Pathogenicity of Cryptocaryon irritans to Larimichthys crocea and Chinese herbal inhibition on the parasitic generation reproduction

The "white spot disease" caused by the parasitic infection of Cryptocaryon irritans has become one of the most serious diseases in the production of Larimichthys crocea. This study aims to investigate the pathogenicity of C. irritans to L. crocea, as well as the effect of Chinese herbal compound HD-2 on inhibiting the generation reproduction of parasite C. irritans. The number, size, the hatching rate and number of hatched theronts of tomonts produced by natural sick L. crocea were investigated. Then the survival time and infectivity of theront was determined in vitro. The effects of Chinese herbal compound HD-2 on theront, trophont, and tomont, and the therapeutic effects of oral HD-2 on diseased L. crocea were studied. The results showed that the total number of tomonts produced by L. crocea was positively correlated to the fish body weight; the average diameter of tomonts were (340.8±64.9) μm and the hatching rate was 76.4%, and each tomont could produce (280±42.5) theronts; the theront could survive for more than 24 h in vitro and lost their infectivity in 20 h after hatching. The 90% lethal dose (LD₉₀) of L. crocea infected with C. irritans in 7 days was 9 712 theronts/fish and the median lethal dose for 50% (LD₅₀) was 4 366 theronts/fish. HD-2 could kill theront effectively. After oral administration with different doses of HD-2 (control, 0; G1, 5 g/kg; G2, 10 g/kg; G3, 15 g/kg) for 30 days, the relative infection intensity of the L. crocea groups challenged with LD₅₀ decreased by 32.0%, 44.5% and 51.8%, respectively. Meanwhile, the volume of tomonts reduced by 35.4%, 36.1% and 37.3% and the hatching rate reduced by 16.3%, 23.3% and 27.9%, respectively. The fish survival rate of G1, G2 and G3 challenged with LD₉₀ was 40.0%, 55.0% and 58.3%, which was significantly higher than the control group (only 8.3%). Using HD-2 oral treatment for 15 d, the tomonts produced from diseased fish reduced 73.1%, 87.7% and 93.8%, and the fish mortality rate of the three groups were 58.3%, 36.7% and 21.7%, which were significantly lower than the control group (83.3%). The physiological status of L. crocea in the treated group was significantly better than that of the control group. Therefore, HD-2 oral treatment could effectively interrupt the secondary infection of C. irritans, suppress the formation and development of tomonts, reduce the tomont hatching rate and the number of hatching theronts, and then inhibit the generation reproduction of parasite C. irritans.