This study highlights the lack of detailed and current data on the distribution of LF in DRC, a country considered to have one of the largest burdens of disease in the world, and second in sub-Saharan Africa, after Nigeria . The national LF Programme in DRC is yet to commence MDA implementation, which is complicated by approximately one third of the country being endemic for loiasis ; a contraindication for any MDA regimen which includes ivermectin. The reason for the lack of information and action may be attributed to the widespread civil unrest affecting many parts of the country for decades, which has left it severely under-resourced in public and private sectors, as well as the absence of efficient transports systems making access to many areas of the country difficult [29, 32].
The insidious nature of internal instability is a major barrier for disease control programmes, and has implications for the national LF programme in terms of accessing remote, rural endemic regions, readily and safely [32, 53]. These problems are not unique to DRC as many of 16 countries still to start MDA in Africa are loiasis endemic and considered to be conflict or post-conflict countries . This group of countries are among the poorest and most fragile in the world , which raises the importance of an integrated effort between international partners, and the various national NTD [8, 16, 36] and vector control programmes [19, 55] to ensure that resources are maximised, and the elimination of these diseases achieved collectively.
The review and mapping of historical data in DRC highlights gaps in our knowledge and the difficulties in fully defining the problem. The value of collating and analysing disparate data sources cannot be underestimated. This activity is recommended for countries planning to start MDA, as it will help to identify high risk areas and key risk factors that are crucial to control efforts even if resources are limited. We also advocate that the new mapping approach of MOM is essential in countries where there is co-endemic loiasis as well as other control programmes which impact on LF implementation. However, it may be important to evaluate if transmission is still ongoing in some areas where CDTi or deworming activities have taken place for several years, since reported coverage may not always be true or may not always have the predicted impact on the targeted diseases. Therefore, before excluding an area for LF control, it may be prudent to rapidly assess the impact of other interventions on the LF endemicity.
The simple reproduction and overlapping of historical data and maps, clearly shows the close association of LF distribution with the Congo River and its tributaries in different regions of the country. This provides key information about the ecology and transmission of W. bancrofti vectors, which is important in DRC where only one study has been carried out in the past 70 years . In Matadi, Bas Congo, An. funestus was identified as the most important vector of W. bancrofti, however, other entomology studies indicate a diverse range of Anopheles across the country [56–58]. An extensive survey found An. moucheti and An. wellcomi most abundant along the central Congo River system, and An. paludis in the surrounding hinterland . In Katanga, the mosquito species were considered to be distinct to other ecological regions , and in the forested Mayombe region, Bas Congo, An. gambaie s.l was found to be much more abundant than An. funestus and An. moucheti, which may explain the lack of LF found in this region by Fain .
The recent maps on the distribution of L. loa in Africa [9, 27] are a useful resource for the national LF programme in DRC. The L. loa maps elucidate SAEs risk areas, where extra precautions and alternative intervention strategies may be required, especially if LF is found to be endemic. Comparisons between the historical and current L. loa maps indicate that its distribution has remained relatively stable for more than half a century, and been a long-standing public health problem, particularly in the Oriental Province. The problem extends across large areas of central Africa affecting many vulnerable people , and poses a major obstacle to the LF and onchocerciasis elimination programmes in Africa. Whilst L. loa is not included in the list of WHO's 17 NTDs  its importance as an impediment to progress of programmes based on preventive chemotherapy cannot be understated.
Overlapping the C. silacea and C. dimidiata distribution data with the L. loa map indicates that both vectors may transmit the infection in the high risk areas. In Central Africa these Chrysops sp are found to be sympatric in tropical rain forests and rubber plantations, and shown to bite throughout the day and have different annual transmission cycles [26, 28]. To eliminate LF in rain forest loiasis co-endemic areas will require new innovative strategies, possibly including Chrysops control as a novel approach. Although the development of villages, clearing of vegetation and the spraying of insecticides of Chrysops breeding sites have shown to produce a degree of control of loiasis transmission [62–65], these methods are impractical. Therefore, it may better to use impregnated/baited traps or trapping methods similar to those used for Human African Trypansomiasis (HAT), which is transmitted by Glossina but shares some biological features of Chrysops biting patterns and habitats . Recent data shown in the global atlas of HAT indicates geographical overlap in some high risk L. loa areas, and the use of MOM may elucidate areas where both vectors may be trapped for control purposes.
Alternatively, a combination of new drug regimens and integrated vector management (IVM)  may be an option in high risk LF areas and co-endemic high loiasis areas. The use of alternative drug strategies such as twice yearly treatment or higher doses of albendazole  or 4-6 week course of doxycycline  may be a better approach to reduce filaria loads, and reduce the risk of disease and SAEs in selected populations. These alternative drug strategies used in combination with ITN/LLINs and/ or indoor residual spraying (IRS) may also significantly reduce W. bancrofti transmission . More synergies between malaria and LF programmes are essential, especially as the distribution of ITN/LLINs is increasing dramatically. Although no large-scale IRS activity is planned for DRC , historical IRS activities using gammexane, show considerable reductions in An. moucheti, An. gambiae s.l and An. paludis densities in DRC , and more recently with DDT in An. funestus in other countries , which suggests that IRS could be targeted with significant impact, especially in Oriental Province. However, it will be important to monitor insecticide resistance of vector control activities [70, 71].
The extent to which W. bancrofti and O.volvulus overlap in L.loa > 40% high risk areas in Oriental Province is unknown, however, Woodman  in neighbouring southern Sudan illustrates that the three diseases are co-endemic across relatively large geographical areas. The use of MOM could elucidate different patterns of co-endemicity within such areas, to determine if certain interventions would be of risk or benefit. The three finer scale maps in this study show that different regions have different spatial and overlapping patterns with different risks and benefits. For example, in the Mayombe region, Bas Congo L. loa was most prevalent, and only overlapped with W.bancrofti and O.volvulus in distinct areas. This region has high bed net/ITN coverage, which is of significant benefit; however, the overlap of the CDTi priority area in the L.loa and O.volvulus co-endemic area is a risk as ivermectin treatment could result in SAEs. In contrast, in the Bandundu (Banningville) region , the overlaps between the CDTI priority areas and W. bancrofti and O.volvulus co-endemic areas will be of considerable benefit as ivermectin could reduce the prevalence of both diseases (as well as STHs) along the river with limited risk of SAEs. However, there is a low or no coverage of bed nets/ITNs in this region [30, 31].
On a broader scale, the country level maps highlighting the overlaps between different disease distributions and interventions provide important insights into the potential risks and benefits of multiple large-scale disease control programmes operating in one country. It clearly identifies high risk and vulnerable populations which need to be targeted with more effective or alternative innovative intervention strategies. Importantly, it also shows the potential large-scale benefits that the combination of the onchocerciasis, malaria, and future STH programmes could provide to the national LF programme and vice versa. Clearly, there are huge benefits of coordinated and overlapping mapping activities so Ministries of Health have a clear picture of the epidemiology for planning purposes.