Geographical distribution of ASF outbreak in China
Outbreak analysis of geographical distribution showed that since August 3, 2018, the first ASF outbreak occurred in Shenyang in China, ASF continued spreading among domestic pigs and wild boars in country. As of August 3, 2019, almost geographical regions total of 31 provinces in China have reported ASF outbreaks except Hong Kong, Macao, and Taiwan (Figure 1A). Outbreak rates of ASF in each province are shown in Figure 1B and Table 1. Apparently, the Liaoning province has the largest cumulative ASF outbreaks, showing 13.9% of cases (21/151), followed by Guizhou with 6.6% (10/151), Anhui and Guangxi both with 5.9%(9/151), subsequently, Hunan, Hubei and Sichuan at 5.2%(8/151), and the other 24 provinces, this ratio is between 4.6-0.6%. Comparison the epidemic tendency among provinces showed that the spread of ASF outbreaks mainly located along the Northeast, Eastern China, and then reached Southern and Southwest, showing an increasing southward in general. Notably, Xinjiang, Tibet and Hainan, were confirmed ASF for the first time in April 2019. Nine months had elapsed since the first ASF outbreak had occurred in China in August 2018 (Appendix Table 1). In this regard, preliminary speculation is that Xinjiang, Tibet and Hainan, different from Eastern China and other areas, of geographical locations are far away from major economic transport routes and of population densities are at a relatively low level, slowing down the spread of ASF. In addition, Xinjiang and Tibet are the major residential areas of ethnic minorities[10]. The consumptions of beef and mutton occupy the major part of the market because of religious practices[11].Therefore, the impacts of ASF on these regions are relatively delayed.
The cumulative number of susceptible pigs in different provinces in China
Subsequently, further analysis of total cases among the 31 provinces in China displayed that Jiangsu province has the maximum cumulative number of susceptible pigs, in which there are 83 752 heads, followed by Heilongjiang province with 74 649 heads and Liaoning province with 52 981 heads. Generally, Jiangsu, Heilongjiang and Liaoning are the main livestock production provinces and important supply bases of animal products for the whole country. The stock of live pigs occupying the top spot increases the number of susceptible pigs. Meanwhile, Ningxia, Tibet and Qinghai have the minimum number of susceptible pigs, showing 179, 129 and 101 respectively (Figure 2A,Table2).Which of them are also the main residential areas of Hui nationals in China[10]. Thus, live pig stocks and pork consumption are keeping at relatively lower level because of religious practice , and then reducing the number of susceptible pigs[11]. In brief, the spread of the ASF epidemics are associated with anthropogenic factors, such as trade of animals, processed meat products and ethnic culture.
The incidence and fatality in different regions of China
This comprehensive analysis has highlighted that in one year period, Shandong province reported 17 infected pigs, substantially fewer than the 5123 infected cases reported in Heilongjiang province, followed by Jiangsu with 3079 cases and Liaoning with 2378 cases. Similarly, the provinces with the highest number of dead pigs are Heilongjiang (4158). Liaoning (2087) and Jiangsu (1466). The lowest number of dead pigs is only 3 in Shandong (Figure1 A, Figure 2B, Table 2). However, it is worth noting that the highest fatality rate of 97.6%(204/209) is in Jilin province, followed by Tibet with 91.7%(55/60). The previous provinces of Heilongjiang, Jiangsu and Liaoning, in which there are the maximum cumulative cases of infected and dead pigs, conversely, showing fatality rates with 81.2%(4128/5123), 47.6%(1466/3079), and 87.8%(2087/2378) respectively (Figure 2B, Table2). Meanwhile, as for incidence, Gansu province has the highest pig incidence rate of 62.5%(187/299), followed by Tibet and Qinghai with the incidence values are 46.5%(60/129) and 45.5%(46/101) respectively. These indicate that there is no consistent tendency between the incidence and fatality (Figure 2B, Table2). For this, it seems that the appropriate prevention and control measures applied in time are fully effective in protecting against ASFV incursion and decreasing the fatality rate.
Characteristics of ASF epidemic in suburban and rural areas
As shown in Figure 3A-3B, the pig numbers of susceptibility, incidence and death in rural areas exceed that of in suburban areas. This can partly be explained that most of commercial pig farms are mainly located in the urban suburbs, they can effectively control the ASF epidemics together with higher level biosecurity and various management measures[12, 13]. Meanwhile, the majority of backyard pig farms with either a low or very low biosecurity level are scattered, correspondingly, the management measures of various norms are relatively primitive resulting in a delayed response to the epidemic, which increases the pig numbers of susceptibility, incidence and death.
It is generally assumed that low level biosecurity farms are at higher risk of introduction the infections. Also, we note that the pig numbers of susceptibility, incidence and death showed an “explosive” increase during the December 2018-February 2019, reaching the peak in January 2019. Despite a small increase during September 2018-November 2018(Figure 3A-3B, Table3). This is likely the result to coincide with the Chinese New Year, which stimulated the nationwide trades of animals and processed meat products due to an increased demand for various pork products across the country, accelerating the incursion of ASFV and the frequency of ASF outbreaks. And subsequently, the pig numbers of susceptibility, incidence and death varied between different month together with a decreasing trend in general during the period March 2019-August 2019 (Figure 3A-3B, Table3), although there were marginal fluctuations within some months.