The community surveillance cohort
In 2017, MEDP enrolled a total of 1,143,126 individuals from 248,825 households through household visits, of which 956,795 individuals participated in this constant contact community-based surveillance cohort during the study period of January 2018 to December 2020. This cohort was nearly equally split between males [480,850 (50.26%)] and females [475,945 (49.74%)]. During the 72 household visits at 15-day intervals over the three-year longitudinal period of the present study, each participant was followed for a median of 37 times (IQR: 19, 55).
Fever and malarial episodes
Out of the 956,795 constant population of the community surveillance cohort, 230,780 individuals (24.12%) reported one or more febrile episodes during 2018 - 2020. There was a total of 322,577 febrile episodes, averaging 1.40 episodes per person during the study period. Over the three-year period of this study, a total of 490 malarial episodes were detected in 422 individuals, with an average of 1.16 episodes per person. Out of 322,577 fever cases, 145,717 (45.61%) were male, 178,860 (54.39%) were female, and out of 490 malaria cases, 303 (61.84%) were male, and 187 (38.16%) were female. In comparison to females, males reported lesser febrile events (OR = 0.68; 95% CI: 0.67 – 0.69; p<0.0001) but a higher risk of malaria infection (OR = 2.62; 95% CI: 2.03 – 3.40; p<0.0001).
Among the parasitaemia individuals, 86.02% (363/422) reported a single episode, while 12.32% (52/422), 1.18% (5/422), and 0.47% (2/422) reported two, three, and four episodes of malaria, respectively. Mean days intervals between second, third and fourth malarial infections irrespective of malaria species were respectively 257 (range: 13 - 819), 164 (range: 108 - 323) and 339 (range: 144 - 535) days. The second repeated infection of P. falciparum was after 296 (range: 13 - 819) days, and of P. vivax was after 205 (range: 23-504) days. The third and fourth repeated infections of P. falciparum were after 180 (range: 108 - 323) and 535 days, respectively. The fourth repeated infection of P. vivax in a single case was after 144 days. The Plasmodium species was consistent across episodes, except in five individual cases where changes between P. falciparum and P. vivax infections were observed. Two cases of P. falciparum infection were turned into P. vivax during the second episode after 20 and 160 days, one case with mixed infection was reported as P. vivax after 130 days, one case of P. falciparum was reinfected or recrudescence with P. falciparum after 21 days and further reported with P. vivax after 157 days, and one case of mixed infection (P. falciparum and P. vivax) was repeated with a mixed infection on 34th day and further reinfected or relapse of P. vivax after 123 days (Table 1).
Multiple episodes of malaria infections were found more in individuals over 30 years of age. Only two children less than five years old reported a second episode of malaria, of which one had a repeated episode of P. falciparum after 149 days, and another had repeated episodes of P. vivax after 40 days. Six children between five and 15 years of age group reported a second episode of malaria infection, three each for P. falciparum and P. vivax after 192 and 228 days, respectively. Further, the two episodes of malaria infection among the adult population of 15-30 years age group were in 15 persons [seven P. falciparum (374 days), seven P. vivax (243 days) and one P. falciparum to P. vivax (20 days)]. In 17 persons of 30-50 years of age group [14 P. falciparum (310 days), one P. vivax (504 days), one P. falciparum to P. vivax (160 days) and one mixed to P. vivax (130 days)] and above 50 years of age groups in 12 persons [8 P. falciparum (259 days), four P. vivax (89 days)]. Third and fourth malarial episodes were found among the 30-50 years and above 50 years of age groups (Table 2).
One woman was diagnosed with repeated P. falciparum infection in her two distinct pregnancies with 365 days intervals, and five non-pregnant women were reinfected with P. falciparum (four cases) after 252 days and P. vivax (one case) after 28 days. Another non-pregnant woman had a third episode of malaria infection where the first infection was mixed of P. falciparum and P. vivax, the second infection after the 34th day was mixed, and the third infection after 123 days was P. vivax; which could be a reinfection or a relapse. Additionally, six women presented second episodes of malaria infection, of which three were non-pregnant at the time of the first infection. During the second episode of infection (all P. vivax), they were pregnant after 359 (range: 233-423) days, and the three parasitaemia pregnant women were reinfected after 323 (range: 20 – 819) days, by which time they were not pregnant anymore (Table 3A, B).
None of the persons of the General Caste group had multiple episodes of malaria infection, and only two persons of the Scheduled Caste group were reinfected with one P. falciparum after 325 days and one P. vivax after 423 days. Amongst Other Backward Caste group, nine persons had a second episode of malaria (six P. falciparum after 187 days and three P. vivax after 40 days), and two persons had four episodes of malaria infection (P. falciparum after intervals of 59-349-535 days and P. vivax after intervals of 77-121-144 days).
A total of 41 persons of the Scheduled Tribes ethnic group were found with second episodes of malaria infection, of which 26 were reinfected with P. falciparum after 320 days, 12 with P. vivax after 229 days, two P. falciparum to P. vivax after 90 days, and one mixed to P. vivax after 130 days. Another five Scheduled Tribes had third episodes of malarial infection, of which three from P. falciparum after 54 and 180-day intervals, one P. falciparum to again P. falciparum after 21 days, then P. vivax after 157 days and one mixed to mixed after 34 days and then P. vivax after 123 days (Table 4).
Further analysis of repeated episodes of malaria infections between socioeconomic strata (SES) found that persons belonging to the higher socioeconomic status (SES) had a smaller number of repeated infections than those in the lower SES group except for one case of four times P. falciparum infections in the upper SES group (Table 5). The repeated episodes of malaria infections between years and seasons could not be presented appropriately because the particular individuals may be reinfected in consecutive months and years of the follow-up (Table 5).
Cumulative incidence (95% CI) of malaria (per 100,000 persons)
The cumulative incidence of fever (322,577/956,795) was 33,714 (33,619.60 – 33,809.15) with a cumulative malaria incidence of 51.21 [490/956795 (46.78 – 55.95)] per 100,000 persons (Table 1). The persons under the age group of 5-15 years were the most susceptible to malaria with a cumulative incidence of 107.30 (82.64 – 137.00), followed by 15-30 years [92.85 (77.30-110.62)], 30-50 years [90.57 (77.86-104.76)] and considerably low in <5 years [32.84 (19.77-51.27)] and >50 years of age [20.01 (16.30-24.31)] (Table 2). A total of 1,160 pregnancies were observed during the study period, and the cumulative incidence during pregnancy [3362.07 (2401.47-4567.59)] was about 44 folds higher than ever married non-pregnant women of the reproductive age group [75.81 (59.10-95.77)] (Tables 3A-B).
Persons belonging to the Scheduled Tribe community had the highest cumulative incidence [64.43 (58.04-71.34)] followed by Scheduled Caste [35.90 (23.85-51.88)], Other Backward Castes [31.00 (24.86-39.19)] and lowest in General Caste group [19.37 (5.28-49.59)] (Table 4). Further analysis of cumulative malaria incidence among the SES of the persons revealed that highest malaria incidence was among lower SES [104.09 (89.79-120.03)], followed by upper lower SES [53.62 (45.22-63.12)], lower middle SES [37.24 (30.87-44.53)], upper SES [33.77 (21.41-50.67)] and lowest in upper middle SES [12.04 (6.58-20.20)] (Table 5).
The incidence of malaria gradually declined over the years. In the year 2018, the malaria incidence was 26.13 (22.99-29.58), which was about a 47% decline during 2018 [13.90 (11.64-16.47)] and further about 20% decline during 2020 [11.18 (9.16-13.51)]. Monsoon months (July – September) [18.60 (15.97-21.55)] and spring (January – March) [14.32 (12.02-16.93)] were higher malarious seasons than winter (October – December) [10.35 (8.41-12.60)] and summer (April – June) [7.94 (6.26-9.94)] (Table 6).
Block-wise data revealed the highest incidence in Mawai, followed by Mohgaon and Bichhiya, with Mandla, Bijadandi, and Narayanganj reporting the lowest (Figure 1). A total of 114 malaria cases were reported from the 369,379 persons residing in low malaria endemic areas of the district, and 376 cases were reported from the 587,416 persons residing in high malaria endemic areas. The cumulative incidence of malaria in low and high endemic blocks was 30.87 (95% CI: 25.47 – 37.09) and 64.01 (95% CI: 57.70 – 70.82), respectively.
There was a significant annual decrease in malaria incidence from 26.13 in 2018 (95% CI: 22.99 – 29.58) to 13.90 in 2019 (95% CI: 11.64 – 16.47) and 11.18 in 2020 (95% CI: 9.16 – 13.51) (p < 0.0001), with the highest incidence during the monsoon season (July – September) [66.54 (95% CI: 57.13 – 77.06)] and the lowest in the summer (April – June) [37.08 (95% CI: 29.22 – 46.41)] (Table 6).
Factors associated with malaria
Univariate and multivariate logistic regression model revealed that the incidence of malaria was highest among 5-15 years of age group (crude OR = 1.68; p<0.0001 and adjusted OR = 2.23; p=0.007) followed by 15-30 years (crude OR = 1.53; p<0.0001 and adjusted OR = 1.23; p=0.001) and 30-50 years of age group (crude OR = 1.51; p<0.0001 and adjusted OR = 1.21; p=0.001) in reference to adults more than 50 years of age. During pregnancy, the risk of malaria was about 45 times higher than ever in married non-pregnant women of the reproductive age group (crude OR = 45.86 and adjusted OR = 45.00; p<0.0001). Scheduled Tribes were the highly susceptible group to malaria infection (crude OR = 3.33; p=0.017 and adjusted OR = 1.02; p=0.037). The population of lower SES showed a significantly higher risk of malaria in the univariate model, but this factor was not found significant in the multivariate model (crude OR = 3.08; p<0.0001 and adjusted OR = 1.05; p=0.905).
Those residing in high endemic areas were likely at greater risk of malarial infection than those in low endemic areas (crude OR = 2.07; p<0.0001 and adjusted OR = 1.60; p=0.036). The decrease in malaria incidence was statistically significant for 2019 (crude OR = 0.53; p<0.0001 and adjusted OR = 0.20; p<0.0001) and 2020 (crude OR = 0.43; p<0.0001 and adjusted OR = 0.48; p=0.007) (Table 7) and monsoon months were highly malarious season than summer (crude OR = 2.34; p<0.0001 and adjusted OR = 1.71; p=0.007) (Table 7).