Antiparasitic effect of in-feed inhibitors of folic acid synthesis and dihydrofolate reductase against ciliate Cryptocaryon irritans infection in the red sea bream Pagrus major and against ciliate Ichthyophthirius multifiliis infection in black pop-eyed g

We investigated the effects of feeding with sulfonamides, dihydrofolate reductase inhibitors or their combinations on Cryptocaryon irritans infection in the red sea bream Pagrus major. We also investigated the effects of an in-feed sulfonamide–diaminopyrimidine combination on Ichthyophthirius multifiliis infection in the black pop-eyed goldfish Carassius auratus. In P. major treated with in-feed sulfonamides (300 mg/kg body weight (BW)), dihydrofolate reductase inhibitors (60 mg/kg BW) and their combination (Sulfadimethoxine (SDMX) 216 mg and Trimethoprim (TMP) 24 mg/kg BW) for 3 days, significantly smaller C. irritans trophonts were found compared to the control group. The smallest trophonts were found in the group treated with the SDMX–TMP combination. In P. major fed with SDMX–TMP (216 and 24 mg/kg BW) or Sulfamethoxazole (SMX)–TMP (429 and 86 mg/kg BW) for long-term, the number of C. irritans cysts was markedly lower compared to the control group at 5 days after exposure to theronts. The cysts in groups treated with these combinations were significantly smaller than those from the control group, and hatching was delayed compared to that of the control group. Further, in the control group, 100% mortality was observed owing to heavy C. irritans infection at 9 days after the exposure. On the other hand, mortality owing to parasite infection was not observed in the groups treated with the combinations, and no parasites were found in any surviving fish at the end of the experiment. In addition, Sulfamonomethoxine (SMMX)–Ormetoprim (OMP) (112.5 and 37.5 mg/kg BW) combination also had a therapeutic effect against I. multifiliis infection in C. auratus. Thus, in-feed sulfonamides, dihydrofolate reductase inhibitors and their combinations will be useful in aquaculture for controlling C. irritans and I. multifiliis infections.

► In-feed folic acid synthesis inhibitors inhibited C. irritans growth.
► In-feed their combinations inhibited C. irritans encystment.
► Feeding with their combinations had therapeutic effect against C. irritans.
► Feeding with their combination had therapeutic effect against I. multifiliis.
► Folic acid synthesis is vital for survival and proliferation of both the parasites.