4.1 Species Distribution Data and Screened Environmental Variables
Twenty-eight L. verrucarum and 12 L. peruensis distribution data points were collected(Fig. 1), the climate data of these points were obtained using the ArcMap sampling tool, and correlation analysis was conducted (Fig. 2). Finally, 4 environmental variables for L.verrucarum(Table 2) and 5 environmental variables for L. peruensis (Table 3)were screened for the maxent. The accuracy of the model is very high. The model AUC value of L.verrucarum was 0.980, while that of L. peruensis was 0.986.
Table 2
The high contributing environmental variables of the potentially suitable area of L.verrucarum
Environmental variables | Connotation | Percent contribution |
bio4 | Temperature Seasonality (standard deviation ×100) | 68.3 |
elev | Elevation | 16.6 |
prec01 | Precipitation of January | 8.6 |
prec05 | Precipitation of May | 8.6 |
Bio7 | Temperature Annual Range (bio5-bio6) | 2.8 |
Bio5: Max temperature of warmest month; bio6: Min temperature of coldest month.
Table 3
The high contributing environmental variables of the potentially suitable area of L. peruensis
Environmental variables | Connotation | Percent contribution |
bio3 | Isothermality (bio2/bio7) (×100) | 62.4 |
elev | Elevation | 19.4 |
prec09 | Precipitation of September | 15 |
bio15 | Precipitation Seasonality (Coefficient of Variation) | 2.3 |
prec10 | Precipitation of October | 0.9 |
Bio2: Mean diurnal range (mean of monthly (max temp - min temp).
Many studies have shown that the growth, reproduction, and behavior of infectious disease vectors, including the vector of carrion’s disease, sandflies, are sensitive to climate factors such as temperature, precipitation, and humidity, and with climate change, many vectors have experienced varying degrees of expansion (C et al., 2010; Caminade et al., 2019; H et al., 2008; Semenza & Suk, 2018). In our study, for L.verrucarum, bio4(temperature seasonality) provided the highest contribution percentage of 68.3%, which is the standard deviation of monthly average temperature, and the larger its value is, the lower the probability of L.verrucarum suitable habitat in the range of 0-500. When it exceeds 500, the probability is close to 0 (Fig. 3); that is, the larger the monthly average temperature difference is, the less suitable it is for L. verrucarum to survive within limits. The study by Luis Fernando Chaves et al. shows that when the temperature suddenly cools or heats up, the average number of sandflies significantly decreases(Chaves et al., 2014). It is probably a sudden high or low temperature that leads to a decrease in sand fly adaptability to the environment. However for L. peruensis, bio3(isothermality) provided the highest contribution percentage of 62.4%, Within the isothermality range of 50–90, as the isothermality increased, the probability of L. peruensis suitability increased (Fig. 4). In other words, the closer the daily and annual temperature ranges are, the more suitable they are for L. peruensis to survive. The main factors affecting the mean diurnal air temperature range are latitude and season, and the lower the latitude is, the greater the mean diurnal air temperature range(Jaagus et al., 2014). The factors affecting the annual range of air temperature are mainly latitude, sea and land distribution; conversely, the lower the latitude is, the smaller the annual range of air temperature. L. peruensis is limited to Peru and certain western Andean valleys(Zorrilla et al., 2017). These regions have low latitudes, which may be the main reason for the high suitability areas in a small number of central African regions in the estimates.
In addition, altitude is another important factor affecting the distribution of suitable habitats for these two species, with altitude having a more significant impact on L. peruensis. Some studies have shown that L.verrucarum can survive in areas from 1100 to 3200 m sea level(Caceres, 1993), while L. peruensis predominates above 2250 m and can extend upward to areas at 3250 m above sea level(Hashiguchi et al., 2018). The high altitude of the Tibet Autonomous Region in China may be the main reason why there was a small number of suitable areas here.
The other environmental variables that affect the distribution of suitable areas for L. peruensis are all related to precipitation, namely prec09 (precipitation in September), prec10 (precipitation in October), and bio15(precipitation seasonality). Studies have shown that the distribution of L. peruensis follows a single peak annual time distribution pattern, with its peak occurring in November before the onset of the rainy season in December(Minnick et al., 2014). However, the results of this study indicate that the higher the precipitation in September is, the lower the probability of L. peruensis adapting, indicating that if the rainy season is abnormally early, it may lead to a decrease in the density of L. peruensis. Since the 1980s, there has been a decrease in precipitation from September to October on a ten-year scale(Espinoza Villar et al., 2009), which may also be one of the reasons why the survival of L. peruensis is more sensitive to September precipitation. Peru has diverse climate and terrain. It is divided into eastern and western parts by the Andes Mountains. The western part has a low altitude and is close to the Pacific Ocean. Affected by the dehumidification effect of the Peru cold current, it has formed a long and narrow tropical desert area. The eastern part has a high altitude and is dominated by tropical rainforest and grassland climates. However, due to the strong influence of the Atlantic Ocean, the seasonal intensity of precipitation in Peru is not particularly strong(Gaudry et al., 2017), so the survival probability of L. peruensis is not significantly affected by precipitation seasonality.
Figure 4 Response curves of environmental variables to the distribution probability of L. peruensis (a) Isothermality (bio2/bio7) (×100) (b) Elevation (c) Precipitation of September (d) Precipitation Seasonality (Coefficient of Variation) (e) Precipitation of October, and the curves shown are averages over 20 replicate runs, blue margins show the ± SD calculated over 20 replicates
4.2 World Distribution of L.verrucarum and L. peruensis Risk Areas
4.2.1 Distribution of Current Risk Areas for L.verrucarum and L. peruensis
The model estimated that L.verrucarum currently has a total area of 16.3 million km2 in the world, accounting for 10.9% of the total continental area, while L. peruensis currently has a total area of 16.4 million km2 in the world, accounting for 11.0% of the total continental area. The estimated results show that both species have the highest risk of adaptation in the Andean regions of northern, central, and southern Peru, consistent with the research of Victor Zorrilla et al(V et al., 2017). For L.verrucarum, the high risk areas are mainly located in the western region of the Andes Mountains in South America, central Africa and the equatorial south central region of Indonesia. The medium risk areas are mainly distributed in parts of central Africa south of the equator. Low risk areas are mainly found in northern South America, covering almost the entire territory of Brazil and South Africa, and are scattered in some Southeast Asian countries (Fig. 5). Compared to L.verrucarum, in western South America, the strip-like high habitat of L. peruensis is generally more southward oriented, with more medium risk suitable areas in Brazil, but the areas of low habitat are significantly smaller than those of L.verrucarum. In Africa, the clustered medium and high risk suitable areas are significantly less abundant, but the low suitability areas in southern Africa are visibly more abundant than those in L.verrucarum. Additionally, it is worth mentioning that there are also a small number of low suitability areas and minute quantities of medium suitability areas in Chinafor L. peruensis (Fig. 6).
4.2.2 Distribution of Future Risk Areas for L. verrucarum and L. peruensis
The future projections of L. verrucarum indicated larger climatic adaptation areas than L. peruensis. Under the lowest radiative forcing of the SSP1-2.6 scenario, the change in L. verrucarum's global adaptive areas is mainly concentrated in the medium risk suitable areas in South America, which gradually decreases. Under the highest radiative forcing of the SSP5-8.5 scenario, the medium and low risk suitable areas in South America and the medium and high risk suitable areas in Africa are gradually reduced. Under the moderate radiative forcing scenarios of SSP2-4.5 and SSP3-7.0, the single trend of change in suitable areas in each region is not significant, but globally, under the SSP2-4.5 climate scenario, the area of suitable habitat slightly decreases, while under the SSP3-7.0 climate scenario, the area of medium and high risk areas first increases and then decreases, while the area of low-risk areas shows a decreasing trend (Fig. 7). Overall, under various climate scenarios in the 21st century, the suitable areas of L. verrucarum worldwide have only increased in a few cases, with most still showing a decreasing trend (Fig. 8).
Figure 8 The future suitable area changes in L. verrucarum compared with the current suitable areas under the four shared socioeconomic pathways: (a) SSP1-2.6, (b) SSP2-4.5, (c) SSP3-7.0, and (d) SSP5-8.5
Compared with the current situation, in the 21st century, the areas of the global suitable habitat of L. peruensis have decreased (Fig. 9), and the most obvious reduction is the low suitable habitat in southern Africa. For the estimation of the suitable habitat of L. peruensis, under the lowest radiative forcing of the SSP1-2.6 scenario, the most crucial thing is the emergence of high suitable habitat in the coastal area of northeastern Brazil, which shows an increasing trend over time, while under the scenario of SSP5-8.5 of the highest radiative forcing, the area of the medium and low suitable habitat in Brazil and the low suitable habitat in Africa increases. Under the moderate radiative forcing scenarios SSP2-4.5 and SSP3-7.0, the change in the mid-2000s is not obvious, but the suitable habitat under the SSP2-4.5 scenarios 2081–2100 and SSP3-7.0 scenarios 2021–2040 is significantly wider than that in the other periods (Fig. 10).
This study indicates that for most of the future of the 21st century, the total suitable habitat of the two species of sandflies will generally show a decreasing trend. However, there was a significant increase in the moderately suitable habitat of L.verrucarum in South America and in the highly suitable habitat of L. peruensis in Brazil, which may be related to climate change factors such as rising global temperatures and increasing extreme weather. Peru will be strongly affected by the El Niño phenomenon; when El Niño arrives, the usually arid areas in northern Peru suddenly experience heavy rainfall. Some studies have shown that the Tumbes, the northernmost part of the Peruvian coast, receives over 200 times the annual rainfall during the El Niño period compared to non-El Niño periods(Bayer et al., 2014). Under the possible impact of climate change caused by El Niño, the number and area of cases of carrion’s disease have significantly increased(E et al., 2004). However, with the implementation of vector control measures, the increase in the number of cases has been alleviated(Maguiña Vargas & Pachas, 2014). These phenomena all indicate that climate change may have a positive impact on the density of the vector of Carrion’s disease, leading to an increase in population risk.
Figure 9 The future suitable area changes in L. peruensis compared with the current suitable area under the four shared socioeconomic pathways: (a) SSP1-2.6, (b) SSP2-4.5, (c) SSP3-7.0, and (d) SSP5-8.5
4.3 Comprehensive Risk of Carrion’s Disease in High-risk African Countries
Both of these sandflies have highly suitable areas in Africa, mainly in The Federal Democratic Republic of Ethiopia, The Republic of Kenya, The United Republic of Tanzania, The Republic of Uganda, Democratic Republic of the Congo, The Republic of Burundi and Republic of Rwanda. The fitness probabilities of these two types of sandflies in these countries are shown in Table 4. Based on the vegetation, landcover, and population data of these countries, the risk level of Carrion’s disease calculated by the model is shown in Fig. 11, and the risk value is shown in Table 3. At this time, the contribution of the probability of the presence of L.verrucarum contributes the most to the risk of disease occurrence, followed by population density(Table 4). The epublic of Rwanda has the highest risk of disease occurrence, followed by The Republic of Burundi.
Table 4
Survival probability of L.verrucarum and L. peruensis and incidence risk of Carrion’s disease in these 7 countries
Country | L.verrucarum | L. peruensis | The risk of disease |
Republic of Rwanda | 0.382 | 0.337 | 0.880 |
The Republic of Burundi | 0.354 | 0.256 | 0.509 |
The Federal Democratic Republic of Ethiopia | 0.142 | 0.104 | 0.284 |
The Republic of Kenya | 0.115 | 0.383 | 0.138 |
Democratic Republic of the Congo | 0.254 | 0.013 | 0.025 |
The United Republic of Tanzania | 0.071 | 0.127 | 0.019 |
The Republic of Uganda | 0.040 | 0.013 | 0.004 |
Table 5
The contribution of variables to the model
Variables | Contribution |
L.verrucarum | 56.7 |
population | 16.7 |
L. peruensis | 15.8 |
landcover | 7.6 |
npp | 3.2 |