Effect of macrophages and serum of fish susceptible or resistant to epizootic ulcerative syndrome (EUS) on the EUS pathogen, Aphanomyces invadans.
The mechanisms by which epizootic ulcerative syndrome susceptible and non-susceptible fish respond to Aphanomyces invadans (Aphanomyces piscicida) was compared. Whether any of the mechanisms are attenuated at the low temperatures at which outbreaks typically occur was also studied. Experimental fish, (striped snakehead, Channa striata; silver barb, Barbodes gonionotus [Puntius gonionotus]; giant gourami,Osphronemus gouramy and Nile tilapia, Oreochromis niloticus), were all from central Thailand. All analyses involved a minimum of 5 fishes and a maximum of 12 individuals per group. Factors which inhibited fungal germination were: macrophages of all groups (P<0.05) except snakeheads, serum of all groups (P<0.05) except low temperature tilapia and heated serum of high temperature snakeheads (P<0.005). Only silver barb and gourami macrophages, serum or heated serum reduced germling growth (P<0.01). At high temperature, snakehead serum was more inhibitory than that of silver barbs (P<0.05). The phorbol myristate acetate (PMA)-stimulated macrophages produced significantly (P<0.05) more superoxide than control macrophages in all cases except that of high temperature silver barbs. Among low temperature fish, silver barb macrophages had the lowest respiratory burst in all 3 treatments (P<0.05). Tilapia macrophages showed higher superoxide production than snakehead and silver barb macrophages in all treatments at high temperature (P<0.05). An assessment of respiratory burst based on control macrophages showed a positive relationship in the case of high temperature tilapia with superoxide production derived from PMA-stimulated macrophages (R2=0.78, P<0.01), and low temperature gourami with superoxide data derived from control macrophages (R2=0.56, P<0.05). Low temperature snakeheads gave positive quadratic regressions superoxide data derived from PMA stimulated (R2=0.95, P0.0005), inoculated (R2=0.86, P<0.01) and control (R2=0.76, P<0.05) macrophages. Anti-A. invadans antibodies were found in all individuals examined with the exception of one low temperature tilapia.