Review articleThe epidemiology and consequences of maternal malaria: a review of immunological basis
Introduction
Malaria, the most important parasitic disease of man, is endemic in 103 countries where more than 2000 million people live exposed to the infection (Menendez, 1995). An estimated 300–500 million cases of malaria each year result in about 1–2 million deaths, mainly in children less than 5 years of age living in sub-Saharan Africa (Snow et al., 1999, Snow et al., 2001). Besides children, some 24 million women that become pregnant each year in malaria-endemic regions are at increased risk of being infected with Plasmodium falciparum malaria and its associated complications (McGregor et al., 1983, Steketee et al., 1996b). In some sub-Saharan African countries endemic for malaria, almost half of all primigravidae will be parasitaemic at their first antenatal visit (Menendez, 1995). The complications of these infections, for the mother and baby are considerable and constitute one of Africa's major public health problems. The reasons for the increased susceptibility to malaria in pregnancy and its clinical manifestations are not as straightforward as has been reported previously. The previously held opinion that pregnancy depresses pre-existing immunity (Blacklock and Gordon, 1925, Ibanesebhor and Okolo, 1992) does not explain the differential impact of maternal malaria infection on primigravidae. The available data suggest that several mechanisms could be involved (Scherf et al., 2001, Beeson and Brown, 2002).
An extensive review of available data on immunological basis of the epidemiology and consequences of maternal malaria was carried out with the view of suggesting some therapeutic and control measures in this high-risk group.
Section snippets
Epidemiology of malaria in pregnancy
There are some distinctive features of the epidemiology of maternal malaria that are important in deciding appropriate control strategies. First of all, in all malaria-endemic areas, the frequency and severity of the infection are greater in pregnant women than in the same women before pregnancy and in their non-pregnant counterparts (McGregor et al., 1952; Gilles et al., 1984). In their analysis of a 14-year follow up study of 15–45-years-old women in the Gambia, McGregor et al. (1952) found a
What happens to maternal immunity during pregnancy
The mechanisms underlying some of the striking epidemiological and clinical features of malaria in pregnancy are subjects of importance for research and proper control strategies. Several immunological hypotheses have been proposed to explain the increased risk of malaria in pregnant women. During pregnancy, a physiological immunosuppression mediated by pregnancy-associated hormones and proteins occurs. This regulation is seen by some as a necessary adjustment to maintain the antigenically
Pathological features
Parasites infecting the placenta are commonly observed in intervillious erythrocytes with or without pigment deposition in the intervillious spaces or intravillious regions. The chorionic villous syncytiotrophoblast and stroma are also commonly observed to contain malaria pigment (Bulmer et al., 1993, Galbraith et al., 1980, Leopardi et al., 1996).
Changes that have been described in parasitised microvilli include proliferation of cytotrophoblast, focal syncytiotrophoblastic necrosis, loss of
Conclusion and challenges ahead
Although other factors may be important in the epidemiology and consequences of maternal malaria, accumulated data suggest strongly that immunological process play a substantial role. A previously immuned woman becomes more susceptible to malaria infection during pregnancy as Th2-biased changes in the placenta and presence of CSA receptors favour the adhesion and sequestration of IRBC in the placenta of unprotected primigravid women. Anti-adhesion antibodies against CSA-binding parasites are
Acknowledgements
We are grateful to Professor Brian Greenwood and Dr Steve Obaro for reviewing this article.
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