This study assessed baseline and future geographic distribution of malaria transmission suitability in Nepal and compared the anticipated future distribution with previously defined areas of malaria risk from two different approaches.
This study found the predicted geographic distribution of malaria transmission suitability of both ASPF and ASPV will increase in the future. For ASPV, in addition to increase, there will be decrease in suitability in all scenarios except in 2030 under RCP 4.5 A study by Hundessa et al. found that the percentage of P. vivax and P. falciparum malaria will increase in China in 2030s, 2050s and 2080s under RCP 4.5 and 8.5 scenario [45]. In addition, the study found that the percent increase of P. falciparum malaria will be higher than for P. vivax malaria and their spatial distributions will differ. In this study, the predicted geographic distribution is nearly the same for both ASPV and ASPF according to the Warren’s I metric.
In this study, the geographical distribution of malaria transmission suitability in 2030 and 2050 did vary compared to baseline. As compared to baseline, there will be emergence of suitability and increase in length of Season for malaria transmission for both ASPF and ASPV in 2030 and 2050, except for ASPV, where in addition to emergence of suitability and increasing length of season, some areas will observe decreasing length of season (endemic to seasonal) in the southwestern part of the country. A similar study carried out for Africa also found that climate change will result in shifting of geographic distribution of malaria transmission suitability in the years 2030, 2050, and 2080 under RCP 4.5 and 8.5 [8]. According to the study, large areas previously unsuitable for malaria transmission in Africa will convert into areas with endemic and seasonal suitability as well as areas that shift from endemic or seasonal suitability to becoming unsuitable for malaria transmission.
Malaria transmission suitability is predicted to emerge and increase in length of season in previously cooler regions of Nepal, towards the northern part of the country in the higher altitudes. This means that malaria will be able to establish in these areas, and local transmission, not just imported cases, can occur. These regions were less suitable or not suitable for malaria transmission in the past. Due to climate change, temperatures will rise, and these regions will become more suitable for malaria transmission. Several other studies have found that malaria already spread in previously cooler places and extended towards higher altitude in various parts of the world. For example, increased number of malaria cases were found at higher altitudes in the highlands of Colombia during warmer years [19]. In Rwanda, there was a substantial increase in malaria cases country-wide, but the rate of increase was greater at high elevations than at medium and low elevations [18]. In Rwanda, malaria migrated to new areas in the late 1980s, to places where it was previously rare or absent due to record high temperatures and heavy rains in 1987 followed by the El Nino event in 1988 [18]. Similarly, some studies have predicted increase in malaria in previously cooler places. For example, Hundessa et al. predicted that both P. vivax and P. falciparum malaria will increase in the previously cooler regions in China in the future under climate change [45]. Similarly, Ryan et al. predicted that due to climate change, exposure to malaria transmission will increase in previously unsuitable regions such as the higher elevation regions of Southern and Eastern Africa in 2030 and will become more concentrated along the Eastern African highlands later (2080) [8].
The results of this study have identified regions with the emergence of suitability, increasing length of season and decreasing length of season, where interventions need to be revisited given the impacts of future climate change. With increase in temperature due to climate change, malaria transmission is expected to occur in some previously unsuitable regions (emergence of suitability), for example in the Karnali province, junction of Sudur Paschim and Karnali province, northern part of Bagmati and Gandaki province, particularly in the Mountainous region of Nepal. The emergence of suitability will put naive population at risk of outbreaks, especially the vulnerable groups such as pregnant women, children, and the elderly. These places need a new malaria control program. Similarly, malaria transmission suitability is expected to increase (increasing length of season) in other previously suitable regions, for example in the junction of Sudur Paschim and Karnali province, Lumbini, Gandaki and Bagmati province, in the Hilly and Mountainous region in Nepal. In these places, malaria seasons are getting longer. This will require different control interventions and management activities than those currently in practice for a shorter malaria season. On the other hand, malaria transmission suitability for ASPV will decrease from endemic to seasonal (decreasing length of season) in the Terai region in Nepal. In these areas, opportunities arise for more targeted intervention and elimination of the disease.
In this study, the changing areas of malaria risk in future were compared with the previously defined areas of risk. First, DoHS risk areas of 2010 and 2021 were compared with the incidence trend clusters from our previous study. ITCs with increasing PF and PV malaria encompass moderate-risk, low-risk, and no-risk 2010 DRDs, but do not encompass high-risk districts. This means that between 2005 and 2018, the PF and PV malaria had increasing trend in some of the moderate, low and no-risk districts and not in the high-risk districts and it may be because vector control interventions were mostly focused on high-risk districts followed by moderate risk districts with little or no interventions in the low and no-risk districts. Comparing the ITCs with DoHS risk wards of 2021, it was found that the high and moderate-risk wards of 2021 do not lie in the ITCs with increasing trend of PF and PV malaria, but they lie in the ITCs with decreasing trend of PF and PV malaria. The increasing trend of PF and PV malaria are forming in places where vector controls are not implemented. Public health officials can use the information of increasing trend of PF and PV malaria and make plans for implementing vector control in those clusters.
It was also found that the emergence of suitability will occur in low and no-risk DRDs and outside of the high and moderate-risk DRWs. Increasing length of season will occur in all categories of DRDs and only few areas with increasing length of season will occur in high and moderate-risk DRWs but mostly outside of them. Thus, this study has identified potential malaria risk areas in the future which are outside of the government’s designated malaria risk areas and thus not in attention of the public health officials for vector control interventions. As, the malaria risk stratification by the government has not included the impact of climate change in their risk designations [27], the results of this study can be helpful in planning for additional areas for vector control interventions.
Similarly, emergence of suitability and increasing length of season are predicted in PF and PV ITCs. This may indicate that the increasing trend of PF and PV malaria is an impact of climate change already affecting malaria transmission in Nepal, which will increase further in the future. Thus, not only are areas of malaria risk already changing in Nepal but will continue to in the future with climate change. Thus, public health officials may need to revisit official risk stratification mapping for the future, incorporating the potential impacts of climate change. Nepal is currently preparing for malaria elimination by 2026, and this may be feasible. However, there is also always the potential for malaria resurgence following elimination because the risk factors for malaria like suitable climate and migration of people (who can introduce imported malaria) still exists. Thus, these potential future scenarios of malaria transmission suitability (emergence, disappearance, increase and decrease) should be incorporated into the planning of vector control interventions and other malaria management programmes, to elimination and beyond. Otherwise, the gains achieved in malaria control in recent decades could be lost.
This study availed itself of the best data accessible to the authors, but parts of the information presented may have been limited by data availability in several aspects of the project. The authors note that after adopting new constitution in 2015, Nepal underwent restructuring of its administrative divisions into 7 provinces, 77 districts, and 753 municipalities and rural municipalities [46, 47]. Before 2015, Nepal was administratively divided into 5 development regions, 14 zones, 75 districts, 53 municipalities, and 3,918 village development committees (VDCs) [48, 49]. In this study, the 75-district presentation was retained. For this study, we are using ward level data for comparison based on the administrative division after 2015. However, for the incidence trend clusters (ITCs), malaria data for 75 districts were used, based on administrative divisions prior to 2015 because epidemiological data was available for 75 districts for most years between 2005 and 2018. The before and after 2015 maps of districts are presented in Additional file 1: Fig. S8 the subdivision of two previous districts did not affect the findings of this study.
The malaria burden data used by Nepal DoHS only include malaria surveillance data from the public health facilities, while malaria information from private sector healthcare is unreported [50]. Thus, the malaria burden data may not represent the actual malaria transmission situation on the ground, which may have affected the official microstratification definitions. The government should focus on strategizing to access a greater proportion of malaria testing information, including prioritizing policy making for including private health facilities in the malaria reporting system. The potential limits to the full geographic scope of Nepal’s reported malaria burden meant that groundtruth models could not be performed on the reported risk definitions. The places where alignment or mismatches occurred could only be commented upon.
The results of this study are based on the temperature response curves of An. stephensi and the malaria parasites because there are not enough studies that have conducted laboratory experiments for identifying thermal responses of An. fluviatilis, the major malaria vector in Nepal. The studies on thermal responses of mosquitoes are mostly for species that are found in Africa for example Anopheles gambiae and An. stephensi [29]. There is a need of more studies on thermal responses of Anopheles species found outside of Africa (for example fluviatilis) because malaria is a major issue in Asia after Africa and An. fluviatilis is a major vector in countries like India, Nepal, Iran, Pakistan, Afghanistan, Bangladesh, and Myanmar [12]. As the entire world is gearing up for elimination of malaria, more information is needed on how climate change might impact the malaria transmission for major malaria vectors not only in Africa but also outside of Africa. In addition, more mosquito surveillance is needed in Nepal to timely update the malaria vectors.
Anopheles stephensi has been reported to be present in Nepal in survey studies [51, 52], and is assumed to be part of the suite of competent malaria vectors. However, a recent entomological study conducted in the eastern part of Nepal did not report An. stephensi [32], and we are not aware of any more recent published or publicly available mosquito survey studies in Nepal in recent years. The absence of records at present are due to under-surveyed conditions, rather than a reflection of its absence, and suggest that understanding the potential for An. stephensi to expand malaria risk in Nepal is an important part of the malaria management strategy for the country.
The results of this study are based on the impact of temperature on malaria. Precipitation, another climatic factor which influences malaria transmission was not included, in this study. Mosquitoes require water as breeding habitats to complete their life cycle. However, precipitation measures such as monthly rainfall totals or cumulative rainfall may not be a good indicator of standing water, as extreme precipitation events are becoming more common with climate change [8]. Heavy rainfall can wash away the mosquito breeding sites disrupting their life cycle. Thus, more rain may not mean more breeding sites and more mosquitoes. In addition, there are more uncertainties in predicting future precipitation with climate change [53].