In last week’s PRRS Corner we explored the need to standardize the terminology we use when discussing the PRRS status of swine breeding herds and the herd classification system proposed by a working group assembled by AASV, NPB and PRRS CAP. The classification system is outlined in a paper authored by Drs. Derald Holtkamp and Dale Polson published in the January/February 2011 issue of JSHAP. In Part 1 of this two part series we discussed the Positive Unstable (Category I), Provisional Negative (Category III) and Negative (Category IV) designations outlined in the paper. This week we want to look a little closer at the more complex Positive Stable (Category II-A/II-B) herd classification.
The Positive Stable classification was divided into two subcategories: II-A and II-B. These herds have an uncertain shedding status and positive exposure status. The exposure status of the breeding herd remains positive. The possibility that animals may still be infected and later shed the virus cannot be ruled out. For this reason, a distinction was made between Category II breeding herds that are not undergoing PRRSV elimination (Category II-A) and those that are (Category II-B). Category II-A is the goal for herds that are trying to control, rather than eliminate, the virus. Confidence that the weaning-age pigs and growing pigs moving from Category II-B breeding herds are not shedding PRRSV increases over time since the breeding herd is undergoing an active PRRSV elimination program.
Category II is meaningful for managing PRRSV both in a production setting and in regional or national elimination programs. In the context of managing PRRSV in a production setting, the absence of viremia is significant for management of pig flow and expectations about improved reproductive and growing-pig performance. In the context of a regional or national elimination program, the distinction between Positive Unstable (Category I) and Positive Stable (Category II) breeding herds and those undergoing elimination versus those that are not may have value for deciding where and when to move pigs.
Hopefully, these two PRRS Corner articles have helped explain the rationale behind the need for standardizing the terminology and provided a brief overview of the classification system. The complete publication (available for review online) offers much more detail about the various categories and the criteria necessary to verify herd classification. The document also establishes the protocols to assess the PRRS virus shedding and exposure status of animals within the various categories.
All swine veterinarians should familiarize themselves with this document and strive to adopt the appropriate terminology and testing protocols outlined as we move forward with herd and regional elimination efforts.