A study of 95 commercial pig farms identified measures and farm characteristics associated with PRRSV outbreaks during the weaning-to-market phase.
The takeaway in one sentence
Biosecurity is not determined by any single measure, but by farm characteristics, exposure pressure, and management decisions, which together influence the likelihood of PRRSV introduction.
Figure 1 Staff changing shoes on a bench at the isolation area
Image source: National Pork Board
Porcine reproductive and respiratory syndrome virus (PRRSV), also known as the virus that causes blue-ear disease, remains one of the diseases with the greatest economic impact on U.S. swine production. The latest estimates put annual losses at approximately 12 billion dollars, with nearly 68% stemming from the wean-to-finish (nursery and finishing) stages. Despite significant investment in improving biosecurity in breeding herds, little is known about the measures currently implemented during the wean-to-finish stages and how effective they are at preventing PRRSV introduction.
Wean-to-finish farms play an important role in maintaining and potentially spreading PRRSV within and between production systems. However, previous studies assessing the relationship between biosecurity measures and PRRSV outbreaks have largely focused on breeding farms, leaving a knowledge gap concerning growing-pig populations. To address this gap, this study aimed to assess the association between biosecurity measures and PRRSV outbreaks on commercial wean-to-finish farms, identifying areas where producers can focus their prevention efforts.
Methods
The study tracked 95 commercial pig farms across eight production systems in six Midwestern U.S. states (Iowa, Illinois, Indiana, Minnesota, Nebraska, and Ohio) from 2023 to 10 through 2024 to 10. Participating farms included 33 nursery farms, 17 finishing farms, and 45 wean-to-finish farms. Each farm was enrolled at the start of one production cycle and followed until the pigs were marketed or transferred. Veterinarians and production teams completed a biosecurity questionnaire with 115 questions covering 17 areas, including animal transport, dead pig disposal, employee movement, feed delivery, manure removal, visitors, and shared equipment. Oral fluid samples were collected every four weeks to monitor PRRSV prevalence and were tested using fluorescent quantitative RT-PCR (RT-qPCR). ORF5 gene sequencing was performed when applicable to distinguish wild-type PRRSV from vaccine-strain-related detections. A total of 273 variables were evaluated to identify factors associated with PRRSV outbreaks.
Study results
Among the 95 participating farms, 55 (57.9%) had PRRSV outbreaks, with significant differences among production types. The proportion of farms with outbreaks was highest among finishing farms (82.4%; 14/17), followed by wean-to-finish farms (66.7%; 30/45) and nursery farms (33.3%; 11/33). These differences may reflect production stage, implementation of biosecurity measures, and differing opportunities for PRRSV exposure throughout the production cycle.
During the screening phase, several routine farm practices were associated with differences in the occurrence of outbreaks (Figure 2). Farms that used rendering to dispose of dead pigs had 6.47 times the odds of an outbreak compared with farms that did not use rendering (95% confidence interval [CI]: 1.62–25.84; p= 0.008). Other practices associated with increased odds of an outbreak included transporting pigs whose PRRSV status had not been confirmed (odds ratio [OR]: 9.79; p=0.009), and allowing employees to live with people who worked in other swine-related jobs (OR: 6.15; p=0.011).
Conversely, several measures were associated with a lower likelihood of a PRRSV outbreak. Farms with designated vehicle parking areas had a lower likelihood of an outbreak (OR: 0.07; p=0.001), as did farms using dedicated manure removal equipment (OR: 0.07; p=0.003). Mandatory overnight downtime for employees working at multiple farms (OR: 0.15; p=0.004) and the use of a bench entry system (Danish entry system) (OR: 0.27; p=0.005) were also associated with a lower likelihood of an outbreak.
Figure 2 Biosecurity measures and farm characteristics associated with PRRSV outbreaks on farms raising pigs from weaning to finishing. The forest plot shows odds ratios (ORs) and corresponding 95% confidence intervals (CIs) from univariable mixed-effects logistic regression models. Red dots indicate factors associated with a higher likelihood of a PRRSV outbreak (OR>1), and green dots indicate factors associated with a lower likelihood (OR<1). Horizontal lines represent 95% CIs; the vertical dashed line indicates no association (OR=1). Results are shown on a logarithmic scale. Association does not imply causation.
In the final analysis, production type and the density of pig farms around a farm remained significantly associated with PRRSV outbreaks. For each additional pig farm within a one-mile (approximately 1.6 kilometers) radius, the odds of an outbreak increased by approximately 62% (OR: 1.62; 95% CI: 1.09–2.41; p= 0.018). Production type was also associated with outbreaks, with finishing farms having higher odds of an outbreak than nursery farms.
These results highlight the need to consider both farm characteristics and operational biosecurity when assessing the risk of PRRSV exposure. Farms located in areas with a higher density of pig farms may face greater exposure pressure, while measures related to vehicle access, employee movement, and equipment sharing offer opportunities to assess and improve biosecurity. Although no single measure can guarantee prevention of PRRSV introduction, understanding these associations can help producers identify potential weak points and prioritize preventive measures.
Practical considerations for producers include:
➢Consider the density of nearby pig farms: Farms located in areas with a high density of pig farms should recognize the increased exposure pressure and strengthen biosecurity measures to reduce the chance of PRRSV introduction.
➢Maintain clear biosecurity boundaries: designate parking areas, implement Danish entry via a bench barrier, and minimize unnecessary entry of vehicles, people, and equipment into the clean zone.
➢Review dead pig handling and animal transport measures: assess the dead pig collection process, limit contact between outside vehicles and farm personnel or equipment, and verify the PRRSV status of incoming pigs.
➢ Strengthen employee and equipment biosecurity protocols: Establish clear quarantine-period requirements for employees working at multiple pig farms, minimize the sharing of equipment between pig farms, and prioritize providing dedicated equipment for tasks such as manure pumping.