[Weiboxin] Japan Reports First Confirmed Dual Infection of PRRSV-1 and PRRSV-2 in a Pig Farm, Presenting New Challenges for Porcine Reproductive and Respiratory Syndrome Control

Recently, a research team from the National Institute of Biomedical Innovation, Health and Nutrition in Japan published a study in the Journal of Veterinary Medical Science that, for the first time, confirmed co-infection with both the European genotype (PRRSV-1) and the North American genotype (PRRSV-2) of porcine reproductive and respiratory syndrome virus (PRRSV) at a commercial pig farm in Kagoshima Prefecture, Japan.

This study was initiated in April 2025, following the emergence of respiratory symptoms in post-weaning pigs at a commercial pig farm in Kagoshima Prefecture that had not been vaccinated against PRRSV-1. Researchers collected a total of 12 samples, including sow serum, piglet serum, and pooled serum samples from piglets of different ages.

Detection was performed using a self-developed RT-PCR assay that effectively distinguishes between the two PRRSV serotypes (with expected amplicon sizes of 403 bp for PRRSV-1 and 438 bp for PRRSV-2) and achieves a sensitivity of approximately 10 TCID50/mL.

Porcine reproductive and respiratory syndrome (PRRS), commonly known as “blue ear disease” in pigs, is a viral disease. Highly contagious infectious disease 。

PRRSV belongs to the Arteriviridae family and is divided into two major genotypes: the European type (PRRSV-1) and the North American type (PRRSV-2), with only about 60% genetic homology between them.

The disease is characterized by reproductive disorders in sows and respiratory illness in piglets, with clinical signs including anorexia, fever, and dyspnea; sows may exhibit abortion, stillbirths, and mummified fetuses.

In recent years, Asian countries such as China and South Korea have reported widespread circulation of PRRSV-1, while Japan first detected PRRSV-1 in 2008 and identified a novel strain in the Yamaguchi–Shikoku region in 2023.

PRRSV is an enveloped single-stranded positive-sense RNA virus, with virions measuring approximately 50–65 nanometers in diameter. The outer layer consists of a lipid envelope studded with small spike-like projections.

The genome is approximately 15 kb in length and contains multiple open reading frames (ORFs) that encode a variety of structural and non-structural proteins. Among these, the GP5 protein is the major structural component of the viral envelope and plays a critical role in viral entry; however, the amino acid sequence similarity between the GP5 proteins of PRRSV-1 and PRRSV-2 is only 50–55%.

The Nsp2 protein is the largest protein encoded by PRRSV and exhibits substantial genetic diversity, with only about 40% amino acid sequence homology between the Nsp2 proteins of PRRSV-1 and PRRSV-2.

The test results showed that PRRSV-1 and PRRSV-2 were detected simultaneously in three samples from sows and two samples from 30-day-old piglets, and the mixed sample from 60-day-old piglets was also positive for both viruses.

Individual analysis confirmed that one 60-day-old piglet was simultaneously infected with two viruses. Sequence analysis revealed that the PRRSV-1 strain belongs to the L1.2 sublineage and exhibits 98.68% sequence homology with the VP-046bis strain of the Unistrain® PRRS vaccine, and is therefore classified as Vaccine strain 。

The PRRSV-2 strain belongs to L4/Cluster III and exhibits 93% sequence homology with the local strain isolated in Kagoshima in 2016, indicating a localized epidemic pattern. The corresponding ORF5 sequences have been submitted to GenBank (accession numbers: LC877719 and LC877720).

Although no significant increase in abortions or stillbirths was observed at the pig farm, clinical signs such as reduced feed intake in sows and respiratory disease in piglets were reported.

The study indicates that vaccine-like strains have been detected in pig farms that have not been vaccinated against PRRSV-1, possibly through Transmission at nearby vaccination sites Detection of the PRRSV-2 L4/Cluster III strain, however, suggests the potential for endemic circulation in the Kagoshima region.

PRRSV infection can cause respiratory signs in pigs of all ages, with particularly severe consequences for pregnant sows and piglets. Infected pigs often develop prolonged viremia or persistent infection with viral shedding, and may continue to shed the virus for up to eight weeks even after clinical signs have resolved.

The RT-PCR method employed in this study effectively distinguishes viral strains, underscoring the critical role of molecular diagnostic tools in PRRSV surveillance.

In the diagnosis of PRRS, the integrated application of serological and etiological diagnostic techniques is essential. ELISA is widely recognized as a highly sensitive and specific method that allows for high-throughput testing, playing a critical role in PRRS outbreak surveillance.

Real-time fluorescent quantitative PCR (qPCR), particularly the TaqMan method, has become one of the most widely used molecular detection techniques due to its high sensitivity and specificity.

Research indicates that PRRSV outbreaks in Japan are becoming increasingly diverse, with a growing risk of vaccine-like strains spreading in non-vaccinated herds, underscoring the need to strengthen biosecurity measures and implement region-specific vaccination strategies.

Pig farms should enhance biosecurity and hygiene management, while veterinary authorities can optimize molecular surveillance and vaccine deployment to address the ongoing evolution of PRRSV.

Building a biosafety system is the foundation for prevention and control, and should be centered on three key elements: eliminating infection sources, interrupting transmission routes, and protecting susceptible animals. Specifically, this entails implementing effective control over the “four flows” associated with production and operations—human movement, vehicle traffic, logistics, and pig movement.

For PRRSV-negative pig farms, the use of live attenuated PRRS vaccines is generally not recommended; inactivated PRRS vaccines may be used for prevention. Piglets and growing-finishing pigs, however, may be immunized with live attenuated PRRS vaccines.

The co-circulation of PRRSV-1 and PRRSV-2 and its clinical implications remain to be thoroughly investigated; future research should increase sample sizes, trace the origins of the viruses, and monitor pathological differences among individual pigs.

In theory, the coexistence of two viruses could lead to interspecies recombination; although no naturally occurring recombinant strains have yet been confirmed, vigilance remains necessary.

As the prevalence of PRRSV-1 continues to rise in China, it is imperative to strengthen surveillance for this strain. When farms observe clinical signs suggestive of porcine reproductive and respiratory syndrome, they should not only test for PRRSV-2 strains but also for PRRSV-1 strains, in order to prevent diagnostic delays that could result in untimely treatment and substantial economic losses.

With the continuous improvement of global animal disease surveillance networks, the genetic diversity of PRRSV is being increasingly elucidated. The reported co-infection of PRRSV-1 and PRRSV-2 in Kagoshima Prefecture, Japan, suggests that future control of porcine reproductive and respiratory syndrome will face even greater challenges. Complex Challenges 。

Viral transmission is no longer constrained by geographic boundaries, with European and North American strains capable of co-circulating within the same pig farm. This dual-infection pattern may increase the risk of viral recombination, thereby complicating vaccine selection and the development of effective control strategies.

In the face of the constantly evolving PRRSV, the global swine industry must strengthen international cooperation and share surveillance data. Optimize diagnostic tools only then can we effectively address this disease challenge that has plagued the swine industry for decades.

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