Figure 2.

Evolution of porcine reproductive and respiratory syndrome (PRRS) status recorded in the Morrison Swine Health Monitoring Project (MSHMP) between 2009 and 2017 (left) and in 12 systems that consistently reported the status during the same time period (right). Breeding herds have been classified using a slightly modified terminology to the one proposed by Holtkamp et al. (15) that incorporates breeding herd shedding and immune status to increase its relevance to industry practices. Category 1 (red): positive unstable breeding herds that are undergoing a PRRSv outbreak and are weaning positive piglets. Category 2fvi (pink): herds that continue to expose breeding replacements to a live field virus strain (e.g., live-virus inoculation). Category 2vx (orange): herds that continue to expose breeding replacements or sows to PRRSv through the use of a modified-live PRRSv vaccine. Category 2 (yellow): herds that have uncertain PRRSv shedding status and positive PRRSv exposure status (animals are seropositive) with no clinical signs, no evidence of weanling piglet viremia and have stopped gilt and sow exposure to live virus. Category 3 (light green): herds that have negative PRRSv shedding status and have introduced breeding replacements that maintain seronegative status for more than 60 days. Category 4 (dark green): PRRSv-naïve herds in which pigs are negative for both shedding and exposure status for at least a year after reaching Category 3.