1. Introduction
African horse sickness (AHS) is a transboundary and non-contagious infectious disease of equids, and its pathogenic virus is the AHS virus (AHSV) of the genus Orbivirus of the family Reoviridae (P. G. Howell, 1962). AHS is a typical arboviral disease, in the field infected Culicoides biting midges (Diptera: Ceratopogonidae) can spread the AHSV (P. S. Mellor, 1996). The clinical signs of AHS are usually classified into four forms (Robin, 2019). Because of the high mortality and the transboundary epidemic potential of AHSV, the World Organisation for Animal Health (OIE) classifies AHS as a listed notifiable disease (OIE, 2020).
AHSV has nine serotypes, all of which are endemic in sub-Saharan Africa, and they periodically invade Europe and Asia (Carpenter, Mellor, Fall, Garros, & Venter, 2017). AHSV serotype 9 (AHSV-9) has caused epizootics in the Asian region (Pakistan and India) and more than 300,000 equines died during 1959–1961 (P. G. Howell, 1960). Then, AHSV-4 caused outbreaks in the Spain, Portugal, and Morocco between 1987 and 1990 (Rodriguez, Hooghuis, & Castano, 1992). In February 2020, the first incidence of AHS was reported in Thailand; this was the first outbreak caused by AHSV-1 outside of sub-Saharan Africa. As of September 1, a total of 17 AHS outbreaks have been recorded with 610 equines infected and with 568 deaths. The total morbidity and mortality of the 17 outbreaks were 22.30% (610/2735) and 20.77% (568/2735), respectively, and the case fatality rate was as high as 93.11%. Recently, Malaysia reported the first AHS outbreak in August 2020. Therefore, the outbreak of AHS in Thailand may pose a major threat to Southeast Asia and even other Asian countries.
The epidemic area and seasonality of AHS occurrence are related to vector epidemiology (Robin, 2019). Culicoides biting midges are important transmission vectors of arboviral diseases worldwide; they transmit the agents that cause diseases in humans and domestic and wild animals. Culicoides imicola (C.imicola ) is the only confirmed field transmission vector of AHSV (P. S. Mellor & Boorman, 1995). C.imicola is present across most of the inhabited world, including Africa, southern Europe, and southern Asia (Guichard et al., 2014; Meiswinkel, 1989). The vector is present round the year, and these regions are also potential risk areas of AHS occurrence. Rawlings et al. (Rawlings, Pro, Pena, Ortega, & Capela, 1997) proposed that global climate change may cause C. imicola to expand northward. This will lead to a wider geographical distribution of the AHSV, thereby increasing the risk of exposure to diseases.
The maximum entropy (MaxEnt) model is a machine learning method, which is used to analyze with the presence-only point data (Phillips, Anderson, & Schapire, 2006). It has become the ecological niche modelling tool for species and epidemic disease distribution studies (Alkhamis, Hijmans, Al-Enezi, Martínez-López, & Perea, 2016; Gao & Cao, 2019; Liu et al., 2019). As the AHS vector, maps of C. imicola distribution have been developed for Senegal, Spain, and Portugal (Ciss et al., 2019; Peters et al., 2014; Ramilo, Nunes, Madeira, Boinas, & da Fonseca, 2017). However, there is limited research on the risk distribution for AHS and the suitable habitat forC.imicola in China. The aim of this study is to identify high risk areas for AHS occurrence in Mainland China and to assess the relationship between the AHS infection and environmental factors. The model could help stakeholders to make decisions for C.imicolamanagement and for the epidemiological control of AHS.