“Early juvenile ayu can survive in Plecoglossus altivelis poxvirus infection and acquire immunity (6920)”

Early juvenile ayu can survive in Plecoglossus altivelis poxvirus infection and acquire immunity

Atypical cellular gill disease (ACGD) in ayu Plecoglossus altivelis caused by P. altivelis poxvirus (PaPV) infection has led to significant economic losses in aquaculture. Ayu is an amphidromous fish with a one-year life cycle. It is known from experience that ACGD has not occurred in aquaculture farms using wild-caught juveniles as seedlings, and a previous paper reported ayu ranging from 0.39g to 0.83g caught near shore or river mouth were positive in PaPV PCR without any symptoms. These imply that wild early juvenile fish have acquired immunity to ACGD. In this study, we examined the mortality of early juveniles experimentally infected with PaPV and immunity against subsequent PaPV infection.

A virus solution was prepared from the homogenate gills of spontaneously ACGD-dead ayu. Ayu (0.5g) were challenged by immersion in rearing water added with the virus solution for the infected group (I) or MEM for the non-infected group (N). The fish were kept at 18℃ for 3 weeks and at 25℃ for about 4 months. Subsequently, the fish (6.2g) were challenged with virus solution (I) or MEM (N), and kept at 18℃ for 3 weeks. The gills were periodically sampled for the quantification of virus DNA copy number, histopathology, and gene expression analysis after infection.

The survival rates were 100% and 97.0% in the non-infected and infected group, respectively after the primary infection. Both the infected and non-infected groups were negative in PaPV PCR 5 months after infection. The secondary challenge test showed survival rates of 100%, 92.9%, 0%, and 100% in the group of N-N, I-N, N-I, and I-I, respectively. Virus DNA copies were at the same level in both N-I and I-I groups up to the middle stage of infection, and those in N-I group increased more than I-I group in the following period of infection. Histopathological examination of the gills showed the occurrence of atypical cells in the secondary lamella in N-I group. The expression level of IFN-γ in the gills was upregulated in I-I group at the early stage of infection.

These results suggest the mortality of early juvenile ayu by PaPV infection is quite low, and the surviving fish can acquire immunity against a subsequent infection. In addition, immunity in surviving fish may involve a cell-mediated response, preventing the virus propagation and consequently the development of ACGD symptoms at the early stage of subsequent infection.

1. Baba, Shuntaro, Tokyo University of Marine Science and Technology, Tokyo, Japan, Presenter
2. Mori, Tatsuya, Tochigi Prefectural Fisheries Experimental Station, Tochigi, Japan, Author
3. Takagi, Yuya, Tochigi Prefectural Fisheries Experimental Station, Tochigi, Japan, Author
4. Wada, Shinpei, Nippon Veterinary and Life Science University, Tokyo, Japan, Author
5. Matsumoto, Megumi, Tokyo University of Marine Science and Technology, Tokyo, Japan, Author
6. Kato, Goshi, Tokyo University of Marine Science and Technology, Tokyo, Japan, Author
7. Sano, Motohiko, Tokyo University of Marine Science and Technology, Tokyo, Japan, Author