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.