<i>In vitro</i> evaluation of the survival and development of <i>Cryptocaryon irritans</i> under ivermectin exposure

Cryptocaryon irritans , a ciliate protozoan parasite, is a major causative agent of marine fish white spot disease, leading to high mortality rates and significant economic losses in marine aquaculture. Current treatments are often ineffective against the tomont stage due to the protective cyst wall, and therapeutic interventions are mostly applied during the theront stage. This study aimed to evaluate the effects of Ivermectin (concentrations from 0.05 to 1.00 ppm) on the development and survival of C. irritans at two key life stages. The experiments were using C. irritans isolates obtained from naturally infected Trachinotus spp. and propagated under controlled conditions (28-30℃). Two independent trials were carried out: (1) assessing the effect of Ivermectin on the transformation of protomonts into tomonts, and (2) evaluating its direct impact on theront survival after exposure. Results revealed that tomont development was significantly inhibited at concentrations ≥ 0.16 ppm, with only 1.67-6.67% successful development after 48 hours. For theronts, mortality reached 100% at concentrations ≥ 0.16 ppm after 60 minutes. This study provides the first evidence of Ivermectin’s potential to inhibit C. irritans development under laboratory conditions.