By Xiaomei Yue, Mariana Kikuti, Claudio Marcello Melini and Cesar A Corzo, University of Minnesota College of Veterinary Medicine, Morrison Swine Health Monitoring Project

Porcine epidemic diarrhea virus was first detected in the United States during the spring of 2013. The initial epidemic spanned approximately 1.5 years, with the prevalence transitioning to endemic levels by the end of 2014 (Figure 1A).

Data from the Morrison Swine Health Monitoring Project, a Swine Health Information Center-funded initiative that tracks the health status of over 55% of U.S. breeding herds, indicates significant trends in PEDV outbreaks. During the 1.5-year epidemic phase, the prevalence of PEDV peaked at 31.6% in April 2014. However, during the subsequent 10-year endemic period, the PEDV prevalence in sow herds remained low and steady, with a peak of 5.5% in April 2022. The reduction in the number of yearly outbreaks has clearly shown that the investments made in biosecurity have clearly paid off.

Figure 1. PEDV occurrence in the U.S. breeding herds. A) Prevalence of PEDV in U.S. breeding herds (1/Red – Unstable herds; 2fvi/pink – Stable herds undergoing gilt acclimation; 2/yellow – Stable herds; 3/light green – Herds undergoing elimination and introducing gilts; 4/ Dark green – PEDV naïve/free herds. Source: Morrison Swine Health Monitoring Project (1).

The proportion of U.S. breeding herds weaning PEDV positive pigs has remained low (i.e., = ≤5.5%) since the beginning of the endemic phase of the disease indicating producers and practitioners have invested time and resources to clean positive herds. We recently estimated how long it takes a sow herd to reach a stable status by consistently weaning PEDV-free pigs (i.e., time-to-stability or TTS), and assessed associated factors. PEDV weekly monitoring data originating from 1,028 U.S. herds between May 1, 2013 and June 30, 2023 were obtained from MSHMP. The TTS was calculated as the number of weeks in positive unstable (Status 1, from outbreak detection through end of shedding) before changing to positive stable (Status 2 - absence of clinical signs and no detectable virus for at least four weeks, or Status 2fvi – positive stable with ongoing gilt field virus exposure). The association of TTS with recorded factors was also assessed for both the PEDV epidemic and endemic periods, with a random effect for herd ID nested with the production system included in the model.

In total, 384 PEDV breaks from 16 production systems were included in this study, with 203 occurring during the epidemic period (May 2013 - December 2014) and 181 during the endemic period (January 2015 - June 2023). The median TTS was 24 weeks (IQR: 18 - 31) during the epidemic and 14 weeks (IQR: 9 - 22) during the endemic period (Figure 2). A log-rank test showed a significant difference (p-value < 0.001) in TTS between the epidemic and endemic period.

Figure 2. PEDV Time-to-Stability in the U.S. breeding herds during the epidemic and endemic periods.

Factors significantly associated with TTS include PEDV status of the herd before the outbreak and herd size. Herds breaking from status 2fvi reached stability faster (hazard ratio or HR, 8.6 and 3.6 for epidemic and endemic periods) than herds breaking from status 4 (negative). Larger herds (> 5,000 sows) took longer to reach stability (HR: 0.41 for epidemic and 0.45 for endemic periods) than small herds (<2,500 sows).

The U.S. industry has effectively been able to attack PEDV as herds are getting cleaned faster thanks to the progressive mindset of producers, pig production companies, their teams and practitioners. Such information and evidence of success is timely as the industry is considering whether this pathogen can be eradicated from the U.S. swine population. In addition, these pieces of information support decision-making in PEDV control and elimination strategies (e.g., forecasting PEDV elimination timeline and herd closure plan, pig flow management and investment timing).

Authors would like to acknowledge MSHMP participating companies, swine veterinary clinics and practitioners for their willingness to share their data. We also would like to acknowledge SHIC for financially supporting MSHMP.

References

1. Morrison Swine Health Monitoring Project. PED aggregate prevalence of sow herd status (last updated April 2024). Report | Morrison Swine Health Monitoring Project (2024) https://mshmp.umn.edu/reports#Charts [Accessed June 21, 2024]