Integrative High-Throughput Screening and Microscopic Evidence Implicates Microsporidia as a Potential Pathogen of "Pus Crab" in the Mud Crab (Scylla paramamosain).

The mud crab (Scylla paramamosain), an economically important crustacean aquaculture species in southern China, is susceptible to infections due to its immune system lacking acquired immunity. An emergent disease locally termed "pus crab" has caused severe muscle lesions in pond-farmed crabs, but its etiology remained unclear. Here, we applied an integrated approach, histopathology, electron microscopy, metagenomic sequencing, and experimental infection to identify the pathogen of "pus crab". Histological staining (H&E, Wright-Giemsa, and Masson) revealed muscle fiber dissolution, disordered fiber arrangement, and abundant interstitial spore-like bodies. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed intracellular spore morphology consistent with microsporidia. Metagenomic profiling showed a pronounced shift in the muscle microbiome, with a marked increase in microsporidian taxa at the genus level and a concurrent decline in bacterial relative abundance. Functional annotation indicated enrichment of pathways related to protein processing, ribosome biogenesis, glycosylation, and the ubiquitin-proteasome system. Isolation of spores from diseased muscle and subsequent injection into healthy crabs reproduced wild-like clinical signs and histopathology, confirming infectivity and implicating microsporidia as the likely etiological agents of "pus crab". These findings establish a multidisciplinary framework for pathogen identification in aquaculture and provide candidate molecular and biochemical markers for early diagnosis and management.