Plasmodium falciparum remains the dominant parasite affecting children despite decades of implementing vector control in two villages of Wolaita Zone, Southwest Ethiopia

Background Malaria is still a significant public health concern, and its prevention and control measures have different impacts in different areas. This study assesses the prevalence of malaria and the effectiveness of routine malaria control programmes such as indoor residual spray (IRS) in two Ethiopian villages. Methods The Kebeles (villages) were purposefully selected based on their malaria prevalence rates. A parasitology survey was conducted in Fango-Gelchecha pre- and post-IRS implementation, whereas in Shochora-Abela it was only conducted post-IRS implementation. The IRS was implemented as part of the routine malaria control programme in August 2017. Every fourth house from the village registration list was systematically selected, resulting in a sample of 300 households per village. A total of 3,075 individuals were enrolled for malaria testing using microscopy. Results After three to four months of application of IRS in August 2017, 59 malaria cases were confirmed, resulting in an overall prevalence of 1.9% (95% CI: 1.5–2.5). Of the positive cases, 18 cases (0.59%: 95% CI: 1.3–1.8) were from Shochora-Abela village, and 41 cases (1.33%: 95% CI: 1.1–1.3) were from Fango-Gelchecha. About age categories, the prevalence of malaria was 10.1% (95% CI: 5.9–15.9) among children under five, 4.7% (95% CI: 3.3–6.4) in children aged 5–14, and only 0.32% (95% CI: 0.13–0.67) in the age group 15 and above. Overall, P. falciparum was the dominant malaria parasite, accounting for 69.5% (95% CI: 56.1–80.8), while P. vivax malaria accounted for 30.5% (95% CI: 19.2–43.8). The malaria prevalence in Fango-Gelchecha village was 3.1% (95% CI: 2.3–4.0) before IRS and 2.6% (95% CI: 1.8–3.5) after IRS application. In the village of Shochora-Abela, the prevalence of malaria post-IRS was 1.2% (95% CI: 0.7–1.9), but the prevalence prior to IRS was not evaluated. Conclusions Plasmodium falciparum is the predominant parasite in the villages, mainly affecting children under five. Therefore, protecting young children should be the top priority for reducing infection burdens.


Introduction
Malaria is a prevalent vector-borne disease causing nearly half a million deaths annually, with Africa accounting for over 90% of all deaths (1,2).Although malaria deaths among children under five have decreased, this age group is still the most vulnerable to infection and fatality in high-transmission regions (2).Five species of Plasmodium can infect humans, with P. falciparum causing the majority of malaria deaths (1).In Ethiopia, the two most important parasites are P. falciparum and P. vivax (3).Anopheles mosquitoes transmit Plasmodium to humans, with An. arabiensis being the primary malaria vector (4)(5)(6) and An.pharoensis playing a minor role (7).
Vector control interventions have significantly impacted malaria epidemiology (8), but their effectiveness varies based on vector behavior, intervention method usage, and human behavior (9).The proper use of tools reduce malaria, but drug-resistant parasites, insecticide-resistant vectors, and changes in mosquito behavior can limit success (10)(11)(12).
Although there have been efforts to decrease malaria-related sickness and deaths, it still remains a common cause for outpatient visits in southwest Ethiopia (13).Wolaita, located in southwest Ethiopia, has a high prevalence of malaria, with P. falciparum and P. vivax being the two most common parasite species.A ten-year trend analysis of malaria from health facilities showed that P. falciparum was the dominant parasite, accounting for 72%, while P. vivax accounted for 24%, and mixed infections of the two parasites accounted for 4% (14).For over 20 years, the region has extensively used indoor residual spraying (IRS) with different insecticides and longlasting insecticide nets (LLINs) as intervention strategies.Since 2005, rapid diagnostic test (RDT) have been used for prompt diagnosis and treatment of patients with effective antimalarial drugs such as Artemisinin-based combination therapies (ACT) at community-based primary health care facilities (3).In the past, P. falciparum was the most common parasite responsible for causing malaria (15).However, due to intensive malaria interventions in some areas, there has been a shift in the dominant parasite species.Recently, there has been a consistent increase in cases of P. vivax malaria, which is caused by various factors, including the unique biology of P. vivax malaria (16).Although children under five and mothers are usually at significant risk of malaria infection, there has been a shift in the burden of cases (17).Therefore, to achieve the country's malaria control and elimination targets, it is important to understand the current dynamics of malaria parasite species and the population at highest risk of infections.Thus, this study aimed to assess the prevalence of malaria among all age groups, identify the proportion of Plasmodium species and the imapct of the IRS intervention implemented by routine malaria control system in reducing malaria prevalence in two malariaendemic Kebeles (villages) located in the Humbo district of the Wolaita zone in southwest Ethiopia.

Description of the study area
The study was conducted in the Humbo district of the Wolaita Zone in the Southern Nation Nationalities and People Regional state (SNNPRs).The district has an altitude ranging from 1,001 to 2,700 meters above sea level (masl).The district comprises two urban and 39 rural villages with an average annual minimum temperature of 15°C and maximum of 31°C.The yearly minimum rainfall averages was 14.3m 3 , while the maximum was 27.46m 3 .
The study includes two rural villages, Fango-Gelchecha and Shochora-Abela (Figure 1), with a total population of 11,075 and 10,975, respectively.Fango-Gelchecha is located at 06°35.565 longitude and 037°33.963latitude, with a village centre elevation of 1,264 masl; while Shochora-Abela is located at 06°35.936 longitude and 037°46.665latitude, with a village centre elevation of 1,456 masl.The primary occupations of the inhabitants are agriculture, trading, and cattle ranching.
Malaria is a prevalent disease in the two villages.The main methods of controlling the malaria vector are IRS and LLINs.Free malaria diagnosis and treatment are offered at each health post.

Study design
A survey was conducted from August to December 2017 to determine malaria prevalence in two Humbo villages.The villages were selected based on prevalence of malaria and households were selected using systematic random sampling with a village registration list as a sampling frame.Six hundred households from two villages were systematically selected for malaria parasite testing.The first household was picked from the registration list, and subsequent households were included by dividing the total number of households by the assigned household number in each village.The parasitological survey was conducted twice in Fango-Gelchecha village: once before IRS in August 2017 and again in November and December 2017, after the IRS.In Shochora-Abela village, the survey was done only after the IRS application (Figure 2) due to logistical constraints.The propoxur IRS was implemented in August 2017 as part of the routine malaria control programme.Although the routine malaria control programme conducted the IRS application, it is worth reporting the impact of the intervention.
Data on malaria prevalence was collected in both villages after the IRS application in November and December 2017.The data collected before the IRS in Fango-Gelchecha was used to assess its effect.Blood samples were collected through house-to-house visits.The questionnaire was designed to gather information about the participants' socio-demographic characteristics, bed net usage, and IRS.The questionnaire was initially developed in English and translated into Amharic.

Sample size determination
The sample size for estimating population prevalence was calculated using Bartlett et al.'s (18) formula.The household was selected as the sampling unit, including all members in the study.n = z 2 P (1-P)/d 2  Where, n = Sample size Z = 1.96 (a confidence level of 95% was used) P = average prevalence of malaria in the area D = marginal error The calculation was conducted by considering a 95% confidence interval for Z-statistics, which is conventionally 1.96% and 5% marginal error.The sample size was calculated using unpublished malaria prevalence data for the Humbo district report of 23.3% in 2016/2017.The sample size was calculated for every village to maximize study participants, which increases representativeness.
Assuming a 5% margin of error (d), a total of 275 households were included in the sample.Additionally, 25 households (9% of the sample) were allocated for contingency and non-response compensation, bringing the total number of households in each village to 300.

Malaria case detection and patient management
Blood film collection was conducted by pricking the finger with a disposable blood lancet and all members of the household were tested for malaria parasites.Treatment was free and followed national and WHO malaria treatment guidelines (19,20).Thick and thin blood smears were collected on the same slide, numbered, fixed using 100% methanol, and stained with 3% Giemsa solution for 20 min.Blood film examination was conducted according to WHO protocol (21) by two laboratory technicians in the Health Center using a light microscope.If there was disagreement between the two Map of the study villages in humbo district, wolaita zone in southwest Ethiopia.

Data management and analysis
The collected questionnaire data was analyzed using descriptive statistical tools such as percentage, mean, standard deviation, and a Chi-square test was performed to observe the impact of IRS on the malaria prevalence in Fango-Gelchecha village.SPSS version 20.0 software was used for statistical tests with a P-value of 0.05 for the significant test.

Malaria prevalence in Fango-Gelchecha village before and after IRS
Before the IRS application, 54 out of 1,736 blood films were positive for malaria (prevalence: 3.1%, 95% CI: 2.3-4.0).Among positive cases, P. falciparum accounted for 79.6% and P. vivax accounted for 20.4%.After the IRS application, 41 out of 1,583 blood films were positive for malaria (prevalence: 2.6%, 95% CI: 1.8-3.5).Among positive cases, P. falciparum accounted for 80.5% and P. vivax accounted for 19.5%.The IRS application did not have a significant impact on the prevalence of malaria or the proportion of parasite species.

Bed nets and IRS interventions
During the study, 536 out of 600 households (89.3%) had at least one mosquito net while 10.6% (64/600) of the households did not have a bed net.56.5% (303/600) of houses had at least one bed net placed over the sleeping area, while 47.8% had no one sleeping under a bed net.During the peak mosquito season, 42.7% (192) of households use bed nets, and 57.3% (344) use them all year.Bed net usage was lowest among children aged 5-14 (4.2%).

Discussion
In this study, children had a higher prevalence of P. falciparum malaria than adults over 15.The use of ITNs was deficient in the age group between 5 and 14 years old despite relatively high coverage.The IRS coverage was considerably low and had no significant impact on malaria prevalence after three to four months of application.
The overall prevalence of malaria was 1.9% after applying IRS for three to four months.Prior to IRS, malaria prevalence was 3.1%; 3-4 months after IRS, it decreased to 2.6%.However, the IRS coverage was below the recommended 80% to significantly impact malaria prevalence (22).Although ITN coverage was high, actual use in the study villages was deficient.This is more important than coverage.Malaria prevalence did not significantly change despite similar bed net coverage and use rates before and after the IRS.This is likely because bed nets protect the user while poor IRS coverage offers little to no protection to individuals in unsprayed dwellings in the neighborhood (22,23).In addition to coverage, the quality of spray and type of wall can affect efficacy of IRS (24).
The study found that P. falciparum was the most prevalent parasite, accounting for 69.5%.This species has been dominant in Ethiopia since Italian malarialogists investigated malaria epidemiology in the country (25,26).The Malaria Eradication Service conducted a study in 30 locations, including southwest Ethiopia, in 1962 as part of a global campaign to eradicate malaria (25).Of the three documented parasites, P. falciparum was most prevalent, accounting for an average of 60%, followed by P. vivax and P. malariae (25).According to the World Health Organization, P. falciparum predominates nationwide (1).Despite decades of malaria control efforts, P. falciparum remains the dominant parasite, with only a few locations where P. vivax is prevalent (3).This indicates active malaria transmission caused by mosquito bites despite the government's targeting of P. falciparum.
The prevalence of P. falciparum malaria varies by age group, with children having a higher infection rate than those over 15.The higher prevalence of malaria in children implies active local malaria transmission, possibly due to insufficient coverage and use of IRS and ITN.Studies show that children who do not use ITNs have a higher prevalence of malaria (27, 28).Moreover, the application of IRS did not significantly reduce malaria prevalence, possibly due to inadequate population coverage.Several studies have shown that the IRS needs to cover more than 80% of the population to succeed (22,23).
This study has both strengths and limitations.Despite being cross-sectional, its large sample size, active case-finding approach via house-to-house visits, and inclusion of all age groups are believed to enhance its representativeness.Analyzing the results of the regular malaria control programme IRS implementation may provide a better understanding of the situation.Since malaria microscopists' capacity is limited, and asymptomatic infections have low parasite density, many cases may be missed through microscopy.Additionally, the impact of IRS was analyzed in only one village, so a more comprehensive analysis could be conducted if it were performed in both villages.

Conclusions
The study found that P. falciparum was the most common malaria parasite in the surveyed villages, affecting children more often than adults.Therefore, children require special attention for protection.Although a regular malaria control program has been put in place through the IRS, there has been no notable decrease in malaria prevalence even after three to four months.To effectively combat malaria, it is essential to expand the coverage of the IRS and conduct a long-term study to evaluate its effectiveness.

FIGURE 2 Study
FIGURE 2Study flow chart.

TABLE 1
Characteristics of the study participants by age, sex and village, August-December 2017.

TABLE 2
The prevalence of P. falciparum and P. vivax malaria by age in Fango-Gelchecha and Shochora-Abela villages after IRS application, November to December 2017.

TABLE 3
The number of participants tested for P. falciparum malaria and association between the outcome and selected variables, November to December 2017.