African swine fever: a global view of the current challenge

Abstract

African Swine Fever (ASF) is an important contagious haemorrhagic viral disease affecting swine whose notification is mandatory due to its high mortality rates and the great sanitary and socioeconomic impact it has on international trade in animal and swine products. This disease only affects porcine species, both wild and domestic, and produces a variety of clinical signs such as fever and functional disorders of the digestive and respiratory systems. Lesions are mainly characterized by congestive-haemorrhagic alterations. ASF epidemiology varies significantly between countries, regions and continents, since it depends on the characteristics of the virus in circulation, the presence of wild hosts and reservoirs, environmental conditions and human social behaviour. Furthermore, a specific host will not necessarily always play the same active role in the spread and maintenance of ASF in a particular area. Currently, ASF is endemic in most sub-Saharan African countries where wild hosts and tick vectors (Ornithodoros) play an important role acting as biological reservoirs for the virus. In Europe, the disease has been endemic since 1978 on the island of Sardinia (Italy) and since 2007, when it was first reported in Georgia, in a number of Eastern European countries. It is also endemic in certain regions of the Russia Federation, where domestic pig and wild boar populations are widely affected. By contrast, in the affected eastern European Union (EU) countries where ASF is currently as epidemic, the on-going spread of the disease affects mainly wild boar populations located in restricted areas and, to a much less extent, domestic pigs. Unlike most livestock diseases, no vaccine or specific treatment is currently available for ASF. Therefore, disease control is mainly based on early detection and the application of strict sanitary and biosecurity measures. Epidemiology of ASF is very complex by the existence of different virus circulating, reservoirs and a number of scenarios, and the on-going spread of the disease through Africa and Europe. Survivor pigs can remain persistently infected for months which may contribute to virus transmission and thus the spread and maintenance of the disease, thereby complicating attempts to control it.

Keywords: Aetiology; African swine fever; Clinical symptoms; Control strategies; Diagnosis; Epidemiology; Prevention; Transmission.

Fig. 1

Clinical signs of acute form of ASF (source: EURL, INIA-CISA, Valdeolmos, Madrid, Spain). Pigs affected by acute form of ASF showing prostration and reddening of the skin at the tips of ears

Fig. 2

Warthog (Phacochoerus africanus). Phacochoerus genera act as the reservoirs of the ASFV in Africa without clinical symptoms. Transmission and maintenance of ASFV can occur in a sylvatic cycle involving warthogs and bushpigs as well as ticks of the genus Ornithodoros

Fig. 3

ASF notifications in Eastern Europe (source: A. Rodríguez (INIA-CISA, Valdeolmos, Madrid, Spain). Geographic map showing notifications of ASF in Eastern Europe since 2007 to July 2015. In green dot notifications in European wild boar. In yellow dot notifications in domestic pigs. Source: OIE

Fig. 4

Geographic distribution of ASF worldwide. In red, countries in which ASF is currently present from 2010 to date. In grey, countries in which ASF was reported in the past. In white, countries in which ASF has not been never reported

Fig. 5

Electron micrograph of ASFV (source, INIA-CISA, Valdeolmos, Spain). By electronic microscopy, viral particles show an average diameter of 200 nm. The virion is formed by several concentric structures with an external hexagonal envelope. The main target cells for ASFV replication are monocytes and macrophage cells

Fig. 6

Distribution of ASF virus genotypes (source: EURL, INIA-CISA, Valdeolmos, Madrid, Spain). Symbols represent the 22 AFSV genotypes determined by partial B646L (p72) sequencing known to be in circulation within that country. Genotypes are indicated in roman numerals

Fig. 7

Clinical signs and lesions of acute form of ASF in a domestic pig experimentally infected with an ASFV genotype II isolate circulating in Eastern Europe (source: EURL, INIA-CISA, Valdeolmos, Madrid, Spain). a Necrotic areas on the skin surface, b subcutaneous haematomas in legs and c melena

Fig. 8

Gross lesions of acute form of ASF in a domestic pig experimentally infected with an ASFV genotype II isolate circulating in Eastern Europe (source: EURL, INIA-CISA, Valdeolmos, Madrid, Spain). Spleen displaying hyperemic splenomegaly (enlarged with rounded edges, friable and dark red to black)

Fig. 9

Gross lesions of acute form of ASF in a domestic pig experimentally infected with an ASFV genotype II isolate circulating in Eastern Europe (source: EURL, INIA-CISA, Valdeolmos, Madrid, Spain). Lymph nodes (LN) enlarged edematous and completely hemorrhagic similar to a blood clot, mainly gastro hepatic and kidney LNs

Fig. 10

Dynamic of ASF infection (source: OIE WRL, UCM, Madrid, Spain and EURL, INIA-CISA, Valdeolmos, Madrid, Spain). The picture summarizes the ASF virus appearance in blood and antibodies after an ASFV infection. In addition, it shows the lethality of the different forms of the clinical disease, which ranges from acute to a subacute, as well as from recovered animals. Antibodies are detectable for a long period of time following the initial exposure

Fig. 11

Summary of current available validated ASF Diagnosis tests (source: EURL, INIA-CISA, Valdeolmos, Madrid, Spain). A significant number of tests are available for ASF diagnosis. Since 2010 several new commercial tests have been incorporated in the market, and several others are coming from companies mainly referred to new PCR commercial kit from Ingenasa, Life technologies, or Quiagen that would require to be validated by International Organisms (OIE, EU…)