Investigation of the distribution and origin of porcine reproductive and respiratory syndrome virus 1 in the swine production chain: A retrospective study of three farms in Thailand

Abstract

Background and aim: Porcine reproductive and respiratory syndrome (PRRS), caused by PRRS virus (PRRSV), is a global issue that affects Thai swine as well. In Thailand, PRRSV-2 predominates over PRRSV-1. The origin of PRRSV-1 transmission remains undiscovered. This study traced the source of infected pigs responsible for disease transmission among three pig-fattening farms and analyzed the spread of PRRSV-1.

Materials and methods: A total of 696 swine samples from breeding and pig-fattening farms in Thailand were screened for PRRSV using open reading frames (ORF7) reverse transcription polymerase chain reaction (RT-PCR). Positive samples were identified as PRRSV-1 using ORF5 RT-PCR. The analysis included the study of nucleotide homology, GP5 amino acid sequences, and N-linked glycosylation patterns to assess the spread of PRRSV-1 across these farms.

Results: Genetic examination identified 28 PRRSV-1-positive samples, of which 13 were chosen as representatives. These strains were categorized into three groups based on breeding farm pig houses and showed distinct distribution patterns across pig-fattening farms. Group 1 included piglets transferred from pig house A to Nakhon Pathom, Chonburi, and Sa Kaeo. Groups 2 and 3 showed transfers from pig houses F and H to Chonburi and Sa Kaeo farms. All 13 PRRSV-1 strains were categorized into PRRSV-1 subtype 1/clade H. N-linked glycosylation analysis revealed that nearly all PRRSV-1 strains exhibited a conserved glycosylation pattern at amino acid positions N37, N46, and N53. This pattern is consistent with the glycosylation profile of the previous Thai PRRSV-1 subtype 1/clade H.

Conclusion: The present study highlights the persistent presence of PRRSV-1 in Thai swine, which leads to sporadic outbreaks. The molecular genetic analysis identified three primary strain groups dispersed throughout the pig production system, emphasizing the importance of regular monitoring for new PRRSV strains in this herd. Understanding the PRRSV-1 distribution in swine farms is vital for veterinarians. This knowledge supports strategies for eradicating the virus and managing swine health effectively in Thailand.

Keywords: Thailand; porcine reproductive and respiratory syndrome virus-1; swine production system; virus distribution.

Figure-1

The pig farms in this study are situated in Central Thailand (Nakhon Pathom) and Eastern Thailand (Chonburi and Sa Keao). The breeding farm (grey area in the map), housing a total of 4,000 sows, is located in Chonburi province and comprises 10 pig houses. After being weaned at 4 weeks of age, the finishing pigs are transferred to pig-fattening farms in Chonburi, Sa Kaeo, and Nakhon Pathom provinces. Each of these farms has a capacity of 500–600 pigs. (Source: This image was created using BioRender scientific illustration software, and the map was constructed using Microsoft Excel in Microsoft Office LTSC Professional Plus 2021).

Figure-2

Phylogenetic tree based on the complete ORF5 gene, demonstrating genetic classification between Thai PRRSV-1 strains and reference PRRSV-1 strains. (a) A total of 107 ORF5 sequences, including 13 from Thai PRRSV-1 strains in this study, 93 PRRSV-1 reference sequences from the GenBank database, and the PRRSV-2 strain VR2332 (U87392) as the outgroup, were used in the phylogenetic analysis. The PRRSVs grouped in subtype 1/clade H are shown with black triangles. (b) PRRSV-1 strains in subtype 1/clade H include 13 Thai PRRSV-1 strains from this study, divided into three major strains, 20 Thai PRRSV-1 strains from previous studies, and four PRRSV-1 strains from Belgium and Korea. Thai PRRSV-1 strains from this study are labeled with red squares. The bootstrap values greater than 50% are shown on the branches. ORF=Open reading frames, PRRSV-1=Porcine reproductive and respiratory syndrome virus-1.

Figure-3

The amino acid alignment of GP5 was conducted among 13 representative PRRSV-1 strains, 11 PRRSV-1 reference strains, and the Lelystad strain (PRRSV-1 prototype). The numbered categories include: (1) PRRSV-1 major strain 1, (2) PRRSV-1 major strain 2, (3) PRRSV-1 major strain 3, (4) PRRSV-1 strain isolated in Chonburi province from our previous study, and (5) PRRSV-1 strain isolated in a different province of Thailand. The boxes labeled 1–6 above the amino acid sequence correspond to: (1) Signal peptide (amino acid positions 1–33), (2) Neutralizing epitope B (amino acid positions 38–54), (3) Hypervariable region 1 (HVR1), (4) Non-neutralizing epitope A (amino acid positions 29–35) in the Lelystad strain, (5) Hypervariable region 2 (HVR2), and (6) Hypervariable region 2 (HVR2). PRRSV-1=Porcine reproductive and respiratory syndrome virus-1.