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 1970-1973indicates the results so far achieved in research on the parasite, on the immune responseof the host, and on the vector; refers to the means of controlling or eradicating malariathat have been developed in recent years; and outlines the present status of the malariacontrol and eradication programme. Although impressive results have already been achievedin malaria research, more systematization and concentration of efforts are required if realbreakthroughs are to be made. The experience gained in this respect is discussed.

One way to assess the present position in malariaresearch is to review the results published so far. Inorder to give participants in the El Salvador sympo-sium some idea of the malaria research carried outduring the period 1951-71, nearly 10 000 recordedabstracts were consulted, together with many origi-nal texts in toto. Many aspects of this research weredealt with in one phrase or even less and no firmrecommendations were made, although generalindications were given with the object of furtherstimulating research. The present symposium wasconvened with the purpose of defining more preciselythe direction that malaria research should take.During the last 10-15 years a great "leap for-

ward" was made in malaria research, particularlyin the fields of immune response and chemotherapyinasmuch as a great many compounds were tested.An analysis of the literature indicated that, during

the period 1971-73, at least 800 papers on malariaresearch were published. The total number of suchpapers is no doubt considerably greater, as a certainnumber, particularly those published in languagesless widely understood, were not included in theanalysis. Furthermore, the number indicated doesnot include the many-over 150-papers presentedat the Ninth Congress of Tropical Medicine andMalaria, 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 ParasiticDiseases, World Health Organization, Geneva.

papers presented at the Walter Reed Army MedicalCenter's Panel Workshop on Malaria Research pub-lished later under the title " Basic research onmalaria ". The impressive number of publicationsshows the many aspects of malaria research that arenow preoccupying research workers and field malari-ologists. Between 1950 and 1960, the literatureprovided valuable information on subjects relevantto the planning and manning of malaria control oreradication programmes, and less attention was paidto the host-parasite relationship. By 1960, thedevelopment of research techniques and laboratoryappliances facilitating the manipulation of cells-and more particularly those dealing with the parasiteat the molecular level-led to a new trend of researchinto the immune response of the host and into theimmune phenomena involved in malaria infection.The numerous scientific group meetings, symposia,and panel workshops have also contributed to thesystematization of malaria research and, in fact, therecommendations made at those meetings facilitatedthe selection of subjects for research. Whereas, in the1950s, 30-50% of the publications related to malariawere devoted to case reports, surveys, the suscepti-bility or resistance of various species of anopheline,and general aspects of control and eradication, inrecent years 75% of them have concerned system-atization. It is realized that any classification of theliterature published may be considered as arbitrary.However, if the criterion of the single predominantaspect 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 211972 9 22 18 8 18 251973 8 20 20 9 17 26

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Among the miscellaneous publications, the re-search carried out on anophelines with moderngenetic techniques is worthy of mention. Clinicalobservations, on the other hand, mostly refer tobasic pathology-more particularly involvement ofthe brain and kidney and its clinical management-and the nephrotic syndrome observed in Plasmodiummalariae 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 allmalaria 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 localimportance accounted for 19 %.

This superficial analysis does not provide per sethe 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 hostUndoubtedly this subject, more than others, has

attracted the attention of research workers and con-sequently the results obtained are more impressivethan those on other aspects of malarial infection.There are probably three reasons for the intensivedevelopment in this field: refined laboratory researchtechniques have been elaborated; great advanceshave been made in immunology in general; andmalarial infection per se offers an excellent model forthe 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; themechanisms involved in the immune response andthe possibility of achieving different levels of pro-tective immunity; and, finally, the prospects ofactive 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 formeasuring protective immunity. Nevertheless, bysimultaneously applying two or three techniques oflongitudinal follow-up, the recorded changes in theintensity 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 beenintroduced in connexion with the measurement ofthe intensity of fluorescence and the preparation ofthe conjugate. Attempts have also been made toexplain the role of different antibodies bound toimmunoglobulin classes, more specifically IgG andIgM, as conjugates in the indirect fluorescent anti-body test. Whereas, in the early years of applicationof this technique, cross-sectional surveys were pre-dominant, in recent years more longitudinal studieshave been undertaken in different epidemiologicalsituations. As a result, valuable experience has beengained in the interpretation of the data obtained.At the same time, the correct place of this techniquein the epidemiological evaluation of malaria controlor eradication programmes has been indicated. Thesampling of sera in the population of a malariousarea is of the utmost importance and the samplingtechnique used for this purpose has been improved.It is hoped that further refinements will facilitatestandardization in all respects.

Until recently, the indirect haemagglutination testwas the privilege of one laboratory that utilizedP. knowlesi as antigen. The test has now been madesuitable for use in the field with P. falciparum-sensitized tanned erythrocytes in lyophilized form.Similarly, further experience has been gained withthe gel-diffusion technique, which appears to be verysensitive in detecting changes in the immune responsefollowing the administration of antimalarial drugs,as shown in field studies in Senegal and in Nigeria.This technique has also proved valuable in studyingthe antigenic structure of plasmodia.

Studies carried out during the last four years onthe mechanism of the immune response of the humanhost, applying one cycle of schizogonic developmentin vitro, demonstrated the mechanism of humoralimmunity. Experiments on monkeys and otherlaboratory animals, on the other hand, provided aninsight into the role of T and B lymphocytes in theinitiation of the immune response. At the same time,research on P. knowlesi in monkeys clearly demon-strated the survival of immunogenically variantpopulations in this species. These findings were of thegreatest importance, not only for our understandingof the peculiarity of the immune response of the hostfollowing malarial infection, but also for the attemptsmade by a number of research workers to develop animmunizing agent.The efforts made towards developing a success-

ful immunizing agent gave encouraging results, to betaken however with some reserve, though much

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remains to be done before immunization becomes areality. For understandable reasons, attempts weremade to prepare the immunizing agent from bloodforms of plasmodia as well as from sporozoites.Blood trophozoites and schizonts were used eitherafter X-irradiation for different periods of timeand intensities of rad or after fractionation of theantigen by different procedures, with or withoutadjuvant. Some methods have proved to be betterthan others, but in all instances considerable delayin incubation was observed. This indicates that animmunizing agent utilizing blood forms, while notproviding total protection, could-if eventuallydeveloped for human use-alleviate the disease andpossibly reduce to zero the mortality caused bymalaria, even though it may not protect from theinfection itself. On the other hand, " vaccine " pre-pared from sporozoites irradiated for a well-definedperiod of time and intensity of rad has proved verypromising inasmuch as it provides total protectionin a certain proportion of animals. In the rodentmodel this protection is either total or nonexistent,but the protected animals have resisted challengewith nonirradiated sporozoites of both homologousand heterologous species. Recent experiments onhuman volunteers have indicated that, whereas theuse of irradiated sporozoites may prove to be theright approach towards developing a malaria" vaccine ", further studies are required before anyattempts are made for their application on a largerscale.

Research on immunization against malaria hasalready led to improved technical procedures for thefractionation, purification, and gradient separationof sporozoites. Through so-called " circumsporozoiteprecipitation ", antisporozoite antibody formationwas also demonstrated. There are, however, a num-ber of technical problems requiring solution, such asin vitro cultivation of blood forms, cultivation ofsporozoites on insect tissue, and mass production ofsporozoites in mosquitos- to mention only a few.Nevertheless, these are grounds for hoping that, inthe foreseeable future, further important advanceswill be made towards the development of a successfulimmunizing agent against human malaria.

The parasite

Significant developments have taken place in thepast few years through research on the parasiteitself-e.g., the discovery and description of the lifecycle of P. silvaticum as a new species. Further workon the characterization of strains of plasmodia (e.g.,

the North Korean strain of P. vivax) has indicatedthat the population of sporozoites of one infectiveinoculum may consist of genetically differentindividuals some of which are apt to develop EEforms within a " normal " limit of time whereasothers develop very slowly, thus necessitating a verylong incubation period. However, this technique iscumbersome and purely empirical. On the otherhand, a real progress in strain identification wasmade recently by introducing the identification ofspecific enzymes in the blood of individuals infectedwith given species of Plasmodium. The availability ofthis technique will certainly further enhance know-ledge of the existence of different strains of the samespecies-a possibility recognized for many years,although the means of proper identification werestill lacking.

Other valuable research work has been carriedout on the ultrastructure of plasmodia, as a resultof which it is now understood how plasmodia invadethe host cells and by what mechanism their meta-bolism takes place. Other studies were carried outon the metabolism of plasmodia, particularly asregards 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 intoresearch on malaria vectors have brought to lightmany important features relating to the sibling spe-cies as well as to genetically distinguishable straincharacteristics coded by specific patterns to chromo-some inversion polymorphism. Further research isrequired for the full interpretation of these findings,as they may reflect differences in bionomics withinthe same species of anopheline. Nevertheless pro-gress made so far is encouraging and should stimulatemore research workers to apply the same techniquesto different species collected in various biotopes,in relation to their feeding and resting habits.Research begun some years ago on the developmentof a suitable methodology and systems for the bio-logical and genetic control of vectors continues toattract the attention of research workers, althoughthere is pehaps less enthusiasm than before. Indeed,the experience gained so far indicates that, whereasbiological control offers better prospects than geneticcontrol, neither of these methods would appear tobe applicable on a large scale. This does not mean

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that such studies should be abandoned. On thecontrary, the biological control of diseases is par-ticularly worthy of further attention, althoughmalariologists should not expect too much in theimmediate future.

Sampling techniques are still not quite satisfac-tory, and it is to be hoped that more attention willbe paid to this aspect, which is so important in theevaluation of malaria control activities.

Another feature of the bionomics of anophe-lines-i.e., the susceptibility or refractoriness to thedevelopment of the sporogony of different speciesof Plasmodium-is also receiving consideration. It isof particular interest to countries and areas fromwhich malaria has already been eradicated but inwhich " anophelism without malaria " persists. It isnot my intention to refer to well-known observationsfrom the past, but simply to motivate some researchworkers, from the Mediterranean countries in par-ticular, to investigate this aspect, which should, infact, be considered as an indication of the receptivityof an area to malarial transmission. Although noepidemics of malaria of local transmission wererecorded as having stemmed from a parasite carrierof an imported case from a distant area, this doesnot prove the refractoriness of the local vector toother strains of Plasmodium.

New means of controlling or eradicating malaria

This heading covers chemical and biological orgenetic control of the vector, chemotherapy, andthe development of immunizing agents against mal-aria.

Reference has already been made to the advancesthat have taken place in the fields of possible immu-nization and biological and genetic control. It isclear from the evaluation of the progress made inthese two fields that, whereas both aspects requirefurther research, priority should undoubtedly go tothe former. However, chemical control of anophe-lines remains the method of choice and most prob-ably will continue to be for some time. There arenot many new compounds that could be consideredas substitutes for DDT. As insecticide resistance incertain anopheline species spreads in various partsof the world, chlorinated hydrocarbon insecticidesneed to be replaced by either organophosphoruscompounds or carbamates, but these insecticides aremuch more expensive than DDT. On the other hand,resistance to malathion and propoxur has alreadybeen observed in certain species and therefore theuse of other insecticides will have to be explored.

WHO spares no efforts in testing new insecticidesand is trying to accelerate this programme, but thereare not many candidate insecticides. It is hoped thatwithin a year the epidemiological evaluation of feni-trothion-a promising insecticide-will be success-fully completed, thus adding at least one insecticideto those that can be selected for different pro-grammes. Furthermore, it is hoped that the pesticideindustry will continue research on the developmentof new compounds and make these available forscreening and testing.

In the field of chemotherapy tremendous effortshave been made in the last few years. In the USAalone, more than 215 000 compounds have beenreviewed and tested for their effect on plasmodia.Unfortunately not many of these compounds couldbe considered as candidate drugs for use. In factonly four of them deserve further clinical and fieldtrials-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 resultshave been meagre, this research has considerablyincreased our knowledge on the mode of action ofantimalarial drugs. Simultaneously, several modelsfor the screening and testing of compounds togetherwith step-by-step procedures were developed. Studieson chloroquine resistance in P. falciparum revealedthat quinine or a combination of long-acting sulfon-amides and pyrimethamine may efficiently replacechloroquine. Studies on the mechanism of chloro-quine resistance indicate that such resistance isdependent on a defect in the high-affinity bindingsites of the resistant parasite.

All in all, important progress has been made inresearch on the various aspects of the parasite, itsvector, and the host. These advances, coupled withthe many research techniques already developed,should be a good stimulus to scientists in maintainingtheir efforts.

STATUS OF THE MALARIA ERADICATIONPROGRAMME

The progress of the malaria eradication pro-gramme has been slow in the past few years and haseven suffered significant setbacks in some large coun-tries, such as India and Pakistan. However, progresshas been made in other countries, notably a numberof 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

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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 itdoes 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 anadditional 592 million are being protected by activeprogrammes. On the other hand, out of the remain-ing 517 million covered by the table, 241 million livein areas where some antimalaria measures are beingapplied and 275 million live in areas-mostly incountries of Africa south of the Sahara-where nospecific antimalaria measures are applied.

Table 1. Populations freed from malaria, protected byactive programmes, or living in areas where few or noantimalaria 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 ofthe programme, inevitably raises some questions,such as: has the progress of malaria eradicationprogrammes reached a threshold beyond which nofurther progress is possible by the means currentlyavailable 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 implementationof the programme, what was thought to be prac-ticable often turns out to be impracticable. Indeed,the revised strategy of malaria eradication allowsfor a flexible and realistic replanning of the pro-gramme based on available financial and manpowerresources. Admittedly, the political and socio-economic situation, coupled with accelerating infla-tion and a reduction in bilateral and internationalassistance, has considerably reduced the potentialof many countries to pursue their antimalaria pro-grammes efficiently. In view of the changed attitudeof a number of governments towards providing thesupport required for their operating malaria eradi-cation programmes, and the technical problems metwith in different countries as a result of resistanceto insecticides or drugs, or the particular bionomicsof the vector, one wonders whether more progresscould 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, andmost countries of south-east Asia and the WesternPacific, as well as in some countries of SouthAmerica. In other countries of these areas significantprogress cannot be expected for some time. Unfor-tunately, little has been done to control malaria inAfrica south of the Sahara, and the internationalcommunity should not remain indifferent to the highmortality caused by malaria among young childrenin that part of the world. The concept of an inte-grated approach to the control of endemic diseasesthrough basic health services, although correct, doesnot seem to have had any impact on the endemicityof malaria in rural areas of Africa. There, tremen-dous efforts will be required in order to reducemalaria transmission significantly. However, no effi-cient means of interrupting transmission in thesavanna areas of Africa exist at present.Thus there is no avoiding the conclusion that

malaria will be with us for quite some time. In orderto cope more efficiently with this world problem,much more research is required. Research per secannot solve all the problems, so more completetraining of malariologist-parasitologists is a conditiosine qua non for the proper application of scientificdevelopment. This would facilitate well-evaluatedfeedback to scientific laboratories in further devel-oping ideas, research, and field techniques as wellas more efficient malaria control methods.

Furthermore, malaria presents a certain threat to

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countries from which it has already been eradicatedor in which it has never existed. The medical profes-sion should therefore be made aware of the possiblyfatal consequences for nonimmune individuals whocontract malaria-particularly P. falciparum ma-laria-in endemic areas. With the tremendous devel-

opment of the tourist trade, which constitutes one ofthe possibilities for economic development of the" third world ", more and more such cases are beingrecorded.

These and other aspects of malaria research aredealt with in the papers presented at the symposium.

RtSUMItLA RECHERCHE SUR LE PALUDISME: REVUE DE LA QUESTION

La litterature consacree A la recherche sur le paludismeest trbs abondante: durant la periode 1951-1971, pres de10 000 articles ont ete publies et de 1971 A 1973, 800 aumoins. Une analyse des travaux parus au cours des 10 A15 demieres annees indique que cette recherche estdevenue plus systematique; la raison en est, tres proba-blement, le grand nombre de rdunions, symposiums etconferences-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 lareponse immunitaire de l'h6te humain. C'est ainsi qu'ona 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 unsucces appreciable, en vue d'immuniser l'h6te contrel'infection paludeenne. Chez le modele du paludisme desrongeurs, les sporozoltes irradies se sont reveles tresefficaces lorsque les animaux ayant requ ce vaccin ontsubi une infection d'epreuve par des sporozoltes nonirradi6s. La protection ainsi obtenue est totale ou nulle.En outre, elle est fonction de la dose et plus forte si l'onutilise un vaccin renfermant un plus grand nombre desporozoites ou si l'on multiplie les doses. Plusieurspreparations obtenues A partir de formes erythrocytairesdes parasites par differents procedds, avec ou sans adju-vant, ont confere une protection dans le paludisme desrongeurs ou des singes. N6anmoins, leur action ne semanifeste que par un allongement de la periode deprepatence et, dans certains cas, par la survie de l'animalA des infections qui, autrement, sont letales pour lasouris.En ce qui regarde plus specialement le parasite, les

recherches des dernieres annees ont amene non seulementla description de quelques especes nouvelles, mais aussiune meilleure caracterisation des souches. On a consacre

beaucoup d'efforts au probleme de la culture in vitro desplasmodiums en etudiant plus particulierement certainsaspects 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 desmoustiques en fonction de caracteristiques chromoso-miques spdcifiques. Pour mettre au point de nouveauxprocedes ou de nouvelles methodes de lutte contre lepaludisme, on a experiment6 de nouveaux insecticideset, en outre, etudie les possibilites de recours A la luttegenetique contre les vecteurs. Neanmoins, l'essentiel desefforts a porte sur la recherche de nouveaux medicaments.Rien qu'aux Etats-Unis d'Amerique, on a examine plusde 215 000 composes et evalue leur action contre lesplasmodiums. Quatre de ces groupes de produits pour-ront eventuellement etre utilises dans les programmes delutte antipaludique des qu'ils auront franchi le cap desessais cliniques et des essais sur le terrain.

Pendant ces dernieres annees, les progres du programmemondial d'eradication du paludisme ont ete relativementlents. Cependant, pres de 800 millions de personnesvivant dans des regions autrefois impaludees ne sontplus actuellement exposees au risque d'infection, et592 millions d'autres beneficient de programmes deprotection efficace. Pour 275 millions de personnes,vivant pour la plupart en Afrique au sud du Sahara, iln'existe malheureusement aucune forme organisee deprotection contre la maladie.Malgre les difficultes de la tache, de nouveaux progres

sont possibles A la condition d'intensifier la recherche etd'assurer aux paludologues et aux parasitologistes laformation indispensable.

DISCUSSION

BRUCE-CHWATT: One of the main problems beingfaced now, and one that will occur even more inthe future, is the gradual attrition of research

centres on human malaria, especially in the USA.No animal model can provide the final verdict onthe value or otherwise of new antimalarial drugs or

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immunological methods of protection against humanmalaria. The WHO Scientific Groups on Chemo-therapy of Malaria and the WHO Expert Committeeon Malaria recommended the continuation of re-

search centres on human malaria. At this criticaltime in the history of the antimalaria campaign, theseriousness of the situation should be emphasizedand the work of such centres should be facilitatedand supported, subject to all necessary ethical andprofessional constraints.

UNGUREANU: I agree. The value of malaria therapystations should not be overlooked.

SCHMIDT: The situation with respect to the continuedoperation of research centres on human malaria inthe USA is grave. I believe that decisions on suchcontinuation might be favourably influenced if theimportant contribution of these centres-especiallyto the development of new antimalarials-wereacknowledged.

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