Background
Malaria is a major public health threat in the Democratic Republic of the Congo (DRC) and places the country among the highest malaria-endemic countries in Africa [
1]. The prevalence of malaria has been high throughout the country, with an estimated 25 M (15.7–38.5 M) confirmed malaria cases and 46.8 K (36.2–57.3 K) estimated deaths in 2017 [
2,
3]. Effective malaria control is affected by conflict situations the country has experienced over the years, especially in the eastern part of the country [
2,
4].
The malaria vectors in DRC include
Anopheles arabiensis, Anopheles coluzzii, Anopheles gambiae sensu stricto (s.s.),
Anopheles melas, Anopheles funestus s.s.
, Anopheles rivulorum, Anopheles leesoni, Anopheles confusus, Anopheles nili and
Anopheles moucheti [
5].
Plasmodium falciparum accounts for the majority of malaria cases in DRC [
6]. Long-lasting insecticide-treated nets (LLINs) are one of the key vector control measures in the country. However, LLINs and other vector control measures are hampered by the development of insecticide resistance. Resistance to insecticides encompasses physiological, biochemical, molecular and behavioural mechanisms [
7,
8]. One of the molecular resistance mechanism against pyrethroids and dichloro-diphenyl-trichloroethane (DDT) are the knockdown resistance (
kdr) mutations. Different substitutions in the amino acid sequence in the voltage gated sodium channel (
Vgsc) can disrupt the activity of these insecticides [
9]. Two well-known
Vgsc point mutations are L1014F, which was first detected in West Africa, and L1014S, which was first detected in East Africa [
10‐
12]. Recent findings confirm that L1014F and L1014S are not geographically limited and also occur in DRC [
13‐
16].
The planning and implementation of a vector control programme requires among other things information on the composition and abundance of the vector species, the proportion of infected mosquitoes and their susceptibility to insecticides [
17]. Due to the conflict situation in DRC, relatively limited information on the malaria vectors and their susceptibility to insecticides is available in comparison with other African countries.
According to the United Nations refugee agency (UNHCR) an estimated 37,000 people a day are forced to flee their homes because of conflict and persecution worldwide. In 2018, 70.8 million people were forcibly displaced at a global scale, of which 41.3 million were internally displaced people (IDP) [
18]. DRC alone has over 3 million IDPs, living in poor conditions [
19]. It is shown that population displacement can have serious implications for malaria transmission, and malaria prevalence is often higher in IDP camps compared to surrounding villages [
20]. Unfortunately, little is known about malaria vectors and their resistance status in these areas, as well as about the opportunities for vector control. As such, the current study was carried out in three different provinces in eastern DRC where Médecins Sans Frontières-Operational Centre Amsterdam (MSF-OCA) operates. All three provinces are characterized by ongoing conflicts that has forced people to flee their homes, resulting in a higher risk of disease outbreaks, poor nutrition status and higher exposure to communicable diseases due to poor housing, or even absence of housing. The collapse in basic health care services and access makes the situation more precarious. MSF has been working in the provinces of North Kivu and South Kivu since the early 1990s and in Katanga Province from 2003 until 2016. In North and South Kivu MSF supports primary and secondary health care in the Baraka and Mweso hospital, in several health centres in the area and via mobile clinics. At the time of the studies MSF was using Fendona
® (active ingredient: α-cypermethrin) for IRS, and distributed different brands of LLINs, mainly PermaNet
® 2.0 (deltamethrin), Olyset
® (permethrin) and Duranet
® (α-cypermethrin). These LLINs were distributed in a targeted way via antenatal care programmes and to patients younger than 5 years old for out and in-patient department. In Shamwana and Baraka, IRS was carried out twice a year in May and November in the MSF supported health structures and compounds. In Kashuga, besides bi-annual IRS in the MSF supported structures, also the entire IDP and indigenous community received IRS once a year in November.
Despite the various interventions (LLIN distributions, IRS, prompt effective treatment with anti-malarials) rolled-out by MSF in its operational areas, malaria transmission remains a public health challenge. In 2012, MSF initiated malaria research studies to understand the persisting high incidence of malaria with indications of increased malaria caseload. These studies included adherence to and efficacy of treatment, coverage and use of bed nets and vector susceptibility to insecticides [
21,
22]. The main objective of these studies was to maximise and better target available interventions. The current study was designed to investigate malaria transmission dynamics under emergency settings, with a focus on key entomological indicators used to characterize the risks of malaria.
Discussion
Malaria transmission was high in all three study sites that were inhabited by internally displaced communities. High levels of insecticide resistance to the currently used compounds in LLIN and IRS programmes was observed from the data reported. The main malaria vectors collected during this study were
An. gambiae s.s. and
An. funestus s.s., but the dominant species differed per study site (Shamwana:
An. funestus, Baraka:
An. funestus and
An. gambiae, Kashuga:
An. gambiae)
. The number of
An. arabiensis collected was low. Knowing the exact species is important for the implementation of effective vector control interventions, since the vectorial capacities of the sibling species differ [
23,
31]. Some samples gave no signal during the PCR analyses, which might be due to poor quality of the DNA extracted or it might result from unidentified species like Stevenson et al
. demonstrated in their study [
32]. It was shown previously that
An. gambiae s.s. is the predominant vector of the
Gambiae complex in eastern DRC [
33]). This seems in contrast with other areas in East Africa where a switch between the strongly anthropophilic and endophilic
An. gambiae s.s. to the more opportunistic and exophilic
An. arabiensis has been observed [
34‐
36]. However, it is debatable whether Eastern DRC should climatically be considered part of East Africa. Mosquito populations will probably be genetically different because of the separation by the big lakes on the borders. The higher presence of
An. gambiae s.s. compared to
An. arabiensis may be explained by the lower selection pressure on the indoor feeding
An. gambiae s.s. due to a lack of indoor vector control tools available in the area [
33,
34]. A Knowledge, Attitudes and Practice survey (KAP) performed by MSF in 2013 showed that Shamwana, Baraka and Kashuga had poor coverage and usage of LLINs (unpublished data).
Anopheles gambiae s.l. tested by ELISA showed a
P. falciparum sporozoite rate of 1.0, 2.1 and 13.9% in Shamwana, Baraka and Kashuga respectively.
Anopheles funestus had a positivity rate of 1.8% in Shamwana and 4.4% in Baraka. In Kashuga no
An. funestus were collected. Except for the high sporozoite positive rate found in Kashuga the overall sporozoite rates are in line with the findings of the President’s Malaria Initiative (PMI) in 2015–2016. They found a mean sporozoite rate of 5.1% in
An. gambiae s.l. and 3.3% in
An. funestus s.l. in seven sentinel sites in different provinces in DRC [
37]. Malaria transmission was highly heterogeneous in the study area. Especially in Baraka, a town with a total surface area of approximately 10 km
2, and Kashuga, a town of approximately 2 km
2, the mosquitoes carrying
P. falciparum sporozoites were concentrated in one specific area of approximately 250 × 250 m (Fig.
2). This is critical information for the implementation of vector control interventions. The high EIRs shows that the entire area is in need of effective vector control interventions, however, uniformly applied vector control interventions will be very challenging in this context. Therefore, targeting these specific high transmission spots will be a good start. However, there is no conclusive evidence that targeting these specific high transmission sites will actually reduce the transmission in the entire area [
38].
What is driving these spatial transmission foci was not further investigated. However, the presence of fishponds and agricultural fields in Ibuga camp (Kashuga) probably contributed to a higher number of mosquitoes present and in combination with a non-immune population that has been displaced to a malaria endemic area, this might have contributed to the high transmission intensity of malaria in this particular area. The presence of brick production sites in Mushimbakye (South Kivu) might have contributed to the higher number of mosquitoes present. It should be noted that the EIR calculated in this study should be treated with caution, since the collection of mosquitoes took place in a specific short time period and, therefore, the numbers were extrapolated to calculate the monthly EIR. Furthermore, CDC light trap collections were used instead of human landing catches.
In all three sites, the mosquitoes showed resistance to pyrethroids. Clear resistance towards permethrin was observed in all three locations. This is in line with the study of Wat’senga et al
. for which they performed pyrethroid resistance intensity tests in 11 provinces in DRC and confirmed pyrethroid resistance, with in general higher resistance levels to permethrin than to deltamethrin [
39]. It is debatable whether this resistance can be explained by the presence of
kdr mutations. Both L1014F and L1014S
kdr mutations were present in this study. Overall, more than 90% of the mosquitoes had at least one of these mutations. However, because of the use of two separate assays in this study to type the different
kdr mutations, many mosquito samples showed unresolved allele combinations, in which more than two alleles were detected. Therefore, these results were not discussed but have been included in Additional File
1. Mixture of the two alleles is increasingly common especially in central Africa including DRC, however these mixtures are the most problematic for the traditional detection methodologies [
15,
16]. This study underscores the need to type both
kdr mutations via a single detection method like described in Lynd et al
. [
15]. Also, some studies have shown the presence of metabolic resistance, as well as an increase in the cuticle thickness in mosquitoes in other parts of DRC [
15,
16,
40,
41]. These resistance mechanisms might also play a role in the mosquitoes collected during this study, but this was not further evaluated. The resistance towards the pyrethroid α-cypermethrin may be explained by the type of LLINs deployed and by the IRS product used in the three areas. To our knowledge, these were the first insecticide susceptibility tests with α-cypermethrin performed in DRC at that time. Resistance towards pyrethroids might also be the result of the use of the same class of insecticides in agriculture [
42,
43]. Bendiocarb, pirimiphos-methyl and malathion caused high mortalities and can therefore be considered as candidate insecticides for vector control activities. However, to prevent rapid development of resistance towards these insecticide classes, a clear rotation strategy of different insecticide families is recommended.
Special attention is needed for IDPs. The living conditions of IDPs are generally very poor and because the majority of them moves from one place to another frequently, it is a challenge to protect them with conventional vector control methods such as LLINs and IRS. Options for vector control methods that focus more on the community level rather than the household level, such as environmental management, (biological) larviciding, push–pull systems and spatial repellents, should be further investigated so they can be used in IDP camp settings as a supplement or alternative to LLINs and IRS [
44‐
46].
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