GENERAL INTRODUCTION: ASFARVIRIDAE

Introduction

African swine fever (ASF) is a devastating disease of domestic pigs caused by a unique virus that originally evolved in East and Southern Africa in an obscure cycle that was confined to its natural hosts, argasid ticks and wild suids. The introduction of domestic pigs into the same region resulted in a population of animals susceptible to the clinical effects of the infection, and in which the virus causes acute haemorrhagic disease, with morbidity and mortality rates that can reach 100 per cent. The character of the disease may change when it becomes endemic in domestic pigs, with considerably reduced mortality regardless of the virulence of the causative virus.

African swine fever was first recognized in East Africa as a disease with many clinicopathological resemblances to classical swine fever (hog cholera) (CSF).³⁶² Studies conducted between 1910 and 1917 in Kenya demonstrated that it was caused by a virus that produced high mortality in domestic pigs, but that the resultant disease differed epidemiologically and immunologically from classical swine fever.³⁶² In particular, outbreaks were related to association of free-ranging pigs with wild suids. Furthermore, inoculated bushpigs (Potamochoerus spp.) and warthogs (Phacochoerus africanus ) developed viraemia, but virtually no clinical signs. Unlike classical swine fever, neutralizing antibody was not detectable in recovered animals,²⁶⁰ and antisera to classical swine fever did not protect pigs against ASF.

In South Africa, a similar disease was first described^{533, 534} in the bushveld of the former northern Transvaal (now Limpopo Province), where there was regular contact between warthogs and domestic pigs. During an initial phase, from about 1900 to 1918, both CSF and ASF apparently occurred in South Africa, but the former had not been recorded since 1918 until an incursion into the Eastern Cape Province in 2004 that was eradicated by 2008.^{7, 427} An outbreak of ASF in the Witwatersrand area of the present Gauteng Province in 1933 spread to the Western Cape, apparently by movement of infected pigs. Control was eventually achieved by slaughter of sick and surviving animals, creation of quarantine zones and restrictions on re-stocking. However, one farm in the Piketberg district suffered repeated outbreaks from 1935 until 1939, ascribed to infection of young pigs by survivors,^{132, 256, 329, 387, 449} although persistent environmental contamination was evidently not ruled out. Periods of apparent freedom from outbreaks occurred in South Africa from 1918 to 1926, 1939 to 1951, and 1962 to 1973, but these may have been at least in part due to small numbers of domestic pigs in the control area and failure by farmers to report disease.⁴⁵⁹

African swine fever was described from Angola in 1932 although it was only confirmed to have been ASF later^{113, 188, 349, 352} and was first diagnosed in northern Mozambique in 1954.³⁵¹ In Angola, an association with warthogs was neither observed nor investigated, and it was suggested that the free-ranging domestic pigs that roamed around the villages acted as a reservoir of the virus.^{113, 352} The disease was subsequently described from most countries in Central and Southern Africa.⁵⁸⁷ In West Africa, Senegal first reported ASF to the World Organisation for Animal Health (OIE) in 1978, but unpublished reports indicated that it was present in southern Senegal and probably Guinea Bissau at least as early as 1959.⁵¹⁴ An unconfirmed outbreak occurred in Nigeria in 1973.^{38, 414}

African swine fever attracted international attention when it reached Portugal in 1957 and again in 1960⁵⁸⁷ and became established in the Iberian Peninsula. Outbreaks subsequently...