Asymptomatic malaria becomes a series challenge in malaria elimination due to low health seeking behaviors of individuals, and is therefore a potential reservoir in sustaining malaria transmission (10).
In this study RDT detected and identified 3% of the asymptomatic Plasmodium spp infection. This is in line with a study in Debre Elias district, Northwest Ethiopia 4.8% (21), Southeast Asia 4% (22), Lao DPR 2.2% (23), and Haiti 1.78% (24). On the other hand, the finding of this study was found to be higher than study done in Malo, Southwest Ethiopia with zero case (17) and Northern Senegal with 0.4% (25). This discrepancy might be due to parasitemia level, quality of different RDT products which may differ in detection limits.
In the current study, microscopy detected and identified 5.2% of asymptomatic infections. This finding is in harmony with study done in Debre Elias district, Northwest Ethiopia 4.2% (21), Sanja, Northwest Ethiopia 6.8% (26), Southern central Oromia, Ethiopia 5% (27) and Southwest Asia 5% (22). However, this figure is lower than previous studies in Kenya 12.6% (28) and Tanzania 8% (29). Lower findings than the current study were reported in Malo, Southwest Ethiopia with no case (17), Myanmar 1.4% (30), and Haiti with 2.8% (24). This difference might be due to parasitemia level, variation in transmission setting, and skills of laboratory personnel.
The polymerase chain reaction (PCR) detected and identified 12% of asymptomatic Plasmodium spp infection. This was higher than previous studies in Ethiopia; with 8.12% (17, 18), Myanmar with 2.3% (30), Zambia with 8% (31). However, it was lower than findings in Southeast Asia with 20% (22), Haiti with 19.1% (24), Kenya; 20% & 33.3% (28,32), India with 22.6% (33), and This difference might be due to variation in transmission settings, difference in case investigation, control and preventive measure taken in each country. The portion of infections that is undetectable by microscopy varies between endemic areas and population groups (8). Furthermore, in the present study, only small numbers of samples were run for PCR due to insufficient resource, and this may contribute for the difference.
Moreover, the present study revealed that PCR invariably detected higher proportion of asymptomatic infection with about 2.7 and 2.3 folds higher than RDT and microscopy, respectively. This finding is in agreement with reports from Brazil (34). Therefore, this substantial difference among diagnostics methods can be due to variation in the detection limit of diagnostic methods, in which PCR detects as low as 0.5–5 parasites /µl of blood while microscopy and RDT detect 50 and 100 parasites /µl of blood respectively (35).
Of overall prevalence of asymptomatic malaria, the predominant Plasmodium spp was P. falciparum with 57.9% and the remaining 42.1% was P. vivax. This finding is in line with Ethiopian nationwide malaria figure reported by Ministry of Health over which P. falciparum account about 60% (2). On the contrary, other studies in Ethiopia reported P. vivax was the predominant species in Hadiya zone, Southern Ethiopia (36), and East Shewa zone, Central Ethiopia (37). This difference relies on the heterogeneity of Plasmodium species distribution as a result of environmental and ecological changes.
Reactive case detection has identified Plasmodium spp infection clustered in households and neighbors of index cases, and is highly valuable in low transmission intensity areas (32). The current study identified high prevalence of asymptomatic infection in index houses with 19.4% (14/72) compared with their neighboring houses with 2.5% (5/198). This finding is consistent with a study done in Jimma zone, Southwest Ethiopia (18). This difference likely reflects family members of index cases are with equal proximity to the breeding sites of vectors along with likely behaviors and occupations within family. Therefore, the index cases justly denote foci of transmission to family members. This study revealed that of the total asymptomatic cases, the prevalence of asymptomatic infection varied with approximately 3 folds higher in index households compared with their neighbors within 200 m radius.
Comparing the conventional diagnostics tools against PCR (taken as gold standard), the current study revealed that RDT and microscopy had lower sensitivity with 36.4% and 45.5%, respectively. This result showed that the sensitivity of RDT is by far lower that WHO recommended sensitivity over 95% at 100 parasites /µl of blood (38). High false negative rate (63.6%) was identified in RDT. This means that 63.6% of asymptomatic individuals who actually had malaria remained undetected, and serve as possible reservoirs to continue the transmission dynamics. Low sensitivity and high false negative rates of RDT might be due to parasitic factors (parasitemia level and variation in parasite antigenic structure) and device related factors (RDT shipment and storage conditions, misinterpretation of results). Evidence suggests that the detection capacity RDT get reduced as antigenic variability occurs at density below 500 parasites / µl of blood (38).
The current study identified previous malaria history increase 4.0 times the odds of having Plasmodium spp infection. This finding is supported by studies done in Ethiopia (18, 21, 26). This might be due to relapse or recurrence of infection. Recurrence of infection is due to the persistence of low level of P. falciparum in the circulation while relapse is due to the reactivation of hypnozoite which leads to initiation of erythrocytic cycle in case of P. vivax (Sastry and Bhat, 2014). However, these studies also underpinned living with index cases and family size ≥ 6 members as potential risk factors. Reasonably, the infection is clustered in index cases’ houses compared with neighbors’ houses and this might be due to equal proximity to vector breeding sites or likely behaviors of family members with the index cases.