EPIZOOTIOLOGICAL SITUATION OF АFRICAN SWINE FEVER IN EUROPE

African swine fever (ASF) is a viral disease of domestic pigs and wild boar. Due to the very serious socioeconomic consequences, the disease is one of the most important ones nowadays. African swine fever is an enzootic disease in many countries in Sub-Saharan Africa, in Sardinia, and Trans Caucasus countries. Aft er its occurrence in Georgia in 2007, ASF spread to Armenia and Russian Federation, and in 2008. to Azerbaijan. Since then, its progressive moving toward the west has been recorded. Despite the number of undertaken preventive and control measures in the European Union (EU), ASF has been still spreading. During 2017, the disease has been reported in domestic pigs in Estonia, Italy-Sardinia, Latvia, Lithuania, Poland, Romania, and Ukraine. ASF cases in domestic pigs have also been reported in Moldova in 2017. Th e number of diagnosed cases in wild boar in 2017 is much higher than in domestic pigs. ASF outbreak in wild boar in the Czech Republic well describes the possible viral „jump“ into a new region. Th e source of infection hasn’t been confi rmed yet, but it is common that such leaps are due to either swill feeding or improperly disposal of food rather than to the animal movements. Since the lack of eff ective vaccine makes eradication even more diffi cult, the prevention of viral entry into the new areas is of the most importance. With the same aim, since 2011.the surveillance of ASF has been implemented in Serbia.


INTRODUCTION
African swine fever (ASF) is a viral disease of domestic pigs and wild boar. Due to the very serious socioeconomic consequences, the disease is one of the most important ones nowadays. Clinical symptoms vary depending on viral virulence and immune status of the host.
In European both domestic and wild pigs, ASF usually has the acute course with mortality 95-100% (Gallardo et al., 2015). Unlike in European species, in African wild suids, the disease is subclinical and asymptomatic enabling them to be the reservoir of the virus.
However, soft ticks from the genus Ornitodoros are considered as the natural host and also vectors and reservoirs of the virus (Plowright et al., 1969). Ornithodoros moubata ticks are the vectors in the east and South Africa and Indian Ocean islands where the disease was confi ned until 2007. Aft er 2007, despite the absence of competent vectors and reservoirs, the disease has been progressively spreading throughout Eurasia (Gallardo et al., 2015).
African swine fever was discovered in Kenya in 1909 aft er the European breeds of domestic pigs had been imported (Penrith et al., 2013). At that time, it was described as a haemorrhagic disease, causing almost total mortality of the domestic pigs. Aft er its discovery, it has been confi rmed that the causal agent had been circulating in the east and south Kenya since long ago in wild suids. Th ough later on the disease was reported in the other parts of Africa, geographically it was confi ned to Sub-Saharan region.
ASF occurred in Portugal in 1957, for the fi rst time out of Africa, incurring from the west of Africa. Aft er the two years of silence, it re-occurred in Portugal from where it has rapidly spread to the rest of Iberian Peninsula and other European countries -France (1964), Italy (1967, 1969, and 1983), Malta (1978), Belgium (1985) and the Netherlands (1986). During this period, ASF has been recorded also in central and south America -Cuba (1971, 1980, Brazil (1978), Dominican Republic (1978) and Haiti (1979). All those countries eradicated ASF, with the exception of Sardinia.

AFRICAN SWINE FEVER VIRUS
African swine fever virus is the only member of the family Asfaviridae and genus Asfi virus. It is a very complex DNA virus with 4 layers envelope (Dixon et al., 2005). Th e genome, 170-193 kbp long, possess 151-167 open reading frames (ORF) and codes 54 proteins and 100 polypeptides (Dixon et al., 2013). Th e capsid protein (p72) and two structural proteins, p30 and p54, are the most important ones. As the polyprotein pp62 is immunodominant, the antibodies generated aft er the natural infection are directed against it (Pastor et al., 1989). As of yet, 22 genotypes and 8 serogroups have been described (Gallardo et al., 2011). ASFV is very resistant to inactivation. However, many disinfectants such as lipid solvents, phenol, and iodide inactivate the virus at pH lower than 4 and higher than 11. Th e virus survives several weeks in the frozen or fresh meat as well as in the meat products. Th e temperature above 70 °C inactivates the virus (Sánchez-Vizcaínoa et al., 2009).

GEOGRAPHIC DISTRIBUTION
African swine fever is an enzootic disease in many countries in Sub-Saharan Africa, in Sardinia and Trans Caucasus countries. Th e virus in Africa circulates between wild suids and soft ticks. Th e disease is inapparent in the warthog, bush pig, and red river hog; the viremia in those species is at very low or even undetectable level but it does enable the infection of ticks (Jori and Bastos, 2009). In Europe, ASF is enzootic in Sardinia in wild boar population despite no soft ticks. Unlike in the past, today the viral circulation is being accomplished through 4 cycles -sylvatic cycle between ticks and wild boars, cycle between ticks and domestic pigs, viral circulation between domestic pigs and viral circulation between wild boars (Pietschmann et al., 2016). African swine fever occurred in 2007 in Georgia and aft erward in other Trans Caucasus countries and Russian Federation as well, having destroyable eff ects on swine production (Rowlands et al., 2008). Th e incursion of ASF in this region has been linked to the international overseas transport between Africa and Georgia and swill feeding of pigs in the area surrounding the ports. Aft er its occurrence in Georgia in 2007, ASF spread to Armenia and Russian Federation, and in 2008 to Azerbaijan. Since then the disease has progressively been moving toward the west: Ukraine

PATHOGENESIS
ASFV infection of ticks in Africa is characterized by low infectious dose, lifelong infection and low mortality rate until the fi rst laying of eggs. Unlike them, Ornithodoros ticks from the central and South America and Caribbean region show relatively high mortality of nymphs while the infection is not lifelong (Kleiboeker and Scoles, 2001). Ornithodoros ticks live up to 20 years enabling continuous viral maintenance in nature and in traditional pig keeping systems (Kleiboeker and Scoles, 2001). However, ticks have no role in long-distance viral spreading but they do link the sylvatic ASF with the domestic pigs in Africa.
Th e most common infection routes in pigs are nasal and oral, with the exception of the dermal route in Africa via infected ticks. Once the disease has been established in a new area, the virus is usually transmitted through direct contact. Carriers, also, signifi cantly contribute to the indirect routes of infection -via vehicles, rodents, equipment, people etc. Usually, incubation takes 4-19 days (Gallardo et al., 2015). Viral excretion in all excretes starts 2 days before the onset of the clinical symptoms and it can last up to 70 days. Th e extremely high viral titer is being found in the blood. Th e acute disease, clinically manifested, terminates within 4-5 days (Gallardo et al., 2015). Peracute and acute courses, with the high lethality, are common for the beginning of the epizootic. With the disease progression, more subacutely and chronically diseased animals are being found. At the same time, the viral virulence and mortality are decreasing. Infected pigs, usually, live several weeks but some of them can survive. Having the subclinical infection, they can live for a longer period of time. Survivals, persistently infected, with no clinical symptoms, play very important role in the disease maintenance in the enzootic regions and sporadic outbreaks in the new areas, as well.

CURRENT EPIZOOTIC SITUATION
Countries with the intensive pig production are the most vulnerable ones. Pigs, due to the fast growth, effi cient feed conversion, and the fast turnover became the most important protein source for the human population. Th e majority of pig production is located in China, south-east Asia, west Europe, central and eastern USA, Americas, and south Brazil. However, there is still a deep gap between traditional and industrial pig production. African pig production is concentrated in Sub Saharan Africa, mostly in small family farms.
Th ough ASF had been existing in the majority of African countries, since 1995. a signifi cant increase of outbreaks in Sub Saharan Africa and spreading into the free countries Madagascar and Mauritius were reported (Beltrán-Alcrudo et al, 2017). Th ose expansion of the disease along with the low awareness were the key factors for the viral incursion in the countries out of Africa in 2007. Th e transmission of ASF in Africa is very complex, depends on reservoirs, ticks, domestic pigs, the breeding system and human habits.
Since the fi rst occurrence in 2007, ASF turned into large-scale epizootic in Europe and the part of Asia.
Beside of that, there are two distinguishable enzootic zones in Russia Federation, in the central and in the southern part of the country (Gogin et al., 2013). Th ere is, also, the evidence of survival wild boars. Such animals have no manifested clinical symptoms but could develop carrier status, enabling viral maintenance and spreading. In those regions, along with the appearance of sub clinically diseased wild boars, the mortality has been decreasing due to the acquired immunity, low infectious dose, viral adaptation to the new host and/ or positive selection of lower virulent strains which normally appear aft er long time circulation in one population. Non-EU countries combat with the ASF trying hard to stop it but the applied measures are apparently not effi cient and do not prevent the viral spreading. Th e trade chains of cheap swine products, originating from the infected regions, are recognized as the major transmission routes of the ASF (Beltrán-Alcrudo et al., 2017). Th e left overs of such products, usually improperly disposed or given to the pigs in swill, are the link between ASF and domestic pigs.
Almost since the fi rst case of ASF in 2007, the preventive measures and surveillance have been implemented in EU (Gallardo et al., 2015). Along with the disease progression, the crisis plans have been updated in order to provide as earliest as possible disease detection. Currently applied preventive measures include regionalisation according to the World Trade Organisation, disinfection of vehicles at borders, strict controls at borders, ban of fairs, strict application of biosecurity measures, awareness campaigns, increase of number of tested animals, protective zones establishment, decrease of wild boar population, establishment of fruitful communication between the fi eld and laboratories, promotion of necessary epizootic investigations etc. Nevertheless, it has been shown that the disease is possible to be put under control, like it has been since 2 decades in Sardinia, with no excursion out from the island.
Unlike in Russian Federation where ASF is mostly found in domestic pigs, wild boars are more aff ected in EU. Th ough it is assumed that ASF in EU has been spreading locally and independently in each, wild boar and domestic pigs, all cases in the domestic swine population were registered in the areas inhabited with both domestic and wild swine despite no contacts between them. Th erefore, low biosecurity and swill feeding are considered as the most important routes of the viral introduction into the pig farms. ASF in domestic pigs occurred in EU in 2014 in Lithuania (Gallardo et al., 2015). It has been shown that for the effi cient disease eradication, the early detection was a crucial factor. At the beginning of the epizootic in Lithuania, only sudden deaths were reported. Th erefore, in high-risk areas, in the case of sudden deaths, ASF needs to be excluded. Viral transmission within the aff ected farm was further directed according to the type of the farm and applied biosecurity measures.
Apart from the sudden deaths, other clinical symptoms must not be overlooked, in particular, fever even in a small number of animals. In EU, considering intensive and modern pig production, the special concern has been put on wild boar in regard to viral maintenance and the spread. However, their exact role has not been fully described so far. In Caucasus and Russian Federation, inhabited by the low dense wild boar populations, ASF was not persisting for a long time despite continuous viral introductions from the domestic pigs. With the disease progression toward the west and the high dense wild boar populations such as in Poland and Baltic countries, wild boars become more important particularly because of continuous disease occurrence (Beltrán-Alcrudo et al, 2017).
Th e majority of cases have been discovered during the summer time. Even in cold climates where the temperature during the winter stays below 0 °C, ASF is usually found in the warmer periods aft er the melting of infected cadavers. Still, there are two peaks of ASF in wild boars -winter and summer one. By using the spatiotemporal analytical methods, it has been determined that ASF spreads 2 km by month in Latvia and Estonia and 1 km monthly in Lithuania and Poland (Abrahantes et al., 2017).
By using the statistic models, it has been shown that both the virus and antibody prevalence have been increasing since 2014 in hunted wild boars in Estonia and Latvia, with the maximum in the winter time. Th e largest number of dead animals has been found during the summer due to the specifi c biology of wild boar. Th e virus prevalence in hunted wild boars is at low level -0.5-3%. In found dead wild boars, it is 60-80% in Estonia, Latvia, and Lithuania but only 0.04-1.42% in Poland (Abrahantes et al., 2017). Since the start of the epizootic, the virus prevalence is higher than the seroprevalence in hunted animals. Th e linkage between ASF occurrence and the factors considered as risky, such as the number of populated places, human population size, number of domestic pigs and farms, roads net, forestation, wild boars habitats, has been investigated. It has been shown that the strongest link exists between ASF occurrence and the size of human population. Th e wild boar population density was considered as an important factor only in Estonia in the period 2014-2016 (Abrahantes et al., 2017). Despite many eff orts and resources put into active surveillance and early detection of ASF, it has been shown that the passive surveillance was more effi cient. All primary cases were discovered within passive surveillance.
Even though carriers in wild boar have not been found so far in EU, there are diff erent ways which enable a long-term viral circulation. Th ey are usually human-mediated: illegal trade of animals and animal products, low biosecurity level, and wild boars feeding (Abrahantes et al., 2017). Beside of those, cadavers of infected animals serve also as a source of the infection (Probst et al., 2017).
Th e most probably, ASF spreads between subpopulations of wild boars by contact with the infectious materials (blood, cadaver, excretes). However, there is moderate to high likelihood that the direct contact between wild boars is crucial for the viral transmission, in particular where the feeders are in place. Moderate to high likelihood is applied also in case of fi nding improperly disposal of food left overs (EFSA, 2015). Considering the consequences of ASF, diff erent strategies to stop its spread have been implemented. One of them is the reduction of wild boar population. But, as every human intervention in nature, this one also produced the certain consequences. Intensive and oft en hunt during the depopulation campaigns lead to the wild boar dispersion and the disease spread. Reduction of a population for the more than 60% drives to the wild boar adaptation, the compensatory growth of the number of animals, and intensive movements out of the hunting area (EFSA, 2015).Th e feeding of wild boars prevents their dispersion but leads, due to their grouping, to the pathogens exchange. More eff ective measure for the control of wild boar population is the ban of feeding along with intensifi ed hunting during the several consecutive years, aiming to reduce the number of females of all categories.
Th erefore, there are two recommended types of managing wild boar populations: 1. Fast control measures meaning depopulation (killing over 70% of wild boars) or rapid disposal of carcasses, 2. Long-term measures meaning ban of feeding and hunting of females (EFSA, 2015).
However, despite all undertaken preventive and control measures, ASF has been still spreading. During 2017 it has been reported in domestic pigs in Estonia (3 cases), Italy -Sardinia (17 cases), Latvia (8 cases), Lithuania (30 cases), Poland (80 cases), Romania (2 cases) and Ukraine (110 cases). ASF in domestic pigs has also been registered in Moldova in 2017.Number of cases in wild boars is even higher -Czech Republic (115 cases), Estonia (542), Italy -Sardinia (28 cases), Latvia (749 cases), Lithuania (794 cases), Poland (398 cases), Ukraine (17 cases) 1 . AFS outbreak in the Czech Republic represents a huge "jump" of the virus into the new region. Th ough the source of the infection has not been documented yet, such events usually happen because of swill feeding or improper disposal of food left overs. Two cases of ASF in domestic pigs in Romania, also, prove this.
Since the eradication of ASF is even more complex due to no vaccine availability, prevention of viral introduction is the most important. Having this as an aim, since 2011 there are implemented active and passive surveillance of ASF in Serbia (19). Considering both small wild boar population in Serbia and traditional pig keeping despite the absence of competent tick vector species (20), ASF would produce destructive consequences for small producers. However , along with the surveillance and having learned lessons from the other countries, additional eff orts are to be put into awareness and education of farmers, producers, and population in general.