Mahmoud Aly Mahmoud

White spot disease is one of the devastating protozoal infections affecting freshwater fish. Commonly known as “Ich”, the Ichthyophthiriasis can infect almost all freshwater fish causing devastating losses in susceptible fish. In the present study, an outbreak of Ichthyophthiriasis erupted in one of the holding tanks of two ornamental fish species, Siamese shark (Pangasius sutchi) and goldfish (Carassius auratus var.bicausatus). Initial observation of the outbreak showed that only Pangasius sutchi was affected by typical white spots associated with mortalities. However, Carassius auratus, a known susceptible species for Ichthyophthirius multifiliis (Ich) in the same aquarium showed only mild erythema that disappeared during the course of infection with no mortalities. To confirm the previous observation, an experimental designs was performed in which infection with Ichthyophthirius was induced in Pangasius sutchi species alone. Cohabitation was performed between the Ich-induced Pangasius sutchi and Carassius auratus. Three days after the induction, Pangasius sutchi started showing the typical clinical signs. Mortalities associated with severe infection were recorded in Pangasius sutchi by 7th day after infection. Associated Carassius auratus showed only mild erythema that disappeared by the end of experiment. Histopathological examination of skin from both species in natural and experimental infection was performed to evaluate the severity of infection on the tissue level. Substantial numbers of typical large size trophonts surrounded by layers of fibrous tissue, melanophores and hemorrhages were detected in dermal and epidermal layers. Underlying myodegeneration was also associated the skin lesions in Pangasius sutchi. In contrary, pathological changes in the skin of Carassius auratus were mild and few numbers of immature trophonts were noticed in the epidermal layers. Possible reasons for such infection discrepancies between the two susceptible species are discussed.