Molecular genetic characterization of porcine reproductive and respiratory syndrome virus outbreak in Assam, India and neighbouring regions.

Porcine reproductive and respiratory syndrome (PRRS) is a significant swine disease with no effective vaccine due to high viral mutation rates. This study investigates a natural PRRS outbreak through molecular, pathological, and serological analyses. Nineteen affected pigs were clinically examined, and 10 underwent post-mortem examination. PRRS virus (PRRSV) presence was confirmed in all tissue samples by RT-PCR targeting open reading frame (ORF) 5 and ORF7 genes. Clinical signs, especially in boars and sows, included fever, appetite loss, movement reluctance, erythematous skin patches, vomiting, and abortions in sows. Post-mortem findings highlighted lung consolidation, severe lymph node enlargement, interstitial pneumonia with mononuclear cells, macrophage accumulation and necrotic cells in alveolar spaces. Multifocal myocarditis, lymphoid follicular degeneration, and follicular necrosis were observed in the tonsil, spleen, and lymph nodes. PRRSV-specific antibodies were detected in 32.75% of serum samples, confirming the outbreak. Phylogenetic analysis of the PRRSV-ORF5 and ORF7 genes revealed a close genetic relationship between the outbreak samples from Assam and recent outbreaks in Idukki, Kerala, India (2018), and neighbouring country China, indicating the circulation of Genotype 2 virus in Assam. However, the sequences showed some differences from the isolates of Mizoram, India. In conclusion, this study provides molecular and pathological evidence of a PRRSV outbreak, confirms the presence of PRRSV-specific antibodies and viral RNA, and shed light on the virus's genetic characteristics in India.