ASFV is a large, icosahedral, double-stranded DNA virus with a linear genome of 189 kilobases containing more than 180 genes. The number of genes differs slightly among different isolates of the virus. ASFV has similarities to the other large DNA viruses, e.g., poxvirus, iridovirus, and mimivirus. In common with other viral hemorrhagic fevers, the main target cells for replication are those of monocyte, macrophage lineage. Entry of the virus into the host cell is receptor-mediated, but the precise mechanism of endocytosis is presently unclear. Based on sequence variation in the C-terminal region of the B646L gene encoding the major capsid protein p72, 22 ASFV genotypes (I–XXIII) have been identified. All ASFV p72 genotypes have been circulating in eastern and southern Africa.
Genotype I has been circulating in Europe, South America, the Caribbean, and western Africa. Genotype VIII is confined to four East African countries. After the re-introduction of ASF Genotype II isolates into Georgia in 2007, the disease spread from Eastern to Western Europe and then jumped first up to Mongolian borders and later into China in August 2018, spreading out of control and reaching different countries of Southeast Asia.
ASF epidemiology is complex with different epidemiological patterns of infection occurring in Africa, Europe and Asia. ASF occurs through transmission cycles involving domestic pigs, wild boars, wild African suids, and soft ticks
- All varieties of Sus scrofa (domestic and wild) are susceptible to the pathogenic effects of ASFV
- African wild suid species: warthogs (Phacochoerus spp.), bush pigs (Potamochoerus spp.), giant forest hogs (Hylochoerus meinertzhageni) are usually inapparently infected and act as reservoir hosts of ASFV
- Ticks of the genus Ornithodoros are the only known natural arthropod hosts of the virus and act as reservoirs and biological vectors
- Direct transmission:
- contact between sick and healthy animals
- Indirect transmission:
- feeding on garbage containing infected meat (ASFV can remain infectious for 3– 6 months in uncooked pork products)
- biological vectors – soft ticks of the genus Ornithodoros
- fomites include, premises, vehicles, implements, clothes
- Within tick vector: transstadial, transovarial, and sexual transmission occur
Sources of virus
- Blood, tissues, secretions and excretions of sick and dead animals
- Animals which have recovered from either acute or chronic infections may become persistently infected, acting as virus carriers; especially in African wild swine, and in domestic pigs and wild boar in endemic areas
- Soft ticks of the genus Ornithodoros
ASF is present in wild or domestic pigs in regions of Asia, Europe and Africa.
ASF is generally characterized by the sudden death of pigs. All ages and both genders may be affected. Animals segregated from the rest of the herd, for example sows with young suckling piglets, may be spared because of the rather low contagiousness of ASF.
The spread of the disease within the herd (and numbers affected) may vary greatly from a few days to several weeks, depending on the type of pig production, management, and biosecurity measures. In fact, ASF, although highly lethal, is less infectious than some other transboundary animal diseases such as foot-and-mouth disease. Also, some indigenous pig breeds in Africa have developed some degree of tolerance to ASF. Wild boar, being the same species as domestic pigs, show the same clinical presentation.
Clinical signs associated with ASFV infection are highly variable (see Table) depending on various factors: virus virulence, swine breed affected, route of exposure, infectious dose, and endemicity status in the area. According to their virulence, ASFVs are classified in three main groups: high virulence isolates, moderate virulence isolates, and low virulence isolates (see Figure). The clinical forms of ASF range from peracute (very acute) to asymptomatic (unapparent). As shown in Figure, highly virulent ASFV isolates produce peracute and acute disease, moderately virulent isolates produce acute and subacute forms of disease. Low virulence isolates have been described in endemic areas (in addition to the virulent viruses circulating) showing milder symptoms, and sometimes associated with subclinical or chronic ASF. Morbidity (i.e. the proportion of animals affected) will depend on the virus isolate and the route of exposure. Although not precisely known, the incubation period in natural infections has been reported to vary from 4 to 19 days. Clinical courses of the disease range from less than seven days post-infection in acute forms, to several weeks, or even months, in chronic forms. The lethality rate depends on the virulence of the isolate, ranging from 100 percent characteristic of highly virulent strains, where pigs of all ages are affected, to less than 20 percent lethality in chronic forms. In the latter the disease may be fatal mostly in pregnant and young animals, and pigs suffering from a concurrent disease, or weakened for other reasons. The survival rate to highly virulent strains observed in some endemic areas may be higher owing to adaptation of the pigs to the virus.
Since there is no vaccine available, rapid and reliable early detection of the disease is essential for the implementation of strict sanitary and biosecurity control measures to prevent the spread of the disease. Diagnosis of ASF means the identification of animals that are, or have previously been, infected with ASFV. An appropriate diagnosis therefore involves the detection and identification of ASFV-specific antigens, or DNA and antibodies, to obtain relevant information to support control and eradication programmes. It is important to consider the course of the disease when choosing the diagnostic test (see Figure). Since each animal could be at a different stage of the disease, both virus and antibody detection tests should be carried out in outbreaks and control/eradication programmes.
The incubation period in natural infections has been reported as varying from 4 to 19 days. About two days before clinical signs develop, ASF-infected animals begin to shed large amounts of the virus. Virus shedding can vary depending on the virulence of the ASFV strain involved. Seroconversion occurs at about 7-9 days post-infection and antibodies can be detected for the rest of the animal’s life (see Figure).
A positive test for the presence of the virus (i.e. antigen) indicates that the tested animal was undergoing infection at the time of sampling. On the other hand, a positive ASFV antibody test indicates an ongoing or past infection, where the animals have recovered (and may remain seropositive for life).
DETECTION OF ASF VIRUS
ASFV genome detection by polymerase chain reaction (PCR) Polymerase chain reaction (PCR) is used to detect the ASFV genome in porcine samples (blood, organs, etc.) and ticks. Small fragments of viral DNA are amplified by PCR to detectable quantities. All validated PCR tests allow viral DNA detection even before the appearance of clinical signs. PCR enables the diagnosis of ASF to be made within hours of sample arrival to the laboratory. PCR provides a sensitive, specific, and rapid alternative to virus isolation for the detection of ASFV. PCR provides higher sensitivity and specificity than alternative methods for antigen detection, such as the antigen enzyme-linked immunosorbent assay (ELISA) and the direct fluorescent antibody test (FAT). However, the extreme sensitivity of the PCR makes it susceptible to cross-contamination, and proper precautionary measures should be taken to minimize and control this risk.
Conventional and real-time PCRs recommended by the OIE in the Manual of Diagnostic Tests and Vaccines for Terrestrial Animals (2016) have been fully validated over time and are useful tools for routine diagnosis of the disease. Other real-time PCR procedures have proved to provide higher sensitivity than OIE-prescribed, real-time PCR methods for ASFV genome detection in recovered animals. Primer sets and probes used in these molecular techniques are repeatedly designed within the VP72 coding region, a well-characterized and highly conserved region of the ASFV genome. A wide range of isolates belonging to all the 22 known p72 virus genotypes can be detected with these PCR assays, even in inactivated or degraded samples.
PCR is the tool of choice in the case of peracute, acute, or subacute ASF infections. Furthermore, since PCR detects the viral genome, it may be positive even when no infectious virus is detected by virus isolation, making it a very useful tool for the detection of ASFV DNA in pigs infected with low- or moderately virulent strains. Although PCR is not informative about the infectivity of the virus, it can provide quantitative information.
ASF was first described almost a century ago, controlling the disease has proven to be a challenge, in particular because no vaccine is available. Following introduction to ASFV-free countries, the only control measures available are strict quarantine and biosecurity, animal movement restrictions and slaughtering affected/exposed animals.
The high lethality of ASFV in domestic pigs, the introduction of massive culling campaigns and pig movement restrictions all contribute to the high socio-economic impact of ASF on pig production, global trade and people’s livelihoods. The impact is often greatest for resource poor livestock farmers in developing countries, who rely on pigs as an additional source of income and a relatively cheap source of protein.
In Eastern Europe and the Russian Federation, the greatest numbers of outbreaks in domestic pigs have been on small scale or backyard farms, which generally have lower biosecurity standards and, in some regions, have closer contact with infected wild boar. From 2014 to August 2017, the highest numbers of farms with ASF outbreaks were in Russia and the Ukraine, and relatively few notifications of ASF were in wild boar in these countries. From 2014 to 2016, most detections in Poland were in wild boar, but in 2017 there were more notifications in domestic pigs on commercial farms, with more than 90 farms infected. In contrast, in the Baltic States, relatively few farms have been affected (approximately 60 in total) and most notifications have been in wild boar. Relatively few large farms have been infected, but such cases result in the death or destruction of substantially larger numbers of pigs. From 2014 to 2017, almost 800,000 pigs have died or been destroyed as a result of ASF in Eastern Europe and the Russian Federation. ASFV infection in wild boar can have an impact on the hunting industry, but perhaps the greatest impact is the threat and economic impact on commercial pig farms.
It is difficult to produce overall figures on the economic costs of ASF and thus estimates can vary substantially. As a result of outbreaks of ASF in 2014 and 2015 in Poland, Lithuania, Latvia and Estonia, the value of exports of pork and pork products was reduced by US$961 million, representing up to 50% of exports. Introduction of ASFV into Denmark could result in losses of US$12 million in direct costs and US$349 million in exports (Halasa et al., 2016). In Russia, ASF was estimated to have cost US$267 million in 2011. The recent outbreak of African swine fever threatens China’s 128 billion pork industry. Rabobank estimates that China could lose up to 200 million pigs (25% of world pig population).