Impact of an insecticide treated net programme on the prevalence of parasitaemia and anaemia in children under two years of age in the Kilombero Valley, Tanzania.

Salim Abdulla,1 Joanna Armstrong Schellenberg,1,2 Rose Nathan,1 Oscar Mukasa,1 Tanya Marchant,1 Tom Smith,2 Marcel Tanner,2 Christian Lengeler.2
1. Ifakara Health Research and Development Centre
2. Swiss Tropical Institute

Correspondence to:

Dr. Christian Lengeler
Swiss Tropical Institute
Basel.

E-mail: Christian.Lengeler@unibas.ch

Abstract

Objective:

To assess the impact of social marketing programme of insecticide treated nets (ITN’s) on parasitaemia and anaemia in children under two years of age in an area of high malaria transmission.

Design:

Annual cross-sectional data was collected at the beginning of the social marketing campaign and subsequent two years. Net ownership and other risk and confounding factors were assessed using a questionnaire. Blood samples were taken from the children to assess prevalence of parasitaemia and haemoglobin levels.

Study participants:

A sample of children under two years of age living in 18 villages in the Kilombero river plain, Southwestern Tanzania.

Main outcome measures:

The presence of any parasitaemia in the peripheral blood sample, and anaemia classified as haemoglobin of less than 8 g/dl.

Results:

There was a rapid increase in net ownership (from 58 to 83%) and ITN ownership (from 10 to 61%). There was an overall increase in the mean haemoglobin (from 8.0 to 8.9 g/dl) of the study children in the successive surveys. Overall, the prevalence of anaemia in the study population decreased from 49% to 26% in two years of implementation. ITN’s had a protective efficacy of 63% (95% CI: 38, 77) on the prevalence of parasitaemia, and 63% (95% CI: 27, 82) on anaemia.

Conclusions

These results demonstrate that ITN’s have a substantial impact on morbidity parameters when distributed in a public health setting.

Key words: Haemoglobin, anaemia, malaria, parasitaemia, insecticide, bednet, social marketing, Tanzania
 

What is already known in this topic

Randomised controlled trials of insecticide treated materials have consistently shown short term impact on malaria morbidity and mortality. It is not known whether these benefits can still be observed when insecticide treated materials are distributed in public health programmes.

What this study adds

This is the first report of health impact of ITN’s within a large scale social marketing programme.

Rapid and high impact of the nets on malaria parasitaemia and anaemia can be obtained in very young children who are most at risk from the disease.

Introduction

Malarial anaemia due to P.falciparum is a major public health problem in Sub-Saharan Africa, especially among women and children. Severe anaemia and cerebral malaria are the two major forms of severe disease leading to death. Compared to the other species, P. falciparum, which is the most common malaria species in Africa, causes more severe malaria anaemia.1 A large part of Sub Saharan Africa has stable malaria transmission.2 Transmission intensity influences the prevalence and incidence of malaria infection and associated complications.3 It has been observed that in areas of very high transmission the severe forms of the disease are concentrated in very young children and frequently present as malarial anaemia. Older children presenting with cerebral malaria are more frequent in moderate to low transmission areas.4 Hospital management of the severe forms anaemia involves blood transfusion5;6 and there is an increasing concern that the transfusions are a potential risk for HIV transmission in children.7;8 In addition, most children at risk of severe anaemia live beyond reach of hospitals able to provide blood transfusions (Font & Nathan, Unpublished data).

The pathogenesis of malarial anaemia is complex and includes the process of haemolysis, sequestration of red cells, and dyserythropoiesis.9 Malarial anaemia may develop rapidly following an acute malaria attack or may develop insidiously over a period of time: many patients fall in between these two extremes.10 Several studies have shown a positive correlation between parasitaemia and anaemia, and further demonstrated that the parasitaemia is the primary cause of anaemia in very young children in Africa.5;11 As a result, in high transmission areas, anaemia is very frequent since malarial infections are the norm. Assessment of the impact of chemoprophylaxis in Tanzanian infants indicated that over 60% of the anaemia could be due to malaria in this age group.12 The emergence and spread of parasite resistance to commonly used antimalarials has exacerbated the problem of anaemia in Sub-Saharan Africa. Persisting parasites after inadequate treatment contribute to the occurrence of anaemia.13

With the increasing incidence of antimalarial drug resistance, alternative cost-effective and sustainable control measures are urgently required. Hopes for malaria and malarial anaemia control have recently been revitalised by the demonstration that ITN’s can reduce morbidity and mortality. A summary of randomised controlled trials showed an average protective effect on mild malaria episodes of 46% in stable malaria areas when controls did not use nets, and a protective effect of 37% when controls used untreated nets. Moreover, protective effects were shown on the prevalence of high parasitaemia (31%) and overall mortality (19%). A modest improvement in packed cell volume (2%) and weight gain was also observed in children sleeping under treated nets.14

Large scale implementation of ITNs is underway in a number of African countries. The challenge is to improve access, extend coverage and promote re-treatment of the nets. However, it is not known whether the impressive impact of ITN’s in the frame of well controlled randomised controlled trials can be replicated under programme conditions.15 This information is urgently required. We report the first impact assessment of a large scale ITN’s social marketing programme on morbidity indicators in children under two years of age in a highly endemic area of Tanzania.

Methods

Study area and population:

Social marketing of ITN’s started in the Kilombero valley in 1997 in the frame of Kilombero Net Project (KINET).16 Formative and market research had identified "ZUIA MBU" (Kiswahili for ‘prevent mosquitoes’) as a suitable brand for nets and insecticide. The products are now being promoted, distributed and sold in the two districts of Kilombero and Ulanga (population 350,000) in Southwestern Tanzania. Distribution channels include both public and private outlets, and a system of community door-to-door distributors and the retail price of the ITN’s was 5.0 US Dollars. The impact of the programme on morbidity and mortality indicators is being monitored in 18 villages under demographic surveillance (DSS).16 Population characteristics of this area of very high perennial malaria transmission have been described elsewhere.17 Moderate anaemia (PCV< 25%) and severe anaemia occur in 61% and 14% of children under five years admitted at the local St Francis District Hospital.6 Chloroquine resistance is common: 65% of malaria infections do not clear within one week of chloroquine treatment.18

Design

Three annual cross-sectional surveys where conducted in a sample of children under two years of age living in the DSS area.16 The first cross-sectional was done at the time of launching the social marketing campaign in June 1997, and two other cross-sectional surveys were carried out at the same period (Mid June to early August) in the subsequent two years. A simple random sample was selected from the DSS database for the first survey, and a two-stage random sampling (sampling 6 villages then sampling children from these) was done for the subsequent surveys, excluding children from two villages where other surveys were carried out. A different sample was selected for each survey.

Procedures

Selected children were visited at home, the aims of the study were explained to the caretakers using the trial information sheet, and a verbal consent obtained from the parent or guardian. A questionnaire was applied to assess ITN use and other potential risk or confounding factors. An inspection of the childrens’ sleeping places was done in 1999. A physical examination of the selected children was performed, and temperature, weight and height measurements were also taken. A finger prick blood sample for haemoglobin estimation and parasitological assessment was then taken. Haemoglobin was measured using the HemoCueÒ system (HemoCue AB, Ängelholm, Sweden). Slides were stained in Giemsa and reading was done using standard procedures as described elsewhere.19

Data management and analysis

Data entry and consistency checks were done in Foxpro version 2.6 (Microsoft corporation). Data analysis was done in Stata version 6 (Stata Corporation, Texas USA). Anaemia was classified as haemoglobin level below 8.0 g/dl since this is the level that has been associated with increased mortality20 and is consistent with earlier studies in the area.12 Parasitaemia and splenomegaly were classified as either present or absent. Use of ITN’s was categorised based on ownership and not on effective use, which could not be measured. Treatment of nets was classified on the basis of the respondent’s answers as either "ever treated" or "not treated", as there are no simple ways to-date to assess insecticide content on the nets. Use of nets in the previous month was included as one of the explanatory variables. Analysis was done for all the three cross-sectional surveys combined. Children older than 24 months at the time of sampling, or whose net treatment status was not known were not included in the analysis. The impact of the bed net on anaemia, parasitaemia and splenomegaly was estimated using multivariate logistic regression models21 taking into account the cluster sampling for year 2 and 3. The effect of different time points of observation (surveys) was included as one of the explanatory variables. Other factors considered in the multivariate models included age of the child, sex, ethnic group, religion, wasting (weight for age Z-scores £ -3), distance (time) to the dispensary, distance (time) to a nearest shop, access to clean water source, treatment with an antimalarial during the last illness, referral to a higher level health facility during the last illness, immunisation status, number of under-fives in the family, whether the study child is a child of the head of the household, literacy of the caretaker, guardian advice to neighbour with a sick child, age of a net, condition of the net in terms of the number of holes, presence of extra net in the family and family income category (quantiles of total family income). Family income was assessed by the sum of the family previous year’s harvest and the estimated monthly income from various money generating activities. The variables were dropped from the model if their likelihood ratio test (LRT) p-value was more than 0.25 and did not alter the estimated net impact odds ratio by more than 15%.

Results

Mothers and guardians of 827 children were interviewed during the three cross-sectional surveys. 68 children were over twenty four months old at the time of sampling, and the ITN status was not known for 11 children. Thus only 748 (91%) children were included in the analysis. Children analysed and those not analysed had similar proportions of anaemia, time (distance) to dispensary and reported ownership of nets (data not shown).

For all children combined, we observed an increase in the mean haemoglobin level from 8.0 to 8.9 g/dl and a decline in the proportion of children under two years of age with anaemia (49% to 26%), any parasitaemia (63% to 38%) and splenomegaly (86% to 49%) in successive surveys (Table 1). 8 (2%) of parasitaemic children had mixed infection with Plasmodium malariae, while the rest had pure Plasmodium falciparum infections. The proportion of children having any net increased from 58% to 83% and the proportion of children having ITN’s increased dramatically during the 3 years (from 10% to 61%) indicating a rapid uptake of the socially marketed ITN’s, especially in the first year of the implementation (Table1).

Net ownership and use

Predictors of net ownership (Table 2) included family income, with those classified in the fourth quartile of income being about 3 times more likely to have a bed net than those in the first quartile. This was expected in a social marketing programme in which the nets are being sold. By the end of the 2nd year of implementation only 16% of the children had no net at all. Children with no access to clean water i.e. not having access to piped water or covered wells, were less likely to have nets (Table 2). Mothers that mentioned that they would advise their neighbours to send their sick children to a formal health facility were more likely to have nets for their own children (OR=2.3 LRT-c2 p value=0.048). On the other hand, those who mentioned traditional healers tended to be less likely to have nets although the numbers were very low and the relationship did not reach statistical significance. This might reflects an association between health seeking behaviour patterns or perceptions about the value of the formal health system and the decision to have a net or not. Other identified factors included appropriate immunisation, literacy of the mother/guardian and ethnicity, although the relationships did not reach statistical significance.

Observation of sleeping places for 171 children in 1999 revealed that 125 (73%) had a net hanging at the sleeping place. No nets were found in those households that reported not having a net. Among those who claimed to be using nets (126/171), only 9 (7%) of them didn’t have a net hanging at the sleeping place. For all these 9 children, we were shown a net which was claimed to be used. Among those that claimed not to be using nets (45/171), 8 (17%) of them had a net hanging at the sleeping place. These observations indicated that reported ownership and use provided a reasonable basis for defining bed net status.

Health impact of ITN’s

Children with ITN’s had a lower prevalence of parasitaemia than those without them. Untreated nets had a protective efficacy(PE) of 51% (95% CI: 0 to 76), while for treated nets the PE was 63% (95% CI:38 to 77%) when compared with children without nets. The Ndamba ethnic group had a lower parasite prevalence compared to either the Pogoro, the Hehe or other tribes. Parasite prevalence was also related to age of the child with the prevalence in those more than 1 year of age being four times higher than those below 7 months. This is consistent with earlier studies in the same area which showed that prevalence of parasitaemia increased with age.22;23 The use of the net in the month preceding the survey was also an important factor with a PE of 47% (95% CI:21 to 65%). Muslims and Catholics had more parasitaemia compared to other religious denominations (Table 3).

The mean haemoglobin level was 7.7 g/dl (95% CI:7.4 to 7.9) for those without nets, 8.6 g/dl (95% CI:8.3 to 8.8) for those with untreated nets and 9.2 g/dl (95% CI: 9.0 to 9.3) for treated nets. The nets had a PE on anaemia of 37% (95% CI:-46 to 73%) for untreated nets and 63% (95% CI:27 to 82%) for treated nets. Those classified as stunted or as having no access to clean water were more likely to be anaemic (Table 4). If the cut off level for anaemia was set at 11g/dl, the nets had a PE of 78% (95% CI:29 to 93%) for untreated nets and 82% (95% CI:42 to 94%) for treated nets. Parasitaemia was associated with anaemia: children with high parasitaemia had lower mean haemoglobin compared to those who had no or few parasites (c2-trend, p<0.001).

The impact of treated nets on prevalence of splenomegaly was also high with a PE of 71% (95% CI:39 to 87%) for untreated nets and 76% (95% CI:52 to 88%) for treated nets. Distance to the dispensary had a strong association with splenomegaly. Those living more than one hour from a dispensary were 3 times more likely to have splenomegaly. Those classified as stunted were twice as likely to have splenomegaly. Having an extra net in the house (an indication of wealth or extra protection) and the use of a net during the last month were protective.

For children without nets, their prevalence of anaemia remained relatively stable over the study period (between 49% and 58%) and this was also the case for the prevalence of parasitaemia (between 68% and 71%), while the mean haemoglobin levels were between 7.5 and 7.9 g/dl in this group. This suggests that there was no major change in the malaria situation during the period under evaluation.

Discussion

These results have demonstrated that the social marketing approach of distributing treated nets was very successful and quickly resulted in more than 80% of children under two years of age having access to a net. The rapid uptake of the intervention further demonstrates the existing demand for the nets and the need for improving accessibility of ITN’s and insecticide for treatment in other areas. Our results suggest a rapid overall impact of social marketing of ITN’s on health outcomes in the community, with an improvement of mean haemoglobin levels in all children under two years from 8.0 to 8.9 g/dl, and a reduction in the proportion of children who had haemoglobin of less than 8 g/dl from 49% to 26%. A decline was also seen in the proportion of children with parasitaemia or splenomegaly. The treated nets had an apparent protective efficacy of over 60% on the prevalence of anaemia, parasitaemia and splenomegaly.

These estimates are higher than those from most controlled trials (Table 5). It is therefore pertinent to question whether this finding may be the result of residual confounding despite the effort made to control for it. The tools used to measure confounding factors like social-economic status and health seeking may not be sensitive enough to allow for proper control. However, factors related to the dynamics of the malaria infection and the associated disease presentation may also explain this finding. For example, it has been observed that variations in transmission strongly affect the estimates of morbidity and mortality in very young children.35 Therefore at a given transmission intensity the age of the study participants may be crucial in determining the level of protection. Our finding of high impact in children under two years is in line with other studies that included very young children (Table 5).

Lower impact estimates than ours were observed in a randomised study near Ifakara in a similar age group.34 This may be due to fact that our study covered a larger geographical area and included study children with lower average haemoglobin (average haemoglobin of those without nets 7.7 g/dl vs. 8.7 g/dl , t-test =3.9: p=0.0001) than the randomised study. It is likely that the more anaemic children may be more likely to benefit from the intervention. In addition, the randomised study was done long after the peak of transmission season (less mosquito bites) and with a relatively short observation period after the initiation of the intervention. Prevalence of parasitaemia and for that matter malarial anaemia is a function of both the rate of new infections (exposure) and the clearance of the parasites.36 ITN’s reduce exposure to infections37 and are therefore likely to alter the age specific equilibrium of the prevalence of parasitaemia and associated morbidity. It is still not clearly known how long it takes for a new equilibrium to be attained with reduced exposure leading to reduced infection.

Antimalarial use and the effectiveness of the early diagnosis and treatment strategy for clinical malarial may also influence the prevalence of parasitaemia and anaemia in young children13 and therefore introduce confounding in our study. Chloroquine was the main drug used for treatment in the private sector and at home (Abdulla et al, unpublished data), and resistance to this drug is high in the study area.18;38 It is therefore unlikely that the use of antimalarials played an important role (those with nets may be more readily treated if they fall sick) in determining the high impact in this study. The overall impact of the ITN’s programmes combined with appropriate use of an effective antimalarial remains to be seen, and may be dramatic if drugs such as artemisinin are used as these may have an effect on transmission.39 Furthermore, ITN’s may help in the effort of containing the rate of development of antimalarial resistance and further enhance malaria control.40 We observed a similar level of prevalence of parasitaemia and anaemia over the 3 years in the unprotected group and the proportion of those without nets got much smaller and therefore it is unlikely that the unprotected get more illness than before the introduction of the intervention..

We conclude that ITN’s distributed through a social marketing programme setting were effective and produced a rapid and high impact on parasitaemia and anaemia prevalence in children under two years of age. This strategy has high potential in the control of malaria in Sub-Saharan Africa.

Acknowledgements

We like to thank the children and guardians who participated in the study, and the Director and staff of the Ifakara Health Research and Development Centre (IHRDC) for facilitating the conduct of the study, Drs. Lwila and Mbena (District Medical Officers) and the health facility staff. Ethical clearance was obtained from IHRDC and the Tanzania Commission of Science and Technology (COSTECH). Financial support was provided by the Swiss Agency for Development and Co-operation and the Government of Tanzania. CL was in receipt of the PROSPER grant 32-41632.94 from the Swiss National Science Foundation.

Reference

1. World Health Organisation (WHO). Severe and complicated malaria. World Health Organization, Division of Control of Tropical Diseases. Trans.R.Soc.Trop.Med.Hyg. 1990;84 Suppl 2:1-65.

2. Craig MH, Snow RW, le Sueur D. A climate-based distribution model of malaria transmission in sub- Saharan Africa. Parasitol.Today 1999;15:105-11.

3. Snow RW, Craig M, Deichmann U, Marsh K. Estimating mortality, morbidity and disability due to malaria among Africa's non-pregnant population. Bull.World Health Organ 1999;77:624-40.

4. Marsh K,.Snow RW. Malaria transmission and morbidity. Parassitologia 1999;41:241-6.

5. Newton CR, Warn PA, Winstanley PA, Peshu N, Snow RW, Pasvol G et al. Severe anaemia in children living in a malaria endemic area of Kenya. Trop.Med.Int.Health 1997;2:165-78.

6. Schellenberg D, Menendez C, Kahigwa E, Font F, Galindo C, Acosta C et al. African children with malaria in an area of intense Plasmodium falciparum transmission: features on admission to the hospital and risk factors for death. Am.J.Trop.Med.Hyg. 1999;61:431-8.

7. Greenberg AE, Nguyen-Dinh P, Mann JM, Kabote N, Colebunders RL, Francis H et al. The association between malaria, blood transfusions, and HIV seropositivity in a pediatric population in Kinshasa, Zaire. JAMA 1988;259:545-9.

8. Holzer BR, Egger M, Teuscher T, Koch S, Mboya DM, Smith GD. Childhood anemia in Africa: to transfuse or not transfuse? Acta Trop. 1993;55:47-51.

9. Weatherall DJ,.Abdalla S. The anaemia of Plasmodium falciparum malaria. Br.Med.Bull. 1982;38:147-51.

10. Abdalla S, Weatherall DJ, Wickramasinghe SN, Hughes M. The anaemia of P. falciparum malaria. Br.J.Haematol. 1980;46:171-83.

11. Kitua AY, Smith TA, Alonso PL, Urassa H, Masanja H, Kimario J et al. The role of low level Plasmodium falciparum parasitaemia in anaemia among infants living in an area of intense and perennial transmission. Trop.Med.Int.Health 1997;2:325-33.

12. Menendez C, Kahigwa E, Hirt R, Vounatsou P, Aponte JJ, Font F et al. Randomised placebo-controlled trial of iron supplementation and malaria chemoprophylaxis for prevention of severe anaemia and malaria in Tanzanian infants. Lancet 1997;350:844-50.

13. Bloland PB, Lackritz EM, Kazembe PN, Were JB, Steketee R, Campbell CC. Beyond chloroquine: implications of drug resistance for evaluating malaria therapy efficacy and treatment policy in Africa. J.Infect.Dis. 1993;167:932-7.

14. Lengeler C. Insecticide-treated bednets and curtains for preventing malaria (Cochrane Review). In The Cochrane Library. Oxford, update software, 1998.

15. Lengeler C, Snow RW. From efficacy to effectiveness: insecticide-treated bednets in Africa. Bull.World Health Organ. 1996;74:325-32.

16. Schellenberg JR, Abdulla S, Minja H, Nathan R, Mukasa O, Marchant T et al. KINET: a social marketing programme of treated nets and net treatment for malaria control in Tanzania, with evaluation of child health and long-term survival. Trans.R.Soc.Trop.Med.Hyg. 1999;93:225-31.

17. Tanner M, de Savigny D, Mayombana C, Hatz C, Burnier E, Tayari S et al. Morbidity and mortality at Kilombero, Tanzania, 1982-88. In Feachem RG, Jamison DT, eds. Disease and Mortality in Sub-Saharan Africa, pp 286-305. Oxford: Oxford University Press, 1991.

18. Hatz C, Abdulla S, Mull R, Schellenberg D, Gathmann I, Kibatala P et al. Efficacy and safety of CGP 56697 (artemether and benflumetol) compared with chloroquine to treat acute falciparum malaria in Tanzanian children aged 1-5 years. Trop.Med.Int.Health 1998;3:498-504.

19. Alonso PL, Smith T, Schellenberg JR, Masanja H, Mwankusye S, Urassa H et al. Randomised trial of efficacy of SPf66 vaccine against Plasmodium falciparum malaria in children in southern Tanzania. Lancet 1994;344:1175-81.

20. Stoltzfus RJ. Rethinking anaemia surveillance. Lancet 1997;349:1764-6.

21. Hosmer DW, Lemeshow S. Applied Logistic Regression. John Wiley & Sons, 1989.

22. Kitua AY, Smith T, Alonso PL, Masanja H, Urassa H, Menendez C et al. Plasmodium falciparum malaria in the first year of life in an area of intense and perennial transmission. Trop.Med.Int.Health 1996;1:475-84.

23. Smith T, Beck HP, Kitua A, Mwankusye S, Felger I, Fraser-Hurt N et al. Age dependence of the multiplicity of Plasmodium falciparum infections and of other malariological indices in an area of high endemicity. Trans.R.Soc.Trop.Med.Hyg. 1999;93 Suppl 1:15-20.

24. D'Alessandro U, Olaleye BO, McGuire W, Langerock P, Bennett S, Aikins MK et al. Mortality and morbidity from malaria in Gambian children after introduction of an impregnated bednet programme. Lancet 1995;345:479-83.

25. Snow RW, Lindsay SW, Hayes RJ, Greenwood BM. Permethrin-treated bed nets (mosquito nets) prevent malaria in Gambian children. Trans.R.Soc.Trop.Med.Hyg. 1988;82:838-42.

26. Snow RW, Rowan KM, Greenwood BM. A trial of permethrin-treated bed nets in the prevention of malaria in Gambian children. Trans.R.Soc.Trop.Med.Hyg. 1987;81:563-7.

27. Alonso PL, Lindsay SW, Armstrong JR, Conteh M, Hill AG, David PH et al. The effect of insecticide-treated bed nets on mortality of Gambian children. Lancet 1991;337:1499-502.

28. Marbiah NT, Petersen E, David K, Magbity E, Lines J, Bradley DJ. A controlled trial of lambda-cyhalothrin-impregnated bed nets and/or dapsone/pyrimethamine for malaria control in Sierra Leone. Am.J.Trop.Med.Hyg. 1998;58:1-6.

29. Binka FN, Kubaje A, Adjuik M, Williams LA, Lengeler C, Maude GH et al. Impact of permethrin impregnated bednets on child mortality in Kassena- Nankana district, Ghana: a randomized controlled trial. Trop.Med.Int.Health 1996;1:147-54.

30. Sexton JD, Ruebush TK2, Brandling-Bennett AD, Breman JG, Roberts JM, Odera JS et al. Permethrin-impregnated curtains and bed-nets prevent malaria in western Kenya. Am.J.Trop.Med.Hyg. 1990;43:11-8.

31. Habluetzel A, Diallo DA, Esposito F, Lamizana L, Pagnoni F, Lengeler C et al. Do insecticide-treated curtains reduce all-cause child mortality in Burkina Faso? Trop.Med.Int.Health 1997;2:855-62.

32. Premji Z, Lubega P, Hamisi Y, Mchopa E, Minjas J, Checkley W et al. Changes in malaria associated morbidity in children using insecticide treated mosquito nets in the Bagamoyo district of coastal Tanzania. Trop.Med.Parasitol. 1995;46:147-53.

33. Njunwa, K. J., Kilimali, V. A., Marero, S. M., Msuya, F. H., and Pilyimo, R. Kamuzora D. Assessment of the efficacy of permethrin incorporated bednets, "OLYSET NET", on malaria transmission after twelve months of their use in three villages of Kibaha District, Coast Region, Tanzania. Osaka, Japan, Sumitomo Chemical Company Limited. 1996.

34. Fraser-Hurt N, Felger I, Edoh D, Steiger S, Mashaka M, Masanja H et al. Effect of insecticide-treated bed nets on haemoglobin values, prevalence and multiplicity of infection with Plasmodium falciparum in a randomized controlled trial in Tanzania. Trans.R.Soc.Trop.Med.Hyg. 1999;93 Suppl 1:47-51.

35. Molineaux L, Gramiccia G. The Garki Project. Reserach on the epidemiology and control of malaria in the Sudan Savanna of West Africa. Geneva, Switzerland: World Health Organisation, 1980.

36. Smith T, Felger I, Kitua A, Tanner M, Beck HP. Dynamics of multiple Plasmodium falciparum infections in infants in a highly endemic area of Tanzania. Trans.R.Soc.Trop.Med.Hyg. 1999;93 Suppl 1:35-9.

37. Dye C, Lines JD, Curtis CF. A test of the Malaria Strain Theory. Parasitology Today 1996;12:88-9.

38. Mshinda H, Font F, Hirt R, Mashaka M, Ascaso C, Menendez C. A comparative study of the efficacies of chloroquine and a pyrimethamine-dapsone combination in clearing Plasmodium falciparum parasitaemia in school children in Tanzania. Trop.Med.Int.Health 1996;1:797-801.

39. White NJ,.Olliaro P. Artemisinin and derivatives in the treatment of uncomplicated malaria. Med.Trop.(Mars.) 1998;58:54-6.