The anti-protozoal activity of bronopol on the key life-stages of Ichthyophthirius multifiliis Fouquet, 1876 (Ciliophora)

Pyceze™ (Novartis Animal Vaccines Ltd.) is licensed as a veterinary medicine to treat fungal infections in salmon, trout and their eggs. The active ingredient is bronopol, which due to its broad-spectrum activity has the potential to be an effective treatment against other important aquatic pathogens. In this study the efficacy of bronopol against Ichthyophthirius multifiliis was tested both in vitro and in vivo. In vitro trials demonstrated a 30 min exposure to 100 mg L−1 bronopol killed 51.7% of the infective theronts. In vitro exposure of the protomonts to bronopol (0, 20, 50 and 100 mg L−1) for 30 min was observed to kill 0%, 76.2%, 97.2% and 100% respectively. Protomonts surviving treatment, demonstrated delayed development with the time taken from protomont until the release of theronts ranging from 28.3 h for 0 mg L−1 exposure, to 70 h for parasites in 20 and 50 mg L−1 exposure groups. These concentrations also caused asymmetric cell division of the encysted tomonts. Exposure of encysted tomonts (min. 8 cell stage) to 100 mg L−1 bronopol for 30 min, killed 50% within this period, with the remainder dying within the subsequent 42 h post exposure. Lower doses of bronopol were less effective in killing encysted tomonts than the higher doses (3.3% of parasites were killed in 20 mg L−1; 10% in 50 mg L−1), but they still delayed theront release significantly (25.7 h for 0 mg L−1 to 46.2 h for parasites exposed to 20-50 mg L−1).Long, low dose (1 mg L−1) exposure to bronopol was also efficacious against theronts. Survival after 12 h was 29% (c.f. 100% in control parasites), and <1% after 24 h exposure (c.f. 74% in control parasites). Theronts surviving these exposures demonstrated reduced infection success compared to control theronts. The findings of this study demonstrate that bronopol (Pyceze™) affects the survival of all free-living stages of I. multifiliis (protomonts, tomont and theronts), thus suggesting that bronopol may serve a useful role in the control of I. multifiliis infections.