Peru has experienced a significant reduction in the reported number of malaria cases (from 50,526 in 2015 to 12,186 in 2020). The current scenario reflects residual malaria with heterogeneous transmission patches with a common occurrence of asymptomatic infections. Asymptomatic infections contribute up to 80% of the community-wide malaria transmission in Peru. The present study analyzed the intra-host evolution of malaria symptoms and the Plasmodium vivax genotypes in recurrent infections (recurrences) in a longitudinal study in the Peruvian Amazon. We aimed to provide insights into the factors associated with asymptomatic recurrences.
We analyzed the socio-demographic data, microscopic and molecular diagnosis, and the genotyping data (14 neutral microsatellites) gathered from a cohort of 302 inhabitants from 25 Amazonian communities with P. vivax malaria, treated with chloroquine plus primaquine. The participants were systematically followed up for up to 2 years.
In total, 609 recurrences were recorded, with 66% (n=198) of the participants experiencing at least one recurrence over time, 23% four recurrences or more, and records of participants experiencing up to 11 recurrences. Out of the total of recurrences, 65% of them were asymptomatic. Regarding the intra-host evolution, the participants had a higher risk to develop symptoms (log-rank p<0.001) over time. However, we observed that the participants become asymptomatic if they previously experienced symptomatic infections (incidence density=32.5 new cases/person-year). Experiencing a recurrence with parasites homologous to the ones present in the initial infection and being children between 6 and 15 years old were associated to experience an asymptomatic recurrence.
The occurrence of asymptomatic infections is associated with host and parasite characteristics, and the knowledge of these factors is relevant for developing tailored interventions in residual malaria areas like the Amazon.