Dis Aquat Org 64: 151–158, 2005

Philasterides dicentrarchi is a protozoan ciliate which causes significant economic losses in fish aquaculture. This study investigated the effects of chitosan microspheres cross linked with glutaraldehyde and containing beta-cyclodextrin (βCD) on the survival of this parasite in 7 d cultures. When used alone in assays, neither chitosan nor βCD showed any activity, whereas free glutaraldehyde was strongly toxic to the parasite. Microspheres were likewise strongly toxic, at total glutaraldehyde concentrations much lower than with free glutaraldehyde: near-100% ciliate death was obtained (1) with 50 μg ml–1 of microspheres prepared with 5% glutaraldehyde and noβCD, or (2) with 10 μg ml–1 of microspheres prepared with 0.15% glutaraldehyde and 0.1% βCD. This suggests that the main active component is glutaraldehyde, but that the presence of small amounts of βCD enhances efficacy. This high efficacy, together with the low toxicity to fish and rapid biodegradability of the individual components, suggest that these microspheres may be an attractive alternative to the formaldehyde baths traditionally used for the control of this parasite.