Virulence of strains/isolates of porcine reproductive and respiratory syndrome virus type 1

The aim of the study is to consider some criteria for assessing the virulence of PRRS virus strains/isolates in mono and coinfection during experimental infection of piglets. The data were analyzed by screening openly available literature on mono and coinfection with PRRS virus strains/isolates. Monoinfection of piglets with PRRS virus strains/isolates of varying virulence resulted in both acute and asymptomatic disease. Virulent strains/isolates caused disease lasting from 3 to 33 days in 75–100 % of experimentally infected piglets, followed by death in 10–100 % of diseased animals. Moderately virulent variants were characterized by the development of a clinical picture in 55–100 % of infected individuals lasting 5–13 days, followed by recovery. Lowvirulence PRRSV strains/isolates resulted in the development of shortterm, mild disease signs of 1–3 days in 10–50 % of piglets. The concentration of virus in blood serum varied, high levels of viremia were observed in both asymptomatic and severe disease. Lesions in internal organs were more pronounced, and the concentration of the virus in them was higher for virulent than for low and moderately virulent PRRS virus strains/isolates. Coinfection of PRRS virus with bacterial and viral pathogens leads to increased severity of the disease, development of atypical clinical signs, and an increase in the mortality rate among infected piglets. Combined infection contributes to the synergistic effect, acting as an "assistant", making the body more vulnerable to secondary infections.