ABSTRACT Aim of the study was to evaluate the virulence, cell mediated immune response and histolesivity of three PRRSV strains, with ORF5 relevant genetic variations, isolated from either respiratory and reproductive field outbreaks. Changes in peripheral lymphocytes showed a cytotoxic natural response more evident than the acquired cytotoxic one. The results, in our experimental conditions, evidenced a different behaviour among the three PRRSV strains. Two of them, the 55/L/2000 and 162/L/2000, were less virulent than the 418/FL/2003 strain and of another PRSSV field strain (114/L/2000) previously investigated. The three PRRSV strains studied had moderate virulence, similar cell mediated immune response, but different histolesivity.
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