The Evolution of African Swine Fever in China: A Global Threat?

Abstract

African swine fever (ASF) is one of the most critical diseases in the pig industry. In Asia, 15 countries have already reported an outbreak as of November 22, 2021. In 2021, China reported the genotype II lower virulent ASF virus (ASFV) and the emergence of genotype I ASFV. ASF is generally known as a contagious and lethal disease, but if chronic infection spreads, then disease control would be more difficult. In the current study, we highlighted the possibility of lower virulent virus distribution throughout China and the subsequent general risk of the virus being released from the country. The kernel density estimation showed that the two highest kernel density areas of ASF notification were located in Northeast and Midwest China. Four of the five provinces where lower virulent ASFV was isolated overlapped with areas of relatively high ASF notification density. In terms of the risk of ASFV spreading from China, eight of the 10 largest airports and three of the 10 largest seaports are located in areas of relatively high ASF notification density. There were flight flow from China to 67 countries and ship flow to 81 countries. Asia had the highest flight flow, followed by Europe, North America, Africa, and Oceania. The highest number of ship flows was also concentrated in Asia, but about 10% of ships head to Africa and South America. Chinese overseas residents were distributed in each continent in proportion to these results. Here, we highlight the potential risk of ASFV spread from China to the world.

Keywords: African swine fever; Asia; China; infectious diseases; lower virulent ASFV; risk assessment; spatio-temporal epidemic modeling; veterinary epidemiology.

Figure 1

Temporal trend of African swine fever (ASF) in China. The number of ASF outbreaks every 3 months, including the details of the event sources, is shown in a bar graph. These information sources are based on official reports to the OIE.

Figure 2

Spatial distribution of ASF in China. Each ASF notification was classified by event source; maps (A) and (B) show the situation of outbreaks in 2018–2019 and in 2020–2021, respectively. These information sources are based on official reports to the OIE.

Figure 3

Density of ASF notifications in China overlapped with the lower virulent ASFV reported area and China's 10 largest airports and seaports. The graduated color shades illustrate the estimated kernel density of ASF notifications (notifications/km²). Each colored area indicates frequency of ASF notification number per >4.61 × 10⁻⁵ (very high), 3.11 × 10⁻⁵ to 4.6 × 10⁻⁵ (high), 1.81 × 10⁻⁵ to 3.1 × 10⁻⁵ (medium), 6.9 × 10⁻⁶ to 1.8 × 10⁻⁵ (low), and <6.9 × 10⁻⁶ (very low). Provinces where ASFV genotype I and CD2v (–) ASFV genotype II were isolated are represented by a diagonal line and a grid line, respectively. The locations of China's 10 largest airports and seaports are represented by airplane and pin symbols, respectively.

Figure 4

The flight and ship flows connected to China combined with the distribution of Chinese overseas residents. The flight flow and ship flow from China are depicted above and below, respectively. The graduated color and width in the map represent the total traffic volume from the highest (darker/thicker) to the lowest (lighter/narrower). Each line indicates total traffic volume of >1.81 × 10⁷ (very high), 8.91 × 10⁶ to 1.8 × 10⁷ (high), 2.91 × 10⁶ to 8.9 × 10⁶ (medium), 9.51 × 10⁵ to 2.9 × 10⁶ (low), and <9.5 × 10⁵ (very low) seats in flight flow. For ship flow, each line indicates total traffic volume of >7.51 × 10⁶ (very high), 3.61 × 10⁶ to 7.5 × 10⁶ (high), 2.21 × 10⁶ to 3.6 × 1 0⁶ (medium), 7.51 × 10⁵ to 2.2 × 10⁶ (low), and <7.5 × 10⁵ (very low) per quarterly deployed capacity (TEU). In each map, the graduated color represents the number of Chinese overseas residents from the highest (darker) to the lowest (lighter).