First report of Dactylogyrus formosus from India: a case of co-existence with Paradactylogyrus catlaius causing mass mortality in Indian major carp, Catla catla.

The family Dactylogyridae, known for its role as a frequent pathogen in Cyprinids, was identified in a case of mass mortality involving Catla catla fingerlings (measuring 6.5 ± 2.0 cm and weighing 40.5 ± 10 g) in West Bengal, India. Microscopic examination revealed the presence of two co-existing parasites, Dactylogyrus formosus and Paradactylogyrus catlaius, attached to different sections of the gill filament. Despite their coexistence, these parasites exhibited marked differences in their haptoral hard parts, genital organs, and preferred habitats. Molecular analysis of the Internal Transcribed Spacer 1 and 4 genes indicated more than 90% similarity between the detected parasite and D. formosus previously reported in China. Histopathological observations illustrated the parasites' specific attachment to the distal end of the primary gill lamellae, gradually causing destruction to a maximum number of secondary lamellae. Internally, infiltration of eosinophilic granular cells was observed in gill and kidney blood vessels, while the liver exhibited hepatocytes filled with hemosiderin. The infected fish were treated for 24 h with a safe dose of common salt (5.6 ppt) and albendazole (62 ppm). The survivability rate was significantly higher (p < 0.05) in both treated groups compared to the control. Interestingly, the group treated with common salt showed superior results compared to the albendazole-treated fish. This study presents the sympatric speciation of D. formosus in a new host (C. catla) and explores its host specificity, histopathology, and treatment methods. This case marks the first report of D. formosus causing substantial mortality in cultured Catla in India, alongside the coexistence with Paradactylogyrus catlaius.

Myxozoan fish diseases: possible treatment and zoonoses.

Myxozoan parasites pose huge threat to wild and cultured fishes and reported to cause heavy mortality, retarded growth and post-harvest quality degradation. It is one of the highly divergent groups of parasites which infects skin, gill, muscle, cartilage and internal organs of host fish and the severity of pathology varies depending on the water temperature, species of fish, site of tissue infection and immune resistance of the individual host. Most infections are difficult to treat, as they can easily evade host cellular and humoral defence mechanisms by proliferating or migrating through immune compromised sites of the host and forming large plasmodia encapsulated by the host cellular elements. This spore-forming parasite is harmless to humans but often detected in faecal samples of immunosuppressed humans. The incidences are mostly associated with the consumption of infected fish having a high concentration of spores which causes diarrhea and stomach pain. Currently, there are no immunostimulants or vaccines available for controlling these parasites, however, fumagillin is the drug of choice in fish for controlling this parasitic infection. Excessive usage of fumagillin causes tissue damage and retarded growth in fish, hence feed incorporation of this antibiotic in proper dose is essential for effective treatment. In this review detailed information on the diseases caused by myxozoan parasites in fishes and their zoonotic potential is discussed.