INTRODUCTION

Peste des petits ruminants (PPR) is an acute, serious, contact infectious disease caused by peste des petits ruminants virus (PPRV) belonging to the genus of morbilliviruses(Li et al., 2018). PPR is characterized by sudden depression, high fever, anorexia, nasal and ocular discharge, mouth erosive lesions, pneumonia and severe diarrhea(Alemu et al., 2019, Alfred et al., 2018). PPRV is mainly transmitted through direct or indirect contact, and the incubation period is typically 2-7 days(Kumar et al., 2014). The morbidity rate can reach 100% with a high case fatality with the acute form of disease(Parida et al., 2016). Both the domestic and wild ruminants are susceptible to PPRV, while the goats and sheep are the most susceptible hosts. The pig was recorded to be infected(Nawathe and Taylor, 1979){Shabbir, 2020 #5;Nawathe, 1979 #16}. At present, the prevention and control measures for PPRV are mainly based on herd immunity by live attenuated vaccine in endemic area(Enchery et al., 2019). In PPR free area, slaughtering of infected animals, environmental sanitation sustainability, prohibition of animal transportation and quarantine are measures adopted generally(Couacy-Hymann et al., 1995). Although many measures have been taken in prevention, PPR is still widely distributed worldwide(Banyard et al., 2010). Above 63% of the domestic small ruminants in the world remains under the threat of PPR, and the risk to wildlife is unknown(Baazizi et al., 2017). The livestock industry has been greatly ruined or decreased by PPR in the affected countries and regions, and even jeopardized the food security system, causing economic recession(Baron et al., 2017). PPR has attracted the attention of international organizations and relevant national authorities, which is listed as a transboundary animal disease need to be controlled and eradicated by FAO and OIE(OIE and FAO, 2015a).
PPR was first discovered in the early 1940s in Cote d’I voire and then expanded cross over nearly the whole world(OIE and FAO, 2015b). Now around 70 countries have either reported infection to the OIE or are suspected of being infected(Shaila et al., 1996). Of these, more than 60% are in Africa and the others are in Asia and the Middle East. There are another 50 countries are considered to be at risk for PPR(Liu et al., 2018). In China PPR first outbreak in the Ngari region of Tibet in 2007 with a 5751 death of sheep and goats accounting for 16% motility rate(Wang et al., 2009). This epidemic was effectively controlled by ways combing slaughter, mass immunity and restriction of animal transportation. However, PPR re-emerged in Xinjiang, China in December 2013 and rapidly spread to much of China in the first half of 2014 through the large live-sheep trading network(Bao et al., 2014). 23 provinces, autonomous regions and municipalities (P/A/M) of PR China (PR China is administratively divided into 34 P/A/M) have been involved into this pandemic and caused a heavy loss of sheep and goat industry of China(Bao et al., 2017).
There are four known lineages of PPRV (I-IV). The lineage I and II are distributed in the West Africa; the lineage III are popular in East Africa, the Arabian Peninsula and southern India; the lineage IV is popular in the Arabian Peninsula, the Middle East, South Asia and Africa(Kwiatek and Olivier, 2011). Phylogenetic analysis revealed that the Chinese 2013–2014 strains and Tibet strains isolated previously in 2007, both belonged to lineage IV but in different sub-branches. Nevertheless, compared with the Tibet strains, the Chinese 2013–2014 strains shared high degree of genetic homology with those from Pakistan and Tajikistan(Xia et al., 2016). As early as 1991, an outbreak of PPR was reported in Pakistan bordering Xinjiang(Anees et al., 2013). Similarly, outbreaks of PPR were reported in Tajikistan bordering Xinjiang in March 2004, and phylogenetic analysis indicated that the isolates belonged to lineage IV as well(Kwiatek et al., 2007). So we consequently speculated that the Chinese PPR epidemic was spread more possibly from bordering countries into China.
The part of the western border of China (N29°54 ’- 44°32’) borders 7 countries, 6 of which are PPR endemic countries (India, Pakistan, Afghanistan, Tajikistan, Kazakhstan and Russia). Though the epidemiological situation in Kyrgyzstan, another neighbor, is not clear, serum positive reports of PPR are existed. In the border areas of China-northern India, China-Pakistan, China-Afghanistan, China-Tajikistan, China-Kyrgyzstan and China-Kazakhstan, crisscrossed huge high mountains and low valleys is the dominant landform. Since ancient times to now, the nomadic nations constitute the main part of local population. Therefore, transboundary grazing is the local typical style of production and life and domestic ruminant exchanges and migration frequently(Padhi and Ma, 2014). Due to the existence of large area of no man’s land and harsh environment, border control between countries is always difficult to be fully realized, and transboundary grazing is still widespread until 1990s here. Moreover, in this region many wildlife protected areas host a large number of wild ruminants (Ovis ammon , Capra ibex , Gazella subgutturosa ,Procapra picticaudata , Pantholops hodgsonii ,Pseudois nayaur ), which provide a big sum of carriers of PPRV for transboundary spreading(Li et al., 2017). Wild ruminants, goats and sheep all are capable to cross the barbed wire mesh on the boundary in the need of foraging, migration and other activities. Also due to the imperfect establishment of the barbed wire mesh in some areas the transboundary spread of PPR has become natural(Liu et al., 2020). Combined with the above possibilities, all susceptible animals in this area contact frequently by common utilization of water sources, food and habitat, so the virus is particularly easy to spread from one side to another(Rahman et al., 2019).
In 2013, after the PPR epidemic which caused huge economic losses to China’s sheep industry, in December 2015, the Ministry of Agriculture of China officially issued the National Plan for the Eradication of Peste des Petits Ruminants (2016-2020). It planned that by 2020, except constructing a 30 km width immune isolation belt in the border counties and plantations along the border line, the whole nation is going to achieve the goal of a national wide ”free zone without immunization”. Considering the impossibility of immunization of wild ruminants, the construction of the free zone should be based on cutting off all possible routes of transmission and eliminating imported cases firstly.
In ecology, resistance coefficient is used to express the willingness of a species to pass through a specific landscape unit or the suitability index of the landscape unit for a species(Zeller et al., 2012). In the process of passing through a specific environment, if the energy and time consumption of individuals is small and the mortality rate is low, it means that the environmental resistance is low(Hashmi et al., 2017). the possibility of animals passing through the area is high. Then the landscape unit is regarded as a corridor (migration paths)(F. et al., 2003). We assume that there are natural migration paths for wild and domestic ruminants in the N 29°54’-44°32’section of the western border of China, and transboundary spreading of PPRV exist. Therefore, this study mainly focuses on the risk analysis of PPR and the verification of transboundary paths in the above-mentioned multinational border areas, and try to block the local transboundary communication of PPR, so as to provide valuable suggestions for the prevention and control for border areas worldwide.