Simple SummaryBased on the fact that Gps are rooted in the upper respiratory tract of pigs, in order to investigate whether secondary infection with HP-PRRSV can exacerbate lung disease and chronic inflammation, our study was designed as follows. Our study randomly divided piglets into four groups: Gps + HP-PRRSV, Gps, HP-PRRSV, and controls. Piglets in the Gps + HP-PRRSV and Gps groups were infected through the intranasal route with the Gps W2 strain. The Gps + HP-PRRSV and HP-PRRSV groups were challenged with the HP-PRRSV HuN4 strain by intramuscular injection and intranasally at 5 days after the initial exposure to Gps. Alternatively, the control group animals received phosphate-buffered saline (PBS). Then, we observed the influence of HP-PRRSV–Gps coinfection on clinical outcomes, pathogen shedding and loading, cytokine production, and specific antibody levels at various time points in infected piglets. Our results revealed synergistic effects in HP-PRRSV–Gps coinfection, which increase the severity of clinical signs compared with single infections. Therefore, in the unavoidable situation of Gps infection in piglets, necessary measures must be made to prevent and control secondary infection of HP-PRRSV, which can save huge economic losses to the pork industry.AbstractGlaesserella parasuis(Gps), Gram-negative bacteria, are a universal respiratory-disease-causing pathogen in swine that colonize the upper respiratory tract. Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus (HP-PRRSV2HP-PRRSV2) and Gps coinfections are epidemics in China, but little is known about the influence of concurrent coinfection on disease severity and inflammatory responses. Herein, we studied the effects of secondary HP-PRRS infection on clinical symptoms, pathological changes, pathogen load, and inflammatory response of Gps coinfection in the upper respiratory tract of piglets. All coinfected piglets (HP-PRRSV2 + Gps) displayed fever and severe lesions in the lungs, while fever was present in only a few animals with a single infection (HP-PRRSV2 or Gps). Additionally, HP-PRRSV2 and Gps loading in nasal swabs and blood and lung tissue samples was significantly increased in the coinfected group. Necropsy data showed that coinfected piglets suffered from severe lung damage and had significantly higher antibody titers of HP-PRRSV2 or Gps than single-infected piglets. Moreover, the serum and lung concentrations of inflammatory cytokines (TNF-α, IL-1β, IL-6, and IL-8) were also significantly higher in coinfected piglets than in those infected with HP-PRRSV2 or Gps alone. In conclusion, our results show that HP-PRRSV2 promotes the shedding and replication of Gps, and their coinfection in the upper respiratory tract aggravates the clinical symptoms and inflammatory responses, causing lung damage. Therefore, in the unavoidable situation of Gps infection in piglets, necessary measures must be made to prevent and control secondary infection with HP-PRRSV2, which can save huge economic losses to the pork industry.