Submitted to: American Journal of Veterinary Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/1998
Publication Date: N/A
Interpretive Summary: Porcine reproductive and respiratory syndrome (PRRS) is currently one of the most costly diseases faced by the swine industry. Its control depends in large part on early and accurate diagnosis. One of the diagnostic concerns is the potential for vaccine virus (i.e. live, cell-culture-adapted PRRS virus) to mask the identification of virulent (i.e.disease producing) field strains of PRRS virus in samples collected from suspected cases of PRRS. The rational is that during laboratory testing vaccine virus will predominate in diagnostic tests that depend on virus isolation in cell culture. In the study described here we demonstrated that virulent virus so completely predominates during replication in pigs that it is likely to be identified even if pigs are vaccinated during a clinical episode of PRRS. This observation confirms the reliability of diagnostic tests that are currently in use and indirectly assists in the control and perhaps eventual eradication of PRRS.
Technical Abstract: To determine the diagnostic implications of dual infection of cell cultures and pigs with attenuated and virulent strains of porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) on subsequent strain identification, cell cultures and pigs were exposed to various concentrations of attenuated (RespPRRS) and virulent (NADC-9) strains of PRRSV. Progeny virus obtained at selected intervals thereafter was tested for strain identity by restriction fragment length polymorphism (RFLP). The strain or strains identified by RFLP of progeny virus obtained from infected cell cultures depended on the relative concentrations of the 2 strains in the inoculum. The attenuated strain was identified either alone or in combination with the virulent strain at all concentrations except those with the virulent strain in great excess (>100,000-fold). At even greater relative concentrations of the virulent strain in the inoculum, only the virulent strain was identified. When pigs were exposed to variou relative concentrations of the same 2 strains, only the virulent strain was identified in samples obtained 7 and 14 days later. The attenuated strain was identified only in samples from pigs exposed to only that strain. These results indicate that from a diagnostic perspective it is unlikely, regardless of vaccination status, that only an attenuated vaccine strain of PRRSV would be identified by RFLP in samples submitted from pigs also infected with a virulent field strain of PRRSV. Nevertheless, the ability of an attenuated vaccine strain to predominate during cell culture passage (the first step in the RFLP testing procedure) suggests that, if possible, samples should be obtained from pigs with no history of direct or indirect exposure to attenuated vaccine virus.