Background
Malaria control efforts remain a priority agenda globally and particularly in Africa where the disease claims hundreds of thousands of lives each year [
1]. Long-lasting insecticidal nets (LLINs), indoor residual spraying (IRS) and environmental management are the most widely used tools for malaria vector control worldwide [
1‐
3]. In Ethiopia, LLINs, IRS, environmental management and rapid diagnostic tests (RDTs) coupled with prompt and effective case management with artemisinin-based combination therapy were the four key malaria control interventions [
4,
5]. The protective efficacy of insecticide-treated nets (ITNs) results from both the physical barrier and the insecticidal action of the net. It provides protection to individual users and to the entire community through mass effect, when widely used [
6]. ITNs can reduce the density, feeding frequency and survival of mosquitoes and wide-scale use can mediate protection of all community members, including those without a bed net [
7,
8].
The use of ITNs in Ethiopia has started since 1997 and the scaling-up began in 2005 with the aim of obtaining a high coverage towards effective malaria control [
9]. According to the central statistical agency (CSA) and Federal Ministry of Health of Ethiopia, 36 million bed nets, targeting about 52 million people at risk, were distributed between the period 2005 and 2010 [
10,
11]. Thus, following the high coverage of LLINs and other preventive measures malaria incidence in the country has decreased by 50–75% in 2015 compared to 2000 [
12]. However, the recent gains from malaria control could easily be unrolled back if the efficacy of LLINs is not regularly monitored and if insecticide resistance management strategies are not implemented.
PermaNet
® 3.0 is a deltamethrin-treated combination net with the addition of synergist piperonylbutoxide (PBO) on the roof section of the net [
13]. Recently, the vector control advisory group of the World Health Organization (WHO) supported Vestergaard’s claim that relative to pyrethroid-only LLINs, the combination net, PermaNet
® 3.0 increased efficacy against malaria vector populations with cytochrome P450-based metabolic pyrethroid resistance, even if co-existing with
kdr in a malaria vector population [
14].
In Ethiopia, widespread insecticide resistance has been reported in the main malaria vector,
Anopheles arabiensis and in few cases in
Anopheles pharoensis [
15]. This resistance is due to both target site and metabolic mechanisms [
16,
17]. Although LLINs in general and PermaNet
® 3.0 in particular is currently being distributed for malaria vector control, their performance should be monitored in the field in different eco-epidemiological settings to assess their durability and long-term effectiveness for malaria prevention and control. Thus, the aim of this study was (1) to assess the susceptibility status of malaria vectors and nuisance Culicine mosquitoes against pyrethroid insecticides (deltamethrin and permethrin), (2) to evaluate the bio-efficacy of PermaNet
® 3.0 using the WHO cone bioassay, and (3) to evaluate the durability of PermaNet
® 3.0 after 3 years of use.
Discussion
According to the WHO framework for malaria elimination, the success in achieving the objective of the SDG 3.3 global targets for malaria depends on public awareness about the value of human health, the use of nets and the provision of effective access to nets [
1]. In line with this, the NMCP of Ethiopia currently relies on strategies targeting mosquito vector control, which involves the use of LLINs, IRS and larval source management [
12]. In this study, the emergence of pyrethroid resistance in Culicine and Anopheline mosquitoes against permethrin and deltamethrin (the two commonly used pyrethroids in net impregnation) was assessed in two districts of Jimma zone, Southwest Ethiopia. Furthermore, net bio-efficacy against local mosquito populations of
Anopheles and
Culex was evaluated. In addition, net possession, handling and usage was assessed through community based cross-sectional survey in Omo-Nada and Tiro-Afeta districts, Southwestern Ethiopia.
Results of the susceptibility test using wild
An. gambiae s.l. (presumably
An. arabiensis; [
16]) and
Culex quinquefasciatus against WHO insecticide-impregnated papers showed that both permethrin and deltamethrin resulted in reduced knockdown and mortality. However, pre-exposure of mosquitoes to synergist PBO resulted in higher mortality for both insecticides. Similar findings were reported by Yewhalaw et al. [
18] in that pre-exposure of pyrethroid resistant
An. arabiensis mosquitoes collected from Jimma area, southwestern Ethiopia for 1 h to PBO, increased the susceptibility of mosquitoes in all four sites to deltamethrin (mortality range 91.8 to 100%) and permethrin (mortality range 73.9 to 100%). Moreover, the vector population in the same area developed multiple resistance for three (organochlorines, pyrethroid and organophosphates) insecticides out of the four classes of insecticides recommended for public health application [
16]. This report on pyrethroid resistance populations of
Culex quinquefasciatus from Jimma area, is the first report in Ethiopia.
In this study, unwashed PermaNet
® 3.0 induced more than 80% mortality, the WHO threshold for standard product efficacy, against wild population of
An. gambiae and
Culex quinquefasciatus However, reduced knockdown effect was recorded against both
Culex and
Anopheles population. Moreover, the efficacy and wash resistance rapidly downgraded as the number of washes increased from 0 to 20. Reduced knockdown effect of unwashed PermaNet
® 3.0 against pyrethroid resistant populations of both
An. gambiae and
Culex quinquefasciatus is the first report from Ethiopia. Other bio-efficacy tests conducted in Nigeria [
23], Mozambique [
24], Tanzania [
25], Côte d’Ivoire [
26], Central and western Africa [
27] showed susceptibility of wild populations to the new brand PermaNet
® 3.0 net with knockdown effect ranging from 95 to 100%. The observed decline in bio-efficacy of PermaNet
® 3.0 in the current study after successive washes is also in consistent with findings from Côte d’Ivoire [
26] and Tanzania [
28], which also reported that the bio-efficacy of PermaNet
® 3.0 declined from 100% mortality when tested at 0 wash to 4 to 15% after 20 washes.
The discriminate killing effect of the roof section and side panels of PermaNet
® 3.0 was evidenced when unwashed PBO-deltamethrin top netting induced significantly higher mean mortality in both
An. gambiae and
Culex quinquefasciatus and compared to the side section. The enhanced killing effect of PBO-deltamethrin coated top netting of PermaNet
® 3.0 is well documented from studies conducted in Ethiopia [
18] and Tanzania [
28] which showed significantly different mosquito mortalities between roof section and side panels against
An. gambiae and
Culex spp.
Used PermaNet
® 3.0 collected from both districts after 3 years of deployment resulted in moderate to low bio-efficacy, below WHO threshold against field collected
An. gambiae, but exposure of susceptible strains of
An. arabiensis to same net samples resulted in mean knockdown and mortality rate above the WHO cut-off point. Comparison of mean knockdown and mortality rates of side and roof sections of the same samples showed enhanced killing effect of PBO coated roof section, which produced greater mortality compared to side panels of the net. The observed low knockdown and mortality against both wild
An. gambiae and laboratory-reared susceptible strains of
An. arabiensis could be due to removal of surface concentration of PBO by human activities, such as washing, rubbing, smoking and overutilization. Similar finding was reported from Kenya by Ochomo et al. [
29], which stated that nets collected from field retained strong activity against a susceptible laboratory strain, but not against f1 offspring of field-collected
An. gambiae.
In this study, at least one LLIN was traced in each of the 150 houses included in the survey. While it was not possible to corroborate the universal coverage of ITN in this survey since we could not determine the exact number of nets distributed at the beginning of the campaign, it was apparently clear that the districts managed to reach 100% target of one ITN per household. Moreover, most of the LLINs used for sleeping purpose prior to the collection day. This was a good trend of LLINs coverage in comparison to national coverage of 64% households with at least one LLINs [
30]. The current coverage was also greater than findings from Amhara region, Ethiopia by Aderaw and Gedefaw [
31] who reported that 84.67% of the households possessed functional bed nets, and 71.4% of them have been slept under bed net a day before the interview took place. Likewise, high LLIN ownership and usage rate was reported from Amhara and Oromia regional states, Ethiopia with 91% of the respondents own at least one ITN prior to the survey date [
32]. The high ITN coverage is not uniform throughout the country. For instance, low ITN ownership and usage rate (62% and 65%) was reported from Eastern Ethiopia by Biadgilign et al. [
33], Gobena et al. [
34], and from western Ethiopia by Tadele et al. [
35] whom documented 69.3% coverage and 64.9% utilization rate. This could be due to priorities given to the severely affected areas by the NMCP.
In this study, nets were washed on average once every 6 month as more than half of the respondents cited it. Most respondents washed their net with local bar soap and dried outdoor under shade. Frequently washing nets, residing indoor with thatched roof, smoking using fire and oil for cooking and the presence of domestic pests such as rodents are the factors attributed to reported net damage and worn-out problem in the study area. Other studies also indicated the effect of smoke from wood fire used in cooking in the same home where bed nets were hanged in contributing to the observed net damage and reduced efficacy [
36]. Children playing with sharp objects near nets, attack from rodents, and damage from sleeping mat are primary source of net damage according a study from Nigeria [
37].
In this study, most nets collected from the community after 3 years of use were with variable number and size of holes. Most of the holes were small and documented from seam, side, roof section and hanging points of the net. Most of the holes were from side section of the net. Damage recorded included horizontal tear at the bottom, tear at hanging points, burn holes, holes from rodents and the nets with open seam with the whole section missing. Based on the proportionate hole index (pHI) assessment of nets had varying degree of damage. Thus, most of nets were good or acceptable, however, 41 (27%) nets were too torn and need immediate replacement. The abundance of small holes in the side section of the net in other studies from Cameroon [
38] and Mozambique [
39] was documented.
Authors’ contributions
AA and DY conceived and designed the study. AA analysed the data and prepared the draft manuscript. AB involved in data collection. DY critically commented on the draft manuscript. All authors read and approved the final manuscript.