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

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

The thesis was published by Robertson, Derek Arthur, in September 2022, University of Stirling.


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
The host response to Ichthyobodo is discussed and it is
concluded that cortisol may suppress the host’s defence mechanism
to Ichthyobodo.

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