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))

The pathogenicity and pathogenesis of Lelystad virus was studied in six 6-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 6 and 8 after inoculation. From day 2 on pigs developed diffuse interstitial pneumonia with focal areas of catarrhal pneumonia, and from this day on splenic red pulp macrophages were enlarged and vacuolated. Lelystad virus was re-isolated from the lungs of infected pigs from day 2 after inoculation. Lelystad virus antigens were detected by immunohistochemical techniques in bronchiolar epithelium and alveolar cells, and in spleen cells of infected pigs from day 2 after inoculation. Ultrastructural examination of tissues by electron microscopy revealed degenerating alveolar macrophages and epithelial cells in lungs and nasal mucosa, with excessive vacuolation of the endoplasmic reticulum. Although the respiratory tract seems to be the target organ for this virus, macrophages in other organs, such as the spleen, can also be infected. This preference for macrophages may impair immunological defences.     

Experimental reproduction of porcine epidemic abortion and respiratory syndrome (mystery swine disease) by infection with Lelystad virus: Koch's postulates fulfilled

Aerosol exposure of eight pregnant sows to cell-culture- propagated Lelystad virus resulted in clinical signs characteristic of so-called mystery swine disease. After an incubation of 4-7 days, all sows were inappetant and listless for 6-9 days. Two sows developed a transient red-blue discolouration of the ears ('abortus blauw' or blue ear disease) accompanied by abdominal respiration, and two had a fever for one day only. One sow aborted at 109 days of gestation. The other seven sows, farrowing between 113 and 117 days of gestation, gave birth to numerous mummified, dead, and weak piglets. Of these seven, the mean number of piglets born dead to each sow was 4.6 and the mean number born alive was 7.7; 3.1 piglets per sow (40%) died within the first week. Lelystad virus was isolated from 31 piglets, which were born dead or died shortly after birth. Antibody was detected in precolostral blood samples or ascitic fluids of 23 piglets, a finding which demonstrated transplacental passage of the virus in six out of eight litters. We conclude that Lelystad virus is the causal agent of mystery swine disease. Since its aetiology is no longer a mystery, we propose the more appropriate name 'porcine epidemic abortion and respiratory syndrome (PEARS)'.