Asymptomatic low-density malaria infections: a parasite survival strategy?
Authors: Anders B Björkman
Journal: The Lancet Infectious Diseases (2018)

Malaria parasitaemias are usually suppressed or eliminated in the human host by the immune system and antimalarial drugs. Low-density asymptomatic infections are therefore expected to be common in high-endemic areas with high herd immunity and premunition in the population. However, it is becoming increasingly apparent, with sensitive molecular PCR-based diagnostics, that submicroscopical asymptomatic infections are also common in low-endemic areas,1Okell LC Bousema T Griffin JT Ouédraogo AL Ghani AC Drakeley CJ Factors determining the occurrence of submicroscopic malaria infections and their relevance for control.Nat Commun. 2012; 3: 1237Crossref PubMed Scopus (410) Google Scholar despite residents having little protective or suppressive immunity. Previously, the interest in these infections was related to whether they affected health,2Greenwood BM Asymptomatic malaria infections—do they matter?.Parasitol Today. 1987; 3: 206-214Summary Full Text PDF PubMed Scopus (99) Google Scholar, 3Chen I Clarke SE Gosling R et al.“Asymptomatic” malaria: a chronic and debilitating infection that should be treated.PLoS Med. 2016; 13: e1001942Crossref PubMed Scopus (197) Google Scholar whereas now there is interest in their role in malaria transmission and elimination. In The Lancet Infectious Diseases, Thuy-Nhien Nguyen and colleagues4Nguyen T-N von Seidlein L Nguyen T-V et al.The persistence and oscillations of submicroscopic Plasmodium falciparum and Plasmodium vivax infections over time in Vietnam: an open cohort study.Lancet Infect Dis. 2018; (published online Jan 22.)http://dx.doi.org/10.1016/S1473-3099(18)30046-XSummary Full Text Full Text PDF PubMed Scopus (81) Google Scholar have studied the existence, persistence, and possible oscillations of asymptomatic low-density infections in an area in Vietnam with low and seasonal transmission of malaria. They used ultrasensitive quantitative real-time PCR5Imwong M Hanchana S Malleret B et al.High-throughput ultrasensitive molecular techniques for quantifying low-density malaria parasitemias.J Clin Microbiol. 2014; 52: 3303-3309Crossref PubMed Scopus (151) Google Scholar to detect Plasmodium falciparum and Plasmodium vivax infections (detection limit 22 parasites per mL) in a cohort of 356 villagers who tested positive for plasmodium among 3362 tested. They were followed up for up to 24 months with surveys every 1–3 months. The infections were evenly distributed between P falciparum or P vivax monoinfections and mixed infections. There was wide variation in infection duration from 1 to 20 months, with median durations of 2 months (IQR 1–3) for P falciparum and 6 months (3–9) for P vivax episodes. These findings confirm that both infections can be quite long-lived.6Felger I Maire M Bretscher MT et al.The dynamics of natural Plasmodium falciparum infections.PLoS One. 2012; 7: e45542Crossref PubMed Scopus (74) Google Scholar, 7Ashley EA White NJ The duration of Plasmodium falciparum infections.Malar J. 2014; 13: 500Crossref PubMed Scopus (151) Google Scholar The reason for varying durations is, however, unclear. These variations might reflect the fact that 190 (53%) of 356 study individuals were treated with dihydroartemisinin and piperaquine at least once (treatment indication not stated) or different natural infection courses related to different, perhaps strain-specific, immunity despite the low-transmission context. Of more interest is the longitudinal variation in parasite densities, although only 16 (7%) untreated infections evolved to parasite densities of more than 200 000 parasites per mL, above the detection limit of microscopy or rapid diagnostic tests. The median amplitude in P falciparum densities in individuals who persistently had parasitaemia for minimum 6 months was log 6·5 (about 500-times), but among 29 participants for whom detailed data were presented, only four developed densities above 1 000 000 parasites per mL (range 3 975 000–25 600 000), all of whom were non-febrile. Among all recorded febrile events, there was little or no correlation to parasite densities. Fluctuations in parasite density persistent infections can occur day to day because of synchronised sequestration of parasite populations,8Farnert A Snounou G Rooth I Bjorkman A Daily dynamics of Plasmodium falciparum subpopulations in asymptomatic children in a holoendemic area.Am J Trop Med Hyg. 1997; 56: 538-547Crossref PubMed Scopus (185) Google Scholar but the high amplitude oscillations observed by Nguyen and colleagues4Nguyen T-N von Seidlein L Nguyen T-V et al.The persistence and oscillations of submicroscopic Plasmodium falciparum and Plasmodium vivax infections over time in Vietnam: an open cohort study.Lancet Infect Dis. 2018; (published online Jan 22.)http://dx.doi.org/10.1016/S1473-3099(18)30046-XSummary Full Text Full Text PDF PubMed Scopus (81) Google Scholar most probably reflect longer fluctuations than those found day to day. A major reason for these persisting and fluctuating infections is probably evasion of adaptive strain-specific immunity by antigenic variation9Scherf A Lopez-Rubio JJ Riviere L Antigenic variation in Plasmodium falciparum.Annu Rev Microbiol. 2008; 62: 445-470Crossref PubMed Scopus (325) Google Scholar and possibly because low-density infections might be less immunogenic. The oscillations therefore probably occur within a persistent infection in this relatively low-transmission context. A sudden increase in parasite density might also reflect a superinfection by a newly acquired infection. This possibility could not be addressed in Nguyen's study because of the small amounts of DNA in these low-density infections. One important issue is the extent to which these persisting asymptomatic infections are transmitted. Individuals with asymptomatic infections can carry parasites until the next transmission peak.10Babiker HA Abdel-Muhsin AM Ranford-Cartwright LC Satti G Walliker D Characteristics of Plasmodium falciparum parasites that survive the lengthy dry season in eastern Sudan where malaria transmission is markedly seasonal.Am J Trop Med Hyg. 1998; 59: 582-590Crossref PubMed Scopus (108) Google Scholar But are persisting infections transmitted only if and when the oscillations peak and do gametocyte counts follow the pattern of asexual parasite fluctuations? There were no tests of the infectivity of these infections or gametocyte presence and densities in Nguyen and colleagues' study. From previous studies, the main conclusion is that submicroscopic infections could be important contributors to transmission in areas with very low transmission intensity.1Okell LC Bousema T Griffin JT Ouédraogo AL Ghani AC Drakeley CJ Factors determining the occurrence of submicroscopic malaria infections and their relevance for control.Nat Commun. 2012; 3: 1237Crossref PubMed Scopus (410) Google Scholar A key question that remains is why P falciparum infections in individuals with no or low immunity can remain asymptomatic often without the development of high parasite densities. A new hypothesis might be that in a low-endemic environment, parasites that are less virulent might not be exposed to high competition from more virulent strains and might therefore persist at a low density, asymptomatic, undetected, and untreated allowing for further transmission over a long time period and therefore eventually selected. Less virulent parasites would thus have an advantage over highly virulent parasites, which would be eliminated earlier by diagnosis and treatment and would possibly be subject to a more potent immune response. The study by Nguyen and colleagues4Nguyen T-N von Seidlein L Nguyen T-V et al.The persistence and oscillations of submicroscopic Plasmodium falciparum and Plasmodium vivax infections over time in Vietnam: an open cohort study.Lancet Infect Dis. 2018; (published online Jan 22.)http://dx.doi.org/10.1016/S1473-3099(18)30046-XSummary Full Text Full Text PDF PubMed Scopus (81) Google Scholar highlights the presence and potential importance of persistent asymptomatic and very low malaria parasite density infections. However, questions remain regarding more precise dynamics and infectiousness of these infections and their implications for malaria elimination. I declare no competing interests. The persistence and oscillations of submicroscopic Plasmodium falciparum and Plasmodium vivax infections over time in Vietnam: an open cohort studyPersistent 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. Full-Text PDF Open Access