Aquaculture plays an important role in food production worldwide. However, the presence of pathogens is currently a major constraint for its development, leading to disease outbreaks and therefore causing severe economic losses. Photobacterium damselae subsp. piscicida (Phdp) is the causative agent of photobacteriosis, a bacterial septicaemia. Phdp is able to infect a wide variety of marine fish with economic relevance such as, gilthead seabream, European seabass, sole and meagre. The aim of this work was to study haematological dynamics and oxidative stress parameters of gilthead seabream (Sparus aurata) juveniles infected with Phdp.
A time-course study was performed, with gilthead seabream (mean weight 9.8 ± 2.2 g) juveniles. Among population, 12 fish were randomly selected and sampled before infection (time 0 h). Thereafter, the remaining animals were randomly selected and intraperitoneally (i.p.) injected with 100 µl of 1 ´ 105 CFU ml-1 of Phdp, while control fish were sham-injected with 100 µl a phosphate buffered saline solution. Afterwards, fish were randomly distributed as a completely randomized design into 6 tanks of 60 l closed recirculation systems (i.e. triplicates per experimental condition). Two animals per tank (n=6) were randomly selected and sampled at 3, 6, 9, 24 and 48 h after i.p. injection. At each sampling point, fish were anaesthetized with 2-phenoxyethanol and blood samples were collected for haematological procedures. Liver was dissected to evaluate oxidative stress parameters. Each recirculation system was kept at 25 °C for 14 days and pathological signs and mortality were observed and recorded.
Mortalities started at 24 h in infected seabream and exhibited a maximum mortality rate of
40 % at the end of infection, whereas no mortality was observed in control groups. Regarding white blood cells, neutrophil counts increased in the infected treatment compared to the control (0 h) whereas monocyte counts decreased in the infected groups at 6h post-infection. Total circulating erythrocytes values decreased in the infected group compared to control and non-infected fish, as well as the haematocrit values that also decreased in the infected treatment. Hepatic catalase activity decreased at 6h and 9h compared to 48h post-infection in the infected group. Lipid peroxidation in the infected group increased consistently until 48 h post-infection.
Acknowledgments: The authors thank FCT, Portugal, for financial support (UID/MAR/04292/2019) and Be4AquaHealth (MAR2020-02.05.01-FEAMP-00139).