Highly cited article published in the Veterinary Quarterly in 1991

In early 1991, the Dutch pig industry was struck by the so-called mystery s wine disease, Large-scale laboratory investigations were undertaken to sear ch for the aetiological agent. We focused on isolating viruses and mycoplas mas, and we tested paired sera of affected sows for antibodies against ten known pig viruses. The mycoplasmas M, hysonoviae M, hyopneumoniae, and Ache loplasma laidlawii, and the viruses encephalomyocarditis virus and porcine enterovirus types 2 and 7 were isolated from individual pigs, An unknown ag ent however, was isolated from 16 of 20 piglets and from 41 of 63 sows, Thi s agent was characterized as a virus and designated Lelystad virus. No rela tionship between this virus and other viruses has yet been established. Of 165 sows reportedly affected by the disease, 123 (75 per cent) seroconverte d to Lelystad virus, whereas less than 10 per cent seroconverted to any of the other virus isolates or to known viral pathogens. Antibodies directed a gainst Lelystad virus were also found in pigs with mystery swine disease in England, Germany, and the United States. We conclude that infection with L elystad virus is the likely cause of mystery swine disease.