Seasonal variation and interspecies dynamics among Plasmodium falciparum and ovale species in Bagamoyo, Tanzania.

Background: Malaria control in sub-Saharan Africa is typically focused on Plasmodium falciparum (Pf), but non-falciparum species like P. ovale curtisi (Poc) and P. ovale wallikeri (Pow) appear to be rising in prevalence, especially in parts of East Africa.

Methods: We conducted polymerase chain reaction (PCR)-based screening of 7,173 asymptomatic individuals over 5 years of age in coastal Tanzania from 2018-2022, employing real-time 18S rRNA PCR assays for P. falciparum and P. ovale, followed by Poc/Pow detection. Plasmodium positivity was compared across seasons and demographic groups, and interactions between species were analyzed via binomial regression.

Results: Pf infection (prevalence 27.4%) was associated with younger age, male sex, and higher recent cumulative rainfall, whereas these associations were not apparent for P. ovale (Po, prevalence 11.5%). Po infections appeared to peak during months with lower Pf prevalence, especially during the long wet season, when Po mono-infections predominated and fewer Pf-Po co-infections were detected than expected by independent assortment. This apparent antagonism was reversed during the short wet season: Pf-Po co-infections were comparatively enriched despite low overall Po prevalence. In contrast, excess mixed Poc/Pow infections were detected across all seasons, composing 23% of the Po-positive isolates in which a specific Po species could be detected.

Conclusions: The epidemiology of P. ovale species in coastal Tanzania suggests they are frequently present when P. falciparum recedes, but also co-infect the same hosts during the short wet season. Meanwhile, the individual Poc and Pow species often co-exist within individuals, perhaps due to co-transmission or concurrent relapse.