The persistence and oscillations of submicroscopic Plasmodium falciparum and Plasmodium vivax infections over time in Vietnam: an open cohort study

Abstract

Background: A substantial proportion of Plasmodium species infections are asymptomatic with densities too low to be detectable with standard diagnostic techniques. The importance of such asymptomatic plasmodium infections in malaria transmission is probably related to their duration and density. To explore the duration of asymptomatic plasmodium infections and changes in parasite densities over time, a cohort of participants who were infected with Plasmodium parasites was observed over a 2-year follow-up period.

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

Findings: Between Dec 1, 2013, and Jan 8, 2016, 356 villagers participated in between one and 22 surveys. These study participants underwent 4248 uPCR evaluations (11·9 tests per participant). 1874 (32%) of 4248 uPCR tests indicated a plasmodium infection; 679 (36%) of 1874 tests were P falciparum monoinfections, 507 (27%) were P vivax monoinfections, 463 (25%) were co-infections with P falciparum and P vivax, and 225 (12%) were indeterminate species of Plasmodium. The median duration of P falciparum infection was 2 months (IQR 1-3); after accounting for censoring, participants had a 20% chance of having parasitaemia for 4 months or longer. The median duration of P vivax infection was 6 months (3-9), and participants had a 59% chance of having parasitaemia for 4 months or longer. The parasite densities of persistent infections oscillated; following ultralow-density infections, high-density infections developed frequently.

Interpretation: Persistent largely asymptomatic P vivax and P falciparum infections are common in this area of low seasonal malaria transmission. Infections with low-density parasitaemias can develop into much higher density infections at a later time, which are likely to sustain malaria endemicity.

Funding: The Wellcome Trust, Bill & Melinda Gates Foundation.

Figure 1

Study design Thin red arrows indicate the days of mass drug administrations. Large grey arrow indicates date of study population count (census). Large green arrows indicate the timing of surveys in which all residents were invited. Small green arrows indicate the timing of surveys for cohort members only. Surveys at months 17, 20, and 23 were cancelled because of logistical reasons. M=month.

Figure 2

Persistence of Plasmodium falciparum and Plasmodium vivax infections over the study period P falciparum group infections included P falciparum, P falciparum and P vivax mixed infections, or unidentified spp. P vivax group infections included P vivax, P vivax and P falciparum mixed infections, or unidentified spp.

Figure 3

Correlation between the duration of a Plasmodium falciparum episode and parasite density Blue dots represent individuals; red line represents the trend line.

Figure 4

Examples of parasite densities over time in two individual cohort members (A) Participant VN300445: 12 months after two rounds of dihydroartemisinin and piperaquine a low-density Plasmodium falciparum infection (31 parasites per mL) was detected, which increased to 58 parasites per mL before expanding to 3 975 000 parasites per mL. The participant remained afebrile throughout the 2-year follow-up period and was not treated after the mass drug administration. (B) Participant VN300699: 9 months after three rounds of dihydroartemisinin and piperaquine, the participant had a low-density P falciparum infection. The density oscillates in the following 13 months between no detectable parasites and 529 200 parasites per mL to increase to 7 296 000 parasites per mL on the last study visit. The participant remained afebrile throughout the 2-year follow-up period and was not treated after the mass drug administration. Red lines represent mass drug administrations. Numbers on the graph represent parasite densities per mL. More individual examples are in the appendix.

Figure 5

Amplitude of parasite density oscillations in relation to the observed duration of plasmodium infections Bars and whiskers are median and IQR.