Ultrastructural findings in lymph nodes from pigs suffering from naturally occurring postweaning multisystemic wasting syndrome.

The aims of this study were to evaluate ultrastructural lesions in lymph nodes from postweaning multisystemic wasting syndrome (PMWS)-affected pigs and to correlate these alterations with detection of viral-like particles (VLPs). Samples of lymph nodes were taken from 4 PMWS-affected pigs and 2 healthy animals and processed by transmission electron microscopy. Significant ultrastructural alterations were only noted in PMWS-affected pigs, mainly in histiocytes and rarely in other cell types. Histiocytes showed severe swelling and proliferation of mitochondria, and proliferation and dilation of rough endoplasmic reticulum and Golgi complex. Infected histiocytes contained large numbers of intracytoplasmic inclusion (ICI) bodies with VLPs; some histiocytes also had intranuclear inclusions (INIs). Small inclusions were surrounded by double membrane, with a granular appearance or containing paracrystalline arrays; icosahedral VLPs were 8-17 nm in diameter. Large ICIs were double- membrane bounded or not and contained VLPs usually forming paracrystalline arrays. ICIs were often found next to mitochondria with severe swelling, and also inside them. INIs were not surrounded by membranes and contained virions of 10-13 nm diameter. Lymphocyte depletion was a striking finding of lymph nodes from PMWS-affected pigs. The inclusion bodies containing VLPs referred to in the present study should be classified as viral factories, suggesting that viral replication is probably a frequent event in macrophages, in which mitochondria might play a role.