African swine fever: how can global spread be prevented?

Abstract

African swine fever (ASF) is a devastating haemorrhagic fever of pigs with mortality rates approaching 100 per cent. It causes major economic losses, threatens food security and limits pig production in affected countries. ASF is caused by a large DNA virus, African swine fever virus (ASFV). There is no vaccine against ASFV and this limits the options for disease control. ASF has been confined mainly to sub-Saharan Africa, where it is maintained in a sylvatic cycle and/or among domestic pigs. Wildlife hosts include wild suids and arthropod vectors. The relatively small numbers of incursions to other continents have proven to be very difficult to eradicate. Thus, ASF remained endemic in the Iberian peninsula until the mid-1990s following its introductions in 1957 and 1960 and the disease has remained endemic in Sardinia since its introduction in 1982. ASF has continued to spread within Africa to previously uninfected countries, including recently the Indian Ocean islands of Madagascar and Mauritius. Given the continued occurrence of ASF in sub-Saharan Africa and increasing global movements of people and products, it is not surprising that further transcontinental transmission has occurred. The introduction of ASF to Georgia in the Caucasus in 2007 and dissemination to neighbouring countries emphasizes the global threat posed by ASF and further increases the risks to other countries. We review the mechanisms by which ASFV is maintained within wildlife and domestic pig populations and how it can be transmitted. We then consider the risks for global spread of ASFV and discuss possibilities of how disease can be prevented.

Figure 1.

Distribution of African swine fever virus (AFSV) genotypes. (a) Map showing African swine fever (ASF) outbreaks between 2003 and 2008. Shading indicates a country within which an outbreak has occurred. Symbols represent ASF genotypes (determined by B646L (p72) sequencing) known to be in circulation within that country (Basto et al. 2003; Lubisi et al. 2005; Boshoff et al. 2007; Rowlands et al. 2008). (b) Phylogram depicting the B646L gene relationships of selected isolates representative of the 22 AFSV genotypes. Because all the Georgian isolates had identical nucleotide sequences, only one isolate is presented in the tree (in boldface). The consensus tree was generated from 1000 replicates; only bootstraps more than 50 per cent are shown. Genotypes are indicated in roman numerals. Moz, Mozambique; Lis, Lisbon; Zim, Zimbabwe; Mad, Madagascar; Bot, Botswana; RSA, Republic of South Africa; Spec, Spencer; Ten, Tengani; Nam, Namibia; Uga, Uganda; Tan, Tanzania; Kab, Kabu. Scale bar indicates number of nucleotide substitutions per site (Rowlands et al. 2008).