Microheterogeneity of Transmission Shapes Submicroscopic Malaria Carriage in Coastal Tanzania

Abstract

Background: Asymptomatic carriage of malaria parasites persists even as malaria transmission declines. Low-density infections are often submicroscopic, not detected with rapid diagnostic tests (RDTs) or microscopy but detectable by polymerase chain reaction (PCR).

Methods: To characterize submicroscopic Plasmodium falciparum carriage in an area of declining malaria transmission, asymptomatic persons >5 years of age in rural Bagamoyo District, Tanzania, were screened using RDT, microscopy, and PCR. We investigated the size of the submicroscopic reservoir of infection across villages, determined factors associated with submicroscopic carriage, and assessed the natural history of submicroscopic malaria over 4 weeks.

Results: Among 6076 participants, P. falciparum prevalences by RDT, microscopy, and PCR were 9%, 9%, and 28%, respectively, with roughly two-thirds of PCR-positive individuals harboring submicroscopic infection. Adult status, female sex, dry season months, screened windows, and bed net use were associated with submicroscopic carriage. Among 15 villages encompassing 80% of participants, the proportion of submicroscopic carriers increased with decreasing village-level malaria prevalence. Over 4 weeks, 23% of submicroscopic carriers (61 of 266) became RDT positive, with half exhibiting symptoms, while half (133 of 266) were no longer parasitemic at the end of 4 weeks. Progression to RDT-positive patent malaria occurred more frequently in villages with higher malaria prevalence.

Conclusions: Microheterogeneity in transmission observed at the village level appears to affect both the size of the submicroscopic reservoir and the likelihood of submicroscopic carriers developing patent malaria in coastal Tanzania.

Keywords: asymptomatic reservoir; malaria; malaria transmission; submicroscopic malaria; subpatent malaria.

Figure 1.

Rapid diagnostic test (RDT)–positive (RDT+) and submicroscopic malaria prevalence in relation to rainfall, seasonality, and age in Bagamoyo, Tanzania. Data are derived from screening 6076 asymptomatic persons aged ≥6 years between 2018 and 2021, using RDTs and 18S ribosomal RNA real-time polymerase chain reaction (PCR). A, Sampling was discontinuous, and months when sampling was not conducted are shaded; on the y-axis, 18–21 represent 2018–2021. Abbreviations: PCR+, PCR positive; RDT−, RDT negative. B, Distribution of parasite densities as determined by 18S PCR in wet and dry season months, with dry season months defined as those with rainfall <33 mm. C, Proportion of participants who tested positive with RDT versus PCR only for each age category.

Figure 2.

Village-level characterization of the submicroscopic reservoir in rural Bagamoyo district. A map of Bagamoyo district in eastern Tanzania depicts the size of the submicroscopic reservoir among 15 villages and the locations of the 2 clinics and 2 schools where study participants were recruited. The submicroscopic malaria ratio is the ratio of polymerase chain reaction–positive (PCR+)/rapid diagnostic test–negative cases to all PCR+ cases in the village. Zemba village is not included in the map but is located nearby Mwavi proper village. based on the Tanzania regional postcode list.

Figure 3.

Village-level characterization of malaria prevalence and the size of the submicroscopic malaria. Each data point represents a village, with size corresponding to the number of participants reporting residence in that village. A, Microscopic malaria prevalence from the sampled villages was correlated with polymerase chain reaction (PCR) malaria prevalence. B, The size of the submicroscopic reservoir for each village, indicated by the submicroscopic ratio (PCR-positive only cases/total PCR-positive cases), increases with decreasing malaria prevalence.

Figure 4.

The natural history of submicroscopic parasite carriers over 4 weeks of follow-up. Flow diagram depicts the parasite status (based on rapid diagnostic test [RDT] and polymerase chain reaction [PCR] results) of those who screened positive for submicroscopic malaria (PCR positive [PCR+]/RDT negative), then followed up without treatment and assessed 2 and 4 weeks later. Those who developed RDT-positive (RDT+) malaria (PCR+/RDT+) were treated and discharged. In total, of those enrolled and followed up to week 4 (n = 219) or became RDT+ before that (n = 47) (total n = 266), 23% of submicroscopic carriers developed RDT+ malaria, 25% continued to have asymptomatic submicroscopic carriage, and the rest had spontaneous resolution (ie, were both PCR and RDT negative) by week 4, though this last group may have continued to harbor low-density infection not detected by PCR. Abbreviation: LFTU, lost to follow-up.

Figure 5.

Parasitological outcomes for untreated submicroscopic parasitemia after 2 and 4 weeks of follow-up, stratified by village malaria prevalence. The proportions of participants with asymptomatic submicroscopic malaria who became rapid diagnostic test (RDT) positive (RDT+), continued to have asymptomatic submicroscopic infection, or became polymerase chain reaction negative are depicted after 2 and 4 weeks of follow-up, with cumulative outcomes depicted in the bottom panel. The minority of participants who developed RDT+ patent malaria were more likely to reside in villages with higher malaria prevalence.