Diagnostic performance of capillary and venous blood samples in the detection of Loa loa and Mansonella perstans microfilaraemia using light microscopy
Authors: Mischlinger Johannes , Manego Rella Zoleko , Mombo-Ngoma Ghyslain , Ekoka Mbassi Dorothea , Hackbarth Nina , Ekoka Mbassi Franck-Aurelien , Davi Saskia Dede , Kreuzmair Ruth , Veletzky Luzia , Hergeth Jennifer , Ndoumba Wilfrid Nzebe , Pitzinger Paul , Groger Mirjam , Matsiegui Pierre Blaise , Adegnika Ayôla Akim , Agnandji Selidji Todagbe , Lell Bertrand , Ramharter Michael 
Published: August 2021

BackgroundLoa loaandMansonella perstans–the causative agents of loiasis and mansonellosis—are vector-borne filarial parasites co-endemic in sub-Saharan Africa. Diagnosis of both infections is usually established by microscopic analysis of blood samples. It was recently established that the odds for detectingPlasmodium spp. is higher in capillary (CAP) blood than in venous (VEN) blood. In analogy to this finding this analysis evaluates potential differences in microfilaraemia ofL.loaandM.perstansin samples of CAP and VEN blood.MethodsRecruitment took place between 2015 and 2019 at the CERMEL in Lambaréné, Gabon and its surrounding villages. Persons of all ages presenting to diagnostic services of the research center around noon were invited to participate in the study. A thick smear of each 10 microliters of CAP and VEN blood was prepared and analysed by a minimum of two independent microscopists. Differences of log2-transformed CAP and VEN microfilaraemia were computed and expressed as percentages. Furthermore, odds ratios for paired data were computed to quantify the odds to detect microfilariae in CAP blood versus in VEN blood.ResultsA total of 713 participants were recruited among whom 52% were below 30 years of age, 27% between 30–59 years of age and 21% above 60 years of age. Male-female ratio was 0.84. Among 152 participants with microscopically-confirmedL.loainfection median (IQR) microfilaraemia was 3,650 (275–11,100) per milliliter blood in CAP blood and 2,775 (200–8,875) in VEN blood (p<0.0001), while among 102 participants withM.perstansthis was 100 (0–200) and 100 (0–200), respectively (p = 0.44). Differences in linear models amount up to an average of +34.5% (95% CI: +11.0 to +63.0) higherL.loamicrofilaria quantity in CAP blood versus VEN blood and forM.perstansit was on average higher by +24.8% (95% CI: +0.0 to +60.5). Concordantly, the odds for detection of microfilaraemia in CAP samples versus VEN samples was 1.24 (95% CI: 0.65–2.34) and 1.65 (95% CI: 1.0–2.68) for infections withL.loaandM.perstans, respectively.ConclusionThis analysis indicates that average levels of microfilaraemia ofL.loaare higher in CAP blood samples than in VEN blood samples. This might have implications for treatment algorithms of onchocerciasis and loiasis, in which exact quantification ofL.loamicrofilaraemia is of importance. Furthermore, the odds for detection ofM.perstansmicrofilariae was higher in CAP than in VEN blood which may pre-dispose CAP blood for detection ofM.perstansinfection in large epidemiological studies when sampling of large blood quantities is not feasible. No solid evidence for a higher odds ofL.loamicrofilariae detection in CAP blood was revealed, which might be explained by generally high levels ofL.loamicrofilaraemia in CAP and VEN blood above the limit of detection of 100 microfilariae/ml. Yet, it cannot be excluded that the study was underpowered to detect a moderate difference.