The frequency and severity of fish diseases is; increasingly, being reported as a limiting factor
in the future development of aquaculture. The control of fish disease is largely performed in
retrospect through curative chemotherapy. However, the development of resistant strains of
fish places the emphasis on preventative, rather than the curative, control of disease.
The freshwater parasite Icthyophthirius multifiliis causes the disease known as
ichthyophthiriasis or white spot. Losses of fish due to this parasite are believed to total over one
million dollars per annum worldwide. The objective of this study was to examine the genetics of
resistance to I. multifiliis,
Comparisons were made between four stocks of the tropical livebearing fish Xiphophorus
maculatus. One of these stocks, the blue platyfish, was found to be less susceptible to white
spot than the yellow comet tail, red or red wag tail platyfish, Comparisons with four other
tropical spades of fish found significant differences between X. maculatus, X. variatus, Ameca
splendens and Ilyodon xanthusi in levels of susceptibility. A. splendens was the most
susceptible species, with the blue platy (X. maculatus) and sunset platy (X. variatus) forming the
most resistant group. All the remaining stocks and species formed an intermediate group.
Examination of resistance to white spot infection in four scale types of related common carp
(Cyprinus auratus) also found variation in resistance, with the fully scaled carp being the most
resistant phenotype (scattered mirror, linear mirror and leather carp being similar in infection
level).
More detailed analysis of the genetics of disease resistance was performed with heritability
determinations in stocks of A. splendens, X. maculatus (yellow comet tail) and X. maculatus
(Vera Cruz). The highest heritability value, based on sire components only, was that for Vera
Cruz platyfish of 0.75, with a value of 0.23 for X. maculatus (yellow comet tail) and 0.00 for A.
splendens. A breeding programme was also performed between X. maculatus (red platy) and
X. variatus (sunset platy) to evaluate the presence of any heterosis. Such was observed, with a
heterosis value, based on actual parasite counts, of 16.2%.
Several factors of the infection process are also discussed, especially the fluctuating yearly
trends in infection levels and parasite strain differences. Finally, the future potential of genetic
manipulation of fish stocks for increased disease resistance is discussed in the light of this
study. It is considered that a useful foundation has been laid for the further development of this
approach to disease prevention in aquaculture.
Date of Award | 1989 |
---|
Original language | English |
---|
Awarding Institution | |
---|
lchthyophthiriasis in fish: genetic variation in resistance to infection
Clayton, G. M. (Author). 1989
Student thesis: PhD