This study determined the prevalence of malaria infection and parasite density in Cameroonian parturient women using maternal peripheral blood microscopy, placental blood microscopy and placental histology. The epidemiological and clinical relevance of the different types of malaria diagnosis as well as some risk factors associated with malaria infection were investigated.
Overall, pregnant women in this semi-urban area of Cameroon were frequently (62.1%) exposed to P. falciparum infection during pregnancy. Using blood smear microscopy, 21.9% of the women were found to harbour malaria parasites at delivery similar to findings reported in Yaoundé [17, 19]. Malaria parasitaemia rate increased to 39.2% when the results of placental histology were added. Furthermore, microscopy of peripheral blood missed more microscopic placental infections, with 49 out of 60 women having a negative peripheral blood film. The observed sensitivity of placental histology is in accord with previous reports [15, 16, 30] and emphasis the gross underestimation of placental malaria infection by peripheral blood microscopy in pregnant women living in endemic areas. It is possible that peripheral parasitaemia may remain below the levels of microscopic detection while parasites may be harboured by the placenta and evade circulation. Nevertheless some studies have reported high sensitivities of peripheral blood immunochromatographic (ICT) strip test that detects P. falciparum histidine-rich protein 2 (HRP-2) [17, 31] and PCR, that amplifies P. falciparum -specific DNA for detection of low-level parasitemia or circulating genetic material  compared to peripheral blood microscopy in detecting microscopically confirmed placental Plasmodium falciparum [19, 31]. Malaria in pregnancy has significant adverse effects on the mother and foetus. Consequently, early and accurate diagnosis of malaria in pregnancy is absolutely imperative. Obtaining results quickly from the examination of blood samples from pregnant women with suspected malaria is now made possible by the use of rapid malaria diagnostic tests (RDTs), although their use in developing countries is limited by their high cost and availability .
There was a significant positive correlation between parasitaemia levels determined by placental blood smear and tissue sections (r = 0.757; p < 0.001), the median placental parasitaemia levels being higher in placental blood smears (0.83%) compared to that detected in placental tissue sections (0.38%). Two concepts are suggested. Firstly, impression smears correlate quite well with histology. Secondly, infection detected only by histology is lower level and potentially less significant than infection found on impression smears.
It is well established that primigravidae are indisputably at greater risk of malaria infection during pregnancy in stable malaria regions . In the present study, parity was associated only with past infections. Similar to previous studies, age (≤ 20 years old) was an independent risk factor for microscopic parasitaemia after adjusting for parity [10, 35–38]. Furthermore, parity had no effect on the prevalence of low grade parasitaemia in pregnant women similar to findings reported elsewhere [16, 19] but women > 20 years had higher prevalence of low-grade parasitaemia than women ≤ 20 years old). This suggests that age-associated immunity may play an important role in limiting P. falciparum to low parasite densities in areas of high and stable transmission . The relationship between the role of age as a determinant of immunity and vaccine studies is not really clear. It is suggested that non-pregnancy-specific vaccines might help teenage mothers , but at present it is not clear whether vaccines like RTS, S is meant rather than VAR2CSA vaccines.
It is generally assumed that in areas of stable transmission, parasitaemic pregnant women are rarely symptomatic, and that severe disease or death from malaria is extremely unusual . However, this assumption has been based on few studies and symptoms suggestive of malaria among pregnant women attending maternity clinics are frequent . Headache, arthromyalgias and history of fever are the most common symptoms reported in pregnant women . In this study, history of fever attack was common (44%) in women during pregnancy and was associated with approximately three-fold increased risk of microscopic parasitaemia at delivery. Future studies are needed to evaluate the clinical presentation of malaria episodes during pregnancy in stable transmission areas to better control the disease. Secondly, the lack of appropriate or timely treatment of symptomatic malaria in pregnancy may lead to adverse pregnancy outcome.
Women with blood smear microscopically detectable malaria were likely to be anaemic and had a mean haemoglobin concentration of 0.98 g/dl lower than that of uninfected women. Similar to findings reported elsewhere [16, 19, 31] blood film microscopy identified women at highest risk of anaemia. This study did not show an association between infection detected by histology only and anaemia nor a change in mean haematocrit levels. Equally, several studies, [16, 19, 37] failed to obtain an association between subpatent P. falciparum infection in both peripheral and placental blood and increased risk of anaemia. On the contrary, in Ghana, as reported by Mockenhaupt et al [31, 43], submicroscopic infections were found to be a risk factor for maternal anaemia. The impact of subpatent malaria infections on anaemia might vary with different malaria endemicity levels and therefore with the level of pre-pregnancy acquired malaria immunity . Anaemia may be caused by an increase in density of parasitaemia rather than the mere presence of parasite infection.
In Malawi, Rogerson et al  showed that birth weights of newborns differed significantly with a diagnosis of malaria. Further, the prevalence of LBW babies was significantly higher for women with blood film microscopically detectable infection compared to those with no parasites by blood film microscopy or placental histology. The prevalence of LBW in the study area was particularly low thus it is possible that the small sample size did not provide the power needed to detect the effect of malaria on LBW. In other areas, LBW among women with malaria infection is frequently around 20% and close to 10% in those without . This may explain the lack of associations. Placental monocytes and fibrin containing malaria pigment have been shown to play important roles in the pathogenesis of pregnancy-associated malaria . There is the need for prospective studies in the study area that will examine the causal roles of monocyte and fibrin containing pigment in the pathogenesis of decreased birth weight and maternal anaemia.
It is not surprising that women who reported to have received one or more doses of SP had a decreased risk of microscopic parasitaemia at delivery than those who had no SP during pregnancy. Furthermore, two or more doses of SP significantly reduce the prevalence of microscopic parasitaemia among pregnant women similar to findings of previous studies [44, 45]. The prevalence of peripheral malaria parasitaemia (5.6%) and placental malaria parasitaemia (25.5%) as detected by blood smear microscopy is low compared to that previously reported in the same community where peripheral and placental malaria parasitaemia (33.2% and 33.6% respectively) were similar . This study was carried out from 1998–2001 when pyrimethamine chemoprophylaxis had been recommended for use in pregnancy. It can be observed from the current study that maternal peripheral and placental malaria infection at delivery has decreased in this area confirming that IPT with SP may be effective in reducing malaria parasitaemia in pregnancy after implementation. Similar to findings reported elsewhere [35, 45–47]. Approximately half of the women in the study received ≤ 1 SP dose. Thus a study to evaluate the coverage of IPTp-SP in this area in order to identify facilitating factors and operational challenges for scaling up IPT delivery is imperative.