interpretation of these findings, as they may reflect differences in bionomics within the same species of anopheline. Nevertheless pro- gress made so far is encouraging and should stimulate more research workers to apply the same techniques to different species collected in various biotopes, in relation to their feeding and resting habits. Research begun some years ago on the development of a suitable methodology and systems for the bio- logical and genetic control of vectors continues to attract the attention of research workers, although there is pehaps less enthusiasm than before. Indeed, the experience gained so far indicates that, whereas biological control offers better prospects than genetic control, neither of these methods would appear to be applicable on a large scale. This does not mean 153 T. LEPES that such studies should be abandoned. On the contrary, the biological control of diseases is par- ticularly worthy of further attention, although malariologists should not expect too much in the immediate future. Sampling techniques are still not quite satisfac- tory, and it is to be hoped that more attention will be paid to this aspect, which is so important in the evaluation of malaria control activities. Another feature of the bionomics of anophe- lines-i.e., the susceptibility or refractoriness to the development of the sporogony of different species of Plasmodium-is also receiving consideration. It is of particular interest to countries and areas from which malaria has already been eradicated but in which " anophelism without malaria " persists. It is not my intention to refer to well-known observations from the past, but simply to motivate some research workers, from the Mediterranean countries in par- ticular, to investigate this aspect, which should, in fact, be considered as an indication of the receptivity of an area to malarial transmission. Although no epidemics of malaria of local transmission were recorded as having stemmed from a parasite carrier of an imported case from a distant area, this does not prove the refractoriness of the local vector to other strains of Plasmodium. New means of controlling or eradicating malaria This heading covers chemical and biological or genetic control of the vector, chemotherapy, and the development of immunizing agents against mal- aria. Reference has already been made to the advances that have taken place in the fields of possible immu- nization and biological and genetic control. It is clear from the evaluation of the progress made in these two fields that, whereas both aspects require further research, priority should undoubtedly go to the former. However, chemical control of anophe- lines remains the method of choice and most prob- ably will continue to be for some time. There are not many new compounds that could be considered as substitutes for DDT. As insecticide resistance in certain anopheline species spreads in various parts of the world, chlorinated hydrocarbon insecticides need to be replaced by either organophosphorus compounds or carbamates, but these insecticides are much more expensive than DDT. On the other hand, resistance to malathion and propoxur has already been observed in certain species and therefore the use of other insecticides will have to be explored. WHO spares no efforts in testing new insecticides and is trying to accelerate this programme, but there are not many candidate insecticides. It is hoped that within a year the epidemiological evaluation of feni- trothion-a promising insecticide-will be success- fully completed, thus adding at least one insecticide to those that can be selected for different pro- grammes. Furthermore, it is hoped that the pesticide industry will continue research on the development of new compounds and make these available for screening and testing. In the field of chemotherapy tremendous efforts have been made in the last few years. In the USA alone, more than 215 000 compounds have been reviewed and tested for their effect on plasmodia. Unfortunately not many of these compounds could be considered as candidate drugs for use. In fact only four of them deserve further clinical and field trials-namely, derivatives of the 9-phenanthrene- methanols; 4-aminoquiline-methanols; 2,4-diamino- 6-sulfur-substituted quinazolines; and 1-benzoxy- alloxy-4,6-diamino-S-triazines. Although the results have been meagre, this research has considerably increased our knowledge on the mode of action of antimalarial drugs. Simultaneously, several models for the screening and testing of compounds together with step-by-step procedures were developed. Studies on chloroquine resistance in P. falciparum revealed that quinine or a combination of long-acting sulfon- amides and pyrimethamine may efficiently replace chloroquine. Studies on the mechanism of chloro- quine resistance indicate that such resistance is dependent on a defect in the high-affinity binding sites of the resistant parasite. All in all, important progress has been made in research on the various aspects of the parasite, its vector, and the host. These advances, coupled with the many research techniques already developed, should be a good stimulus to scientists in maintaining their efforts. STATUS OF THE MALARIA ERADICATION PROGRAMME The progress of the malaria eradication pro- gramme has been slow in the past few years and has even suffered significant setbacks in some large coun- tries, such as India and Pakistan. However, progress has been made in other countries, notably a number of countries in west Asia and South America. It was customary in the past to present the pro- gress of malaria eradication in terms of the popula- tions freed from malaria, protected by programmes 154 REVIEW OF RESEARCH ON MALARIA at different phases, or not yet benefiting from spe- cific antimalarial measures. The same type of presen- tation is used in this review because, although it does not reflect the actual status of the programme, it does indicate the extent of the problem. The figures in Table 1 show that nearly 800 million people living in originally malarious areas have al- ready been freed from the risk of malaria and that an additional 592 million are being protected by active programmes. On the other hand, out of the remain- ing 517 million covered by the table, 241 million live in areas where some antimalaria measures are being applied and 275 million live in areas-mostly in countries of Africa south of the Sahara-where no specific antimalaria measures are applied. Table 1. Populations freed from malaria, protected by active programmes, or living in areas where few or no antimalaria measures are applied Population (in millions) Type of programme 1957 1962 1967 1973 Malaria Eradication Programme maintenance 220 329 654 787 consolidation - 243 355 286 attack 569 461 277 306 preparatory - 33 42 4 Total 789 1 066 1 328 1 383 Control measures - - - 241 No specific measures 421 406 364 276 Total 421 406 364 517 Grand total 1 210 1 472 1 692 1 900 The slow progress of malaria eradication pro- grammes in the past few years, following the spec- tacular successes achieved in the first ten years of the programme, inevitably raises some questions, such as: has the progress of malaria eradication programmes reached a threshold beyond which no further progress is possible by the means currently available for the control or eradication of the dis- ease? Is malaria eradication as it is conceived a practicable proposition? The WHO Expert Committee on Malaria dis- cussed the latter question in some detail and con- cluded that there was nothing wrong with the con- cept. However, when it comes to the implementation of the programme, what was thought to be prac- ticable often turns out to be impracticable. Indeed, the revised strategy of malaria eradication allows for a flexible and realistic replanning of the pro- gramme based on available financial and manpower resources. Admittedly, the political and socio- economic situation, coupled with accelerating infla- tion and a reduction in bilateral and international assistance, has considerably reduced the potential of many countries to pursue their antimalaria pro- grammes efficiently. In view of the changed attitude of a number of governments towards providing the support required for their operating malaria eradi- cation programmes, and the technical problems met with in different countries as a result of resistance to insecticides or drugs, or the particular bionomics of the vector, one wonders whether more progress could be expected at present. However, despite the difficulties, there are good prospects for further progress, particularly in coun- tries of the Mediterranean Basin, west Asia, and most countries of south-east Asia and the Western Pacific, as well as in some countries of South America. In other countries of these areas significant progress cannot be expected for some time. Unfor- tunately, little has been done to control malaria in Africa south of the Sahara, and the international community should not remain indifferent to the high mortality caused by malaria among young children in that part of the world. The concept of an inte- grated approach to the control of endemic diseases through basic health services, although correct, does not seem to have had any impact on the endemicity of malaria in rural areas of Africa. There, tremen- dous efforts will be required in order to reduce malaria transmission significantly. However, no effi- cient means of interrupting transmission in the savanna areas of Africa exist at present. Thus there is no avoiding the conclusion that malaria will be with us for quite some time. In order to cope more efficiently with this world problem, much more research is required. Research per se cannot solve all the problems, so more complete training of malariologist-parasitologists is a conditio sine qua non for the proper application of scientific development. This would facilitate well-evaluated feedback to scientific laboratories in further devel- oping ideas, research, and field techniques as well as more efficient malaria control methods. Furthermore, malaria presents a certain threat to 155 T. LEPES countries from which it has already been eradicated or in which it has never existed. The medical profes- sion should therefore be made aware of the possibly fatal consequences for nonimmune individuals who contract malaria-particularly P. falciparum ma- laria-in endemic areas. With the tremendous devel- opment of the tourist trade, which constitutes one of the possibilities for economic development of the " third world ", more and more such cases are being recorded. These and other aspects of malaria research are dealt with in the papers presented at the symposium. RtSUMIt LA RECHERCHE SUR LE PALUDISME: REVUE DE LA QUESTION La litterature consacree A la recherche sur le paludisme est trbs abondante: durant la periode 1951-1971, pres de 10 000 articles ont ete publies et de 1971 A 1973, 800 au moins. Une analyse des travaux parus au cours des 10 A 15 demieres annees indique que cette recherche est devenue plus systematique; la raison en est, tres proba- blement, le grand nombre de rdunions, symposiums et conferences-ateliers ou l'accent a ete mis sur les princi- paux domaines A explorer. Les acquisitions nouvelles concemant l'infection palu- d6enne sont en grande partie le fruit des etudes sur la reponse immunitaire de l'h6te humain. C'est ainsi qu'on a mis au point plusieurs epreuves serologiques relative- ment fiables, elucide le mecanisme de l'immunite humo- rale et procede A de nombreux essais, parfois avec un succes appreciable, en vue d'immuniser l'h6te contre l'infection paludeenne. Chez le modele du paludisme des rongeurs, les sporozoltes irradies se sont reveles tres efficaces lorsque les animaux ayant requ ce vaccin ont subi une infection d'epreuve par des sporozoltes non irradi6s. La protection ainsi obtenue est totale ou nulle. En outre, elle est fonction de la dose et plus forte si l'on utilise un vaccin renfermant un plus grand nombre de sporozoites ou si l'on multiplie les doses. Plusieurs preparations obtenues A partir de formes erythrocytaires des parasites par differents procedds, avec ou sans adju- vant, ont confere une protection dans le paludisme des rongeurs ou des singes. N6anmoins, leur action ne se manifeste que par un allongement de la periode de prepatence et, dans certains cas, par la survie de l'animal A des infections qui, autrement, sont letales pour la souris. En ce qui regarde plus specialement le parasite, les recherches des dernieres annees ont amene non seulement la description de quelques especes nouvelles, mais aussi une meilleure caracterisation des souches. On a consacre beaucoup d'efforts au probleme de la culture in vitro des plasmodiums en etudiant plus particulierement certains aspects de leur metabolisme: proteines, acides amines, purines, folates, lipides et hydrates de carbone. Dans le domaine de la bionomie des vecteurs, l'emploi des techniques genetiques a donne des resultats interes- sants et importants, l'objectif etant d'identifier des popu- lations germaines ou d'etudier le comportement des moustiques en fonction de caracteristiques chromoso- miques spdcifiques. Pour mettre au point de nouveaux procedes ou de nouvelles methodes de lutte contre le paludisme, on a experiment6 de nouveaux insecticides et, en outre, etudie les possibilites de recours A la lutte genetique contre les vecteurs. Neanmoins, l'essentiel des efforts a porte sur la recherche de nouveaux medicaments. Rien qu'aux Etats-Unis d'Amerique, on a examine plus de 215 000 composes et evalue leur action contre les plasmodiums. Quatre de ces groupes de produits pour- ront eventuellement etre utilises dans les programmes de lutte antipaludique des qu'ils auront franchi le cap des essais cliniques et des essais sur le terrain. Pendant ces dernieres annees, les progres du programme mondial d'eradication du paludisme ont ete relativement lents. Cependant, pres de 800 millions de personnes vivant dans des regions autrefois impaludees ne sont plus actuellement exposees au risque d'infection, et 592 millions d'autres beneficient de programmes de protection efficace. Pour 275 millions de personnes, vivant pour la plupart en Afrique au sud du Sahara, il n'existe malheureusement aucune forme organisee de protection contre la maladie. Malgre les difficultes de la tache, de nouveaux progres sont possibles A la condition d'intensifier la recherche et d'assurer aux paludologues et aux parasitologistes la formation indispensable. DISCUSSION BRUCE-CHWATT: One of the main problems being faced now, and one that will occur even more in the future, is the gradual attrition of research centres on human malaria, especially in the USA. No animal model can provide the final verdict on the value or otherwise of new antimalarial drugs or 156 REVIEW OF RESEARCH ON MALARIA immunological methods of protection against human malaria. The WHO Scientific Groups on Chemo- therapy of Malaria and the WHO Expert Committee on Malaria recommended the continuation of re- search centres on human malaria. At this critical time in the history of the antimalaria campaign, the seriousness of the situation should be emphasized and the work of such centres should be facilitated and supported, subject to all necessary ethical and professional constraints. UNGUREANU: I agree. The value of malaria therapy stations should not be overlooked. SCHMIDT: The situation with respect to the continued operation of research centres on human malaria in the USA is grave. I believe that decisions on such continuation might be favourably influenced if the important contribution of these centres-especially to the development of new antimalarials-were acknowledged. 157