Seasonality and transmissibility of Plasmodium ovale in Bagamoyo District, Tanzania

Abstract

Background Plasmodium ovale is a neglected malarial parasite that can form latent hypnozoites in the human liver. Over the last decade, molecular surveillance studies of non-falciparum malaria in Africa have highlighted that P. ovale is circulating below the radar, including areas where Plasmodium falciparum is in decline. To eliminate malaria where P. ovale is endemic, a better understanding of its epidemiology, asymptomatic carriage, and transmission biology is needed. Methods We performed a pilot study on P. ovale transmission as part of an ongoing study of human-to-mosquito transmission of P. falciparum from asymptomatic carriers. To characterize the malaria asymptomatic reservoir, cross-sectional qPCR surveys were conducted in Bagamoyo, Tanzania, over three transmission seasons. Positive individuals were enrolled in transmission studies of P. falciparum using direct skin feeding assays (DFAs) with Anopheles gambiae s.s. (IFAKARA strain) mosquitoes. For a subset of participants who screened positive for P. ovale on the day of DFA, we incubated blood-fed mosquitoes for 14 days to assess sporozoite development. Results Molecular surveillance of asymptomatic individuals revealed a P. ovale prevalence of 11% (300/2718), compared to 29% (780/2718) for P. falciparum. Prevalence for P. ovale was highest at the beginning of the long rainy season (15.5%, 128/826) in contrast to P. falciparum, which peaked later in both the long and short rainy seasons. Considering that these early-season P. ovale infections were low-density mono-infections (127/128), we speculate many were due to hypnozoite-induced relapse. Six of eight P. ovale-infected asymptomatic individuals who underwent DFAs successfully transmitted P. ovale parasites to A. gambiae. Conclusions Plasmodium ovale is circulating at 4–15% prevalence among asymptomatic individuals in coastal Tanzania, largely invisible to field diagnostics. A different seasonal peak from co-endemic P. falciparum, the capacity to relapse, and efficient transmission to Anopheles vectors likely contribute to its persistence amid control efforts focused on P. falciparum. Graphical Abstract

Non-falciparum malaria infections are as prevalent as P. falciparum among Tanzanian schoolchildren

“…infections identified. There is precedent for this approach 17,25,31 , as low density parasitemia is characteristic of P. ovale spp. infections, making them challenging to detect.…”

“…Decades of concentrated malaria control interventions have helped lower the national prevalence from 18% in 2008 to 7% in 2017 23 . The vast majority of malaria cases reported in Tanzania are attributed to P. falciparum 9,21 , although recent studies have also identified evidence of P. malariae, P. vivax , and P. ovale transmission 4,18,24,25 . Given widespread use of P. falciparum- specific HRP2-based RDTs for malaria diagnosis, the propensity for missed detection or misclassification of non-falciparum species in Tanzania is high, and large-scale, geographically representative studies to assess spatial distributions of these species are lacking.…”

Micro-heterogeneity of transmission shapes the submicroscopic malaria reservoir in coastal Tanzania

“…Malaria transmission occurs throughout the year, with peaks typically during the long (March to June) and short (October to November) rainy seasons. Study data were collected as part of a large observational study called Project TranSMIT (Transmission of Submicroscopic Malaria in Tanzania) that prospectively screened and enrolled participants over three years for mosquito feeding studies and performed weekly follow-up over four weeks for a subset of participants with submicroscopic parasitemia (Markwalter et al, 2021; Tarimo et al, 2022).…”

“…Screened participants reported their age, gender, and village in which they reside. Approximately 200 μL of finger-prick blood was used to make thick and thin blood smears, perform a dual antigen HRP2 and pLDH RDT (rapid diagnostic test, SD Bioline), and create 2-3 dried blood spots (DBS) on Whatman 3MM filter paper for analysis by real-time polymerase chain reaction (PCR) targeting P. falciparum 18S rRNA as previously described (Markwalter et al, 2021; Tarimo et al, 2022). We previously determined the limit of detection (LOD) for the P. falciparum PCR test performed onsite as approximately 1-5 parasites/uL, which is more sensitive than both microscopy (approximately 50-500 parasites/uL) and RDT (approximately 100 parasites/uL), but not as sensitive as ultrasensitive PCR tests (Berzosa et al, 2018; Leski et al, 2020; Markwalter et al, 2021; Mfuh et al, 2019).…”

Malaria species positivity rates among symptomatic individuals across regions of differing transmission intensities in Mainland Tanzania

“…This possibility is particularly salient in light of current malaria elimination goals stemming from the WHO Global Technical Strategy[9], which aims by 2030 to reduce the global malaria burden by 90% from 2015 levels. Non-falciparum malaria species have notably different biology than P. falciparum and are not necessarily controlled by the same measures, due to the potential for transmission by different vectors, differing seasonality[10], potentially earlier gametocytogenesis[11–13], presence of hypnozoite stages/persistent infection[12,14], generally lower levels of parasite carriage/density[15,16], and greater asymptomatic infection and transmission[10].…”