The North American Swine industry is engaged in a conversation on a national porcine reproductive and respiratory syndrome virus (PRRSV) strategy. The parties involved in the conversation clearly recognize that effectively monitoring the PRRSV status of herds over time is a prerequisite for successfully controlling the virus. Because diagnostic testing can be expensive, the challenge is to design effective monitoring strategies for minimal cost. Regional PRRSV control and elimination projects are adopting and testing various monitoring strategies.

This project developed a data collection tool, methods of analysis, and standardized reports to help regional PRRSV control and elimination projects monitor the PRRSV status of the region and benchmark their monitoring efforts against each other. The project is designed to help participants and leaders of regional control and elimination project answer, “How much testing is enough?” A description of the region and information on each swine herd is needed. This can be submitted using the data collection tool, which is available as an excel spreadsheet, or by electronic file transfer. These data are collected initially and can be updated at any time. Diagnostic and clinical data describing the PRRSV circulation can be submitted at any time.

The developed methods classify each herd as PRRSV circulation positive, negative, or unconfirmed. The status is updated each week. A core principle of this project is that while a single positive test, where the possibility of a false positive has been ruled out, proves a herd is PRRSV circulation positive it is difficult to prove a herd is PRRSV circulation negative. To do so would require a perfect test be applied to every animal on a continuous basis. A major focus of this project is the process and caveats used for ongoing classification of herds as virus circulation (VC) negative.

Herds are initially classified as PRRS VC-negative after conducting diagnostic testing equivalent to the guidelines to classify a breeding herd as positive-stable (IIA, IIB, III or IV) or a growing herd as negative as described in the American Association of Swine Veterinarians (AASV) herd classification system (Holtkamp, et al., 2011). Once classified as VC-negative, herds must conduct ongoing testing to maintain this status. Ongoing testing is required because information is discounted over time to account for the chance the herd may have become infected since testing. The amount of ongoing testing may vary by the type of herd (breeding vs. growing) and type of production (commercial vs. genetic). It is determined by the desired level of confidence in the negative status and is communicated to participants as a baseline threshold. Establishing the standards for ongoing claims of VC-negative status is novel and helps address the problem of when to consider a herd’s status and regional maps outdated.

Each month the prevalence of PRRS VC-negative herds in the region is estimated along with 95% confidence intervals. Higher participation rates narrow the confidence intervals so this gives perspective on the estimate’s precision. In regions with no known positive herds, the probability the region is PRRS VC-negative is calculated. The probability the region is PRRS VC-negative increases with the proportion of known PRRS VC-negative herds in the region and with the intensity of ongoing testing of herds in the region.

While the methods developed and described in this report are relatively technical, the herd-level reports are designed to return meaningful information to participants so that regional activity can be considered in each herd’s monitoring decisions. The regional reports developed for this project are designed to deliver monitoring information that will help regions track trends and progress toward goals. Because the process is standardized, the reports can be compared between PRRSV regional control and elimination projects to stimulate discussion and innovation. This is facilitated by an inter-regional report which is generated quarterly and can be opted into by interested regional control and elimination projects.

Participation in PRRSV regional control and elimination projects is voluntary and herds vary in the frequency and intensity of PRRSV monitoring. These methods are designed to use all diagnostic and clinical information about PRRSV circulation without restriction on choice of test, timing or amount of sampling. This allows participants to select the best monitoring regime for their herd and provides them with feedback to use in future monitoring decisions. The regional reports will help determine if a region is conducting enough monitoring to meet its goals, to identify herds where more testing is needed, and to evaluate if monitoring dollars are being spent effectively
This project leveraged prior work and expertise of a working group established by the PRRS Coordinated Agricultural Project (PRRS-CAP) project (USDA NIFA Award 2008-55620-19132). The project also built on existing programs in Canada that have been supported by the Canadian Swine Health Board (Development of a PRRS-Free Certification Project & Sustainability and Enhancements of the PRRSV Free Certification Project).

This project provides value to PRRSV regional control and elimination projects. Freedom from disease modelling is a rapidly developing field that is becoming accepted in international trade. The reports provided by these methods will be required by regions achieving and claiming PRRSV freedom. Until that happens, the reports will provide useful information to regions striving to control PRRSV.