Prevalence of asymptomatic Plasmodium infections with sub-microscopic parasite densities in the northwestern border of Thailand: a potential threat to malaria elimination

Abstract

BackgroundAsymptomatic infections with sub-microscopic Plasmodium serve as a silent reservoir of disease, critical to sustaining a low level of remanent malaria in the population. These infections must be effectively identified and targeted for elimination. The sensitivity of light microscopy, the traditional method used for diagnosing Plasmodium infections, is frequently insufficient for detecting asymptomatic infections due to the low density of parasitaemia. The objective of this study was to explore the current prevalence of asymptomatic sub-microscopic Plasmodium carriages to evaluate the parasite reservoir amongst residents from 7 hamlets in Tak Province in northwestern Thailand using a highly sensitive molecular method.MethodsMalaria infection was screened in a real-world setting from 3650 finger-prick blood specimens collected in a mass cross-sectional survey using light microscopy and loop-mediated isothermal amplification (LAMP). LAMP results were later confirmed in a laboratory setting in Bangkok using nested PCR, restriction enzyme digestion and DNA sequencing. The association of malaria infection with demographic factors was explored.ResultsParasite prevalence was 0.27% (10/3650) as determined by microscopy. Sub-microscopic infection prevalence was 2.33% (85/3650) by LAMP. Of these, 30.6% (26/85) were infected with Plasmodium falciparum, 52.9% (45/85) with Plasmodium vivax, 2.4% (2/85) with Plasmodium malariae, 4.7% (4/85) with mixed P. falciparum and P. vivax, and 9.4% (8/85) had parasite densities too low for species identification. Asymptomatic carriages (T < 37.5 °C) accounted for 95% (76/80) of all sub-microscopic cases with the highest prevalence occurring in the subjects 31–45 years of age (p ≤ 0.035). Participants working on plantations or as merchants had an increased infection risk. Evaluation by microscopy identified 10.53% (10/95) of all Plasmodium infected participants.ConclusionParticipants carrying asymptomatic Plasmodium infections with sub-microscopic parasite densities are considerable in this area. These findings provide the true disease burden and risk factors in this region. This information helps to direct policy makers towards better schemes and delivery of targeted interventions. Moreover, this is the first study to use LAMP in mass screening for sub-clinical and sub-microscopic infections in a field setting in Thailand. LAMP proves to be a sensitive and field-deployable assay suitable for national malaria control screening campaigns.

Malaria in North-East India: Importance and Implications in the Era of Elimination

“…Asymptomatic carriers are individuals who carry the malaria parasites without manifesting any clinical symptoms. However, they remain infectious to mosquitoes and continue to spread the disease [92], and they are potential threat to malaria elimination efforts [93]. It is estimated that 20%-70% of the infections cannot be detected by conventional microscopy and RDTs [94].…”

Spatial distribution of asymptomatic Plasmodium vivax and Plasmodium falciparum infections in low, moderate and high endemic settings in Ethiopia

“…Moreover, the effect size was comparatively large with approximately 5-fold higher detectability of infections in high endemic settings compared to low endemic settings. The trend of increasing detectability with increasing transmission intensity may be attributable to the fact that asymptomatically infected individuals have higher average parasite densities in high transmission settings [44,46]. Moreover, in low endemic settings individuals will receive fewer infectious bites with, due to the absence of super-infections, lower parasitemia over the course of infection [40,47].…”

Evaluation of Visual LAMP detection of Plasmodium falciparum&nbsp;and non-Plasmodium falciparum

“…Sensitivity of LAMP is several orders of magnitude higher than by microscope and RDT [21] . As worldwide countries set targets of malaria elimination, accurate diagnosis, optimal treatment, stop of transmission, and reducing mortality of malaria are becoming critical [25,26] . As a result of the sudden decline in local patients and the gradual elimination of malaria, there are a trend of degradation in the clinical diagnosis and screening capacity of malaria.…”

“…Particularly, asymptomatic malaria infection and lowdensity malaria parasite emias are hard to be detected by microscope examination and RDT due to below the detection threshold [11,24] , causing misdiagnosis or delayed diagnosis [3,21] . But they could maintaining transmission of malaria as infected intermediate host [24,26] . It has been reported that subclinical malaria accounts for 70-80% of all malaria infections [11] .…”

“…Stopping transmission of malaria requires detecting methods with high sensitive. The LAMP shows high sensitivity and specificity, by which asymptomatic and asymptomatic malaria infections could be diagnosed accurately, and low-density infections that cannot be detected by conventional diagnosis methods could be identified [26] . At the same time, LAMP is fast, easy to operate and effective which provides the possibility for clinical diagnosis and monitoring of malaria.…”