Cell-mediated immunity against P. salmonis: a path to fewer antibiotics

Piscirickettsia salmonis, the agent behind SRS (piscirickettsiosis), is an intracellular pathogen: it invades the host’s own cells, where antibodies cannot reach it. That is why humoral immunity alone struggles against SRS, and why the disease drives heavy antibiotic use in salmon farming. SRS mortality reaches up to 90% in severe outbreaks and causes annual losses of US$500–700M in Chile alone.

In the study published in Aquaculture (Romero et al., 2021; Universidad Austral de Chile and INCAR), juvenile Atlantic salmon fed NatControl^® (0.5 kg/ton) for 14 days and then subjected to a cohabitation challenge with P. salmonis showed:

• 79.1% survival versus 52.7% in controls 70 days after the challenge, a relative percent survival (RPS) of 48.1%, with the outbreak delayed by 5 days.
• Higher expression of IL-12, CD8 and MHC-I in head kidney after 14 days of feeding, consistent with a potential modulation of the cell-mediated immune response, including mechanisms mediated by cytotoxic T-lymphocytes.
• 10-fold increase in IL-12 and 4-fold increase in IFN-I in vitro in salmon SHK-1 cells, the innate cytokines that orchestrate the cell-mediated response, together with a protective effect on the cells against infection.

The mechanism follows the cell-mediated route: NatControl^® modulates macrophage activity, improving phagocytosis without inducing inflammation, and favors the production of cytokines that increase the detection and destruction of intracellular pathogens, along with potential modulation of mechanisms mediated by cytotoxic T-lymphocytes.

For producers, the practical consequence is fewer outbreaks and lower mortality. NatControl^® can thus be part of health management strategies aimed at reducing dependence on antibiotics and contributing to the industry’s sustainability goals.