*Article* **Pathology, Enzyme Activity and Immune Responses after** *Cryptocaryon irritans* **Infection of Golden Pompano** *Trachinotus ovatus* **(Linnaeus 1758)**

**Hua-Yang Guo 1,2,3,†, Wen-Fu Li 4,†, Ke-Cheng Zhu 1,2,3, Bao-Suo Liu 1,2,3, Nan Zhang 1,2,3, Bo Liu 1,2, Jing-Wen Yang 1,2 and Dian-Chang Zhang 1,2,3,\***


**Abstract:** Golden pompano (*Trachinotus ovatus*) has become an economically important fish in China in the past decade. However, *Cryptocaryon irritans*, a parasitic ciliate, causes considerable economic losses in the mariculture of *T. ovatus*. To characterize the pathogenesis of *C. irritans* in *T. ovatus*, the pathological properties, immune-related enzyme activity and expression of the *NEMO* gene was analysed. The results from the histological sections showed that there was considerable metamorphosis and hyperplasia in the parasitized sites (skin) with leukocyte aggregation and mucous cell increases after *C. irritans* infection. Moreover, the activities of four enzymes, that is, alkaline phosphatase (AKP), acid phosphatase (ACP), superoxide dismutase (SOD) and lysozyme (LZM), were significantly increased in different tissues after *C. irritans* infection. Furthermore, the ORF of *T. ovatus* NF-kappa-B essential modulator (*ToNEMO*) measures 1650 bp, encoding 548 amino acids. The *ToNEMO* transcripts were universally expressed in all examined tissues, with higher levels being observed in the immune-relevant and central nervous tissues. The mRNA levels of *ToNEMO* after *C. irritans* infection were significantly increased in the gill, skin, liver, spleen and head kidney. These results suggested that *ToNEMO* might be involved in immune responses and helped to elucidate the physiological response after the *C. irritans* infection of fish.

**Keywords:** *Cryptocaryon irritans*; *Trachinotus ovatus*; histologic section; *NEMO*; enzyme activity
