Vesicular exanthema of swine

Vesicular exanthema of swine

N J KNOWLES

Introduction

Vesicular exanthema of swine (VES) is a disease of pigs that is clinically indistinguishable from other vesicular diseases of pigs, such as foot-and-mouth disease, and is caused by an immunologically diverse group of caliciviruses whose primary hosts are marine-dwelling mammals and fish.

Vesicular exanthema of swine was first recognized in California in 1932 and became a serious problem in that state during the 1940s. In 1952, the disease occurred outside California for the first time, and by the end of that year had spread to 31 states in the USA. This resulted, in August 1952, in the declaration of a national state of emergency, which provided legislation for a vigorous campaign of eradication based on the slaughter-out of affected herds, with compensation, control of the movement of pork products derived from swill-fed pigs, and the prohibition of feeding uncooked swill to pigs.1 These measures resulted in the eradication of VES in the USA by 1956. However, since a marine reservoir of these viruses was discovered in 1972,22 vesicular disease has appeared in free-living and captive marine mammals along the Pacific coast of the USA and on islands in the eastern Pacific.

Aetiology

The 13 VES virus (VESV) serotypes derived from pigs along with 16 serologically distinct caliciviruses isolated from marine- dwelling mammals and opaleye fish (Girella nigricans) which are referred to as San Miguel sea lion (SMS) viruses, form a single complex.4, 12, 31 Seven serologically distinct caliciviruses isolated from cetaceans,26 cattle,28 primates,29 skunk,18 walrus,27 reptiles30 and humans32 have also been shown to be part of this complex,6, 10, 12 bringing the total number of serotypes in this group to at least 35.31

Caliciviruses are non-enveloped, single-stranded RNA viruses with icosahedral symmetry (T=3) which have characteristic depressions on the virion surface.7 Virions have a diameter of 35 to 40 nm and a mass of approximately 15 × 106 Daltons.7 The capsid is made up of a major (Mr 58–60 × 103) and a minor (Mr 10 × 103) polypeptide.7 The positive-sense RNA is 7,4 to 8,3 kb in length and has a small polypeptide (VPg; Mr 10−15 × 103) covalently linked to its 5’ end, while the 3’ end of the genome is polyadenylated. 7

Vesicular exanthema of swine and the other related viruses belong to the genus Vesivirus in the family Caliciviridae. 7, 8 Within the vesivirus genus three genetic groups of viruses are recognized; the first group consists of VESV, SMSV and related viruses, the second is feline calicivirus, and the third is a newly recognized canine calicivirus.17 The relationships of caliciviruses isolated from mink9 and white tern15 to the vesiviruses remain to be elucidated. This family also contains three other genera, Lagovirus, Norovirus (Norwalk-like viruses) and Sapovirus (Sapporo-like viruses).7, 8 The lagoviruses are rabbit haemorrhagic disease virus (sometimes referred to as rabbit calicivirus) and European brown hare syndrome virus, while the Norwalk- like and Sapporo-like viruses are made up of noncultivatable human and animal gastrointestinal diseasecausing viruses.7, 8

Vesicular exanthema of swine and SMS viruses replicate with the production of cytopathic effects in a wide variety of cell cultures derived from pig tissues (such as kidney, lung, testicle and amnion); however, their replication in nonporcine cells is variable depending upon serotype.1, 24 There is a distinct correlation between the size of plaques produced by a strain in cell culture and its virulence for pigs.1 In pigs, different serotypes of VESV may differ markedly in their virulence,1 and some strains do not produce clinical disease.

Epidemiology

Marine caliciviruses have been isolated from California sea lions (Zalophus californianus), Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus), northern elephant seals (Mirounga angustirostris), captive Atlantic bottle-nosed dolphins (Tursiops truncatus), Pacific dolphins (Tursiops gillii), Pacific walruses (Odobenus rosmarus) and opaleye fish.4, 21, 27 A calicivirus was also isolated from a macerated liver fluke (Zalophatrema sp.) from a California sea lion, and indirect evidence suggests that sea lion lungworms, which parasitize opaleye fish as part of their life cycle, may transmit caliciviruses.19 Recently, the reptile calicivirus Crotalus 1 has been isolated from feral pinnipeds. 3

At least some serotypes of SMS viruses are pathogenic for California sea lions, in which they produce vesicles on the hairless parts of the flippers and, less frequently, around the mouth and on the hairy regions of the body.4 One SMSV isolate was made from a California sea lion with flipper lesions and which was in the process of aborting.

Antibody prevalence to SMS viruses is variable,20 but is sometimes high in free-living California and Steller sea lions.4 Up to 3 per cent of northern fur seals may be serologically positive.14

Apart from being found in marine animals, pigs, and mink fed on seal meat, caliciviruses (broadly classified as VES or SMS viruses) have also been recovered from or identified in snakes, toads, skunks, five species of primates, and cattle.21 Feline caliciviruses which cause an important disease in cats are distinct from the VES/SMS viruses. Antibodies to caliciviruses have been detected in the sera of...

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