[Objectives] Bufo gargarizans, a nationally protected amphibian species with significant ecological and medicinal value in China, faces critical conservation challenges due to declining wild populations. Its artificial breeding is essential for sustainable resource utilization, particularly for producing Bufonis Venenum—a key component in Chinese patent medicines such as Liushen Pills and Shexiang Baoxin Pills. However, outbreaks of mass mortality in captive breeding facilities threaten conservation and pharmaceutical efforts. This study investigated the etiology of explosive mortality events occurring in June-July 2024 at a B. gargarizans farm in Shandong Province, China, where environmental conditions (26﹣35 ℃) coincided with seasonal rainfall. [Methods] Eighty-four moribund individuals of B. gargarizans were subjected to necropsy, pathological examination, and molecular diagnostics. Gross examination revealed systemic abnormalities: pulmonary edema, hepatomegaly with pallor, hydrops of the gallbladder, and intestinal edema. Histopathological analysis identified severe pulmonary lesions, including multifocal calcification, extensive tissue vacuolization, and nuclear vacuolization (Fig. 9), indicating profound structural damage. Nematodes were exclusively localized in the lungs, with no detection in the liver, kidneys or gastrointestinal tract. Morphological characterization integrated light microscopy (LM; Fig. 3, 4) and scanning electron microscopy (SEM; Fig. 5). The parasites exhibited fusiform bodies (2.866﹣8.817 mm length × 0.177﹣0.320 mm width), transverse cuticular annulations, funnel-shaped buccal capsules, cylindrical esophagi, and conical tails with terminal papillae, which were consistent with those of Rhabdias spp. Comparative morphometrics against other Rhabdias species (Table 2) further supported taxonomic assignment. [Results] Molecular identification confirmed the pathogen as R. nipponica. Genomic DNA was extracted from nematodes, and ITS (730 bp) and COI (436 bp) genes were amplified with primers NC5/NC2 and COI-F/COI-R, respectively (Table 1). BLAST alignment of the obtained sequences (GenBank accession number: PQ669203 for ITS; GenBank accession number: PQ669209 for COI) showed 99.65% similarity to R. nipponica ITS (GenBank accession number: AB818377) and 98.66% similarity to R. nipponica COI (GenBank accession number: LC671279). Phylogenetic analysis (neighbor-joining method, 1 000 bootstrap replicates) placed the isolates within the R. nipponica clade with strong support (95% for ITS, 63% for COI; Figs. 7, 8). Crucially, PCR screening excluded viral co-infections (LMBV, MRV, RGV, and FV3; Fig. 2). [Conclusion] Epidemiological analysis revealed high parasitism: 65 of 84 (77.38%) B. gargarizans individuals were infected, harboring 283 nematodes (mean intensity: 4.4 nematodes/toad). The extensive pulmonary damage observed—attributed to mechanical obstruction, inflammatory responses, and impaired gas exchange—directly correlated with clinical symptoms (lethargy, anorexia, and dyspnea) and mortality. This study provides the first conclusive evidence of R. nipponica as a primary pathogen causing epizootic mortality in farmed B. gargarizans. The integration of histopathology, morphometrics, and molecular phylogenetics establishes a robust diagnostic framework for lung nematodiasis. Our findings underscore the urgent need for targeted anthelmintic protocols (e.g., ivermectin or fenbendazole, validated in related studies) and environmental disinfection strategies to mitigate transmission. This work lays a foundation for biosecurity measures essential for sustainable farming of B. gargarizans, aligning with national policies promoting non-food wildlife utilization for medicinal conservation.