Combating Malaria Morbidity and Mortality by Reducing Transmission

c Reviews

mate choice by males? Aiibn. Beliuv. 40,870-876 sitaires de Bordeaux 42 (1979) 47 (1994) Burley, N. and Moran, N. The significance of age and De Clercq, D. et nl. Schistosomiasis in Dogon country, reproductive experience in the preferences of feral pigeons, Mali: identification and prevalence of the species responsible 27, 686-698 Coliciiibn livia. Aiziiii. Belinzi. for infection in the local community. Trniis. R. Soc. Trop. Med. 43 (1994) 88,653-656 Owens, I.P.F. et nl. Sex differences, sex ratios and sex HI&. roles. Proc. R. Soc. Loildoti Ser. B 258,93-99 48 Woolhouse, M.E.J. (1991)On the application of mathematical 44 (1987) 116,80-88 Combes, C. et nl. Les schistosomes.Porir In Sckiice models of schistosome transmission dynamics. I. Natural 45 Southgate, V.R. (1978) On factors possibly restricting the transmission. Actn Trop. 49,241-270 distribution of Scliistosoiiia intercnlatuiii Fisher, 1934. Z. Pnm- 49 May, R.M. and Woolhouse, M.E.J.(1993) Biased sex ratios and siteiikiriide 56, 183-193 .. parasite mating probabilities. Pnrnsitologj 107,287-295 46 (1987) 50 (1990) Doumenge, J.P. et nl. Atlas of tlie Globnl. Distri6utioii of Basch, P.F. Scliistosoiiies: Developmiit, Reprodttctioii, niid Schistosoiiiinsis, pp 41-49. CEGET-CNRS-WHO, Presses Univer- Host Relntioiis, pp 152-153, Oxford University Press

Focus

Combating Malaria Morbidity and Mortality by Reducing Transmission

J-F. Trape and C. Rogier - Jean-François Trape aiid Christophe Rogier present epideini- viduals living all their lives in a given endemic area ological data aizd an nizalysis of the relatioizship between depends on transmission intensity and age. This has trarzsnzission, morbidity ancl mortality from inalaria zvhich marked consequences for the absolute and relative suggest that any intervention aiming to reduce trans- importance of the burden of malaria at a given age7, nzissioii will not, on a long-tem basis, reduce the burden of but also, probably, on tlie immediate and delayed malaria in the majority of epidemiological coiztexts observed evolution of the incidence of malaria morbidity and iiz tropical Africa. mortality after a reduction in transmission.

I Malaria control in tropical Africa is principally based Transmission and mortality on the presumptive treatment of fever cases using The results of a large study covering the 500000 anti-malarial drugs. In the past decade, the rapid inhabitants of Brazzaville (Congo) provide an initial spread of chloroquine resistance has stimulated the indication that extreme differences in malaria trans- exploration of other control methods. Several studies mission may be associated with only minor differ- have now shown that insecticide-impregnated bed- ences in malaria mortality rates8-11. This study is the nets can reduce morbidity and mortalityl-4, and this only published comparison of malaria mortality rates method is generally considered to be an efficient between populations that were identical in their gen- means of combatting malaria. Aided by substantial etic and socio-cultural backgrounds and that bene- funding from several international agencies, interven- fited from equal opportunities for therapeutic care, tion programmes based on insecticide-impregnated while differing dramatically in their exposure to ma- bednets and curtains are either under way, or being laria. Depending on the district of Brazzaville, the planned, in many African countries. Other strategies entomological inoculation rate varied from more than aimed at reducing malaria transmission (such as the 100 infective bites per person per year to less than one genetic manipulation of mosquito vectors5 or the de- infective bite per person every three years, which rep- velopment of a transmission-blocking vaccine9 are resents almost the entire scale of malaria transmission also actively being explored. rates observed in Africag. Despite this, the incidence Generally speaking, can we hope that interven- of severe malaria cases was essentially identical for all tions that aim to reduce malaria transmission can re- the districts, the only significant difference being the zi duce, on a long-term basis, malaria morbidity and younger average age of severe malaria attacks in the0 P mortality whatever the epidemiological context? The high-transmission districts (Fig. 1). It is important to + .a answer lies within the general framework of relation- note that the parasite rate in schoolchildren varied ships between the entomological inoculation rate, the from 3% to 81%, depending on the district of the0o= a incidence rate of malaria attacks and the frequency of town, and that two-thirds of the schoolchildren from .f! severe forms of the disease. The average level of the low-transmission districts had no detectable anti- transmission varies considerably with the endemic Plasnzodiunz antibodies at the age of seven, which C - area, from about to lo3 infective bites per person clearly indicated that the circulation of children be- a, -7-1 lper year. The degree of acquired iminun6y i; indi- tween different districts was limited and could not, E 2- 0- 0- $- therefore, explain the homogeneity of the risk of se- 2 g-g vere malaria". Recently, other studies have compared c) o ======CI Jean-FrançoisTrape is at the Labot-atoirede Paludologie, 'I severe malaria rates in areas with different entomo- Cl Ic=b-~ ORSTOM, BP 1386, Dakar, Sénégal. Christophe Rogier is u-0 .. E7_1 -0 I logical inoculation rates. Similar levels of severe $ "====Oat the Service d'Epidémiologie, Institut Pasteur, BP 220, Dakar, O- O- -? Sénégal. Tel: +22I 32 O9 62, Fax: +22I 32 I6 75, ? malaria were observed in two areas of markedly c U i c- c- m-0= e-mail: [email protected] different malaria transmission in East Africa: one in ,f Y- Y- 0- O VOI. 12, no. - 236 Copyright 1996. Elsevier Science Ltd All rights reserved O1 69-4758/961 6, I996 - '$1500 PII SOl69-4758(96)lOOl5-6 Parasito/ogyToday, , . .. F~OCUS

Tanzania, where transmission 5 reached 300 infective bites per per- .-cL1 son per year; the other in Kenya, 2 where transmission was about 10 24 infective bites per person per year12. As in the case of Brazzaville, the E $3 only important differences con- v)a, cerned the age distribution and the c clinical patterns of the severe forms a,02 o of the disease. Similarly, the inci- C dence of severe malaria was not a, 21o associated with transmission level at -C nine different sites in the Kilifi Dis- O 01 trict in Kenya13. Finally, comparison 01.3-2 2-12 12-52 52-11 of malaria mortality rates observed in 28 studies in Africa revealed that Entomological inoculation rate (EIR) these estimated rates were generally Fig. I. Incidence of severe malaria as the number of cases per 10000 people per year of of only limited variability and observation in children aged 0-14 years living in Brazzaville (Congo) according to the showed no relationship with the entomological inoculation rate (EIR) in the district of residence. Cases occurring before transmission level in the 11 studies age five are shown in black, and those occurring after age five are hatched. where entomological data were available14. ence, night and day, of a medical team in the village to diagnose and treat any pathological episode. As Transmission and morbiditv shown in Fig. 2, these two populations differ markedly If the level of transmissioh is not an important risk in terms of the pattern of age-dependent variations in factor for malaria mortality in Africa, is the same true malaria attack incidence rates. From these data, it can for malaria morbidity? In highly malaria-endemic be estimated that, at the age of 60, the Dielmo inhab- areas, distinguishing malaria from other causes of itants, who are exposed to about 200 infective bites fever poses difficult methodological problems be- per person per year, average a total of 43 attacks since cause of the high frequency of asymptomatic infec- birth, with only 23% of these arising during adult- tions and the lack of specificity of the signs and symp- hood. For the Ndiop inhabitants, who are exposed to toms of the disease's. It is only recently that methods about 20 infective bites per person per year, one can have been developed that permit a precise estimation estimate an average total of 62 malaria attacks by the of the incidence of malaria attacks in areas of moder- age of 60, of which 41 % occur during adulthood. ate and high transmission15-18. By using these The comparison of these three Senegalese popu- methods, we have compared the malaria morbidity of lations suggests that there is little difference in the total three Senegalese populations (from Dakar, Ndiop and number of attacks over an entire lifetime in individ- Dielmo) exposed to approximately 1, 20 and 200 uals residing in areas that vary by as much as a factor infective bites per person per year, respectively. of 200 in transmission intensity. If this is correct, how In Dakar, among individuals that have lived since do we explain the decrease in malaria morbidity and birth in a district of the town where the transmission mortality following the implementation of bednets intensity was about one infective bite per person per programmes which has been observed in most stud- year, the clinical incidence rate was identical to the ies in Africa? Figure 3 compares the fluctuations of parasitological incidence rate in children aged seven the entomological inoculation rate and the incidence to 11 (Ref. 7) and was three times less than the para- density of malaria attacks in chidren in Dielmo. sitological incidence rate in adults (T-F. Trape and Clearly, these fluctuations are closely correlated, and L. Konate, unpublished). These observations show that a tenfold decrease or increase in malaria transmission a high proportion of infections are symptomatic in is associated, in the following weeks, with a twofold individuals of all ages exposed since birth to about decrease or increase in malaria morbidity. The de- one infective bite each year. By the age of 60, these crease of malaria transmission in Dielmo each year at individuals have probably accumulated an average of the end of the rainy season can be compared to the -27 to 30 malaria attacks silice birth, of which about implementation of a bednet programme, as a tenfold half will have occurred in adulthood. reduction of transmission is close to the maximum Silice 1990 (Dielmo village) and 1993 (Ndiop vil- reduction in malaria transmission that has been lage), we have uninterruptedly followed up the popu- achieved by impregnated bednets in tropid Africa. i.latioii of two villages in Senegal where malaria trans- In the short term, it is followed by a decrease of ma- . mission intensity differs considerably1g,20. In the first laria morbidity. There are no existing data on the me- .-!hdlage, transmission is intense and perennial due to dium- and long-term efficacy of bednet trials. How- -,the presence of a stream which serves as a permanent ever, they can be predkted using data from areas breeding site for AizoylzeIes gmbine s.1. and An. funestus. where malaria transmission is lower because of natural -In the second village, transmission is about ten times conditions. '-lower, as the Aizoylzeles-breeding sites only exist during the rainy season, which lasts about four months. For Relationships between transmission, morbidity and these two populations, identical and strict clinical sur- mortality , .,veillance programs have been carried out. These in- We have attempted to quantify the relationships ., clude a daily home visit to each person and the pres- between transmission, the incidence of clinical attacks ;d

Parasitology Today, vol. 12, no. 6, I996 237 * t & Focus

a Ndiop (EIR = 20) 3

1 Ym O -EO(d (d <1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15- 20- 25- 30- 40- 50- 260

Õ 63- Dielmo (EIR = a, 200) O c .- .i O -K I 4

<1 1 23 4 . 5 6 7 8 9 10 .11 12 13 14 15- 20- 25- 30- 40- 50- 260 Age (years)

P.Fig. 2. Annual incidence of malaria attacks according to age in permanent residents of two Senegalese villages with markedly different falciporum transmission: Ndiop (EIR 20 infective bites per person per year) (a), and Dielmo (EIR 200 infective bites per person per year) (b).

which is acquired by a person

Y & exposed to malaria, is lost after sev- 60-- I' -- 0.6 5 eral years without exposure. Thus, 5c I.. -- c . I -. E" for low levels of transmission, --I ie. 0.01 and 0.1 infective bites per .. FL- Q nn -- 0.4 person per year, the incidence of c malaria attacks is probably directly . O r to '. a, proportional the level of trans- . Q mission, in adults as in children. For -- 0.2 $ Q levels of transmission of 1, 10, 100 Yffl and 1000 infective bites per person mo per year, tlie data that we have col- o1 .-a, lected in Senegal suggest that global Jul- Oct- Jan- Apr- Jut- Oct- Jan- Apr- Jui- Oct- Jan- Apr- .S Iu inalaria morbidity, which is always ca, Sep Dee Mar Jun Sep Dee Mar Jun Sep Dee Mar Jun -?i -C very high, varies at maximum by 1990 1991 1992 1993 2 a factor of two to three according to the level of transmission (Table 1). 'ig. 3. Entomological inoculation rate and incidence of malaria attacks in children under ;even years old in Dielmo (Senegal), 1990-1 993. Quantifying the relationship between transmission levels and potential malaria mortalitv is a and potential malaria mortality, on the basis of avail- much more uncertain exercise as almost all of available data for six different levels of transinission repre- able data, even old data, deal with populations who senting, on a logarithmic scale, tlie complete range of had access, albeit varying, to antimalarial drugs. For epidemiological situations observed in endemic areas populations benefitting from identical possibilities of (Table 1). At least half of the bites from an Anopheles treatment, we have previously seen that all available which carries sporozoites of P1nsinodiunz fizlcipnrtliiz in data in Africa suggest that there is no marked vari- its salivary glands will give a blood infection followed ation in malaria mortality according to transmission by a clinical attack in non-iinm ie subjects21, and it is when this is transmission of at least one infective bite generally acknowledged that t rotective immunity, per person per year. In tlie case of populations with no. 238 Parasitology Today, vol. 12, 6,I996 ,y--- * .) i

t Focus

of of of O00 Table I.(125 Annual number malaria attacks and malaria deaths according to the level transmission in an hagined population IO people individuals per year of age 0-79 years) who would have no available means of treatment

Entomological inoculation rate (EIR) (no. infective bites per person per year)

0.0 I o. I I IO I O0 I O00 O00 8800" 5800a Number of attacks I O0 I 3700" ? 2-20 a20 25 25 25 25 Number of malaria deaths

"Estimated figure based on studies carried out in Pikine7. Ndiop2'J and Dielmo19.20. bHypothesis:no more than 20% of newborns are at risk of death from malaria. 'Maximum estimate derived from previous field studies in holoendemic areas of central Africa27and from the prevalence of sickle-cell trait in these area+. no access to antimalarial drugs, we have attempted to Concluding comments estimate the maximum or minimum malaria mortal- It is generally estimated that over SO,% of the deaths ity rates at different levels of transmission. In people due to malaria in the world occur in tropical Africa, without immunity, such as tourists, cases of severe or although this region represents only one quarter of complicated malaria (always fatal without treatment) those populations exposed to P. frzZcipnrirm3'J.Clearly, are observed in 1% to 5% of clinical infections. How- a huge mortality from malaria exists in tropical Africa, ever, historical data suggest that the complications of and is often attributed to the very high transmission untreated malaria in a non-immune subject often levels, since the entomological inoculation rate ranges occur several weeks after the onset of clinical symp- generally from five to 1000 infective bites per person tom$, and it is relatively infrequent nowadays that a per year in rural areas, whereas in other endemic diagnosis would be so delayed. To our knowledge, areas in the world (except in New Guinea) the ento- the most documented data on malaria mortality in mological inoculation rate is almost always less than non-immune populations with little or no access to one. anti-malaria drugs are those of the epidemics of Our analysis suggests that in most epidemiological Mauritius in 1867 (Refs 23,24), Rio Grande do Norte contexts observed in tropical Africa, only a consid- and Ceara (Brazil) in 1938 (Ref. 251, and Ethiopia in erable reduction of transmission (much higher than 1958 (Ref. 26). Data from these three epidemics are that which it is presently possible to obtain on a large consistent in suggesting that in the absence of any treat- scale, or to maintain for more than several years) ment, lethality due to P. fdcipmm in non-immune would be able to reduce, on a long-term basis, the people occurs between 5% and 20% of cases. This lat- burden of malaria for the whole community. ter rate is close to the maximum estimates of global The health sectors in African countries have few malaria mortality that were reported in the most means at their disposal, are often badly managed and highly endemic regions of Central Africa27 or which their staff frequently lack motivation. Hoping to work are derived from the frequency of the carriers of the round these difficulties, the main funding agencies sickle-cell gene in these areas2*.For these populations are now strongly encouraging the setting up of pro- exposed since birth to numerous malaria infections, grammes to combat malaria with insecticide-impreg- there is strong evidence that genetically determined nated bednets, basing such programmes on the re- factors prot'ecting against the severe forms of malaria sults of short-term studies, and thus reflecting the have been selected and that in that way the risk of general disarray with regard to the continual aggra- death following a malaria attack may vary consider- vation of the problem posed by chemoresistance. We ably according to individuals. However, even in re- believe that the only effective ways of fighting taining the low hypothesis that only 2% of malaria malaria in Africa with currently available means are attacks are potentially lethal in a non-immune African (1) improvements in health services, and (2) health population29 and that this lethality is in fact concen- education to facilitate better use of antimalarial drugs. trated in only 20% of genetically susceptible individuals, Table 1 suggests that it is necessary to reduce Acknowledgements transmission to very low levels - probably one infective We thank Pierre Druilhe, David Fidock and Louis Molineaux for bite per person every 10 years, or even less - to hope helpful suggestions and Guy Charmot for supplying helpful historical to obtain a long-term impact on potential malaria documents. mortality. References In this succinct quantitative approach of the re- 1 Rozendaal, J.A. (1989) Impregnated mosquito nets and curtains 56, lationships between transmission, morbidity and for self-protection and vector control. Trop. Dis. Bid[. Rl-R41 2 Alonso, P.L. et al. (1991) The effect of insecticide-treated bed- mortality from malaria, several data and hypotheses nets on mortality of Gambian children. Laficet 337,1499-1502 that we used were approximate or uncertain. Further- 3 Bermejo, A. and Veeken, H. (1992) Insecticide-impregnated more, the real populations' age structure and the bed nets for malaria control: a review of the field trials. Bull. competing causes of death within these populations WHO 70,293-296 are also to be considered, because they are funda- 4 D'Alessandro, U. et al. (1995) Mortality and morbidity from malaria in Gambian children after introduction of an impreg- mental to ascertaining the number of potential deaths nated bednet programme. Lailcet 345,479-483 due to malaria in high- and moderate-transmission 5 Spielman, A. (1994) Why entomological antimalaria research areas. However, whatever hypothesis is considered, should not focus on transgenic mosquitoes. Parasitology Todny it appears clearly that variations of the burden of 10,374-376 morbidity and potential mortality from malaria are 6 Meuwissen, J.H.E.T. (1989) Current studies related to the devel- 1 opment of transmission-blocking malaria vaccines: a review. weak compared with the considerable range of tr 7ns- 7"s. R. Soc. Trop. Med. Hyg. 83 (Suppl.), 57-60 al. mission levels. 7 Trape, J-F. et (1993) Malaria morbidity among children Parasitology Today, vol. 12, no. 6, I996 239 '4 b. 'i Focus exposed to low seasonal transmission in Dakar, Senegal, andJ. 17 Rogier, C., Commenges, D. and Traye, J-F. Evidence for an age- its implications for malaria control in tropical Africa. Alii. dependent pyrogenic threshold of Plnsiiiodiztm fnlcipnrziriz 48, Trop. Med. Hyg. 748-756 parasitaemia in highly endemic populations. Alii. J. Trop. Med. 8 Trape, J-F. et 01. (1987) Malaria aiid urbanization in Central Hyg. (in press) Africa: the example of Brazzaville. V. Pernicious attacks and 18 Smith, T., Armstrong Schellenberg, J.R.M. and Hayes, R. (1994) mortality. Trniis. R. Soc. Trop. Med. Hyg. 81 (Suppl. 2), 34-42 Attributable fraction estimates and case definitions for 9 Trape, J-F. and Zoulani, A. (1987) Malaria and urbanization in malaria in endemic areas. Stnt. Mid. 13,2345-2358 nl. Central Africa: the example of Brazzaville. II. Results of ento- 19 Traye, J-F. et (1994) The Dielnio Project: a longitudinal mological surveys and epidemiological analysis. Trntrs. R. Soc. study of natural malaria infection and the mechanisms of pro- Trop. Md.Hyg. 81 (SUPPI.2),10-18 tective immunity in a community living in a holoendemic 10 Trape, J-F. and Zoulani, A. (1987) Malaria and urbanization in area of Senegal. Am. J. Trop. Md.Hyg. 51,123-137 Central Africa: the example of Brazzaville. III. Relationships 20 Rogier, C. and Trape, J-F. (1996) Etude de l'acquisition de la between urbanization and the intensity of malaria trans- prémunition en zones d'holo et mésoendémie palustre à mission. Tmtts. R. Soc. Trop. Med. H!/g. 81 (Suppl. 2), 19-25 Dielmo et Ndiop (Sénégal). McLf. Trop. 55 (Suppl.), 71-76 11 Trape, J-F. (1987) Malaria and urbanization in Central Africa: 21 Rickman, L.S. et nl. (1990) Plnsiiiodiilin fnlcipnriiiiz-infected the example of Brazzaville. IV. Parasitological and serological Atzopheles stepliemi inconsistently transmit malaria to humans. surveys in urban and surrounding rural areas. Trails. R. Soc. Aiir. J. Trop. Med. Hys. 43,441-445 Trop. Med. Hyg. 81 (Suppl. 2), 26-33 22 Chauliac, G. (1964) Coiztribirtiorr ri ['Etde Médico-Militnire de nl. 12 Snow, R.W. et (1994) Severe childhood malaria in two areas I'Expédifioii de Mndngnscnr eir 2595, Tananarive of markedly different falcipnrriin transmission in East Africa. 23 Davidson, A. (1892) Geogrnphicnl Pntliology, Y.J. Pentland Ach Trop. 57,289-300 24 Julvez, J. (1993) Aiztluopisntiori et Pnlidisirie, University of Toulouse 13 Mbogo, C.N.M. et nl. (1995) Relationships between Plnsiiro- 25 Soper, F.L. and Wilson, D.B. (1943) Anopheles gambiae iiz Brazil, dizim fnlcipnrzm transmission by vector populations and the 2930 to 1940, Rockefeller Foundation incidence of severe disease at nine sites on the Kenyan coast. J. 26 Fontaine, R.E., Najjar, A.E. and Prince,J. J.S. (1961) The 1958 Ain. Trop. Med. Hyg. 52,201-206 malaria epidemic in Ethiopia. Ain. Trop. Med. Hyg. 10, 14 Snow, R.W. and Marsh, K. (1995) Will reducing PInsitzodiriitz 795-803 falcipaniin transmission alter malaria transmission among 27 Duren, A.N. (1951) Essai d'étude sur l'importance du paludisme African children? Pnrnsitology To@/ 11,188-190 dans la mortalité au Congo Belge. Atzit. Soc. Belge Med. Trop. 31, 15 Trape, J-F., Peelman, P. and Morault-Peelman, B. (1985) Criteria 129-147 for diagnosing clinical malaria among a semi-immune popu- 28 Molineaux, L. (1985) in Ln Liilte Contre Ln Mort (Vallin, J. and lation exposed to intense and perennial transmission. Trnizs. Lopez A. eds), pp 11-40, Presses Universitaires de France R. Soc. Trop. Med. Hyg. 79,435-442 29 Sudre, P., Breman, J.G. and Koplan, J.P. (1990) Delphi survey of 16 Armstrong Schellenberg, J.R.M. et nl. (1994) What is clinical malaria mortality and drug resistance in Africa. Lnizcet 335,722 malaria? Finding cases definitions for field research in highly 30 WHO (1993) World malaria situation in 1991. Weekly Epideiniol. endemic areas. Parmitology Todny 10,439-442 Rec. 68,245-252

Control of Lymphatic Filariasis by Annual Single=dose Diethylcarbamazine Treatments E, Kimura and J.U. Mataka

It has long been stressed that diethylcnrba~iiazirecitrate For more than 40 years, DEC has been used, world- nztist be giveii nt n total dosage of 72iizg per kilogram of wide, as the most effective and safest drug. The stand- body weight in 12 divided doses of 6 irig lcg-1 to obtniii iriaxi- ard treatment scheme recommended by WHO* is to intiiri effect against Wuchereria bancrofti. However, recent administer the drug at a dosage of 6mg per kilogram studies sevenled thnt only n single dose nt 6 mg kg-1 could of body weight daily, weekly or montldy for a total of reduce nricrofilaria CMf} counts by 90%, aiid tlint the effect 12 times in ord& to obtain the required overall dose would persist for 12-18 nioiitlzs. The aizniinl repeat of the of 72 mg kg-1 for the treatment of Wuchereria baiicrofti a single-dose mss trentiizent zuns shown to be effective ìiz infection, or total dose of 36-72mgkg-1 for Brtigin seducing Mf prevnleizce atid deizsity in large-scale, long-tenn spp infections. The standard treatment can effectively field trials. The scheine is simple arid economic, and cozild be reduce filariasis and suppress its transmission, but stistaiiinble iii many eirdeinìc areas, diese health inaiipozuer ensuring that 12 doses are given poses considerable and sesources are often not sufficieiit. Aiiiziinl siiigle-dose practical difficulties in a large-scale campaign. Recently, inass treatinelits can be niz effective weapon ngaiiist lziiniaii annual single-dose treatments with DEC at 6 mg kg-1 lymphatic filariasis, CIS discussed liese by Eisnku Kirnaira were reported to be effective in reducing the micro- aiid Jorra Mntnika. filaria (Mf) prevalence and density. The effect of each single dose is not very strong but is steadily progres- There are an estimated 78.6 million cases of lymphatic sive in a course of repeated treatments. In a filariasis filariasis in the worldl, and only a small proportion control campaign in Samoa involving 160 O00 people of them is fortunate enough to be treated with the over eight years, three single-dose treatments de- first drug of choice, diethylcarbamazine citrate (DEC). creased the Mf prevalence from 5.6% to 2.5%, show- ing that single-dose chemotherapy is a practical strat- egy for filariasis control. Eisaku Kimura is at the Department of Parasitology, Aichi Medical Univenity, Nagakute-Cho, Aichi-ken,Japan 480-I I. lona What is the aim? Mataika isTel: at the Wellcome Virus Laboratory, Tamavua Hospital, Suva, Fiji. +8 I 56 I 6233 I I, Fax: +8 I 56 I 633645, Annual single dose is given for mass treatment, e-mail: [email protected] eliminating the laborious and costly step of blood no. 240 CopynghtQ 1996. Elsevier Science Ltd All nghts reserved 0169-4758/96/$15.00 PII. SOl69--1758(96)1OOl4--1 POr~SitOìOgyToday, VOI. 12, 6, I996