PATHOLOGICAL, ULTRASTRUCTURAL AND IMMUNOHISTOCHEMICAL CHANGES CAUSED BY LELYSTAD VIRUS IN EXPERIMENTALLY-INDUCED INFECTIONS OF MYSTERY SWINE DISEASE [SYNONYM - PORCINE EPIDEMIC ABORTION AND RESPIRATORY SYNDROME (PEARS)] (REPRINTED FROM THE VETERINARY QUARTERLY, VOL 13, PG 137-143, 1992)

The pathogenicity and pathogenesis of Lelystad virus was studied in si x six-day-old SPF piglets. A third passage of the agent was propagated on porcine alveolar macrophages and intranasally inoculated into pigs . Pigs were killed at hours 24, 48, 60 and 72, and on days six and eig ht after inoculation. From day two on pigs developed diffuse interstit ial pneumonia with focal areas of catarrhal pneumonia, and from this d ay on splenic red pulp macrophages were enlarged and vacuolated. Lelys tad virus was re-isolated from the lungs of infected pigs from day two after inoculation. Lelystad virus antigens were detected by immunohis tochemical techniques in bronchiolar epithelium and alveolar cells, an d in spleen cells of infected pigs from day two after inoculation. Ult rastructural examination of tissues by transmission electron microscop y revealed degenerating alveolar macrophages and epithelial cells in l ungs and nasal mucosa, with excessive vacuolation of the endoplasmic r eticulum. Although the respiratory tract seems to be the target organ for this virus, macrophages in other organs, such as the spleen, can a lso be infected. This preference for macrophages may impair immunologi cal defences.