MYSTERY SWINE DISEASE IN THE NETHERLANDS - THE ISOLATION OF LELYSTAD VIRUS (REPRINTED FROM THE VETERINARY QUARTERLY, VOL 13, PG 121-130, 1991)

In early 1991, the Dutch pig-industry was struck by the so-called myst ery swine disease. Large-scale laboratory investigations were undertak en to search for the etiological agent. We focused on isolating viruse s and mycoplasmas, and we tested paired sera of affected sows for anti bodies against ten known pig viruses. The mycoplasmas M. hyosynoviae, M. hyopneumoniae, and Acholeplasma laidlawii, and the viruses encephal omyocarditis virus and porcine enterovirus types two and seven were is olated from individual pigs. An unknown agent, however, was isolated f rom 16 of 20 piglets and from 41 of 63 sows. This agent was characteri sed as a virus and designated Lelystad virus. No relationship between this virus and other viruses has yet been established. Of 165 sows rep ortedly afflicted by the disease, 123 (75%) seroconverted to Lelystad virus, whereas less than 10%) seroconverted to any of the other virus isolates or to the known viral pathogens. Antibodies directed against Lelystad virus were also found in pigs with mystery swine disease in E ngland, Germany and in the United States. We conclude that infection w ith Lelystad virus is the likely cause of mystery swine disease.