Abstract

Fungal infections in fish, although considered "secondary" to environmental and/or biological stressors or agents, nonetheless can result directly in significant losses in the aquarium fish industry. Malachite green, considered one of the most effective fungicides to date, is illegal for use in food species due to teratogenic and carcinogenic potential. Consequently, alternatives to malachite green have been widely sought. Formalin has FDA approval as a fungicide, but can cause significant damage to skin and gills if used improperly. Other low regulatory priority chemicals, such as hydrogen peroxide and salt, also have their limitations. Pyceze® (Novartis Animal Vaccines Ltd) is a clear liquid containing bronopol, a broad spectrum anti-microbial agent, at 50% w/v (2-bromo-2-nitropropane-1,3-diol). It has demonstrated marked effectiveness, and has been approved for use in Europe against saprolegniasis in salmonids and is under investigation for approval in a number of countries including the U.S.

Chemical stability at three different water quality conditions common in the freshwater aquarium hobby were examined for each of 1 mg/L, 5 mg/L, and 20 mg/L of Pyceze®: A) hardness 30-50 mg/L CaCO3, pH=6.5-6.9, salinity less than 0.1ppt ; B) hardness 100-120 mg/L CaCO3, pH=7.1-7.4, salinity less than 0.1 g/L; and C) hardness 150-180 mg/L CaCO3, pH=7.8-8.2, salinity 1.5 g/L (adjusted with NaCl). Three replicates were run for each condition. Pyceze® was considerably more stable in water quality condition A (approximately 2% and 40% degraded for 5 and 20 mg/L), less so in B, and the least stable in C (approximately 70% and 90% degraded for 5 and 20 mg/L) after 24 hours.

Survival of three different, Florida-produced, tropical fish species (neon tetras, Paracheirodon innesi; marigold platys (Xiphophorus maculatus); and common suckermouth catfish (Hypostomus sp.) at each of 7 different Pyceze® treatment regimens (water quality conditions: hardness: 188.1 mg/L, pH: 8.0, salinity less than 0.1 g/L, temp 23 degrees C) was also examined. Three replicates of 25 fish each were tested for each treatment regimen (1, 3, and 5 mg/L each at 8 and at 24 hours; 20 mg/L for 30 minutes; and a control). There was no significant difference in survival (P>0.05) among any treatments and associated controls. Histological samples have been saved for evaluation.

One experimental field trial was run with male Siamese fighting fish (Betta splendens) that were breaking down with saprolegniasis 2-3 days post-shipment. A new shipment was divided into 4 treatment groups, 48 fish each, as follows: 1) 3 g/L NaCl and ~ 8 mg/L Kanamycin (an antibiotic used because of evidence of bacterial infection), at 20-21 degrees C; 2) 3 g/L NaCl and ~ 8 mg/L Kanamycin, 24-25 degrees C; 3) 5 mg/L Pyceze® at 20-21 degrees C; all, for 24 hours; 4) control (no treatment, 20-21 degrees C). The Pyceze® treatment (group 3) had the best survival, with only 1 mortality by 5 days post treatment, compared to group 1(=20 dead); group 2 (=7 dead); and group 4 (=21 dead).

In summary, Pyceze® breaks down relatively rapidly at the more common Florida production water quality parameters, has no toxicity at the concentrations and regimens tested in neon tetras, marigold platys, and common suckermouth catfish, and shows promise as an effective tropical fish fungicide in at least one field trial. Toxicity testing in other tropical fish species will help provide additional important information, and this work is currently underway at North Carolina State University by Greg Lewbart and his staff. More complete and thorough efficacy testing using different regimens on a variety of tropical fish species is warranted and necessary, giving these preliminary results.

Acknowledgements

This research was funded through a grant from Novartis Animal Vaccines Ltd, UK. The authors would like to thank Derek Thompson, Stewart Howlett, John Pitts, and Hugh Mitchell for their assistance with this study.