African swine fever virus

African swine fever virus
Electron micrograph of a virus particle
Virus classification
Group:	Group I (dsDNA)
Family:	Asfarviridae
Genus:	Asfivirus
Species:	African swine fever virus

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.^[1]

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.^[2] ASFV infects domestic pigs, warthogs and bushpigs, as well as soft ticks (Ornithodoros), which likely act as a vector.^[1]

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'.

Virology

Diagram of ASFV and other members of Asfarviridae

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.^[3] 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.^[4]

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 at 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.

Macrophage cell in early stages of infection with ASFV

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.^[2]

Genotypes

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.^[5] 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.

Evolution

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).^[6] 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.^[7] They were introduced into southern Africa from Europe and the Far East by the Portuguese (300 years ago) and Chinese (600 years ago), respectively.^[8] 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.

Signs and symptoms

Reddening of the ears is a common sign of African swine fever in pigs.

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.^[9] 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.^[10]

Diagnosis

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.^[10]

History

The swelling around the kidneys and the muscle hemorrhages visible here are typical of pigs with African swine fever.

The first outbreak was retrospectively recognized as having occurred in 1907 after ASF was first described in 1921 in Kenya.^[11] 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.^[12]

Cuba

In 1971 an outbreak of the disease occurred in Cuba, resulting in 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.

Conspiracy theory

Six years after the event the newspaper Newsday, citing untraceable sources,^[13][14] claimed that 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 six weeks prior to the outbreak 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.^[15][16]

The Caribbean

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).

Eastern and Northern Europe

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.^[12][17][18]

In August 2012, an outbreak of African swine fever was reported in Ukraine.^[19] In June 2013, an outbreak was reported in Belarus.^[20]

African swine fever had become 'endemic' in the Russian Federation since spreading into the North Caucasus 'in November 2007, most likely through movements of infected wild boar from Georgia to (Chechnya)', said a 2013 report by the Food and Agriculture Organization, a United Nations agency.^[21] The report showed how the disease had spread north from the Caucasus to other parts of the country where pig production was more concentrated the Central Federal District (home to 28.8% of Russia's pigs) and the Volga Federal District (with 25.4% of the national herd) and northwest towards Ukraine, Belarus, Poland and the Baltic nations. In Russia, the report added, the disease was 'on its way to becoming endemic in Tver oblast' (about 106 km north of Moscow - and about 500 km east of Russia's littoral neighbours on the Baltic. Among the vectors for the spread in Russia of African swine fever virus was the 'distribution' of 'infected pig products' outside affected (quarantined and trade restricted) areas, travelling large distances (thousands of kilometers) within the country.

'Wholesale buyers, particularly the military food supply system, have been implicated multiple times in the illegal distribution of contaminated meat' were vectors for the virus's spread, said the Food and Agriculture Organization report - and evidence of that was 'repeated outbreaks in Leningrad oblast'. The report warned that 'countries immediately bordering the Russian Federation, particularly Ukraine, Moldova, Kazakhstan and Latvia, are most vulnerable to [African swine fever] introduction and endemic establishment, largely because the biosecurity of their pig sector is predominantly low. Preventing the spread of [African swine fever] into Ukraine is particularly critical for the whole pig production sector in Europe. Given the worrisome developments in the Russian Federation, European countries have to be alert. They must be ready to prevent and to react effectively to [African swine fever] introductions into their territories for many years to come'...To stop the virus's spread, 'the current scenario in the Russian Federation suggests that [prevention] should be particularly stressed at the often informal backyard level and should involve not just pig keepers, but all actors along the whole value chain - butchers, middlemen, slaughterhouses, etc...They need to be aware of how to prevent and recognize the disease, and must understand the importance of reporting outbreaks to the national authorities...It is particularly important that [African swine fever]-free areas remain free by preventing the [re]introduction of the disease and by swiftly responding to it when it occurs'.

In January 2014, authorities announced the presence of African swine fever in Lithuania and Poland,^[22] in June 2014 in Latvia, and in July 2015 in Estonia.^[23]

Estonia in July 2015 recorded its first case of African swine fever in farmed pigs in Valgamaa county on the country's border with Latvia. Another case was reported same day in Viljandi county, which also borders Latvia. All the pigs were culled and their carcasses incinerated.^[24] Less than a month later, almost 15,000 farmed pigs had been culled and the country was 'struggling to get rid of hundreds of tons of carcasses'. The death toll was 'expected to rise'.^[25]

Alternative theory

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.^[26] However, the realization that the human immunodeficiency virus (HIV) causes AIDS, discredited any potential connection with ASF.

See also

• Animal viruses

References

External links

• CFIA Animal Disease Information
  • African swine fever fact sheet
  • Biosecurity training video
  • Farm-level biosecurity practices
• World Organisation for Animal Health (OIE)
  • Current status of African Swine Fever worldwide at OIE. WAHID Interface - OIE World Animal Health Information Database
  • Disease card