Population increase of Gyrodactylus derjavini on rainbow trout induced by testosterone treatment of the host

Abstract

Field observations indicating a possible effect of testosterone on the susceptibility of teleosts to gyrodactylid infections were tested experimentally. Female rainbow trout Oncorhynchus mykiss (O+) were injected with testosterone (5 or 10 pg) or vehicle alone (placebo) and subjected to infection with Gyrodactylus derjavini Mikailov, 1975 through cohabitation with infected fish. The number of parasites on testosterone-treated fish increased significantly compared to parasites on untreated fish. The site selection of parasites differed shghtly between experimental groups. Infected trout used for cohabitation expenenced a marked decrease in infection, indicating acquired resistance. This was further supported by the different site selection of parasites on these fishes. The superficial mucous cell density on the tail fins of testosterone-treated fish was not significantly lower than in the placebo trout, whereas previously Infected fish with a low parasite burden showed a significantly higher mucous cell density. A poss~ble immunosuppressive effect of testosterone is discussed, but although testosterone is likely to act on the trout immune system, the possibhty that the male sex hormone affects the parasites' reproduction directly cannot be excluded.

Gyrodactylus salaris (Monogenea, Gyrodactylidae) infections on resident Arctic charr (Salvelinus alpinus) in southern Norway

“…A positive correlation between increased testosterone levels and susceptibility to Gyrodactylus has been shown in salmonids (Buchmann 1997), and this could also potentially contribute to the observed pattern.…”

Associations between fish reproductive cycle and the dynamics of metazoan parasite infection

“…When considering the observation on the relationship between GSI and steroid hormones (Jackson and Sullivan 1995), the higher intensity of viviparous Gyrodactylus in our study could suggest a potential immunosuppressive effect of steroid hormones, the level of which could increase during the breeding period in both sexes. Increasing intensity of Gyrodactylus species connected with fish steroid hormones was previously reported in experimental studies in female fish (Buchmann 1997).…”

Rainbow trout leucocyte activity:influence on the ectoparasitic monogenean Gyrodactylus derjavini

“…Second, immunoreactivity of trout is better between 10 and 20°C compared to the interval 1-5°C (O'Neil 1985, Andersen & Buchmann 1998); and third, rainbow trout are capable of mounting a significant response to G. derjavini at 1 1 "C (Buchmann 1997, Buchmann & Bresciani 1998. Under these conditions it was shown that peritoneal macrophages were able to colonize G, de javini within 3 h whether or ruled out that the squalene injection used for the isolation activates the peritoneal macrophages to a certain extent.…”

“…Complement has been shown to possess a devastating effect on gyrodactylids (Harris et al 1997, Buchmann 1998 and is known to be produced by macrophages at least in mammals (Lappin & Whaley 1993). As gyrodactylids are seen to escape surface areas rich in mucous cells and macrophages during the response phase (Buchmann 1997, Buchmann & Bresciani 1998, Lindenstrnn & Buchmann 1998, it could be suggested that macrophages in the skin have some influence on the amount of complement and mucous cells in the fish skin and thereby affect the microenvironment of these ectoparasites. Some studies have shown that fish leucocytes are able to interact with both protozoans (Cross & Matthews 1993, Steinhagen & Hespe 1997 and metazoan parasites as cestodes (Sharp et al 1991, Hoole 1994, Taylor & Hoole 1995, Morley & Hoole 1997, digeneans (Wood & Matthews 1987, Whyte et al 1989, nematodes (Ramakrishna et al 1993) and acanthocephalans (Hamers et al 1992).…”