Pathogenicity associated with an infestation of the marine leech parasite Pterobdella arugamensis in farmed fish

The marine leech Pterobdella arugamensis is a hematophagous parasite, and the extent of injury to the host largely depends on the number of attached leeches. This study aimed to assess the pathogenicity of marine leeches in Asian seabass (Lates calcarifer) and tiger grouper (Epinephelus fuscoguttatus) fingerlings under laboratory conditions. Five groups of healthy Asian seabass and tiger grouper were exposed to varying numbers of marine leeches (0, 1, 10, 30, or 70 per fish) for 7 d. Infested Asian seabass and tiger grouper both showed pathological changes even with only 1 leech, manifesting as clinical signs like haemorrhages. The cumulative mortality at 7 d post-exposure (dpe) was 11 or 33% for Asian seabass infested with 1 or 10 marine leeches, respectively. Fish with 30 or 70 marine leeches showed higher rates of mortality (56%). A similar trend was seen in tiger grouper, with mortality rates reaching 78% in fish with 30 or 70 marine leeches, and 56 or 33% in fish with 10 leeches or 1 leech, respectively. Factorial analysis of mortality after 7 dpe between both species showed significant differences (2-way ANOVA p = 0.001) when exposed to varying numbers of marine leeches. The haematocrit values differed significantly between Asian seabass or tiger grouper infested with either 0 or 1 marine leech and those infested with 10, 30, or 70 marine leeches (1-way ANOVA, p = 0.0001). This suggests that marine leech infestation has a measurable impact on both species. Consequently, fish farmers should promptly address leech infestation upon discovery in their cages.