Temporal Changes in the Genetic Diversity and Relatedness of Plasmodium vivax Clinical Infections in Eastern Cambodia.

BACKGROUND: Elimination of Plasmodium vivax is challenging due to the dormant liver stages (hypnozoites), which can reactivate weeks or months after the primary infection, causing relapses and ongoing parasite transmission. Despite these challenges, P. vivax clinical case numbers have declined over the past decade in Cambodia. We used parasite genotyping to assess whether declines in reported case numbers were accompanied by changes in parasite diversity and relatedness among symptomatic infections. METHODS: Genotyping was conducted on 296 symptomatic P. vivax isolates collected in eastern Cambodia in 2014, 2015, 2019, 2021, 2022, and 2023. A panel of 93 microhaplotype markers (vivaxGEN panel) was genotyped using Illumina. Population genetic measures were applied to determine infection diversity and relatedness (identity-by-descent [IBD]) each year. RESULTS: A total of 289 (20-94 per year) samples were successfully genotyped. The percentage of polyclonal infections was 5% in 2023 compared to 29%-55% in earlier years (P < .05). The cases collected in 2023 also had the highest proportion of infections with IBD >0.95 with 1 or more other infections (81.4% vs 0%-22.3% in 2014-2022). In contrast, cases in 2022 showed higher frequencies of polyclonal infections (55%) and increased population diversity. CONCLUSIONS: Our findings illustrate the potential of microhaplotype genotyping to characterize temporal changes in the genetic composition of symptomatic P. vivax infections and to complement conventional epidemiological surveillance.