Porcine reproductive respiratory syndrome emerged in the U.S. in the late 1980’s and since then the disease has been prevalent in the country. The causative agent of PRRS is a highly mutant RNA-virus with two main linages, genotype 1 and genotype 2 , which is resilient to low temperatures and hence adaptable to the US Midwest, where most of the US swine industry is located. High animal density, the use of live PRRSV vaccines, collection of dead pigs rather than incineration, early weaning age, and close proximity to or frequent contact with infected premises, also referred as farms, are factors suggested or demonstrated to promote PRRS spread. While the only natural host for PRRSV is pigs, the sources of spread include direct and indirect contacts between infectious and susceptible animals. Because PRRSV can be excreted via multiple fluids facilities and vehicles used for pig transportation may be contaminated, contributing to disease spread within and between regions. PRRS imposes more than $550 million in losses annually, increasing prices to consumers. Furthermore, within a global environment where the US swine industry competes with producers in other countries and with other sources of meat, e.g., beef and poultry, profitability is reduced for the industry. The World Organization for Animal Health has listed PRRS as a “notifiable terrestrial animal disease” , and the US has reported the presence of PRRS in the country. However, since it is considered a production disease, daily reporting is not mandatory and implementation of control activities are voluntary. Several diseases, such as classical swine fever or Aujeszky’s disease, have been eradicated from the US by implementation of official programs and by means of agreements reached among different levels of decision-makers. Because PRRS has become endemic in the US , in the absence of an official regulatory framework, regional strategies are emerging to control the disease. One initiative, led and funded by swine producers and supported by the University of Minnesota, curing cannabis was launched in 2004 with approximately 90 premises in Steven County, MN. It evolved into what may have been the first regional control project in the US. This RCP is referred to as the N212 Minnesota Voluntary Regional PRRS Elimination Project .
In 2014 it has expanded to include swine producing premises in 39 counties in MN. The RCP-N212 initiative was followed by others and currently there are more than 30 RCPs, also referred as area regional control projects or ARCs, throughout the US and Canada, including, RCPs in Southeast Iowa, Western Michigan, Northwest Indiana, and Pennsylvania. RCPs promote communication among producers regarding disease prevalence and efforts to control it, with the expectation that such information will lead to the development and adoption of strategies for disease control within a geographical area. RCPs function through the voluntary participation of swine producers who enroll in the program and who agree to share the disease status of their farms. As an incentive to participate, participants receive an exclusive, weekly report of regional disease status. RCPs initially focused on PRRS, but more recently have also included porcine epidemic diarrhea. RCP programs have, arguably, strengthened the US swine industry by encouraging communication between and among producers and research institutions. However, few evaluations ofRCP goals and achievements have been carried out. This paper proposes a methodological framework to evaluate the progress of an RCP, using data collected at the RCP-N212 for years 2012–2014. We analyzed demography of premises, disease communications, short-term trends in PRRS incidence, and disease distribution, based on information shared by participants. We anticipate that established benchmarks will facilitate comparisons among RCPs and the establishment of control objectives.This paper used a confidential dataset with information collected from July 2012 to June 2014 of swine premises enrolled in the RCP-N212 , and public sources with information to account for the total number of premises per county and area per county. The RCP-N212 dataset contained information at premises level including geographical location, day in which premises was enrolled in the RCP-N212, type of premises, and PRRS status.
Type of premises indicates the phase of swine production, e.g., farrow-to-wean, wean-to finish, finishing, etc.. In this study, for simplicity, premises that have breeding herds or sows were referred to as sites with sows , whereas premises without sows were referred to as sites with no sows . Participating producers are requested to report any PRRS status changes to the coordinator of the RCP-N212 as soon as it occurs, i.e., within a day. Additionally, at least once a month, the coordinator directly contacts producers to obtain a PRRS status update for their farm. Premises that voluntarily share PRRS status were categorized following the American Association of Swine Veterinarians guidelines. Briefly, the AASV guidelines assign to SSs one of five mutually exclusive status categories: 1, 2A, 2B, 3 and 4. Positive unstable indicates virus detection in the premises and clinical signs compatible with PRRS. Positive stable are premises in which breeding herds are PCR positive but do not present clinical signs of PRRS and weaning pigs have passed at least four consecutive PCR negative tests, one every 30 days using a sample size of 30 weaning pigs, to demonstrate lack of viremia. This category is divided into two subgroups: 2A for SS that are not undergoing elimination, and 2B for premises undergoing elimination of PRRS. In the latter, at a certain point in time, neither vaccinations nor exposure to the live virus to achieve immunity is allowed, and additional restrictions on cross-fostering and herd access to replacements are applied. Provisional negative denotes a premises that is continuously introducing negative replacement gilts, with results of ELISA negative for breeding herds after 60 days of introduction. Negative indicates consistently negative results to serologic and PCR testing. For NSSs AASV guidelines assign one of two mutually exclusive status categories: Positive , similar to category 1 for SSs, and negative in which the premises must have ELISA negative results in growing pigs. RCP-N212 dataset was protected on a codified database at the University of Minnesota. From an analytical perspective, this study may be regarded as an observational, longitudinal, retrospective cohort study.First, demographics of the RCP N212 were measured by the monthly proportion of premises enrolled, using the number of premises enrolled per county as numerator and the total number of swine premises per county as denominator. Then, among premises enrolled in the RCP-N212, a repeated measures analysis of variance was used to assess changes in the proportion of premises types over a 24-month period.Although the proportion of premises enrolled slightly decreased over the study period, both the number of enrolled premises and the geographical coverage of RCP-N212 increased from 427 premises in 34 counties in July 2012 to 500 in 39 counties in June 2014 . Among premises enrolled, the proportion of SSs and NSSs did not change significantly , through time, but proportions between SSs and NSSs were statistically different , ranging from 0.23 to 0.24 for SSs and from 0.76 to 0.77 for NSSs. The most parsimonious mixed-effects logistic regression model contained time and type of premises as fixed effect, using premises ID nested by county as random effects. Final results suggested a significant increase of active participation in RCP-N212 over the 24-months period . The monthly increase in the odds of sharing PRRS status by premises was 4.2 , although NSS enrolled premises were less prone to report than SS enrolled premises . There was a larger variability in the observed participation data between than within counties, as demonstrated by the estimated random effects . The normal time-space scan test identified 2 significant clusters of high and low probability of sharing PRRS status, compared to the expected null hypothesis of an even distribution of cases in the assessed area . The cluster of high-probability of sharing PRRS, located in the northern area of the RCP N212 , cannabis dryer was detected at the second half of the study period , whereas the low probability cluster was located in the southern area in the RCP N212 in the first half of the study .Among premises enrolled that shared PRRS status , 175 outbreaks of PRRS were reported in 168 premises during the 24 months of the study period. This roughly represents 35% of all enrolled premises and 44% of premises that shared PRRS status .
The univariate logistic regression analysis indicated that time, probability of sharing PRRS status , type of premises, proportion of stable premises in the county, and density of premises in the county showed a p-value <0.25 in their associations with PRRS incidence and were consequently incorporated and tested in the multivariate mixed effects models.However, the most parsimonious mixed-effects logistic regression model contained only time, probability of sharing PRRS status, and density of premises in the county as fixed effects, whereas premises ID nested into county was included as a random effect. A significant monthly decrease of odds of PRRS incidence was identified . The probability of sharing PRRS status was negatively associated with PRRS incidence , whereas the density of big and medium-sized premises in the county was positively related with PRRS incidence . Significant temporal aggregations of incidence of PRRS were observed over the study period, and at the same time a decreasing trend on temporal densities was detected . This result suggest that PRRS incidence was grouped in time, as an initial outbreak increases virus shedding within a region, which leads to disease spread, but that leads to a corresponding increase in disease control, resulting in a decrease in shedding and spread. The probability of outbreaks is trending down slightly over the period. At the same time, and coincidently with temporal manifestation of PRRS, spatial aggregation in a number of locations, mainly in the mid-western region of the RCP N212 was revealed . In turn, cluster for the spatio-temporal point process given by KST > πu2 v was consistent with findings by pair correlation function.Results of this study demonstrate the application of a systematic approach to assess the evolution of RCPs. We have demonstrated that farmers’ enrollment in a voluntary regional control program is not necessarily an accurate estimate of participation, as farmers may enroll, but not share information on disease status, which may be critical in PRRS control. Results on organization of the RCP-N212 program and on PRRS control are encouraging. While 40% of those enrolled in July 2012 did not share information, this figure went down to 20% in June 2014. Although active participation did not reach 100% among premises enrolled in the program, the statistical increase in sharing PRRS status suggested a growing interest of participants to share disease status. This information was incorporated in a regression model, which suggested a significant negative relationship between probability of sharing disease status and PRRS incidence in the RCP-N212. In 2014, premises enrollment had reached 34% of all premises in the counties included in RCP-N212. NSSs account for roughly three quarters of all enrolled premises , but the principal strategies to control PRRS are focused on SS premises, which attempt to ensure that only PRRSV-negative pigs are weaned. In areas in which premises density is high, immunization of the population through vaccination or live-virus exposure has been the preferred control strategy leading premises to stay in AASV category 2A, whereas in regions with low premises density, undergoing elimination of PRRSV is preferred. However, despite the relevance of upstream PRRSV control for maximizing returns at the system level, status of PRRS in NSSs is also important if the objective is to control the disease at a regional level, due to their potential importance of NSSs as sources of infection. The above may be particularly important in controlling PRRS at a regional scale, in which a large proportion of the premises present might be NSSs, as is the case for the RCP-N212 where direct and indirect contacts due to exchange of inputs between NSS-NSS, SS-SS and SS-NSS might facilitate transmission dynamics of PRRSV. Considering all premises located in counties in the RCP-N212, active participation increased from 22% in July 2012 to 27% in June 2014 . While only about 60% of enrolled premises shared information in 2012, 80% shared in 2014, suggesting a significant increase in willingness to participate among producers . However, the extent at which information was effectively shared was heterogeneous in premises type , time, and space.