Nguyen TN, et al. - Experts observed a cohort of participants from 4 villages in Vietnam who were infected with Plasmodium parasites over a 2-year follow-up period to study the duration of asymptomatic Plasmodium infections and changes in parasite densities over time. In this area of low seasonal malaria transmission, persistent largely asymptomatic P vivax and P falciparum infections were common. At a later time, infections with low-density parasitaemias could develop into much higher density infections, which were likely to sustain malaria endemicity.

Methods

• In this open cohort study, researchers invited the inhabitants of 4 villages in Vietnam to participate in baseline and subsequent 3-monthly surveys up to 24 months, which included the collection of venous blood samples.
• They batch-screened the samples using ultra-sensitive (u)PCR (lower limit of detection of 22 parasites per mL).
• During any of these surveys, participants found to be infected by uPCR were invited to join a prospective cohort and provide monthly blood samples.
• The persistence of Plasmodium falciparum and Plasmodium vivax infections and changes in parasite densities were estimated over a study period of 24 months.

Results

• Findings suggested that between Dec 1, 2013, and Jan 8, 2016, 356 villagers participated in between 1 and 22 surveys.
• As per results, these study participants underwent 4,248 uPCR evaluations (11·9 tests per participant).
• Data demonstrated that a plasmodium infection was indicated in 1,874 (32%) of 4,248 uPCR tests; P falciparum monoinfections were 679 (36%) of 1,874 tests, P vivax monoinfections were 507 (27%), 463 (25%) were co-infections with P falciparum and P vivax, and 225 (12%) were indeterminate species of Plasmodium.
• Authors noted that 2 months was the median duration of P falciparum infection (IQR 1–3); after accounting for censoring, participants had a 20% chance of having parasitaemia for 4 months or longer.
• They observed 6 months (3–9) to be the median duration of P vivax infection, and participants had a 59% chance of having parasitaemia for 4 months or longer.
• Results suggested that the parasite densities of persistent infections oscillated; following ultralow-density infections, high-density infections developed frequently.