ABSTRACT
External signs of the disease are variable, but the most consistent pathology is seen in the gills (Fig. 3.3). They are usually discoloured, necrotic, inflamed and mucus may trail from them. Ttiere may be erosion of primary lamellae, fusion of secondary lamellae, and the tips of the primary and secondary lamellae may be swollen (Bretzinger et al, 1999; Walster, 1999; Hedrick et al, 2000; Gray et al, 2002; Denham, 2003; Perelberg et al, 2003; Sano et al, 2004a, b; Tu et al, 2004a, b; Sunarto et al, 2004; Hutoran et al, 2005). Enophthalmia (sunken eyes) is another typical feature of the disease. There may be a pale discolouration or
Disease Aetiology
However, Kortet and colleagues have extensively studied epidermal hyperplasia in roach populations in Finland. Eight out of 11 populations studied exhibited epidermal hyperplasia, and the highest prevalence was 62% (Kortet et al, 2002). The peak prevalence was during spawning time (April/May) when severe disease occurred, with a second, lower, peak in August. The highest prevalence was in polluted areas. There was a higher prevalence in males compared to females (Kortet et al, 2002; Korkea-aho et al, 2006). There was a negative correlation between the number of frontal breeding tubercles (a feature of males in breeding condition) and the intensity of epidermal hyperplasia (Vainikka et al, 2004), but a positive correlation between testosterone (which may have an immunosuppressive effect) and elaborate breeding tubercles and disease (Kortet et al, 2005). However, there was no correlation between Cortisol levels, which increase during stress and can also have an immunosuppressive effect, and disease intensity in roach (Vainikka et al, 2004). Any correlation between fish length or age and prevalence of epidermal hyperplasia varied between locations. In some areas there was no correlation but in others there was a positive correlation (Kortet et al, 2002, 2004; Korkea-aho et al, 2006). There was no correlation between epidermal hyperplasia and condition factor, nor the dominance ranking of males (Kortet et al, 2004) and no correlation between severity of disease and disease prevalence (Korkea-aho et al, 2006) or mortality (Kortet et al, 2003).
