Pathogenesis of porcine reproductive and respiratory syndrome virus-induced increase in susceptibility to Streptococcus suis infection

Eighty 3-week-old crossbred pigs were randomly assigned to six groups (13-1 4 pigs/group). Group 1 pigs served as uninoculated controls, group 2 pigs w ere inoculated intranasally (IN) with Streptococcus suis serotype 2, group 3 pigs were inoculated IN with a modified live porcine reproductive and res piratory syndrome virus (PRRSV) vaccine, group 3 pigs were inoculated IN wi th the same vaccine and with S. suis, group 5 pigs were inoculated IN with VR-2385 (a high-virulence strain of PRRSV), and group 6 pigs were inoculate d LN with VR-2385 and S. suis. Pigs exposed to both PRRSV and S. suis were inoculated with PRRSV 7 days prior to S. suis inoculation. The pigs were 26 days old when inoculated with S. suis. Respiratory disease was significant ly more severe in groups 5 and 6. Mortality rate was the highest in group 6 (87.5%). This rate was significantly higher than that observed in all othe r groups except group 4 (37.5%). The mortality rate in group 2, inoculated with S, suis alone, was 14.3%. No pigs from groups 1, 3, or 5 died prior to the scheduled necropsies at 10 and 28 days postinoculation with PRRSV (DPI ). To study the effect of PRRSV and/or S. suis on pulmonary clearance by pu lmonary intravascular macrophages, six pigs from each group were intravenou sly infused with 3% copper phthalocyanine tetrasulfonic acid in saline prio r to necropsy at 10 DPI. Mean copper levels in the lungs of pigs in groups 2,, 5, and 6 were significantly lower than those in control pigs. The mean percentage of lung tissue grossly affected by pneumonia at 10 DPI was 0%, 1 %, 0%, 3%, 64%, and 62% for groups 1-6, respectively. Both gross and micros copic interstitial pneumonia lesions were significantly more severe in the VR2385-inoculated groups (5 and 6). PRRSV was isolated from bronchoalveolar lavage fluid collected at necropsy from 100% of the pigs in groups 5 and 6 , 71.4% of pigs in group 4, 38.5% of pigs in group 3, and none of the pigs in groups 1 or 2. Streptococcus suis serotype 2 was cultured from the inter nal tissues of 7.7%, 28.6%, and 78.6% of the pigs in groups 2, 4, and 6, re spectively. Streptococcus suis serotype 2 was isolated from whole blood at necropsy from 7.7%, 35.7%, and 78.6% of pigs in groups 2, 4, and 6, respect ively. Significantly more pigs in group 6 had S. suis isolated from whole b lood and internal tissues. In summary, both high-virulence PRRSV and S. sui s decreased copper clearance, and the incidence of isolation of S. suis and PRRSV was higher in dually inoculated pigs. PRRSV-induced suppression of p ulmonary intravascular macrophage function may in part explain PRRSV-associ ated increased susceptibility to S. suis infection.