The study provided baseline information for malaria elimination program in the targeted low transmission districts of Ethiopia. The overall prevalence of malaria by RDTs in this survey was 1.17% (339/28983). In this survey, malaria prevalence ranged from zero to 4.7% at district level by malaria RDTs. In half (50%) of the surveyed districts, the prevalence of malaria was less than 1% by RDTs. The difference in malaria prevalence among the districts was supported by the findings reported from baseline malaria surveys conducted elsewhere in Asia [1–4, 23].
The large scale deployment of RDTs for malaria diagnosis has greatly improved access to confirmatory diagnosis in malaria endemic countries, contributing to the success of malaria control programs in recent years . RDTs are convenient tests for the screening of large number of samples in national surveillance studies or other large-scale malaria elimination programs such as mass screening and treatment programs. However, population surveys using traditional diagnostic techniques including microscopy or RDTs may miss low-grade infections that are below the detection limit of these tools. Studies in high transmission areas have shown that as many as two-thirds of microscopy-negative patients may have low grade parasites .
False-negative RDT results commonly occur because of professional errors, inconvenient storage situations, histidine rich proteine deletions, low parasitaemia and RDT types with poor performance. The detection capacity of malaria RDTs varies in different transmission settings and false negative RDT results are common in lower malaria transmission settings . Therefore, highly sensitive and specific malaria diagnostics such as PCR or highly sensitive RDTs are critically needed in community surveys, particularly in low transmission and elimination target settings to explore more precise prevalence estimates .
In this study, among the 2,510 febrile individuals having axillary temperature > 37.5oC, only 84(3.35%) were malaria positive while 2,426 (96.65%) were negative. This may be so because fever is not specific to malaria and it might have been caused by other illnesses. Fever was not fully explained by malaria as reported by other studies in low-resource areas . Although the proportion of infection positivity is lower among febrile cases, statistics shows significant association between fever and malaria positivity (P-value: <0.001).
In our study, afebrile malaria cases were ‘cases tested positive by RDTs but were with axillary temperature of < 37.5oC as measured by a digital thermometer or/and those who had reported no fever history or no malaria like symptoms within 48 hrs before field data collection. Afebrile malaria cases accounted for 0.88% (255/28973) among the total tested study participants and 75.2% (255/339) among all malaria positives. The afebrile or asymptomatic malaria cases in low transmission and elimination-targeted areas are potential reservoirs sustaining uninterrupted malaria transmission in the area . With regard to the elimination strategy, WHO remarks that asymptomatic malaria infections should be confirmed by standard diagnostic tests. WHO states that any case confirmed by a diagnostic test is malaria infection whether it is symptomatic or asymptomatic. Active monitoring of malaria infections is necessary in settings where malaria transmission has currently declined, and national efforts are under way to achieve malaria elimination.
In Zambia , among 3,863 household members tested in the elimination target areas, 2.6% of individuals were found positive by either of RDTs, microscopy or PCR. Of all positives, 47% (48) had sub-patent parasitemia and 85% of sub-patent parasitemia cases were asymptomatic. The study recommended further need of active or reactive case detection approaches to identify more asymptomatic individuals at the community levels during declining malaria. Most countries with low transmission settings and striving to eliminate malaria demonstrated a large proportion of asymptomatic Plasmodial infections, particularly submicroscopic parasitemia cases [30–32]. Asymptomatic and subpatent malaria reservoirs can maintain continuous transmission even in low transmission settings unless they are strictly monitored and detected by a highly sensitive diagnostic method, and immediately treated by an effective antimalarial drug . The current study findings forward the necessity of use of more sensitive molecular diagnostic tools to give up more accurate prevalence of infections in community surveys occurring at low-transmission settings.
Malaria RDT positivity was higher among the age group 5–11 years (1.8%) followed by 12–17 years (1.4%), while the positivity rate was low among the age group of 18–59 years (0.9%). However, the difference in malaria prevalence among age groups was not significant. Among the total afebrile malaria patients (255), the highest proportion was observed in 18–59 yrs of age, which was 38.4% (98). This means afebrile or asymptomatic malaria prevalence was 10.2% (26/255) in under-five children and 89.8% (229/255) in age groups above 5 years. This may be so because the older ages might have developed acquired immunity from repeated previous exposures that could block malaria symptom development as reported by other studies .
Malaria prevalence among males (1.4%) is significantly higher than among females (1%). Males usually stay in outdoor activities more frequently than females and hence more exposed to mosquito bites. Currently studies reported that mosquitoes change their behavior from indoor biting to outdoor or probably new mosquitoes with outdoor-biting may have arrived at the study areas. The current malaria prevention and control intervention program gives higher priority for pregnant women and children; however, malaria prevalence among pregnant women (1.9%) was higher when compared to non-pregnant women and those who did not know their pregnancy status. A possible reason for this may be immune suppression and loss of acquired immunity among the pregnant, and the hormonal, immunological and hematological changes during pregnancy period. High malaria prevalence (59% of all positives) was seen at an altitude of 1501-2000m ASL, while 23% at 1000-1500m and 18% at > 2000 m. Statistically significant relationship was observed between lower altitude and malaria positivity (P-value: <0.001). Finally, because of lacking reagents, we are unable to produce molecular and sero-prevalence (ELISA) prevalence despite a large number of dried blood spots (29,085 DBS samples) collected in this baseline assessment.