Research Article | Volume 5 - Issue 1 | Article DOI :
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Hyunju Kim1, Oyoung Kwon2, Jooyoung Lee2, Siyeoung Choi3, Hongyao Lin4, Leonardo Ellerma5*, Sungbo Cho6 and In Ho Kim7
1Pig Management and Clinic, South Korea
2MSD Animal Health Korea Co., Ltd, South Korea
3Department of Animal Biotechnology, Dankook University, Cheonan, South Korea
4MSD Animal Health Innovation Pte Ltd, Singapore
5MSD Animal Health (Phils.), Inc, Philippines
6Department of Animal Biotechnology, Dankook University, Republic of Korea
7Smart Animal Bio Institute (SABI), Dankook University, Cheonan, Republic of Korea
Corresponding Author:
Leonardo Ellerma, MSD Animal Health (Phils.), Inc, Philippines, Tel: +639178386170
Keywords
PRRS; NADC34-like virus; PrimePac PRRS; Vertical Transmission; Modified Live Vaccine; Farrow-to-Finish Herd.
Abstract
Porcine Reproductive and Respiratory Syndrome (PRRS) causes major economic losses, and cross-protection of Modified-Live Vaccines (MLV) against emerging heterologous strains remains uncertain. This study reports a field based, longitudinal before after intervention study evaluating the impact of a Lineage 7 PRRS MLV (PrimePac® PRRS) during an acute NADC-34 like PRRS outbreak in a commercial 1200-sow farrow-to-finish farm in South Korea. Following a confirmation of an NADC-34 like PRRSV outbreak, a whole-herd mass vaccination program was implemented using intradermal administration (0.2ml) of PrimePac® PRRS. Reproductive performance indicators (farrowing rate, abortions, stillbirths and mummification), vertical transmission dynamics (PRRSV qPCR testing of processing fluids), and post-weaning health outcomes (nursery and grow-finish morbidity and mortality, piglet viremia by qPCR) were monitored longitudinally before and after vaccination. After implementation of the vaccination program, key breeding-herd performance parameters progressively returned toward pre-outbreak baseline levels. PRRSV became undetectable in the processing fluid samples within one month, indicating rapid interruption of vertical transmission. Improvement in nursery and grow-finish pig performance occurred more gradually, consistent with ongoing virus circulation in a continuous-flow production system. This findings demonstrate that, although complete virus elimination was not achieved, a strategic mass vaccination program using a Lineage 7 PRRS MLV substantially mitigated the reproductive and early-life impacts of an NADC-34 like PRRS outbreak. Vaccination combined with biosecurity measures remains a critical component of integrated PRRS control strategies in farrow-to-finish systems where eradication is difficult.
Abstract
Porcine Reproductive and Respiratory Syndrome (PRRS) causes major economic losses, and cross-protection of Modified-Live Vaccines (MLV) against emerging heterologous strains remains uncertain. This study reports a field based, longitudinal before after intervention study evaluating the impact of a Lineage 7 PRRS MLV (PrimePac® PRRS) during an acute NADC-34 like PRRS outbreak in a commercial 1200-sow farrow-to-finish farm in South Korea. Following a confirmation of an NADC-34 like PRRSV outbreak, a whole-herd mass vaccination program was implemented using intradermal administration (0.2ml) of PrimePac® PRRS. Reproductive performance indicators (farrowing rate, abortions, stillbirths and mummification), vertical transmission dynamics (PRRSV qPCR testing of processing fluids), and post-weaning health outcomes (nursery and grow-finish morbidity and mortality, piglet viremia by qPCR) were monitored longitudinally before and after vaccination. After implementation of the vaccination program, key breeding-herd performance parameters progressively returned toward pre-outbreak baseline levels. PRRSV became undetectable in the processing fluid samples within one month, indicating rapid interruption of vertical transmission. Improvement in nursery and grow-finish pig performance occurred more gradually, consistent with ongoing virus circulation in a continuous-flow production system. This findings demonstrate that, although complete virus elimination was not achieved, a strategic mass vaccination program using a Lineage 7 PRRS MLV substantially mitigated the reproductive and early-life impacts of an NADC-34 like PRRS outbreak. Vaccination combined with biosecurity measures remains a critical component of integrated PRRS control strategies in farrow-to-finish systems where eradication is difficult.
Keywords
PRRS; NADC34-like virus; PrimePac PRRS; Vertical Transmission; Modified Live Vaccine; Farrow-to-Finish Herd.
Citation
Kim H, Kwon O, Lee J, Choi S, Lin H et al, (2026) Field Evaluation of a Lineage 7 PRRS MLV in Mitigating the Impact of Nadc34-Like PRRS Infection in Korean Pig Farm. JSM Vet Med Res 5(1): 9.
INTRODUCTION
Porcine Reproductive and Respiratory Syndrome (PRRS) remains one of the most economically devastating diseases affecting the global swine industry. Caused by the PRRS virus (PRRSV), an enveloped, single stranded RNA virus of the Arteriviridae family, this pathogen emerged in the 1980s and rapidly spread to major pig-producing regions such as the USA [1], China [2], and Korea [3]. Despite extensive control measures, PRRSV continues to persist on many farms due to its high mutation rate and extensive genetic diversity, which frequently leads to the emergence of heterologous strains capable of evading the immunity induced by conventional vaccines [3,4]. Two species exist in PRRSV: Betaarterivirus suid 1 (PRRSV-1) and Betaarterivirus suid 2 (PRRSV-2). In 2010 a phylogenetic classification based on 8,624 global ORF5 sequences was proposed to describe PRRSV-2 genetic diversity. Based on this, nine lineages (L1–L9) and 37 sublineages were proposed [5]. The emergence of new lineages or sub-lineages is usually associated with increases in outbreaks and mortality until herd immunity emerges [6]. As of 2025, lineage 1 has become the most prevalent and diverse lineage in Asia [7] and the United States [8]. In South Korea [9], sub-lineages 1.5 and 1.8 are dominant. Both sub-lineages 1.5 (NADC-34) and 1.8 (NADC-30) have demonstrated varying levels of pathogenicity, depending on isolate. For example, a study by Tu et al [10]. Demonstrated that a NADC-30 isolate in challenge studies was able to cause prolonged fever and reduced appetite, leading to decreased weight gain and growth rates. Other studies have also similar results in NADC-34 isolates [11]. Different results were found however in a study by Zhou et al, where a challenge was not only able to cause clinical symptoms but also mortality [12], suggesting the highly varied nature of pathogenicity of PRRS isolates, even within the same sub-lineage.
Vaccination is considered a cornerstone of PRRS management, and various PRRSV Modified-Live Vaccines (MLV) have been shown to reduce clinical symptoms, lung lesions, viremia, and the duration of viral shedding [13,14]. Several authors have advocated for MLV selection based on genetic or antigenic matching between field strains and the MLV strain [15,16], commonly performed by determining genetic similarity based on ORF5 sequences. In practical terms, this is increasingly difficult to perform in the field. Additionally, the period for a vaccine to obtain a commercial license is long, in some instances up to 5-10 years [17]. This means that in the field, commercial vaccines are not able to match with emerging genetically distinct isolates such as Lineage 1 NADC34-like PRRSV [7-9]. Previous studies have found that vaccination of swine with MLV was able to induce good protection in pigs exposed to genetically diverse PRRSV strains post vaccination [7-18]. Other authors have found that in the face of heterologous challenge, vaccination is able to reduce clinical signs but cannot stop vertical transmission or reduce viral shedding significantly [19-21]. In the face of conflicting findings, it is therefore of value to veterinary practitioners to evaluate PRRS MLV vaccines individually to assess their ability to cross-protect against contemporary PRRS field isolates.
Apart from vaccination, biosecurity measures are another important measure to control PRRSV infection on farm. Measures such as reducing the movement of infected animals, increased disinfection of facilities and moving to All-In-All-Out (AIAO) production are indicated. In farrow-to-finish farms in Asia, the management of facilities on AIAO basis is often not possible and continuous flow production is a more common situation [22]. Producers in such situations thus rely more on vaccination compared to biosecurity.
PrimePac PRRS (MSD Animal Health, Rahway, NJ, USA) is a PRRS Modified-Live Vaccine (MLV) that has recently obtained a commercial licence in South Korea. This vaccine can be administered either via intramuscular or intradermal route through a purpose designed injector. Intradermal administration of vaccine has several important safety advantages, namely reducing needles and iatrogenic transfer of disease [23-25], and reducing shedding of PRRS virus compared to intramuscular injection [24]. By administering vaccine in proximity to dermal antigen-presenting cells, the volume of vaccine needed can be reduced while still providing similar or improved efficacy [26].
Our study objective was therefore to evaluate the efficacy of the Lineage 7 PRRS MLV (PrimePac PRRS) vaccine in a commercial Korean pig farm challenged by a heterologous NADC34-like PRRSV outbreak. Specifically, the study aimed to improve breeding production parameters by assessing the vaccine’s impact on abortion rates, premature births, mummification, and stillbirths; to evaluate its efficacy in blocking vertical transmission through PCR testing of processing fluids collected from litters; and to enhance respiratory health in nursery and finisher pigs by monitoring morbidity and mortality rates following vaccination. By integrating these objectives into a comprehensive herd health management program, this investigation seeks to determine whether a vaccination strategy using PrimePac PRRS and biosecurity can restore production metrics and reduce virus circulation in the face of an acute PRRS outbreak.
MATERIALS AND METHODS
Animal Welfare and Ethical statement
Approval to use the animals on farm was obtained from the farm owner and all procedures were done in full consultation with the farm consulting veterinarian. The Animal Care and Use Committee of Dankook University, Cheonan, Korea, authorized the research protocol (DK-4 2439) of the study.
Vaccine
The vaccine used was a commercially available lineage 7 PRRS MLV (PrimePac® PRRS, MSD Animal Health, Rahway, NJ, USA). The vaccine was prepared according to the instructions of the manufacturer and injected via Intradermal (ID) injection (reconstituted to 0.2ml, minimum 4.0 log10 TCID50 per dose) using a purpose built intradermal injector supplied by the vaccine manufacturer (IDAL®, MSD Animal Health, Rahway, NJ, USA).
Farm Profile and Study Period
The study was a single-herd, longitudinal before-after field intervention conducted in a commercial sow farm located in Gyeonggi Province, South Korea, with approximately 1,200 sows managed in a farrow-to-finish style production system during an acute PRRS outbreak. The farm operated under a two-step pig movement protocol, with piglets weaned at 4 weeks and reared in nursery and grow-finish units in close proximity to the sow herd. After confirmation of an NADC34-like PRRSV outbreak, a whole-herd mass vaccination program using a lineage 7 PRRS modified-live vaccine was implemented. Production and virological data obtained before vaccination served as the pre-intervention reference, and the same parameters were monitored longitudinally after vaccination to evaluate changes in reproductive performance, vertical transmission, and post-weaning health. Because of the severity of the outbreak and the need for immediate herd-level disease control, an untreated contemporaneous control group was not included. Production data was collected from the period of January 2023 (Week 1, January 2023) until December 2024 (Week 52, December 2024). The breeding section of the farm was operating on a one-week batching system. Therefore, records were kept of all sows and gilts mated on a weekly basis. Animals that successfully farrowed at least one piglet were marked as successfully farrowing in the batch. Animals that experienced an abortion were marked as having an abortion in the batch. The total numbers of piglets born to each batch were also recorded and numbers of mummies, stillborn piglets and live piglets were recorded on a weekly basis. For the purpose of recording, a stillborn piglet was classified as a piglet that was not alive at birth and confirmed by visual inspection by the farm staff. Similarly for the suckling piglet mortality, piglets born to a batch of sows were recorded, and at weaning all piglets that died from that batch would be recorded as mortality. The nursery, grower and finisher were managed on a monthly continuous flow basis. Over the course of one month, animals would be continuously added into the nursery, grower and finisher barns. Any deaths from animals associated with this group would be recorded as a mortality. Because this study was conducted as an emergency field intervention in a single commercial herd, analyses were primarily descriptive and exploratory.
During the period of January 2023 to January 2024, the herd veterinarian confirmed that no major outbreaks of PRRS occurred. This farm was previously using a different MLV PRRS vaccine (ORF5 Lineage 5), administered every three months to all sows, gilts and boars on farm and as a continuous program to 2-week-old piglets. From February 2024, a sudden spike in abortions and sow deaths was observed. 8 sows died in Feb 2024 and 30 sows experienced abortions. In the nursery and grow finish area, a combined peak mortality of 32.9% was observed in the month of February 2024. The vaccination program started on Week 9 after the onset of the outbreak and all sows, gilts and boars were concurrently vaccinated each time on the same day. The previous PRRS MLV vaccine was last administered in January 2024 and was not administered again after the onset of the outbreak in February 2024
Sow Vaccination and Sampling Protocol
The herd underwent two initial rounds of mass vaccination, administered four weeks apart followed by serological testing and production monitoring to assess stability. For the purposes of this trial, sow herd stability was defined as a lack of PRRS virus detection in the processing fluid samples. If stability was confirmed, a subsequent mass vaccination was scheduled three months later. However, if stability was not achieved, a contingency plan was implemented, administering a third vaccination four weeks after the second dose. Once stability was established, mass vaccinations continued every three months, maintaining herd immunity. The vaccination and sampling schedule implemented in sows is shown in Figure 1.
Figure 1: Schedule of vaccination and samplings in sows
A sample size of thirty sows was chosen based on standard veterinary epidemiological practices, allowing for the detection of disease at a 10% expected prevalence with 95% confidence in a large herd. During each sampling timepoint, blood samples were collected through jugular venipuncture from thirty sows in the gestation building to monitor PRRS viremia. In addition to serological testing, production parameters, including abortion rates, premature births, stillbirths, and mummy rates, were recorded according to the established schedule. To assess vertical transmission, processing fluids [27,28], from thirty litters were collected monthly and tested by PCR to quantify PRRS levels. Processing fluids were collected during tail docking and castration, as close to birth as possible and all efforts were made to process piglets immediately to avoid the possibility of infection by the sow in the interim time period
Piglet Vaccination and Sampling Protocol
Piglets received their primary vaccination at 2 weeks of age with 0.2 mL of PrimePac PRRS, administered intradermally. To homogenize the immunity inside the nursery, an additional mass nursery vaccination protocol was implemented for pigs already inside the nursery that had already received the previous MLV used on farm. The vaccination and sampling schedule for piglets is outlined in Figure 2.
Figure 2: Schedule of vaccination and samplings in piglets
The processing fluids from 30 litters were collected monthly, and groups of at least 30 piglets were sampled at each age/time point. Also, groups of at least thirty piglets were subjected to blood sampling testing at 6, 9, 12, 15, and 18 weeks of age, with qPCR to assess blood viremia levels of PRRSV. Where piglets tested positive for PRRS (with Ct values below a predetermined threshold), PCR sequencing was conducted to confirm the identity of the virus as NADC34 like using the method outlined by Tu et al [18]. The forward primer was N34-F (5′-CCTGTGTTGACTCATATTGTCTCC-3′); and the TaqMan probe was N34-P (FAM-5′-CGCCCTCACCACCAGCCATTTCCT-3′-BHQ1); the reverse primer was N34-R (5′-CGGCGTAAATGCTACTCAAGAC-3′). The length of the amplicon was 130 bp. Mortality rates were monitored and compared between the pre- and post-vaccination periods for both the farrow-to-wean and wean-to-finish stages. Additionally, processing fluids at birth were gathered from at least thirty litters monthly to monitor vertical transmission of the virus via qPCR. If any samples showed Ct values below 25, they were subjected to sequencing to confirm the specific strain of the virus present.
RESULTS
Pre-Vaccination Analysis
In early February 2024, sows began showing signs of reproductive failure, including elevated abortion rates. Subsequent testing of lung tissue from aborted piglets isolated a strain of PRRSV with 95.85% ORF5 homology with the NADC34 strain of PRRSV. In addition, pooled processing fluid samples collected from stillborn piglets tested positive for PRRSV. During this same period, 6- and 9-week-old piglets were also found to be PRRSV-positive by PCR, with sequencing also confirming 95.85% homology to the NADC34 strain. In practical terms, this means that the field virus was closely related to NADC34 but not identical to it; therefore, it was classified as an NADC34-like PRRSV strain. These findings collectively indicated a farm-wide outbreak of NADC34-like PRRS.
Post-Vaccination Viremia Analysis
Mass vaccination of sows commenced on Week 9, 2024. At the first processing-fluid sampling after implementation of the vaccination program (1 March), 30/30 samples (100.0%) were PRRSV qPCR-positive. At subsequent monthly sampling points, PRRSV RNA was no longer detected in processing fluids, with 0/30 samples (0.0%) testing positive on 4 April, 6 May, 4 June, and 3 July (Figure 3). Clinically, symptoms such as respiratory distress and reproductive failures began to subside in parallel with the loss of detectable PRRSV in processing fluids.
Figure 3: Viremia in processing fluid samples at different timepoints. The average qPCR cycle threshold (Ct) values for PRRSV from testicular fluid at five sampling dates (Mar. 01, Apr. 04, May. 06, Jun.04, and Jul. 03). Higher Ct values indicate lower levels of viral RNA, while lower Ct values reflect higher viral loads. The limit of detection of the assay was a CT value of 45.
Over four specific sampling dates, February 29, July 3, October 19, and December 27—qPCR cycle threshold (Ct) values for PRRSV were measured in five piglet age groups, specifically 6, 9, 12, 15 and 18 weeks (Figure 4).
Figure 4: qPCR Detection of PRRSV Viremia Post-Vaccination in Different Ages. The average qPCR cycle threshold (Ct) values for PRRSV were measured at four sampling dates (Feb. 29, Jul. 3, Oct. 19, and Dec. 27) in piglets aged 6, 9, 12, 15, and 18 weeks. Higher Ct values indicate lower levels of viral RNA (reduced viremia), while lower Ct values reflect higher viral loads.
At the first-sampling time point, PRRSV was detected in 30/30 (100.0%) of 6-week-old pigs and 30/30 (100.0%) of 9-week-old pigs, whereas 3-, 12-, 15-, and 18-week-old pigs were negative (0/30, 0.0% each). On 3 July, the same pattern was observed, with positivity restricted to the 6- and 9-week-old groups (30/30, 100.0% each). On 19 October, PRRSV was detected in 3-, 6-, and 12-week-old pigs (30/30, 100.0% each), whereas the 9-, 15-, and 18-week-old groups were negative (0/30, 0.0% each). On 27 December, PRRSV was detected in 3-, 6-, and 9-week-old pigs (30/30, 100.0% each), while 12-, 15-, and 18-week-old pigs were negative (0/30, 0.0% each). These findings indicate that PRRSV circulation persisted mainly in specific younger post-weaning age groups rather than uniformly across all sampled ages.
Citation
Kim H, Kwon O, Lee J, Choi S, Lin H et al, (2026) Field Evaluation of a Lineage 7 PRRS MLV in Mitigating the Impact of Nadc34-Like PRRS Infection in Korean Pig Farm. JSM Vet Med Res 5(1): 9.
