Host parasite interactions between Ichthyobodo necator (Henneguy 1883) and farmed Salmonids

Abstract

The literature on Ichthyobodo necator is reviewed. The prevalence and intensity of Ichthyobodo infestations on farmed salmonids was investigated on three farms over a period of two years. The infestations were found to be markedly age dependent. Peak infestations and related mortalities occurred in the first eight weeks after first feeding. Both mortalities and infestations declined to zero shortly after this period with no chemotherapy. Ichthyobodo reappeared on 0+ and appeared for the first time on I+ fish after a drop of water temperatures to less than 10*C. Many of the 1+ fish had started to mature. It is suggested that some form of host defence mechanism operates which limits the Ichthyobodo infestations in farmed salmonids. The sequential pathology of Ichthyobodo infestations of the skin of 0+ and 1+ salmon and rainbow trout was studied. Areas of greatest shelter from water currents were found to be most commonly infested and no parasites were found attached to the epidermis on the head of the fish. The parasite caused hyperplasia of the malphigian cells and exhaustion of the goblet cells below infestations, followed by spongiosis of the underlying epidermis. The epidermal plaque then sloughed off leaving a single layer of cells attached to the basement membrane. Cell kinetic studies showed that Ichthyobodo caused the cells immediately below infestations to divide, a markedly different pattern from that of normal teleost epidermal cell proliferation. The possibility that the parasite secretes some form of digestive enzyme is postulated. In areas where sloughing had occurred, the remaining malphigian cells were seen to be in the process of division. Various endocrinological aspects of Ichthyobodo infestations were investigated. Three corticosteroids and one androgen were injected or implanted into 1 year old rainbow trout. Implantations of hydrocortisone led to very heavy ichthyobodo infestations. Radio immune assays showed that the level of cortisol and testosterone in the serum of implanted fish was similar to that which would occur when salmonids mature. There appears to be a clear link between cortisol levels in the serum and Ichthyobodo infestation. The host response to Ichthyobodo is discussed and it is concluded that cortisol may suppress the host's defence mechanism to Ichthyobodo.