Cortisol and dexamethasone increase the in vitro multiplication of the haemoflagellate, Cryptobia salmositica, possibly by interaction with a glucocorticoid receptor-like protein

[Display omitted] ► In vitro parasite (Cryptobia salmositica) replication is enhanced by cortisol and dexamethasone. ► Mifepristone inhibits the stimulatory effects of both glucocorticoids (cortisol and dexamethasone). ► The host’s stress hormone (cortisol) is used as a reproductive hormone by the pathogen. ► The study indicates the presence of glucocorticoid receptors on/in the parasite. Cryptobia salmositica is a pathogenic haemoflagellate of Pacific salmon, Oncorhynchus spp., on the west coast of North America. The in vitro multiplication of the parasite was significantly enhanced by the addition of cortisol (within a range consistent with physiological levels in salmonid fishes; 10–50ngml−1) to the culture medium (MEM supplemented with FBS). However, higher cortisol concentrations (100 and 200ngml−1) either had no enhancing effects or resulted in lower replication rates compared with the controls. The synthetic glucocorticoid, dexamethasone (Dex), also stimulated the replication of the parasite and mifepristone (RU486), a synthetic steroid that has glucocorticoid receptor (GR) antagonist properties, inhibited the stimulatory actions of both cortisol and Dex, when added to the medium at a concentration of 100ngml−1 co-culture with cortisol or Dex. Furthermore, the dose-dependent effects of glucocorticoids (cortisol and Dex) on the multiplication of the haemoflagellate were correlated with the initial size of the inocula. The study revealed a novel relationship between the parasite and its host, in which the host’s cortisol is used by the parasite to enhance its replication. Also, since C. salmositica responds to both native and synthetic glucocorticoids and to the GR antagonist, RU486, and exhibits a biphasic (hormetic) response to the amount of cortisol in the medium, we propose that the glucocorticoid exerts its effects via an interaction with GR-like proteins in C. salmositica that are functionally similar to those present in vertebrate cells.