Women show increased susceptibility to malaria infection before and after giving birth

Pregnant women may be more susceptible to malaria in the later stages of pregnancy and during the two months following birth, according to a new study published recently (New England Journal of Medicine, 2000, 343: 598–603). Researchers have found that in areas with high rates of malaria, susceptibility increased during the second and third trimesters, reaching a maximum in the 60 days after delivery when women were four times more likely to have an episode of the disease than they were in the year before the pregnancy. The study also found that the duration of fever was longer, and that the density of asymptomatic parasites was significantly higher during the pregnancy and in the following two months.

In areas where malaria is endemic, pregnant women are the main group of adults at risk. Infection by Plasmodium falciparum, one of four parasites that cause malaria in humans, increases the chances of maternal anaemia, abortion, stillbirth, prematurity, growth retardation in the womb, and low birth weight.

In sub-Saharan Africa, where 90% of deaths from malaria occur, it is infants who are most likely to die as a result of maternal infection. Malaria is one of the main reasons why 3.5 million low-birth-weight infants are born each year in the region.

Precisely why the risk of malaria increases in pregnancy is not clear, and several theories have been put forward, including changes to the cellular immune responses affecting protection.

In the new study, researchers analysed 71 pregnancies in 38 women in a village in Senegal. The women were monitored for two years, from the year before conception through to 12 months after the birth. In an accompanying editorial (New England Journal of Medicine, 343: 651–652), Dr Bernard Nahlen of the World Health Organization in Geneva, challenged one of the conclusions of the researchers, namely that the increased susceptibility found during the immediate post-birth period may have severe consequences. ‘‘ [The authors] provide no clinical data to support this conclusion. Thus, it is difficult to accept their recommendations that weekly chemoprophylaxis against malaria should be continued for at least two months after delivery, and it unlikely that extension of this strategy to the postpartum period would result in a major decrease in the burden of malaria among mothers,’’ he said.

He also pointed out that the emergence of chloroquine resistance and problems of compliance have limited the effectiveness of weekly chemoprophylaxis with chloroquine. The latest studies in Kenya and Malawi (American Journal of Tropical Medicine and Hygiene, 1998, 59: 813–822; Annals of Tropical Medicine and Parasitology, 1998, 92: 141–150) have found that a new and simpler control strategy of two doses of sulfadoxine–pyrimethamine has proved effective in decreasing maternal anaemia and low birth weight.

Dr Nahlen added that malaria during pregnancy is potentially the most controllable part of the global malaria problem. ‘‘However, despite the availability of an effective control strategy, the implementation of programmes to prevent malaria in pregnancy has been rare. WHO’s Roll Back Malaria Programme has brought a new emphasis to malaria-control efforts and has identified the need for control strategies in areas of intermittent or unstable P. falciparum transmission, where women have little acquired immunity, as well as in areas of P. vivax transmission.’’

WHO now recommends intermittent preventive treatment with an effective, preferably one-dose, antimalarial drug such as sulfadoxine–pyrimethamine in areas where P. falciparum is resistant to chloroquine and sensitive to sulfadoxine– pyrimethamine. Such intermittent treatment should be made available to women in highly endemic areas, especially for women in their first and second pregnancies. Treatment should be started from the second trimester onwards at intervals of not less than one month apart.

Roger Dobson, Abergaveny

World Health Organization Genebra - Genebra - Switzerland