INFECTION KINETICS OF Kudoa yasunagai AND EFFECTIVE ULTRAVIOLET IRRADIATION DOSE FOR PREVENTION
Kudoa yasunagai is a myxozoan parasite that infects the brains of various marine fishes, including important aquaculture species. The fish-infective stage of K. yasunagai (and any Kudoid myxozoans) remains unknown until date and very little information exists regarding their infection process in the fish host. We monitored parasite abundance in different organs/tissues (gills, skin-muscle, intestine, blood, spinal cord, and brain) of juvenile Seriola lalandi for one month using K. yasunagai-specific quantitative PCR assays. In addition, we investigated the minimum ultraviolet (UV) irradiation dose to rearing water to prevent K. yasunagai infection.
K. yasunagai DNA was first detected in the intestine after a week, and attained a plateau phase after 3 weeks. Parasite DNA was detected in the brain after 3 weeks and increased to an estimated intensity equivalent to 105 myxospores mg-1 tissue by 5 weeks. In contrast, the estimated parasite intensity remained below 1 myxospore mg-1 in other organs throughout the experiment. These findings indicate that K. yasunagai infection in fish likely initiates from the intestine and the parasite then migrates to the target organs such as the brain and spinal cord for further development and multiplication. The infection prevalence in Seriola lalandi reared in untreated water reached 45 % while that in fish reared in UV-treated water at 5 mJ/cm2 remained below 10%. Additionally, 5 mJ/cm2 UV irradiation significantly reduced spore formation in the brain. No infection was detected when the water was treated with UV at the doses 15 and 30 mJ/ cm2. These results indicated that K yasunagai actinospores are relatively vulnerable to UV irradiation and the estimated minimum effective dose lies somewhere between 5 and 15 mJ/ cm2.