African swine fever virus
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|African swine fever virus|
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African swine fever virus (ASFV) is the causative agent of African swine fever (ASF). The virus causes a haemorrhagic fever with high mortality rates in pigs, but persistently infects its natural hosts, warthogs, bushpigs, and soft ticks of the Ornithodoros genus, with no disease signs.
ASFV is a large, double-stranded DNA virus which replicates in the cytoplasm of infected cells, and is the only member of the Asfarviridae family. ASFV infects domestic pigs, warthogs and bushpigs, as well as soft ticks (Ornithodoros), which likely act as a vector.
ASFV is the only virus with a double-stranded DNA genome transmitted by arthropods. The virus causes a lethal haemorraghic disease in domestic pigs. Some isolates can cause death of animals within as quickly as a week after infection. In all other species, the virus causes no obvious disease. ASFV is endemic to sub-Saharan Africa and exists in the wild through a cycle of infection between ticks and wild pigs, bushpigs, and warthogs. The disease was first described after European settlers brought pigs into areas endemic with ASFV and, as such, is an example of an 'emerging infection'.
Signs and symptoms
In the acute form of the disease caused by highly virulent strains, pigs may develop a high fever, but show no other noticeable symptoms for the first few days. They then gradually lose their appetites and become depressed. In white-skinned pigs, the extremities turn blueish-purple and hemorrhages become apparent on the ears and abdomen. Groups of infected pigs lie huddled together shivering, breathing abnormally, and sometimes coughing. If forced to stand, they appear unsteady on their legs. Within a few days of infection, they enter a comatose state and then die. In pregnant sows, spontaneous abortions occur. In milder infections, affected pigs lose weight, becoming thin, and develop signs of pneumonia, skin ulcers, and swollen joints.
The clinical symptoms of ASFV infection are very similar to classical swine fever virus, and the two diseases normally have to be distinguished by laboratory diagnosis. This diagnosis is usually performed by an ELISA or isolation of the virus from either the blood, lymph nodes, spleen, or serum of an infected pig.
ASFV is a large, icosahedral, double-stranded DNA virus with a linear genome containing at least 150 genes. The number of genes differs slightly between 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 haemorrhagic 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.
The virus encodes enzymes required for replication and transcription of its genome, including elements of a base excision repair system, structural proteins, and many proteins that are not essential for replication in cells, but instead have roles in virus survival and transmission in its hosts. Virus replication takes place in perinuclear factory areas. It is a highly orchestrated process with a least four stages of transcription - immediate-early, early, intermediate, and late. The majority of replication and assembly occurs in discrete, perinuclear regions of the cell called virus factories, and finally progeny virions are transported to the plasma membrane along microtubules where they bud out or are propelled away along actin projections to infect new cells. As the virus progresses through its lifecycle, most if not all of the host cell's organelles are modified, adapted, or in some cases destroyed.
Assembly of the icosahedral capsid occurs on modified membranes from the endoplasmic reticulum. Products from proteolytically processed polyproteins form the core shell between the internal membrane and the nucleoprotein core. An additional outer membrane is gained as particles bud from the plasma membrane. The virus encodes proteins that inhibit signalling pathways in infected macrophages and thus modulate transcriptional activation of immune response genes. In addition, the virus encodes proteins which inhibit apoptosis of infected cells to facilitate production of progeny virions. Viral membrane proteins with similarity to cellular adhesion proteins modulate interaction of virus-infected cells and extracellular virions with host components.
Based on sequence variation in the C-terminal region of the B646L gene encoding the major capsid protein p72, 22 ASFV genotypes (I-XXII) 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.
The virus is thought to be derived from a virus of soft tick (genus Ornithodoros) that infects wild swine, including giant forest hogs (Hylochoerus meinertzhageni), warthogs (Phacochoerus africanus), and bushpigs (Potamochoerus porcus). In these wild hosts, infection is generally asymptomatic. This virus appears to have evolved around 1700 AD.
This date is corroborated by the historical record. Pigs were initially domesticated in North Africa and Eurasia. They were introduced into southern Africa from Europe and the Far East by the Portuguese (300 years ago) and Chinese (600 years ago), respectively. At the end of the 19th century, the extensive pig industry in the native region of ASFV (Kenya) started after massive losses of cattle due to a rinderpest outbreak. Pigs were imported on a massive scale for breeding by colonizers from Seychelles in 1904 and from England in 1905. Pig farming was free-range at this time. The first outbreak of ASF was reported in 1907.
The first outbreak was retrospectively recognized as having occurred in 1907 after ASF was first described in 1921 in Kenya. The disease remained restricted to Africa until 1957, when it was reported in Lisbon, Portugal. A further outbreak occurred in Portugal in 1960. Subsequent to these initial introductions, the disease became established in the Iberian peninsula, and sporadic outbreaks occurred in France, Belgium, and other European countries during the 1980s. Both Spain and Portugal had managed to eradicate the disease by the mid-1990s through a slaughter policy.
With at least the tacit backing of U.S. Central Intelligence Agency officials, operatives linked to anti-Castro terrorists allegedly introduced African swine fever virus into Cuba in 1971 for the purposes of destabilizing the Cuban economy and encouraging domestic opposition to Fidel Castro. The virus was allegedly delivered to the terrorists from an army base in the Panama Canal Zone by an unnamed U.S. intelligence source. Six weeks later, an outbreak of the disease forced the slaughter of 500,000 pigs to prevent a nationwide animal epidemic. The outbreak was labeled the "most alarming event" of 1971 by the United Nations Food and Agricultural Organization. However, this claim only emerged six years after the original outbreak, and was made by the newspaper Newsday citing untraceable sources. ASFV crossed the Atlantic Ocean and outbreaks were reported in some Caribbean islands, including the Dominican Republic. Major outbreaks of ASF in Africa are regularly reported to the World Organisation for Animal Health (previously called L'office international des épizooties). Outside Africa, an outbreak occurred at the beginning of 2007 in Georgia, and subsequently spread to Armenia, Azerbaijan, Iran, Russia, and Belarus, raising concerns that ASFV may spread further geographically and have negative economic effects on the swine industry.
The appearance of ASF outside Africa at about the same time as the emergence of AIDS led to some interest in whether the two were related, and a report appeared in The Lancet supporting this in 1986. However, the realization that the human immunodeficiency virus (HIV) causes AIDS discredited any potential connection with ASF.
In August 2012, an outbreak of African swine fever was reported in Ukraine. In June 2013, an outbreak was reported in Belarus. In January 2014, authorities announced the presence of African swine fever in Lithuania and Poland, and in June 2014 in Latvia.
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- "Россельхознадзор запретит ввоз свинины из Литвы из-за АЧС в ближайшее время" [Rosselkhoznadzor bans the import of pigs from Lithuania immediately on account of ASF] (in Russian). ITAR-TASS. 2014-01-24. Retrieved 2014-01-26. "[...] глава ветеринарно-пищевой службы Литвы Йонас Милюс представил российской стороне факты о том, что у двух диких кабанов на территории страны был выявлен вирус АЧС, который стал причиной их гибели [...]"
- "Latvia extends emergency zone for African swine fever" (in English). AFP. 2014-07-22. Retrieved 2014-07-28. "[...] Latvia on Tuesday declared a state of emergency in a second area of this Baltic EU state as efforts continued to contain an outbreak of deadly African swine fever in its pig population.[...] Straujuma blamed wild boar crossing in from Russia for Latvia's first-ever outbreak of the disease, detected on June 26. [...]"