Comparison of pathogenicity of Vietnamese highly pathogenic and Japanese typical PRRS virus in experimentally infected sows in late gestation

Porcine reproductive and respiratory syndrome (PRRS) is characterized by late abortion and early farrowing in sows. In 2006, highly pathogenic PRRS (HP-PRRS) was occurred and posed a great concern to the global swine industry. Highly incidence of reproductive failures induced by HP-PRRS compared to typical type PRRS were reported. However, differences between HP- and typical PRRS in reproductive failure are not clear. The aim of this study was to compare the pathogenicity of HP- and typical PRRS in late pregnant sows.

Seven SPF pregnant sows purchased from PRRSV-free farm were used on 90 days of gestation. Sows were divided to two groups. Group 1 (4 sows) and group 2 (3 sows) inoculated intranasally with 1x10⁵ TCID₅₀ HP-PRRSV (100186-614 strain; isolated in Vietnam, 2010) and typical PRRSV (YK09 strain isolated form aborted sow in Japan), respectively. All animals were monitored daily for clinical signs. Serum samples were also collected. Sows were euthanized when they aborted or gave birth prematurely, and tissue samples were collected from sows and fetuses/piglets. Viral RNA in serum samples and several organs were measured by quantitative real time RT-PCR. Histopathological examination was performed on collected tissues, and immunohistochemistry was conducted for same sample with anti-PRRSV monoclonal antibody.

In group 1, body temperature increased from 2 dpi, and peaked in 8 dpi (over 40 ℃) with mild respiratory distress. All sows aborted from 11 to 17 days post-inoculation (dpi). In group 2, sows showed no clinical signs during experimental periods, and farrowed suddenly between 20 to 22 dpi.

Histologically, mild to moderate interstitial pneumonia and small necrotic foci in lymphoid organs with viral antigens were observed in both groups of sows. In fetuses, only a few animals showed small necrotic foci in lung and/or mild lymphadenitis in both groups. Small necrotic foci were detected in the tip of second folds of fetal placenta of group 1 fetuses. Viral antigens were also detected in fetal placental lesions of group 1 fetuses. Viral RNA was detected in serum samples from 1 dpi in group 1 sows and 3 dpi, in group 2 sows. Amount of viral RNA in group 1 and 2 shown approximately 10⁵ to 10⁶ and 10³ to 10⁴ TCID₅₀/ml at 5-10 dpi, respectively. In uterus of sows and fetal organs from both groups, viral RNA was also detected.

In this study, fetal placental lesion with viral antigen was only observed in group 1. Although, mechanism underlying the development of this lesion is still unclear, the lesion might be involved in abortion of HP-PRRS.

Disclosure of Interest: None Declared.