An epizootic of African swine fever in Belgium and its eradication

Management of a Focal Introduction of ASF Virus in Wild Boar: The Belgian Experience

African swine fever (ASF) is a fatal disease of suids that was detected in wild boar in Belgium in September 2018. The measures implemented to stop the spread and eliminate the African swine fever virus consisted of creating restriction zones, organising efficient search and removal of carcasses, constructing wire fences, and depopulating wild boar in the area surrounding the infected zone. The ASF management zone included the infected and the white zones and covered 1106 km² from which 7077 wild boar have been removed. A total of 5338 wild boars have been qPCR-tested and 833 have been detected ASF-positive. The search effort amounted to 60,631 h with a main focus on the infected zone (88%). A total of 277 km of fences have been set up. The main cause of mortality in the infected zone was the virus itself, while hunting, trapping, and night shooting were used together to reduce the wild boar density in the surrounding white zones. After continuous dispersion of the virus until March 2019, the epidemic wave stopped, and the last fresh positive case was discovered in August 2019. Hence, Belgium was declared free of the disease in November 2020.

Semi-quantitative risk assessment by expert elicitation of potential introduction routes of African swine fever from wild reservoir to domestic pig industry and subsequent spread during the Belgian outbreak (2018-2019)

Since the introduction in Georgia in 2007 of an African swine fever (ASF) genotype 2 virus strain, the virus has rapidly spread to both Western European and Asian countries. It now constitutes a major threat for the global swine industry. The ongoing European transmission cycle has been related to the 'wild boar habitat' with closed transmission events between wild boar populations and incidental spillovers to commercial and non-commercial (backyard) pig holdings. During the epidemic in Belgium, only wild boar were infected and although the introduction route has not yet been elucidated, the 'human factor' is highly suspected. While ASF was successfully contained in a small region in the Southern part of Belgium without affecting domestic pigs, the risk of spillover at the wild/domestic interface remains poorly assessed. In this study, we used a semi-quantitative method, involving national and international experts, to assess the risk associated with different transmission routes for ASF introduction from wild boar to domestic pig holdings and subsequent dissemination between holdings in the Belgian epidemiological context. Qualitative responses obtained by our questionnaire were numerically transformed and statistically processed to provide a semi-quantitative assessment of the occurrence of the hazard and a ranking of all transmission routes. 'Farmer', 'bedding material', 'veterinarian' and 'professionals from the pig sector' were considered as the most important transmission routes for ASF introduction from the wild reservoir to pig holdings. 'Animal movements', 'farmer', 'veterinarian', 'iatrogenic', 'animal transport truck' and 'animal care equipment' were considered as the most important transmission routes posing a risk of ASF spread between pig holdings. Combined with specific biosecurity checks in the holdings, this assessment helps in prioritizing risk mitigation measures against ASF introduction and further spread in the domestic pig industry, particularly while the ASF situation in Western Europe is worsening.

African swine fever: A permanent threat to Indian pigs

India has 9 million pigs, of which 45% are in the North eastern (NE) states of India. Viral diseases affecting pigs are a major concern of mortality causing huge loss to the pig farmers. One such disease is African swine fever (ASF) that has already knocked the porous borders of NE states of India. ASF is a highly contagious devastating disease of pigs and wild boars causing 100% mortality. The causative agent African swine fever virus (ASFV) belongs to the genus Asfivirus, family Asfarviridae. Pig is the only species affected by this virus. Soft ticks (Ornithodoros genus) are shown to be reservoir and transmission vectors of ASFV. Transmission is very rapid and quickly engulfs the entire pig population. It is very difficult to differentiate classical swine fever from ASF since clinical symptoms overlap. Infected and in contact pigs should be culled immediately and buried deep, and sheds and premises be disinfected to control the disease. There is no vaccine available commercially. Since its first report in Kenya in 1921, the disease has been reported from the countries in Europe, Russian federation, China, and Myanmar. The disease is a threat to Indian pigs. OIE published the first report of ASF in India on May 21, 2020, wherein, a total of 3701 pigs died from 11 outbreaks (Morbidity - 38.45% and mortality - 33.89%) in Assam and Arunachal Pradesh states of India. ASF is non-zoonotic.

Clinical and Pathological Study of the First Outbreak Cases of African Swine Fever in Vietnam, 2019

African swine fever (ASF) is a devastating disease of swine and the most important disease for the pork industry. Since the outbreaks in 2007 in the Caucasian region, it has been spreading to the West and East quite swiftly. In this study we have analyzed the clinical signs and pathological features of the first outbreaks on ASF in Vietnam in 2019, caused by an isolate with 100% similarity to the genotype II (p72) isolates from Georgia in 2007 and China in 2018. The disease onset with a peracute to acute clinical course with high mortality. Some animals showed very unspecific clinical signs with other showing severe hyperthermia, respiratory distress, diarrhea, or vomit. Hemorrhagic splenomegaly and lymphadenitis were the main lesions observed at post mortem examination, with histopathological changes confirming the lymphoid depletion and multiorganic hemorrhages. Monocyte-macrophages were identified by means of immunohistochemical methods as the main target cell for the ASF virus in tissue sections.

African Swine Fever: Lessons to Learn From Past Eradication Experiences. A Systematic Review

Prevention, early detection, prompt reaction, and communication play a crucial role in African swine fever (ASF) control. Appropriate surveillance capable of early detection of the disease in both domestic and wild animals, and the implementation of consolidated contingency plans, are currently considered the best means of controlling this disease. The purpose of this study was to understand the lessons to be learned through the global disease eradication history. To establish which strategies were successful for prevention, control, and eradication of ASF, and which errors should not be repeated, we conducted a systematic review. A query was defined to search for surveillance and control strategies applied by countries worldwide for ASF eradication in the past. Inclusion and exclusion criteria were defined. Decisions on study eligibility and data extraction were performed by two independent reviewers and the differences were resolved by consensus or by a third reviewer. From 1,980 papers, 23 were selected and included in the qualitative analysis. Reports from Belgium, Brazil, Cuba, the Dominican Republic and Haiti, France, mainland Italy, Malta, Portugal, and Spain were included. Despite the economic resources allocated and the efforts made, eradication was possible in only eight countries, between the 50s and 90s in the twentieth century, in different epidemiological and cultural contexts, in some instances within <1 year, and in others in about 40 years. Classical surveillance strategies, such as active and passive surveillance, both at farm and slaughterhouse levels, targeted surveillance, together with conventional biosafety and sanitary measures, led to eradication even in countries in which the tick's epidemiological role was demonstrated. Historical surveillance data analysis indicated that eradication was possible even when technological tools either were not available or were used less than they are currently. This emphasizes that data on surveillance and on animal population are crucial for planning effective surveillance, and targeting proper control and intervention strategies. This paper demonstrates that some strategies applied in the past were effective; these could be implemented and improved to confront the current epidemiological wave. This offers encouragement for the efforts made particularly in Europe during the recent epidemics.

Comparative Pathology and Pathogenesis of African Swine Fever Infection in Swine

African Swine Fever (ASF) is a viral disease that affects animals of the Suidae family, and soft ticks from the genus Ornithodoros can also be infected by the ASF virus (ASFV). The disease was first described in Africa at the beginning of the twentieth century as an acute disease characterized by high mortality and fatal hemorrhages. ASF has caused outbreaks in numerous countries and it continues to be devastating nowadays for the porcine sector in those countries affected, and a massive threat for those free of the disease. ASF can follow clinical courses from peracute to chronic in domestic pigs (Sus scrofa) depending on a variety of factors, including the immune status of the animals and the virulence of the ASFV strain. The key features of the pathogenesis of the disease in domestic swine are a) a severe lymphoid depletion including lymphopenia and a state of immunodeficiency, and b) hemorrhages. However, African wild swine like bushpigs (Potamochoerus larvatus), red river hogs (Potamochoerus porcus), and warthogs (Phacochoerus africanus) can be infected by ASFV showing no clinical signs of disease and acting as natural reservoir hosts. In this article we review the key features of the gross and microscopic pathology together with a description of the pathogenesis of ASFV infection in domestic pigs following the different clinical courses. The pathogenesis of ASF in wild and domestic swine is also described, what can provide important information for the design of control strategies, such as vaccines.

African Swine Fever: Fast and Furious or Slow and Steady?

Since the introduction of African swine fever (ASF) into Georgia in 2007, the disease has been spreading in an unprecedented way. Many countries that are still free from the disease fear the emergence of ASF in their territory either in domestic pigs or in wild boar. In the past, ASF was often described as being a highly contagious disease with mortality often up to 100%. However, the belief that the disease might enter a naïve population and rapidly affect the entire susceptible population needs to be critically reviewed. The current ASF epidemic in wild boar, but also the course of ASF within outbreaks in domestic pig holdings, suggest a constant, but relatively slow spread. Moreover, the results of several experimental and field studies support the impression that the spread of ASF is not always fast. ASF spread and its speed depend on various factors concerning the host, the virus, and also the environment. Many of these factors and their effects are not fully understood. For this review, we collated published information regarding the spreading speed of ASF and the factors that are deemed to influence the speed of ASF spread and tried to clarify some issues and open questions in this respect.

Risk of African Swine Fever Virus Sylvatic Establishment and Spillover to Domestic Swine in the United States

African swine fever virus (ASFV) causes a high-consequence foreign animal disease that has emerged along international trade routes. Owing to high lethality and resulting trade sanctions, establishment of this disease in the United States would have devastating economic consequences. ASFV can be transmitted by soft ticks in the genus Ornithodoros or directly between swine, including domestic, feral, and wild swine. Consequently, the spatial risk of ASFV establishment depends on where susceptible animals, with or without competent vectors, co-occur. We synthesized county-level historical records of soft tick occurrence, current maps of feral swine distribution, and domestic swine inventory to evaluate the risk of ASFV establishment and spillover in the United States. Areas of California, Florida, and much of the southwestern United States were classified as high risk for ASFV establishment and spillover should an introduction event occur. Our analyses indicate that California, Texas, Georgia, and Florida are high-priority candidates for proactive risk reduction strategies. Domestic swine are often produced in high-biosecurity environments, mitigating health risks associated with contacting infected hosts and vectors. However, small-scale and organic pig producers in much of the southern United States remain more vulnerable to disease emergence.

African Swine Fever Status in Europe

African Swine Fever (ASF) is a highly contagious disease that affects the domestic pig and wild boar population. The aim of this study was to describe the introduction and spread of the ASF virus in Western Europe (1960⁻1995) and in Eastern Europe (2007⁻2018), with particular emphasis on the current ASF situation in Poland and its challenges and future perspectives. The first ASF outbreak in Europe was reported in Portugal in 1957, with the virus spreading over most of Western Europe over the next 30 years. In Eastern Europe, the virus was first observed in Georgia in 2007, from where the disease spread quickly to other neighboring countries, reaching Poland in 2014. Since then, there have been 3341 confirmed cases in the wild boar population in Poland. Although there have been no confirmed cases of wild boars coming into contact with domestic pigs, the first notified case concerning domestic pigs was reported in July 2014. Since then, there have been a total of 213 confirmed outbreaks of ASF on Polish pig farms. Given the virulence of the ASF virus and the myriad of transmission routes across Europe, the monitoring of this disease must be a priority for Europe.

Transboundary spread of pig diseases: the role of international trade and travel

As globalization increases the interconnectedness between nations, economies, and industries, the introduction of diseases will continue to remain a prominent threat to the livestock sector and the trade of animals and animal products, as well as the livelihoods of farmers, food security and public health. The global pig sector, with its size and dichotomy between production type and biosecurity level, is particularly vulnerable to the transmission of transboundary animal diseases such as African and classical swine fever, foot and mouth disease, or porcine reproductive and respiratory syndrome. All of the above pose a constant threat to swine health, mainly as a result of both formal and informal international trade.Inspired in the risk assessment methodology, this paper classifies and provides an overview of the different pig disease introduction and exposure pathways, illustrated with abundant examples. Introduction pathways are classified as formal international trade (by product), informal international trade (by product), and spread through fomites. Formal trade of pigs and pork products is regulated by legislation and measures protecting animal populations from exotic diseases. Much more difficult to control is the transboundary swine disease transmission originating through informal trade, which entails illegal smuggling, but also the informal cross-border transfer of animals and products for personal use or within informal market chains. Meat products are most commonly mentioned, although fomites have also played a role in some cases, with live pigs, being more difficult to smuggle playing a role less frequently. The main exposure pathways are also described with the oral route playing a prominent role.Risk assessments can aid in the identification of pathways of pathogen introduction and exposure. However, quantitative information on informal disease introduction pathways remains very scarce and often incomplete, making it difficult to estimate the actual magnitudes of risks. Nevertheless, this knowledge is deemed essential to set up risk based awareness, prevention and surveillance programs that correspond to reality.

African Swine Fever Virus Biology and Vaccine Approaches

African swine fever (ASF) is an acute and often fatal disease affecting domestic pigs and wild boar, with severe economic consequences for affected countries. ASF is endemic in sub-Saharan Africa and the island of Sardinia, Italy. Since 2007, the virus emerged in the republic of Georgia, and since then spread throughout the Caucasus region and Russia. Outbreaks have also been reported in Belarus, Ukraine, Lithuania, Latvia, Estonia, Romania, Moldova, Czech Republic, and Poland, threatening neighboring West European countries. The causative agent, the African swine fever virus (ASFV), is a large, enveloped, double-stranded DNA virus that enters the cell by macropinocytosis and a clathrin-dependent mechanism. African Swine Fever Virus is able to interfere with various cellular signaling pathways resulting in immunomodulation, thus making the development of an efficacious vaccine very challenging. Inactivated preparations of African Swine Fever Virus do not confer protection, and the role of antibodies in protection remains unclear. The use of live-attenuated vaccines, although rendering suitable levels of protection, presents difficulties due to safety and side effects in the vaccinated animals. Several African Swine Fever Virus proteins have been reported to induce neutralizing antibodies in immunized pigs, and vaccination strategies based on DNA vaccines and recombinant proteins have also been explored, however, without being very successful. The complexity of the virus particle and the ability of the virus to modulate host immune responses are most likely the reason for this failure. Furthermore, no permanent cell lines able to sustain productive virus infection by both virulent and naturally attenuated African Swine Fever Virus strains exist so far, thus impairing basic research and the commercial production of attenuated vaccine candidates.

Simulating the epidemiological and economic effects of an African swine fever epidemic in industrialized swine populations

African swine fever (ASF) is a notifiable infectious disease with a considerable impact on animal health and is currently one of the most important emerging diseases of domestic pigs. ASF was introduced into Georgia in 2007 and subsequently spread to the Russian Federation and several Eastern European countries. Consequently, there is a non-negligible risk of ASF spread towards Western Europe. Therefore it is important to develop tools to improve our understanding of the spread and control of ASF for contingency planning. A stochastic and dynamic spatial spread model (DTU-DADS) was adjusted to simulate the spread of ASF virus between domestic swine herds exemplified by the Danish swine population. ASF was simulated to spread via animal movement, low- or medium-risk contacts and local spread. Each epidemic was initiated in a randomly selected herd - either in a nucleus herd, a sow herd, a randomly selected herd or in multiple herds simultaneously. A sensitivity analysis was conducted on input parameters. Given the inputs and assumptions of the model, epidemics of ASF in Denmark are predicted to be small, affecting about 14 herds in the worst-case scenario. The duration of an epidemic is predicted to vary from 1 to 76days. Substantial economic damages are predicted, with median direct costs and export losses of €12 and €349 million, respectively, when epidemics were initiated in multiple herds. Each infectious herd resulted in 0 to 2 new infected herds varying from 0 to 5 new infected herds, depending on the index herd type.

The Assessment of African Swine Fever Virus Risk to Belgium Early 2014, using the Quick and Semiquantitative Pandora Screening Protocol

A risk assessment was organized during the early EU ASF outbreaks of early 2014 (February-April) and performed in cooperation with 15 Belgian and European experts on ASFV and its epidemiology in pigs/wild boar. African swine fever (ASF) is considered as one of the most dangerous infectious pig diseases, causing many outbreaks. Since the end of 2013 - early 2014, several outbreaks within the European Union (Lithuania, Poland, Estonia and Latvia) were reported to OIE, which prompted several risk assessments by (inter)national bodies and scientists. In this study, the open source, semiquantitative Pandora risk assessment tool was used for a quick overall screening of the risk posed by ASF to Belgium early 2014. A set of integrated risk scores was calculated within the Pandora framework. Experts scored the questions and uncertainty levels in the Pandora modules individually, after which the calculations were performed and averaged scores were used within pre-defined risk scales to define and visualize the ASF risk to Belgium. Emergence risk was considered low (Pandora score 0.29), while disease consequences were deemed high (0.93); the resulting multiplicative overall risk of ASFV for Belgium was low (0.27). The Belgian experts tended to give lower risk scores than the European experts, especially for entry risk and trade/public opinion consequences. These risk scores are further interpreted with a due consideration of the qualitative data in the expert remarks and of other ASF risk assessments. The results are similar to more extensive and elaborate risk assessment models/procedures which may require more time and resources. The Pandora tool allows sequential updates to monitor (rates of) increasing risk and provides information for risk managers to organize targeted control.

A congenital persistent infection of bovine virus diarrhoea virus in pigs: clinical, virological and immunological observations

We report on a lifelong 'carrier' state of non-cytopathic bovine virus diarrhoea virus (BVDV) in an otherwise healthy pig. Three out of 13 pigs of a litter congenitally infected with BVDV survived for more than 3 months. One pig was BVDV seropositive at 1 month, the second seroconverted between 6 and 8 months, and the third remained viraemic and BVDV-immunotolerant until slaughter at 26 months. The latter pig, a boar, excreted virus in oropharyngeal fluid, urine and semen. Ejaculates, however, did not contain spermatozoa. The persistently infected pig had leucopenia from 3 months onwards, though differential white cell counts and thrombocyte counts remained normal. The antibody response of this pig after vaccination against foot-and-mouth disease and after infection with porcine parvovirus was not affected. The antibody response after vaccination against hog cholera, however, was delayed. In contrast, superinfection with antigenically related cytopathic BVDV strains did not alter the status of BVDV immunotolerance, nor did it induce clinical signs resembling mucosal disease as observed in cattle persistently infected with BVDV. Although the role of pigs in the epizootiology of bovine virus diarrhoea infections may be limited, the existence of healthy BVDV carrier pigs should be noted wherever control or eradication measures are applied.

Identification of a variable region of the African swine fever virus genome that has undergone separate DNA rearrangements leading to expansion of minisatellite-like sequences

Nucleotide sequencing identified a tandemly repeated sequence array 22 x 10(3) base-pairs from the right-hand DNA terminus of the African swine fever virus (ASFV) genome. The sequence of the repeat array and sequences closely flanking it were compared in the genomes of four groups of ASFV isolates that had very different restriction enzyme site maps. Arrays present in one group of ASFV isolates from East Zambia/Malawi varied in length and contained between 8 and 38 copies of a 17-nucleotide repeat unit. Repeat arrays in a second group of ASFV isolates from Europe were less variable in length but consisted of different types of repeat unit that were divergent in sequence. A third genetically diverse ASFV isolate. LIV 13 from a South Zambia Game Park, contained repeat unit types that were similar to those of European viruses. MFUE6 isolate from an East Zambia Game Park contained a shorter version of the European repeat unit. An eight-base-pair core sequence was conserved between the East Zambia/Malawi and European and LIV 13 repeat units. These tandemly repeated sequence arrays share a number of properties with chromosomal minisatellite DNA. Similar tandem repeat arrays have not been described in poxviruses.