The Development of a Small-scale Laboratory System to Study Cryptocaryon irritans Infection in Seawater-adapted Guppies, Poecilia reticulata

Abstract

The protozoan parasite Cryptocaryon irritans is responsible for significant economic losses of commercially farmed marine fishes. In this study we report, for the first time, C. irritans infection in the seawater-adapted guppy Poecilia reticulata. We describe a simple method for experimental propagation and maintenance of the infection in a laboratory, without apparent loss of infectivity, for 15 months. Fish were gradually adapted to increasing concentrations of artificial seawater by successive transfer to incremental steps of 10 ppt every 2 days and then transferred to disease propagation aquaria (salinity 30 ± 2 ppt) where the infection was maintained. Between 2- 3 fish were held at any one time in each aquaria, each fish being replaced once it succumbed to the infection and died. Gross observations of heavily infected fish revealed a uniform distribution of trophonts visible on the skin and fins as white surface lesions or ‘spots’ after 4-5 days. Death of the host usually occurred after 7-8 days. Heavily infected fish were fixed and processed using routine histological techniques and histological examination revealed infective trophonts, round or pyriform in shape, invading the fish epithelium as well as larger, feeding trophonts located beneath the epithelium. A thickening of the epithelial layers and a proliferation of mucous secreting cells was evident around embedded parasites.