Association Between Age andPlasmodium falciparumInfection Dynamics

Abstract

Few data exist on the incidence or duration of natural Plasmodium falciparum infections in high transmission settings. School-aged children (SAC) carry a disproportionate burden of infections, suggesting either increased incidence or duration. We estimated the incidence and duration of unique infections by age groups. The Mfera Cohort Study (2012-2017) had two years of follow-up with 120 participants tested monthly and during sick visits. Blood samples were collected to detect P. falciparum by microscopy and polymerase chain reaction. Positive samples underwent genotyping. Simulation was used to account for high non-detection of infection among low parasitemia infections, which increase in frequency with age. Adults had significantly fewer unique infections per person per year (median, 2.5) compared to SAC and under-five children (6.3 and 6.6, respectively). Over half of all genotypes were persistent. Infections lasted significantly longer in adults (median, 180 days) and SAC (median, 163 days) compared to under-five children (median, 97 days), after accounting for age-dependent, non-detection of infection. SAC acquired new infections at the same rate as under-five children, but maintained these infections for longer periods of time, similar to adults. This study provides new insights into P. falciparum infection dynamics that should be considered when designing malaria control strategies.

Sources of persistent malaria transmission in a setting with effective malaria control in eastern Uganda: a longitudinal, observational cohort study

“…3 Asymptomatic infections with Plasmodium falciparum are particularly prevalent in school-aged children, who have a higher complexity of infection and more persistent parasitaemia than in younger children or adults. 4,5 In persistent infections, parasite densities might fluctuate over time, and are often below the threshold of detection by microscopy or conventional malaria rapid diagnostic tests. 2,6 These submicroscopic infections are disproportionately prevalent in areas of low endemicity and in settings that have reported a decline in malaria burden.…”

Asymptomatic Plasmodium falciparum malaria prevalence among adolescents and adults in Malawi, 2015–2016

“…Even if an individual is tested, parasite densities in asymptomatic infections are usually low and may be undetectable by microscopy or rapid diagnostic test (RDT) 3 , 4 , further reducing the likelihood of diagnosis and treatment. Asymptomatic infections can persist for months and older age is associated with an increased duration of persistent infection, allowing the parasite the opportunity to transmit for a prolonged period of time 5 . The annual P. falciparum parasite rate was estimated to be 18–19% in children ages 2–10 years in Malawi in 2015–2016 6 , however, little is known about the prevalence of adult asymptomatic P. falciparum infection.…”

Maximizing Impact: Can Interventions to Prevent Clinical Malaria Reduce Parasite Transmission?

“…Additionally, the duration of time that gametocytes are present is essential for quantifying transmission potential because a longer duration increases the opportunities for mosquito bites [5]. The duration of an asymptomatic infection remains difficult to quantify but likely ranges from a few weeks [17] to many months [18,19] and depends on factors such as the level of host immunity. Symptomatic infections are truncated by effective treatment, although the extent to which this occurs depends on the likelihood of treatment-seeking behavior, access to effective drugs, and the effect of the drugs on gametocytes.…”

“…The extent to which a disease-targeting malaria intervention reduces transmission will depend on the contribution of the targeted population to the infectious reservoir ( Figure 1). A number of studies over the last decade have shed light on the relative contribution of different age groups to the infectious reservoir, based on the prevalence of infection in the population, the relative infectiousness and duration of those infections, and the frequency of bites by malaria vectors [13,19,20,25,26]. Interventions targeted at reducing clinical malaria disease in high-burden age groups may also reduce parasite transmission by decreasing the number of infections that develop gametocytes, decreasing the density or duration of gametocytemia, or reducing exposure to mosquito bites.…”