Parasite prevalence
During the first survey (dry season), conducted in February 2018, the parasite prevalence by microscopy and qPCR was 18.5% (113/610) and 37.4% (228/610) respectively. A significant reduction in prevalence was recorded during the second cross-sectional survey (wet season) by both microscopy and qPCR. For instance, parasite prevalence reduced to 14.2% (105/737) (χ²= 5.117, df = 1, p = 0.02) by microscopy and 25.1% (185/737) (χ² =23.08, df = 1, p < 0.0001) by qPCR. Further decline in parasite prevalence was reported in the third survey with microscopy and qPCR recording 3.3% (24/720) and 10.4% (75/720) respectively. During the last survey, malaria parasite prevalence was finally reduced to 1.3% (11/849) and 6.7% (57/849) by microscopy and qPCR respectively. Overall, the reduction in parasite prevalence during the 4 cross-sectional surveys was significant by microscopy (χ²= 186.9, df = 3, p < 0.0001) and by qPCR (χ²= 266.2, df = 3, p < 0.0001) as indicated in Table 1. Parasite prevalence as determined by microscopy, varied significantly among age groups during the first (χ² = 28.6, df = 2, p < 0.0001), second (χ² = 25.1, df = 2, p < 0.0001) and third survey (χ² = 8.6, df = 2, p = 0.01) (Figure 2). While the children aged 5 -14 years old had the highest peaks of Plasmodium infections during the first, second and third survey, the least affected by malaria parasites infection during the same study period was those aged ≥15 years old (figure 2A). Due to lowest Plasmodium infection rate during the last survey, there was no significant variation in parasite prevalence among age groups (χ² = 1.9, df = 2, p = 0.39). A similar pattern of variation in parasite prevalence among age groups was recorded by qPCR during the 4 cross-sectional surveys as shown in figure 2A. The detection of Plasmodium species infection rate by qPCR was significantly higher than that of microscopy as illustrated in figure 3.
Table 1
Demographic characteristics of the study participants, parasite density, asymptomatic and submicroscopic Plasmodium infections during the 4 cross-sectional surveys
Variable | Survey period |
| Feb 2018 (dry season) | June 2018 (wet season) | Feb 2019 (dry season) | June 2019 (wet season) |
Total population screened (N) | 610 | 737 | 720 | 849 |
Microscopic malaria parasite prevalence % (n) | 18.5 (113) | 14.2 (105) | 3.3 (24) | 1.3 (11) |
Malaria parasite prevalence by qPCR % (n) | 37.4 (228) | 25.1 (185) | 10.4 (75) | 6.7 (57) |
Geometric mean parasite density (parasites/µl of blood) | 2229 | 353 | 578 | 184 |
Submicroscopic % (n) | 50.4 (115) | 43.2 (80) | 68.0 (51) | 80.7 (46) |
Asymptomatic infections by microscopy % (n) | 64.6 (73) | 67.6 (71) | 75.0 (18) | 83.3 (9) |
Asymptomatic with submicroscopic infections % (n) | 48.2 (110) | 41.6 (77) | 61.3 (46) | 75.4 (43/57) |
Plasmodium falciparum parasite density category % (N) | 100% (113) | 100% (102) | 100% (24) | 100% (11) |
Low % (n) | 33.6 (38) | 77.8 (77) | 58.3 (14) | 90.9 (10) |
Moderate % (n) | 36.3 (41) | 19.2 (19) | 41.7 (10) | 9.1 (1) |
High % (n) | 27.4 (31) | 3.0 (3) | 0.0 (0) | 0.0 (0) |
Hyper % (n) | 2.7 (3) | (0) | 0.0 (0) | 0.0 (0) |
Plasmodium species distribution and P. falciparum gametocyte prevalence
Three malaria species were found in the study population with Plasmodium. falciparum being most prevalent, followed by P. malariae and P. ovale as the least prevalent as shown in Table 1. Both P. malariae and P. ovale occurred at low parasite densities hence mostly detected by PCR. The proportion of Plasmodium species varied during the 4 cross-sectional surveys. For instance, Plasmodium falciparum accounted for 94.3% (215/228), 81.6% (151/185), 68% (51/75) and 83.3% (50/60) during the first, second, third and fourth survey respectively. Similarly, the proportion of P. malariae during survey 1, 2, 3 and 4 was 0.9% (2/228), 8.6% (16/185), 10.7% (8/75) and 8.3% (5/60) respectively. The infections due to P. ovale only occurred during the first (0.4% (1/228), second (1.6% (3/185and fourth survey (3.3% (2/60). Mixed species infections due to P. falciparum and P. malariae mostly occurred throughout the study period. Mixed infections involving P. ovale was rare and was reported during the first and the second survey as illustrated in Table 2 below. Results further indicated lowest levels of gametocyte prevalence during survey periods. The declining trend of gametocyte prevalence was 1.3% (8/610), 0.5% (4/737), 0.14% (1/720) and 0.12% (1/849) during the first, second, third and the fourth survey respectively (χ² = 12.97, df = 3, p = 0.005). The geometric mean gametocyte density was: 66.6, 35.9, 48 and 16 gametocytes/ µl of blood during survey 1, 2, 3 and 4 respectively (ANOVA, F = 1.24, df = 3, 10, p = 0.35).
Table 2
Plasmodium species composition during the 4 cross-sectional survey as detected by qPCR
Survey period | Plasmodium species composition | |
| Pf | Pf+Pm | Pf+Pm+Po | Pf+Po | Pm | Pm+Po | Po | Total |
Feb 2018 | 215 | 7 | 2 | 1 | 2 | 0 | 1 | 228 |
Jun 2018 | 151 | 12 | 0 | 0 | 16 | 3 | 3 | 185 |
Feb 2019 | 52 | 15 | 0 | 0 | 8 | 0 | 0 | 75 |
Jun 2019 | 50 | 3 | 0 | 0 | 1 | 0 | 3 | 57 |
Prevalence Of Asymptomatic And Submicroscopic Infections
The results indicated a varying proportion of both asymptomatic and submicroscopic infections during the 4 cross-sectional surveys. The asymptomatic cases increased with a decrease in Plasmodium infection rates. For instance, 64.6% (73/113), 67.6% (71/105), 75% (18/24) and 83.3% (9/11) of all participants infected with malaria parasites during the first, second, third and fourth survey respectively, reported no fever during or up to 48 hours before sample collection. This increase in asymptomatic Plasmodium infections during the four surveys was not significant (χ² = 1.349, df = 3, p = 0.7). Similarly, the findings indicated an increasing trend in proportion of Plasmodium submicroscopic infections. For example, submicroscopic infections accounted for 50.4% (115/228), 43.2% (80/185), 68% (51/75) and 80.7% (46/57) of all infections during survey 1, 2, 3, and 4 respectively (χ²= 31.98, df = 3, p < 0.0001). The study also reported the occurrence of Plasmodium infections which were both submicroscopic and asymptomatic. For example, during the first survey, 48.2% (110/228) of submicroscopic infections were asymptomatic. Similarly, during the second, third and fourth survey, 41.6% (77/185), 61.3% (46/75) and 75.4% (43/57) of submicroscopic infections respectively, were asymptomatic. The findings indicated a significant difference among the 4 cross-sectional surveys (χ²= 24.00, df = 3, p = 0.0002) Table 1.
Plasmodium falciparum parasite density and category
The 4 cross-sectional surveys indicated significant reductions in parasite burden among the study population. The overall geometric mean parasite densities (GMPD) during first, second, third and fourth survey were 2229 infected red blood cells (irb)/µl of blood, 353 irb/µl of blood, 578 irb/µl of blood and 184 irb/µl of blood respectively (ANOVA, F = 28.95, df = 3, 243, p < 0.0001) (Table 1). Geometric mean parasite density varied significantly among age groups during the first (ANOVA, F = 4.142, df = 2,110, p = 0.018) and the third survey (ANOVA, F = 5.463, df = 2, 21, p = 0.012 (Figure 2B). During the first and third surveys, children aged below 5 years old, had highest mean parasite densities compared to other age groups. However, during the second and fourth surveys, geometric mean parasite density did not vary significantly among age groups (ANOVA, F = 1.361, df = 2, 96, p = 0.26) and (ANOVA, F = 0.67, df = 2, 8, p = 0.54) respectively. The parasite density during each survey was categorized into low, moderate, high and hyperparasitaemic. The findings indicated elimination of high and hyperparasitaemia during the 4 cross-sectional surveys. For example, the study reported zero prevalence rates of hyperparasitaemia during the second, third and the fourth survey. Equally, during survey 3 and 4, there were no cases of high parasite density. With the onset of indoor residual spray program, most infections were due to low parasite densities as illustrated in Table 1.