DISCUSSION
Xinjiang and Tibet, as well as neighboring countries are all contaminated regions of PPR and resulted in large scale of deaths(Wu et al., 2016). Though this area is an important geographical region for PPR spreading from the west to the east, there is not an integrated study on the spatial distribution of PPR here. For the first time, the spatial distribution risk of PPR in this region was accurately assessed by multiple eco-geographical, anthropoid and meteorological variables. The results show that western border of Xinjiang and the northwest border of Tibet are high risk areas for PPR, which are consistent with the historical distribution of PPR. High-risk areas of PPR was also verified in Pakistan, Afghanistan, Kazakhstan, Kyrgyzstan and other bordering countries, while the distribution area is larger than that in China, showing the possibility of risk export. Previously, it was thought that there had been PPR communication between China and its western neighboring countries, but no evidence was found. Though the genetic relationship of PPRV isolated in countries within this region can be determined by phylogenetic analysis, the origin country of PPRV from the aspect of epidemiology can not be answered yet(Sahu et al., 2017). Then the source of PPR in China is still a mystery. In this paper, the validation of the available transboundary paths for PPR spreading are proved first time by spatial and habitat analysis techniques. Combining with the migration of wildlife and transboundary grazing, the integrated transboundary situation of PPR in this region was effectively revealed(Stewart and Belote, 1976). Currently, different elevation areas are dealt with separately to overcome the incapability of the model in dealing with DEM with great difference(Himeidan et al., 2009). We also categorized the target areas into two and taken 1500 m as the threshold. After analysis, the accuracy of these two models is optimal and the prediction is reliable. The predicted high-risk area completely covered all known historical epidemic areas. To adopt 1500 meters as the threshold to distinguish the high altitude areas from the low altitude areas is based on the internationally standard for altitude division.
In our research, the densities of wild and domestic small ruminants animals are not used because of their uneven distribution and the difficulty in population size assessment, which is influenced greatly by the migration habits of wild small ruminants animals and nomadic lifestyle. Such behaviors of migration and nomadism are sensitive to temperature, precipitation, altitude, slope and human population distribution(Carlson et al., 2019). They are natural responses to the natural seasonal factors, or in other words, regulated by the above factors. Therefore, the risk analysis based on the above variants can effectively reflect the actual distribution of PPR.
The LCP analysis is the most commonly used method to assess the connectivity between habitats(Cushman et al., 2013). It designs a landscape resistance surface based on the assumed ”cost” of landscape components versus species movement, and determines the minimized cumulative cost path between sites(Ntassiou et al., 2015). The minimum cost model can integrate the geographic information and behavior information. In our maximum available path’s analysis, based on special purposes, we did not assign a value to DEM and LC after reclassification according to the expert experience as usual. For transboundary spreading of PPR, except the unavailable paths, all paths can be utilized by small ruminants and responsible for PPR spreading. Therefore, all available paths should be kept without further classification. In terms of the number of transboundary path, we focus on the calculation on risk areas close to the border on both sides. A band of paths that did not cross the boundary itself and finally partially overlapping with the transboundary paths were removed. Correspondingly, the transboundary paths were kept priority.
In 2018 in Xinjiang the 3 parallel border separation fences were built(Liu et al., 2020), which severely restricted the transboundary migration of wildlife and grazing of livestock, and to some extent, it is beneficial to the control the spreading of PPR(Cui et al., 2017). However, from the experience of the construction and lesson of damaging of the border fences in northeast border of China, it is impossible to build a large-scale border separation fences without any escape intervals for wildlife, even in the hilly and plain areas such as the northeastern China; secondly, in some special environments, such as rivers which can not be completely enclosed and remain for wildlife migration; Finally, the capability to damage fences of wild animals can not be ignored, such as the usual seen damage of boar to the border fences. Therefore, we can neither ignore nor be too optimistic about the obstructive role of the border fence for the migration of wild animals. To barrier the transboundary communication of human and animals does reduce the transboundary spreading of PPR(Didelot et al., 2015). While in the long run, the changes of the behaviors of human and animal will certainly lead to the changes of the landscape and then to the spatial distribution of wildlife diseases at the landscape scale(M. and Munir, 2014). Therefore, in the future, we will pay attention to the spatiotemporal dynamics of PPR in this region, in order to provide experience for the study of PPR distribution and decision making in other similar regions in the world.
Density of population is the most important factor affecting the distribution of PPR, in pastoral areas, herdsmen mainly raise small ruminants as their economic source, and the number of small ruminants is closely related to PPR epidemic. In the border areas of western China, the nomadic nations are dominant. In the area above 1500 m, with the human population density increasing gradually, the risk of PPR increases slowly, and followed by a platform period. This is because most of the high-altitude areas are pastoral areas and with the increase of small ruminants raised by people, the risk of disease also increased. In the area below 1500 m, with the human population density increasing, the risk of disease increases first, and then decreases rapidly. This is because the low altitude areas are mostly occupied by cities and towns. In small towns (less urbanization), the risk of disease is higher in accordance with the density of livestock where small ruminants are still kept as economic sources(Keesing et al., 2010). While in urban areas, the lower risk of disease is the reason of the unsuitability of livestock raise.
Some studies have pointed out that temperature rise plays a key role in diseases outbreak and spreading(Aguilar et al., 2018). In this study, the minimum temperature of June is the main variable in the category above 1500 m. In June with the gradually increase of the average minimum temperature (within 0-13℃), the PPR risk increases gradually; The mean temperature of September is the main factor in the category below 1500m. In September with the gradually increase of the average temperature (within 0-31℃), the probability of PPR increases gradually. It has been found that with the increase of temperature, the risk of animal disease increases(Nicholls, 2015). Richard Kock pointed out that temperature rise plays a key role in disease(Kock et al., 2018).
China share a long border with many neighboring countries and facing a great risk of PPR cross-border import by the cross-border activities of small ruminants. Five cross-border import paths are predicted in this study, among which path h is from Kazakhstan through the junction of Khorgos River and Ili River into Xinjiang, China. The homology of PPRV strains isolated from Kazakhstan and China is 99.8%, which reflects the cross-border communication between the two countries(Kock et al., 2015). Huocheng County, Ili Kazak Autonomous Prefecture, Xinjiang, China is located in the open area in the northwest of Ili River Valley. It borders Kazakhstan, and the Khorgos river is in the west. Planting and animal husbandry are the main local production mode, and there are bilateral cross-border grazing. It has been reported that Saiga antelopes which widely distributed in Kazakhstan migrates to the north and south twice a year, and they migrate to Xinjiang, China through Kazakhstan to give birth, which also provides the possibility for PPR cross-border transmission(Cui et al., 2017, Bekenov et al., 2010). The cross-border paths a.b.c enters China from the west of Pamir Plateau. Western Pamir has abundant rainfall, abundant vegetation and wide distribution of wild small ruminants. As there are many mountain passes, it can lead to India in the south and to Afghanistan in the west, and may also be a natural channel for PPR transmission. On the Pamir Plateau, China’s Tashkurgan Wildlife Nature Reserve, which borders Pakistan, is where rare wildlife such as Procapra przewalskii ,Ovis ammon polii, Pseudois Nayar and Capra sibiricaroam. Wild animals are easy to cross into China. Therefore, Pamir Plateau is also an important cross-border passage for wildlife, which is a high-risk area for PPR cross-border import. The cross-border paths d is from Kashmir along the Bank of Bangongcuo lake into Ritu County, Tibet, China. Bangongcuo Lake runs through Kashmir and Tibet, China. It is a natural ranch, among which Equus Kiang, Pantholops hodgsoniiand Ovis ammon are widely distributed. These wild animals can adapt to the harsh environment and survive in the area of 4600-6000 altitude. They can live in the area of ultra-high altitude. The natural channel carries out cross-border activities. There are abundant grasslands in Ritu County in China. Most of them are semi-agricultural and semi pastoral nomads. Wild animals and domestic animals share the same water source, food and frequent contact in the activity area. It is suspected that PPR has been introduced into Ritu County in Tibet since 2005. It has also been reported that wild animals in this area have died of small ruminant epidemic(Bao et al., 2011b).
PPR has been transmitted to China twice. The western border of China is long and borders many countries, and there are many transboundary channels. In order to control the introduction of diseases from abroad, it is necessary to strengthen the monitoring on the available transboundary paths. Especially during the suitable seasons for PPR spreading, to forbid grazing and set barriers within the path range should be done. Compulsory immunization should be carried out in densely populated border towns and pastoral areas to reach a highly effective immune response in case of contacting with infected wild animals(Bao et al., 2011a). Due to the reason of broad distribution of PPR, multiple hosts and strong migration of hosts, large-scale vaccination is impossible for prevention, so prevention and control has been difficult. This research not only provides more accurate prevention and control strategies for the prevention of the introduction of PPR into China, but also provides better suggestions for the prevention and control of PPR in neighboring countries, and provides an important rationale for the complete elimination of PPR in China in 2020(Albina et al., 2013).