54 SEPTEMBER 2024 • WORLD AQUACULTURE • WWW.WAS.ORG Damage to fish skin due to capsalids infestation In many cases, fish mortality in aquaculture may not be caused directly by these parasites but by injuries or secondary infections associated with lesions caused by monogeneans. An example is the case of Neobendenia girellae, which feeds on the epidermis and mucus of its host and, sometimes, can devour tissue until it reaches the bone. This feeding activity could cause lesions such as irritation, thinning of the epidermis, desquamation, and ulcers. These lesions create conditions favorable for the proliferation of bacteria and other opportunistic organisms, leading to secondary infections which eventually can cause anemia, kidney damage, and, ultimately, death. Our research team at the Centro de Investigación en Alimentación y Desarrollo (CIAD), Mazatlán, Mexico, has observed fish species such as the yellow snapper (Lutjanus argentiventris) with a high parasite load (300 Neobenedenia sp. per organism; Figure 2A and B), where some fish could develop skin damage. Therefore, the histopathological damage and the presence of bacteria have been evaluated to evaluate their severity and to identify effective treatments scheduled according to the life cycle of the monogeneans for control in culture conditions (Figure 2). Findings include the absence of epidermis with exposure to Monogeneans are tiny worms that parasitize fish, often living on their skin and gills. While many move freely, some attach permanently to a single location. They can even invade the fish’s internal organs, such as the rectal cavity and blood vessels. These parasites can cause significant health problems in fish farms, where it is easy for parasites to spread rapidly, leading to outbreaks of disease and death. This problem is especially severe in floating cages and poorly maintained tanks, where water quality is difficult to control. Monogeneans are common in fishes from oceans, rivers, lakes, and lagoons, with about 5,000 species worldwide. However, a subgroup known as Capsalids is particularly problematic in aquaculture. For example, Neobenedenia girellae, in severe infestations, can cause a decrease in condition factor and substantial fish mortality, resulting in economic losses for stakeholders. These problems can be associated with a simple life cycle: adult worms lay eggs that release larvae, immediately looking for fish to infest, making it very likely to find monogeneans on any captured fish. Furthermore, previous studies (Hutson et al. 2022, Valles-Vega et al. 2019, Tubbs et al. 2005) have indicated that temperature affects this life cycle. For example, in warm temperatures (30 ºC), it is shorter, around 15 days, compared to cold temperatures (18 and 20 ºC), at which it spans 30 days (see Figure 1). Capsalid Monogeneans as a Potential Cause of Fish Disease: A Threat to Aquaculture Isabel Valles-Vega, Rosa María Medina-Guerrero, Selene María Abad-Rosales, Rodolfo Lozano-Olvera, Francis Isela Marrujo-López and María Isabel Abdo-de la Parra FIGURE 1. Schematic of the effect of temperature in the life cycle of Neobenedenia sp., including adult, eggs, and larvae (blue letters indicate time in cold temperature, and red letters indicate time in warm temperature) based on Valles-Vega et al. 2019. FIGURE 2. Yellow snapper infested with Neobenedenia sp. A) The yellow circle indicates selected skin that presents monogeneans. B) Neobenedenia sp. The yellow rectangle indicates the selected skin sections for histological analyses: C) Epidermis regions with lesions, such as congestion. D) Inflammatory response with a slight proliferation of bacteria (yellow square) in the damaged tissue.
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