Diagnostic Implications of Concurrent Inoculation with Attenuated and Virulent Strains of Porcine Reproductive and Respiratory Syndrome Virus

Objective: To determine the predominant strain of progeny virus in samples obtained from cell cultures and pigs exposed simultaneously to attenuated and virulent strains of porcine reproductive and respiratory syndrome virus (PRRSV).

Sample Population: Cell cultures and twenty 4-week-old pigs.

Procedure: Cell cultures and pigs were simultaneously exposed to various relative concentrations of an attenuated, cell-culture-adapted vaccine strain and a virulent field strain of PRRSV. Progeny virus obtained at selected intervals thereafter was tested to determine strain identity by use of restriction fragment length polymorphism (RFLP) analysis.

Results: Progeny virus from infected cell cultures comprised the attenuated strain, alone or in combination with the virulent strain, except when cultures had been exposed to a large excess (> 100,000-fold) of the virulent strain. Progeny virus from infected pigs comprised only the virulent strain regardless of the relative concentrations of the 2 strains to which the pigs had been exposed.

Conclusions: During concurrent replication in cell cultures, the attenuated strain quickly predominated. Conversely, during concurrent replication in pigs, the virulent strain quickly predominated.

Clinical Relevance: It is unlikely that only an attenuated strain of PRRSV would be identified by RFLP testing of samples obtained from pigs concurrently infected with a virulent strain of PRRSV. Nevertheless, the ability of a cell-culture-adapted attenuated strain of PRRSV to predominate during cell culture passage (the first step in the current RFLP testing procedure) indicated that, if possible, samples should be obtained from pigs that do not have a history of direct or indirect exposure to attenuated-virus vaccine.