Activation of the alternative complement pathway also seems to
be a general effect of b-glucan administration in fish. For example,
Chettri et al. [64] observed c3 up-regulation in rainbow trout fry
after bath exposure to b-glucan. This is also in accordance to the
studies conducted by Pionnier and colleagues [31,32], in which the
authors demonstrate an up-regulation of c3 in response to MacroGard®
feeding in juvenile carp. In their study c3 up-regulation
was accompanied by enhanced activation of the alternative
complement pathway. In the present study it can therefore be
assumed that MacroGard® feeding activated the complement
pathway and hence increased opsonisation of pathogens. This increase
in immunity most probably contributed to the elevated
survival rates observed in the present study. In future studies
endpoint analyses of cellular immunity such as phagocytic activity
as well as pathogen resistance should also be included to elucidate
the overall effects of b-glucan on immuncompetence.