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Transmission of PRRSV by direct, close, or indirect contact

Method: Groups of pigs were housed under conditions simulating current swine production systems. Five trials were conducted, each using 13 3- to 5-week-old pigs, housed in nursery decks fitted with raised wire-mesh flooring. Three decks were placed parallel to one another in an isolation room. The decks were placed 46 cm (18 inches) apart for the first three trials and 102 cm (40 inches) apart for trials 4 and 5. A single sheet of aluminum was suspended equidistantly in the space between one side deck and the center deck to inhibit the direct passage of biological materials from pigs in the center deck to the side deck. On the first day of each trial, three pigs were placed in the center deck and inoculated intranasally with PRRSV (ATCC VR-2402) (primary exposure group). Two days later, three pigs (direct contact group) were placed in the center deck with the inoculated animals; three pigs (close contact group) were placed in the side deck with no barrier; and four pigs (indirect contact group) were placed in the side deck separated from the center deck by the sheet of aluminum. Transmission was considered to have occurred if PRRSV was isolated from, or anti-PRRSV antibodies were detected in, serum collected on day 31 of the study. Results: Transmission of PRRSV was demonstrated between the primary exposure and the direct contact groups in all five trials. In contrast, the close contact group became infected in three trials and the indirect contact groups in two trials. Implications: Transmission of PRRSV can occur when susceptible pigs are not in direct contact with infected pigs. Separation by relatively short distances may be used to reduce exposure levels of PRRSV. Minor separation of animals also may contribute to the development of subpopulations of noninfected pigs. Airborne transmission may be less likely than previously believed.