Antifungal Activity and Protective Effects of Cetylpyridinium Chloride Against “Milky Disease” in Chinese Mitten Crab (Eriocheir sinensis)

The milky disease caused by Metschnikowia bicuspidata has severely affected the farming of Chinese mitten crabs (Eriocheir sinensis), yet there are still no effective and environmentally friendly control measures available. This study employed a drug-repositioning strategy, combined with virtual screening, in vitro antifungal tests, and in vivo experiments, to evaluate the effect of cetylpyridinium chloride (CPC) on the pathogen. Sterol 14α-demethylase (CYP51), the key enzyme in the ergosterol biosynthetic pathway, was selected as the candidate target for molecular docking. The docking results showed that CPC had a strong binding affinity (score −9 kcal/mol) to the predicted CYP51 active site, but its use as a direct target still requires further biochemical verification. In the in vitro experiment, CPC exhibited potent antifungal activity (MIC = 1.56 μg/mL, MFC = 3.125 μg/mL) and significantly reduced the formation of biofilms at the tested concentrations (p < 0.05). After feeding at a dose of 800 μg/g feed for 7 days, the activities of immune-related enzymes (ACP, AKP, LZM) and antioxidant enzymes (CAT, SOD, GPX) in the hepatopancreas of crabs significantly increased (p < 0.05), and no oxidative stress was induced. In the challenge test, the original cumulative mortality rate of group C (challenge + 800 μg/g CPC bait) was 59.72%, lower than that of group B (challenge + basal feed) at 75.00% (absolute risk reduction of 15.28 percentage points). The corrected survival rates were 40.0% (group C) and 24.4% (group B), but the group differences after correction by the generalized linear mixed model did not reach a statistically significant level (p = 0.068). The histopathological score showed that CPC significantly alleviated the severity of lesions in the hepatopancreas, gills, muscles, and heart (p < 0.001). Although the targeting mechanism of CPC against CYP51 requires further experimental validation, our findings suggest that CPC holds promise as a drug-repositioning candidate for managing milky disease in E. sinensis, particularly by boosting host immunity and mitigating tissue damage.