Molecular and serological evidence of Nairobi sheep disease virus in Haemaphysalis longicornis and sheep in eastern Shandong Peninsula, China.

Abstract

BACKGROUND: Nairobi sheep disease virus (NSDV), a highly pathogenic tick-borne orthonairovirus affecting ruminants, represents an emerging zoonotic threat. While endemic to East Africa, recent reports confirm its presence in Asia. HYPOTHESIS/OBJECTIVES: This study investigated NSDV circulation in the eastern Shandong Peninsula. ANIMALS: A total of 745 Haemaphysalis longicornis ticks (grouped into 139 pools) and 246 ruminant serum samples were collected during 2021 field surveillance in Weifang and Yantai regions. METHODS: Molecular detection employed quantitative reverse transcription PCR and nested PCR assays, with genomic characterization through complete and partial segment sequencing. Serological analysis used ELISA and immunofluorescence assays (IFAs). Phylogenetic reconstruction was performed using maximum-likelihood methods. RESULTS: Nairobi sheep disease virus RNA was detected in 9 tick pools (minimum infection rate: 1.2%). Genomic analysis revealed one complete genome (L: 11 999 bp; M: 5013 bp; and S: 1469 bp) and 4 partial S segment sequences (975 bp) showing > 98% nucleotide identity with Chinese reference strains and 100% identity among themselves. Phylogenetically, Shandong isolates clustered with other Chinese NSDV variants, indicating local evolutionary adaptation. Seroprevalence reached 5.7% (14/246) among ruminants, suggesting local virus exposure. CONCLUSIONS AND CLINICAL IMPORTANCE: This molecular and serological evidence of NSDV in eastern Shandong suggests the potential presence of the virus beyond its historical range. The close genetic relationship with other Chinese strains suggests regional spread rather than independent introductions. These findings underscore the need for enhanced surveillance in tick-ruminant systems and the development of control strategies to mitigate economic and zoonotic risks.