<![CDATA[Classical swine fever (CSF) is a highly contagious viral disease of pigs caused by classical swine fever virus (CSFV) and often results in significant economic losses in endemic regions. This retrospective study investigated the histopathological, immunohistochemical, molecular, and phylogenetic characteristics of CSFV infection in archived formalin-fixed, paraffin-embedded (FFPE) porcine tissues collected in Malaysia between 2018 and 2024. A total of 375 FFPE tissue blocks from 100 pigs were retrieved. Histopathological examination of 13 pigs with lesions consistent with CSF revealed lymphoid depletion, histiocytic hyperplasia, vascular congestion, haemorrhage, and necrosis, predominantly affecting lymphoid and vascular organs. CSFV antigen localisation was demonstrated by immunohistochemistry in 5/13 pigs, predominantly in macrophages, epithelial cells, and vascular endothelial cells across several tissues. In one immunohistochemically positive case, the presence of CSFV RNA was confirmed using two-step PCR targeting the 5′ untranslated region (5′ UTR). Phylogenetic analysis of this partial sequence placed the isolate within genotype 1; however, this finding is based on a single case and should be interpreted with caution. The limited PCR detection rate is likely attributable to RNA degradation inherent in FFPE tissues. This study demonstrates that archived FFPE tissues can serve as a valuable resource for retrospective CSFV diagnosis, particularly when histopathology and IHC are combined. However, molecular detection from archival material remains challenging, and epidemiological inferences should be considered preliminary.]]>
