An article in Malaria World by Alemayehu et al., Asymptomatic malaria in pregnancy and associated risk factors in Majang Zone, Gambella Region, Southwest Ethiopia: a hard-to-reach malaria hotspot, examines the association of various factors with the occurrence of asymptomatic malaria in pregnant women.
I am suspicious of the description of illnesses as asymptomatic especially since such reports were used to raise fears during the COVID19 pandemic. In this paper three methods were used to identify asymptomatic cases. The overall prevalence of Asymptomatic malaria in pregnancy (AMiP) was 15.3%. It was 11.3% measured by rapid diagnostic tests (RTD), 11.8% by microscopy and 17.6% by PCR. (With PCR the prevalence of ‘asymptomatic illness’ is significantly greater than the other two methods of detection. It is unclear how it can be considered a valid diagnostic test).
Those who tested positive were associated with not utilizing insecticide-treated net (ITN) within the previous week, having a history of malaria within the previous year and lack of indoor residual spraying (IRS) within the previous year. The second observation is obvious. The other two observations imply that if they had used the treated nets or sprayed their homes, they would have been less likely to contract their asymptomatic ‘illness’. But it does not say what other factors might be associated with these two factors and be the more likely reason. Poverty and malnutrition are probably associated with those who had less access to the interventions.
What was also interesting was the underlying justification for the research, that preventing asymptomatic malaria in pregnancy is a worthy enough goal to justify insecticidal and pharmaceutical interventions. Two articles were cited in the background for this. The first by Saito et al (2020) contains no evidence to support this. The second by Fried and Duffy (2017), a review article has a supporting reference, McGready et al (2012) with data that may support the goal.
This study of 17,613 chosen from 48,424 pregnant women in Thailand used microscopy to screen for malaria and determine if it was Plasmodium falciparum or Plasmodium vivax. Of the 17,613 pregnant women 80% delivered babies and 20% miscarried. 95% did not have malaria in the first trimester. The data state that 32% of those with asymptomatic malaria miscarried and 47% of those with symptomatic malaria miscarried. This does suggest that there is an association of asymptomatic malaria to increased risk of miscarriage, although notes in the article state than many data are missing.
But what does this mean? The microscopy analysis finds plasmodia in the blood. These organisms clean up dead cells and so indicate possible recent illness. Clearly those without symptoms are not as unwell as those with obvious symptoms but may have just recovered. As such it is not surprising that they were slightly more likely to have adverse pregnancy outcomes, but better than those with symptoms. It is also worth noting that most of those with symptomatic malaria in the study were treated with chloroquine, quinine or artesunate and were still very likely to miscarry.
And none of this research in any way supports the underlying mosquito-malaria hypothesis or proves that treatment will improve outcomes.