REGIONAL MEDICAL RESEARCH CENTRE FOR TRIBALS

Pregnant women are more susceptible to malaria and its adverse consequences than non-pregnant women. Insecticide-treated bednets (ITNs) have emerged as an efficacious and cost-effective malaria control strategy, and are a key technical element of the Roll Back Malaria strategy, which targets children <5 years of age and pregnant women. This manuscript reviews the available evidence for the efficacy of insecticide-treated nets in the prevention of malaria in pregnancy. To date six randomized controlled studies have been conducted to determine the impact of insecticide treated nets (ITNs) on malaria in pregnancy (N=7122). In addition one nonrandomized study of the effect of socially marketed ITNs assessed the impact in pregnant women (N=505). These studies cover a wide spectrum of malaria endemicity including stable-low to moderate transmission, high and markedly seasonal malaria transmission, and intense perennial transmission. These studies show that ITNs reduce maternal malaria parasitaemia and anaemia, and increase the birth weight of the newborn. ITNs, at least in the context of efficacy trials, can provide significant health benefits to pregnant women and their newborns in a wide range of areas with stable malaria transmission. For Africa, the greatest challenge is now to make optimal use of the available resources and make ITNs a key part of malaria control and reproductive health programs in malaria endemic areas. INSECTICIDE TREATED NETS(ITNs) One of the main components of the Roll Back Malaria (RBM) initiative is the use of mosquito nets or curtains treated with insecticides such as permethrin or deltamethrin. Numerous studies, mostly from sub-Saharan Africa have shown that ITNs are able to reduce the incidence of malaria morbidity in children < 5 years of age (reviewed by Lengeler). Five of these studies were large–scale population based randomized controlled mortality trials. The studies showed that ITNs reduce under-five mortality by 14% to 30% and save approximately 6 lives per 1000 pre-school children annually in African villages. ITNs act in several ways to reduce human-vector contact. First, they provide personal protection by acting as a physical barrier to mosquitoes, at least in intact nets. The insecticides such as permethrin also have exitorepellent properties that affect the 1 Tribal Health Bulletin : Vol. 9 (1&2) Jan & July 2003 1 Child and Reproductive Health Group, Liverpool School of Tropical Medicine, Pembroke Place, Liverpool L3 5QA. behavior of mosquitoes by reducing the rate of entry into houses and increasing the rate of early exit from houses. Lastly, the insecticide has a toxic killing effect on mosquitoes that attempt to feed and rest. With the community-wide use of permethrintreated bed nets and curtains this may result in an area-wide reduction in malaria transmission through a decline in parity and sporozoite rates as well as the overall vector population suggesting that mortality of vectors is an important mode action for ITNs. This has been called the ‘mass’ or ‘community’ effect of ITNs. MALARIA IN PREGNANCY Pregnant women are more susceptible to malaria than non-pregnant women, and its adverse consequences vary based on the pregnant woman’s preexisting level of immunity against malaria. In areas of low-stable or unstable transmission such as in many part of Asia, including India, women do not acquire substantial antimalarial immunity and are susceptible to episodes of acute and sometimes severe malaria, which may result in stillbirths, spontaneous abortions, or maternal death. In areas with higher and stable malaria transmission, Plasmodium falciparum infection in pregnancy is characterized by predominantly low-grade, sometimes sub-patent, persistent or recurrent parasitemia. These frequently do not result in acute symptoms but are a substantial cause of maternal anemia and of prematurity and intrauterine growth retardation. Because most of these infections remain asymptomatic, undetected and therefore untreated, prevention of malaria in pregnancy is especially important. Primi, and secundi gravidae are most at risk, but women of higher gravida are also affected in areas with a high prevalence of HIV or in areas with low or unstable malaria transmission. It is estimated that malaria in pregnancy in Sub-Saharan Africa is responsible for up to 30-35% of preventable LBW, and contributes to 75,000 to 200,000 infant deaths each year . In Asia, P. vivax malaria has also been shown to cause significant maternal morbidity, maternal anemia and reduced birth weight, although the overall effect appears to be less severe than with P. falciparum. A recent series of studies have shown P.falciparum to bind selectively to chondroitin sulfate A (CSA) expressed in the placenta. Unlike primigravidae, women who have had placental malaria in previous pregnancies, possess anti-adhesion antibodies that inhibit placental sequestration of this distinct sub-population of parasites with the CSA-binding phenotype. If this is the main mechanism of the parity specific adverse effects of malaria in pregnancy then vaccination against these specific parasites may one day be possible. Until then, malaria control in pregnancy relies on a three-pronged approach: (1) use of intermittent preventive treatment provided as part of antenatal care (IPT), (2) insecticide treated nets (ITNs), and (3) malaria case management. 28 2 Tribal Health Bulletin : Vol. 9 (1&2) Jan & July 2003 Here a summary is provided of the available scientific evidence from controlled trials in Africa and South East Asia that suggest that ITNs are a promising tool in the control of malaria in pregnancy. Some of the remaining research questions are also discussed.