African swine fever (ASF) is a viral disease that only domestic pigs and wild boar are susceptible to. The disease occurs primarily in Africa, more specifically in the countries south of the Sahara. From 2007, outbreaks were observed in Georgia / Eastern Europe and from 2014 also in the European Union.
The African swine fever (ASF) virus is the sole member of the Asfarviridae family. The disease was first described in 1921, in Africa. It is the only DNA virus known that is able to infect arthropods (certain soft-bodied ticks of the genus Ornithodoros), as well as mammals.
Survival in the environment
The virus is able to survive for several days in the environment. However, this may be extended in the presence of protein (blood, meat) to weeks or months, and even a year. For example, the virus can remain infectious in dried hams (Serrano and similar) for up to 4-5 months. This may even rise to years in frozen meat.
The virus can be rendered inactive by heat treatment (at least 20 minutes at >60 °C, 70 minutes at >56 °C), pH <3.9 of >11.5 and responds well to most disinfectants.
The natural hosts for the virus are suids. This historically refers in particular to warthogs (Phacochoerus spp.), but also for example to bushpigs (Potamochoerus spp.) and to the giant forest hog (Hylochoerus meinertzhageni), all of which are endemic to Africa. Domestic pigs and the wild boar (Sus scrofa) are also susceptible to this virus.
Infection of Soft-Bodied Ticks
Certain ticks can also become infected with the virus. These are the soft-bodied ticks of the Ornithodoros genus. These ticks occur only in tropical and subtropical regions. For example, in Europe only around the Mediterranean Sea.
Harmless to Humans
Other animals are not susceptible under natural conditions. The virus is also completely harmless to humans.
African swine fever, as the name suggests, occurs primarily in Africa, more specifically in the countries south of the Sahara.
Between 1957 and the mid-1990s, the disease spread to Europe, and there were outbreaks in the 1970s in the Caribbean and Brazil. By the mid-1990s, the virus had been eradicated everywhere outside of Africa, with the exception of the Italian island of Sardinia.
Recorded in Europe again since 2007
In 2007 the virus turned up in Georgia, in the Caucasus region. From there it spread to surrounding countries, including Russia. Over subsequent years, the virus spread across virtually the whole of Western Russia up to the border with Finland. In 2012, an outbreak was recorded in Ukraine, followed by one in Belarus in 2013. In 2014, the first cases were observed in the European Union, in Lithuania and Poland. The virus then soon spread to Latvia and Estonia as well. In 2016, it spread to Moldavia, and in 2017 to the Czech Republic and Romania. In Poland, the virus moved to the Warsaw region. From 2018, outbreaks have also been registered in Hungary. In 2018 the virus made a big leap and hundreds of wild boars in Belgium became infected. In 2020, the first case of ASF was detected in wild boar in Germany and the number has been rising ever since.
Soft-bodied ticks, warthogs and the cycle in Africa
In Africa, the reservoir of the African swine fever virus is found in the wild. The warthog and the soft-bodied tick (mainly of the species Ornithodoros moubata) maintain the virus cycle there. Soft-bodied ticks feed on infected warthogs, propagate the virus and pass it on to another warthog at their next blood meal. As warthogs and ticks cohabit closely in warthog burrows, this is an efficient method of spreading the virus. Direct mutual contact between warthogs probably plays a more limited or even no role in spreading the virus.
Distribution among wild boar
Wild boar, as known in Eurasia, can also become infected with the African swine fever virus. Their role can certainly not be compared to that of the wild African suids. In wild boar, the disease is usually acute, and virtually all infected wild boar die. Over recent years we have seen the virus being found increasingly in wild boar, and it seems that under certain circumstances, the virus can continue to circulate among wild boar over the longer term or even become endemic.
Infection of pigs and wild boar may occur via direct contact between an infected animal and a susceptible (as yet uninfected) animal. This ensures distribution within an infected pigsty, for example. Contact between wild boars and pigs kept outside is also an example of this.
All outbreaks outside Africa probably started with feeding kitchen waste (swill) from for example ships or planes arriving from Africa.
The virus can easily end up in the food chain through the slaughter and processing of infected pigs. This is not unusual, particularly in small-scale pig-keeping, where people keep just one or a couple of pigs. The virus can survive for a long time in products of this kind. When these products are moved, the virus can be spread over large distances, with the result that it is extremely difficult to suppress. Products like dried sausage or salami can for example be a source of infection, if fed to pigs, taken to a pig farm or left in an environment where they can be reached by wild boars. The sources may be tourists, workers/employers or truck drivers on long routes, alternatively hunters carrying trophies or wild boar products.
Contact with Carcasses
Contact with carcasses or remains left from a hunter gutting a wild boar may result in further infections in wild boar. In addition, as stated above, food remains left in the environment can infect wild boar.
Contaminated materials, such as tools, boots and also stable bedding may cause infection in pigs. The quantitative risk of the virus ending up in pigs via this route is not entirely clear at present. Livestock transport vehicles in which infected pigs are being moved, also pose a risk.
Present long term in surviving pigs and wild boar
Pigs that survive an initial infection become carriers of the virus. They can carry the virus for several months. These pigs are much less infectious than pigs in the disease’s acute phase. Nevertheless, they may have an important role in the epidemiology, because they ensure that the virus can remain latent over the longer term.