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ASF Virology


The disease

 

African Swine Fever (ASF) is a highly contagious, generalized disease of pigs caused by an Iridovirus of family Asfarviridae that exhibits varying virulence between strains and is very hardy to physical and chemical inactivation. The agent can remain viable for long periods in blood, faeces and tissues. It can also multiply in its vectors. In view of this, the control of ASF is dependent on stamping out policy and strict quarantine enforcement. It most commonly appears in the acute form as a haemorrhagic fever. Subacute and chronic forms of the disease also exist. Mortality is usually close to 100 percent and pigs of all ages are affected.

 

The ASF virus

 

The causative agent of ASF is a unique, enveloped, cytoplasmic, double-stranded DNA arbovirus, which is the sole member of the family Asfarviridae. Although it was generally considered that there is only one serotype of ASF virus, recent studies have reported the classification of 32 ASFV isolates in eight different serogroups based on a hemadsorption inhibition assay (HAI) (Malogolovkin et al., 2015). However, genetic characterization of all the ASF virus isolates known so far has demonstrated 23 geographically related genotypes with numerous subgroups, illustrating the complexity of ASF epidemiology (Figure 4). The genotype is the reflection of the variability of a segment in a single gene and protein (VP-72) and is used for mainly phylogenetic and molecular epidemiological purposes (e.g. to identify the source of outbreaks). As far as is known, it does not determine the virulence, or other disease parameters.

 

Animals affected

 

In the natural sylvatic cycle, the soft-bodied, eyeless Ornithodoros ticks (also known as tampans) are, together with African wild suids, the natural reservoir hosts of ASFV. They can transmit the virus through their bites. All members of the pig family (Suidae) are susceptible to infection, but clinical disease is only seen in domestic and feral pigs, as well as in the closely related European wild boar. Wild African suids are asymptomatic carriers of ASF and act as the reservoir of the virus in parts of Africa. These include warthogs (Phacochoerus africanus and P. aethiopicus), bushpigs (Potamochoerus porcus and Potamochoerus larvatus) and giant forest hogs (Hylochoerus meinertzhageni).

 

Transmission

 

The ASF virus persists in distinct cycles – traditionally, the sylvatic cycle, the tick-pig cycle and the domestic (pig-pig) cycle. More recently, a wild boar cycle has been described, which may sometimes be involved in the latter. The sylvatic cycle occurs only in parts of Africa and involves warthogs and ticks of the Ornithodoros moubata complex. The tick-pig cycle involves pigs and Ornithodoros spp. ticks, which have been described as infesting parts of Africa and the Iberian Peninsula. Transmission from the sylvatic cycle (African wild suids) to the domestic cycle (farmed pigs) occurs via indirect transmission by ticks. This can happen where pigs and warthogs share common grounds, particularly when warthogs establish burrows on farms, or when ticks are brought back to villages through the carcasses of warthogs killed for food.