African swine fever

African swine fever

M-L PENRITH, G R THOMSON AND A D S BASTOS

Introduction

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

African swine fever was first recognized in East Africa as a disease with many clinicopathological resemblances to classical (European) swine fever (hog cholera). 156 Studies conducted between 1910 and 1917 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.156 In particular, outbreaks were related to association of freeranging pigs with wild suids, and not movement of domestic pigs or fomites. Furthermore, inoculated bushpigs (Potamochoerus spp.) and warthogs (Phacochoerus africanus) developed a viraemia, but virtually no clinical signs. Unlike classical swine fever, neutralizing antibody was not detectable in recovered animals, and antisera to classical swine fever did not protect pigs against ASF.

In South Africa, a similar disease was first described 240, 241 in the bushveld of the former Transvaal (now including Limpopo Province), where there was regular contact between warthogs and domestic pigs. During an initial phase, from about 1900 to 1918, both classical swine fever and ASF apparently occurred, but the former has not been recorded since 1918. An outbreak of ASF in the Witwatersrand area in 1933 spread to the Western Cape 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, apparently due to infection of young pigs by survivors. 56, 113, 139, 162, 187 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.200

African swine fever was also described from Angola50, 79, 150 and was first diagnosed in northern Mozambique in 1954.152 In Angola, there was no known association with warthogs, and it was suggested that the free-ranging domestic pigs that roamed around the villages acted as a reservoir of the virus.50, 153 The disease was subsequently described from most countries in Central and southern Africa.286 In West Africa, Senegal first reported ASF to the OIE in 1978, but unpublished reports indicate that it was present in southern Senegal and probably Guinea Bissau at least as early as 1959.227

African swine fever attracted international attention when it reached Portugal in 1957 and again in 1960 286 and became established in the Iberian Peninsula. Outbreaks subsequently occurred in several European countries as well as Cuba, the Dominican Republic, Haiti, and Brazil. For the first time the devastating effects of the disease in countries with well-developed commercial pig industries were appreciated. Eradication proved difficult and expensive. The arrival of the disease in Europe sparked considerable research, and concerted efforts were made to obtain a vaccine. These attempts were unsuccessful, although, probably as a result of attempts to vaccinate pigs using attenuated virus, strains of low virulence emerged, and subacute and chronic forms of the disease, with a relatively high proportion of survivors, occurred. Perhaps the most important result of the research carried out in Europe was the discovery that argasid ticks, or tampans, of the genus Ornithodoros could maintain the virus for long periods and transmit it to pigs.221 Investigation of this possibility in Africa revealed that infected Ornithodoros tampans occurred in the burrows of warthogs,194 and that they transmit the infection to each other by various mechanisms and are essential to the presence of the virus in warthog populations.194, 195, 197, 201 There is good evidence for a cycle of virus transmission between neonatal warthogs and eyeless tampans that occurs in burrows occupied by breeding female warthogs.251, 252

Table 98.1 Outbreaks of African swine fever in sub-Saharan Africa from 1978 to 2001

COUNTRY 1978– 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001
Angola + + + + + + + + + + +
Benin + + - - +
Botswana + +
Burundi + +
Cameroon + _ + + ± +
Cape Verde (+) (+) - - -
Chad +
Congo + + - - - - - - - - +
(Brazzaville)
Congo (DR) + + +
Côte d’Ivoire - - - - - + - - - - -
Gambia (+) - - - - - + - - + -
Ghana - - - - -

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