Associations between genetics, farm characteristics and clinical disease in field outbreaks of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is a disease of domest ic swine characterized by exceptionally high clinical variability. This stu dy addresses the question of whether clinical variability in PRRS results f rom (a) genetic variation among viral isolates and/or (b) variation in mana gement practices among farms on which isolates are found. Genetic data (ope n reading frame 5 gene sequences) and data on farm characteristics and asso ciated clinical disease signs were collected for 62 PRRS virus (PRRSV) fiel d isolates, representing 52 farms. Clinical disease signs were interrelated - confirming that a true reproductive syndrome exists (involving abortions , infertility in sows, deaths of sows and preweaning mortality). Pairs of farms experiencing deaths in their sow populations also tended to share viral isolates which were more similar to one another than expected b y chance alone. This implies that sow death (one of the more-seven manifest ations of PRRS) is under genetic influence. Large herd size was a significa nt risk factor for the death of sows and for respiratory disease in nursery pigs. All-in-all-out management practices in the nursery were protective a gainst reproductive signs in the sow herd. All-in-all-out management practi ces in the finishing stages of production were protective against respirato ry disease in nursery pigs - but were paradoxically associated with an incr eased risk of infertility in sows. These results suggest that farm-manageme nt practices can also influence which PRRS clinical signs are manifested du ring an outbreak. In general, signs associated with PRRS appear to result f rom a combination of genetic factors and herd-management characteristics. T he relative contributions of these two influences differ depending on the s pecific clinical sign in question. (C) 2000 Elsevier Science B.V. All right s reserved.