Bioscience Methods 2025, Vol.16, No.3, 162-172 http://bioscipublisher.com/index.php/bm 164 The disease progresses slowly but has a high mortality rate. It has broken out in many farmed fish, including California bass, yellow catfish and hybrid snakehead fish, causing significant economic losses to the aquaculture industry. Aeromonas hydrophila is widely present in water bodies and is prone to multiply in large numbers when the water temperature rises or when organic matter is abundant. After being infected with Aeromonas hydrophila, snakehead fish will show obvious symptoms of bleeding and ulceration, often accompanied by ascites and gill anemia, with a rapid course of disease and rapid death (Liu et al., 2021). In terms of viral diseases, snakehead fish vesicular virus (also known as snakehead fish rhabdovirus) has attracted attention in recent years. The virus belongs to the family of rhabdoviridae and can cause symptoms similar to carp spring virus disease, including lymphatic hemorrhage, abdominal distension, slow swimming, etc. Experiments have shown that after snakehead fish cells are infected with the virus, viral proteins use the host ubiquitin system to promote their own replication (Qin et al., 2024). In addition, snakehead fish is also susceptible to some parasites and fungi. For example, parasites such as wheelworms and ringworms can attach to the gills of snakehead fish, causing breathing difficulties and secondary infections. Myxosporidia caused by dinoflagellates also causes losses in the juvenile stage (Ferdous et al., 2022). Among fungal diseases, the most typical one is fulminant ulcerative syndrome (EUS) caused by invasive Saprolegnia invadans. After infection, snakehead fish develop large areas of ulcers and necrosis on the skin, with a high mortality rate. It is listed as a reportable disease by the World Organization for Animal Health (Qin, 2023). 2.2 Overview of immune response mechanism As a teleost fish, the immune system of snakehead fish includes two major parts: innate immunity and adaptive immunity. It has both similarities and differences with higher vertebrates. Innate immunity is the first line of defense for snakehead fish against pathogens, and is composed of skin mucus barrier, phagocytes, lysozyme and complement (Figure 2) (Mokhtar et al., 2023). When pathogens invade, pattern recognition receptors (PRRs) in the innate immune system of fish, such as Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), and NOD-like receptors (NLRs), can recognize pathogen-associated molecular patterns (PAMPs) such as bacterial lipopolysaccharides and viral RNA, triggering downstream signaling pathways (Fraslin et al., 2020). Figure 2 Schematic representation of the fish mucosal immunity (Adopted from Mokhtar et al., 2023)
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