(Continued from the previous issue)
epidemiology
ASF has been endemic in 25 Sub-Saharan African countries, with different epidemiological characteristics and wetness. In Europe, ASF is endemic in Sardinia (Italy) and in some parts of Eastern Europe (Gogin et al., 2013). In June 2007, the virus was detected in the Caucasus after an outbreak in Georgia by ASF. The outbreak then spread to Armenia, Azerbaijan and the Russian Federation, reaching the border with Ukraine and near the Baltic and Barents Seas in northwestern Russia. Since 2011, ASF has spread to the north-west, reaching new regions of the Russian Federation (around Moscow), Ukraine (2012), Belarus (2013), Estonia (2014), Latvia (2014), Lithuania (2014), Poland (2014), Moldova (2016), Czech Republic (2017), Romania (2017), Hungary (2017), Bulgaria (2018), Belgium (2018). Since August 2018, ASF has rapidly expanded to many provinces and cities in China. The outbreak of ASFV in eastern Europe was caused by the invasion of single gene II, which is endemic in southeastern Africa, into Europe. Two genetic variants were found isolated from numerous Eastern European countries to African swine fever strains (Gallardo et al., 2014). The mortality rate caused by the outbreak gradually changed, and the number of surviving pigs with antibody-positive or atypical symptoms gradually increased. This phenomenon is at least somewhat associated with moderately virulent strains found in some parts of Europe (Gallardo et al., 2018).
The natural host of ASFV is a wild boar or a domestic pig. European wild boars are more susceptible to ASFV infection and present clinical symptoms and mortality rates similar to those of domestic pigs (McVicar et al., 1981; Sánchez‐Botija,1982).。 Instead, there are three species of African wild boar that can be negatively poisoned and become a reservoir of viruses: warthogs, giant forest pigs, and African bush boars (De Tray 1957).
In Africa, there is a more complex cyclical pattern of ASFV transmission, including African wild boar, soft tick and domestic pig. In the eastern and southern regions, virus transmission follows an ancient forest circulation pattern involving soft ticks and infected asymptomatic warthogs and jungle pigs. Two other modes of transmission have been found in endemic areas, one is domestic pig/tick transmission without the participation of warthogs, and the other is pig/pig transmission.
In Europe, transmission of healthy animals including domestic pigs and direct contact between wild boar and ticks is the most common route of transmission. AsFV is transmitted through mouth and nose in domestic pigs (Colgrove et al., 1969). Pigs have other forms of infection, such as tick bites (Plowright et al., 1969), skin wounds, and injection routes (intramuscular, subcutaneous, intraperitoneal, intravenous) (McVicar 1984). Natural infections of European wild boar have been found in the Iberian Peninsula, Sardinia and eastern Europe. Wild boars currently play an important role in assimilation and survival of ASFV in Eastern Europe, for reasons that are unclear (EFSA, 2015). In stark contrast, in Spain and Portugal, wild boars are not a major ASFV-carrying population and do not pose a major obstacle to virus eradication. The difference may be that wild boar density is much higher than in the past (Arias and S á nchez-vizca í no2002). Evidence from Sardinia suggests that if the disease is eradicated from domestic pigs in a certain area, the disease also disappears from wild boar populations (Addomada et al., 1994).
A variety of soft ticks are carriers and propagators of ASFV, including the African blunt-edge tick (Plowright et al., 1969) and the wandering blunt-edged tick of the Iberian Peninsula (S á nchez ‐ Botija, 1963). In the Iberian Peninsula there is also indirect transmission of organisms such as the wandering blunt-edge tick, especially outdoor pig production. The dangers of these virus carriers in Eastern Europe are unclear.
By comparing the replication of ASFV by soft ticks in Europe and Africa, an important difference in the epidemiology of the disease was found. In Africa, ASFV has been found to be transmitted through eggs or menstruation in the migratory blunt-edge tick (Plowright et al., 1970), and only menstrual transmission has been found in Europe. In Africa, the wandering blunt-edged soft tick is able to transmit ASFV to domestic pigs under experimental conditions (Mellor and Wilkinson, 1985), a phenomenon that has not been detected in field conditions. Many species of ticks, which are widespread in South and North America, can carry and transmit ASFV (Groocock et al., 1980). All species of blunt-edge ticks have been tested to be susceptible to ASFV (EFSA, 2010).
The incubation period of ASFV is 4-19 days, depending on its type and the route of transmission. Domestic pigs infected with wild strains detoxify the outside world during the incubation period, i.e., before clinical symptoms appear. After clinical manifestations are observed, ASFV is able to peak detoxification through secretions and excrement, including nasal discharge, saliva, feces, urine, conjunctival exudates, genital excrement, and blood flowing from wounds. At the same time, recovered pigs can maintain high antibody levels and prolonged viremia, and the virus can survive in tissues for weeks to months. Therefore, once the ASFV infected domestic pigs survive, the poisonous domestic pigs will become an important source of infection of the disease, so it is a key consideration in ASFV culling.
Infection with ASFV in African wild boars contains a lower titer of the virus in the tissues, and viremia is not apparent or detectable (Plowright, 1981). The host's genetic factors and immune response may be related to this low viral load, but it is unclear. This virus is enough to spread domestic pigs through soft ticks, but it does not cause mutual infection between pigs. This mode of transmission makes it very difficult to eradicate ASFV in Africa.
ASFV is relatively stable in the environment, able to remain infectious for more than 3 days in contaminated pig pens and maintain infective performance in pig manure for several weeks. ASFV is capable of surviving for 18 months in serum or blood preserved at room temperature and 15 weeks in decaying blood (EFSA, 2009). ASFV is capable of surviving in frozen or raw meat for weeks to months. In products that are cured, such as Parma ham, no infectious virus is found after 300 days of curing (McKercher et al., 1987). Spanish cured pork products, such as Serrano ham and elbows, are free of ASFV after 140 days, and ASFV is free of ASFV after 112 days of Iberian tenderloin (Mebus et al., 1993). No infectious ASFV has ever been found in cooked or canned ham made at 70°C (158°F). ASFV loses infectivity in boneless flesh, bone-in, and ground meat for 110 days, and in smoked boneless flesh for 30 days (Adkin et al., 2004).
ASFV is easily deactivated in lipid solvents, detergents, oxidants such as hypochlorite and phenol, as well as some commercial disinfectants that are time- and temperature-dependent. For example, ASFV is inactivated after 30 minutes of contact with 2.3% chlorine and 3% o-phenylphenol, or a synthetic iodine solution. Other effective agents that inactivate ASFV include formalin, sodium hydroxide, proprolactone, glyceraldehyde, and acetidimide (EFSA 2010). In short, soaps, detergents, and alkaline substances are very effective in disinfecting livestock houses, utensils, clothing, vehicles, and human dwellings. Disinfectants on aircraft are recommended for Vilkon®. Feed, drinking water and manure contaminated with the virus should be buried or burned. Pig manure contaminated with ASFV can be treated for 3 min at 4 °C (39 °F) with 1% sodium hydroxide or calcium hydroxide or 0.5% sodium hydroxide or calcium hydroxide for 30 min. Insecticides (organophospholipids and synthetic pyrethroids) are recommended for culling ticks.
(To be continued)
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