Comparison of infection sequence of porcine reproductive and respiratory syndrome virus (PRRSV) and Mycoplasma hyorhinis to potentiate PRRSV-induced pneumonia.

The objective of this study was to evaluate the effect of infection sequence between Mycoplasma hyorhinis and porcine reproductive and respiratory syndrome virus (PRRSV). Thirty-six pigs were randomly allocated into six groups (six pigs per group) according to the order of infection: pigs inoculated first with M. hyorhinis followed by PRRSV (Mhr-PRRSV group), pigs co-inoculated simultaneously with both pathogens (PRRSV+Mhr group), pigs inoculated first with PRRSV followed by M. hyorhinis (PRRSV-Mhr group), pigs inoculated with either PRRSV or M. hyorhinis alone, and a negative control group. The Mhr-PRRSV, PRRSV+Mhr, and PRRSV-Mhr groups exhibited more severe clinical signs and a significant reduction in growth performance compared with the other three groups. These same groups also showed higher PRRSV loads in the blood, which corresponded with a greater severity of interstitial pneumonia. Furthermore, serum tumor necrosis factor-α concentrations were significantly elevated in these groups relative to the singly infected or control groups. The results of this study demonstrated that M. hyorhinis potentiates PRRSV-induced interstitial pneumonia in pigs, irrespective of infection sequence.