Abstract: There are two objectives of the study. First is to explore the relationship between HP-porcine reproductive and respiratory syndrome virus (HP-PRRSV) and pathological changes of pulmonary microthrombus, the other is to clarify the effect of endothelial nitric oxide synthase (eNOS) expression on thrombosis. Fifty 70-day-old healthy piglets were employed, control group consisted of four piglets and 46 piglets were infected with HP-PRRSV. Lungs were collected, HE technique was then applied to observe histopathological changes and to evaluate thrombus grade. Mild thrombus group, moderate thrombus group and severe thrombus group were assigned based on state of their thromboses according to the statistics, four piglets in each group were selected for follow-up study. Immunohistochemistry, Western blot, real-time PCR were employed in examining the distribution, expression and changes of HP-PRRSV and eNOS in the lungs of piglets. The results of histopathological studies showed that the alveolar space of the lung tissue was significantly widened, different degree of blood stasis and bleeding in blood capillary occurred, alveolar epithelial cells, macrophages peeled off in the alveolar cavity and amounts of microthrombus in the alveolar wall were also observed. HP-PRRSV was found mainly located in cytoplasm of alveolar macrophages and fewer in lymphocytes. The degree of thrombosis mounted associated with the increase of viral content and then lead to a higher mortality rate. In addition, eNOS was found mainly located in the cytoplasm of alveolar type Ⅱ cells, macrophages, vascular endothelial cells and bronchial smooth muscle cells. The expression of eNOS mRNA and protein in infected pig lungs was significantly lower compared with controls but there was no obvious difference between the three infected groups. In summary, HP-PRRSV develops the mortality of infected pigs by reducing the expression of eNOS and participated in the formation of microthrombosis.

Key words: HP-PRRSV, Etongliangtouwu pig, lung, microthrombus, eNOS