Pathological Characterization of an Outbreak of Porcine Reproductive and Respiratory Syndrome in Northern Vietnam

In 2007, a highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) emerged in Vietnam and spread to nearly all regions of the country by 2010. Ten representative pigs of different age groups, infected naturally with HP-PRRSV in northern Vietnam in 2010, were used to characterize the pathological features of the infection. Infection was confirmed using reverse transcriptase polymerase chain reaction and viral isolation. The clinical signs and gross findings in these pigs included high fever (>40.2°C), red skin, blue ears, anorexia, respiratory distress, diarrhoea, haemorrhagic pleurisy and lymphadenopathy. Reproductive failure was the main clinical feature in sows. PRRSV infection-associated microscopical lung and lymph node lesions were observed frequently, regardless of age of the animals. Lung lesions were characterized by interstitial pneumonia and were occasionally associated with haemorrhage and fluid exudation following alveolar collapse. Lymph nodes exhibited characteristic haemorrhage and apoptosis, lymphocytic depletion and disorganization secondary to fibrosis and capillary formation. Haematoxylin and eosin staining or caspase-3 immunohistochemistry revealed apoptosis induction in various tissues and organs, particularly the lymph nodes and lungs. Primarily haemorrhagic microscopical lesions were observed commonly in other organs including the spleen, liver, heart and kidney. Immunohistochemical examination revealed HP-PRRS antigen in the lung, lymph node, liver and kidney macrophages, and lung and kidney epithelial cells. Pigs infected naturally with HP-PRRS in the field have multisystemic disease characterized by marked apoptotic cell death.