A polysaccharide extracted from the green seaweed Ulva Lactuca raised survival in rainbow trout vaccinated against Yersinia ruckeri from 73.3% to 86.7%. The result point to ulvan as a potential natural adjuvant for strengthening vaccines used against yersiniosis, a bacterial disease capable of causing substantial mortality on trout farms.

Researchers from Gorgan University of Agricultural Sciences and Natural Resources, the University of Tehran and other Iranian institutions evaluated an experimental vaccine made from inactivated Y. ruckeri. The trial involved 450 trout with an average weight of 52.9 grams, divided between a vaccine without an adjuvant, the same formulation supplemented with ulvan and an unvaccinated control group.

The fish received the vaccine by intraperitoneal injection, followed by a booster dose 21 days later. After 60 days, some of the animals were challenged with the bacterium. Two weeks later, survival reached 88.7 among trout vaccinated with ulvan, compared with 73.3% among those receiving the vaccine alone with and 16.7% in the unvaccinated group.

The ulvan formulation also produced the strongest response across several immune and antioxidant indicators. The researchers recorded increases in white blood cells, lysozyme activity, immunoglobulins and several enzymes associated with protection against oxidative stress. Antibody titres against Y. ruckeri were higher in both vaccinated groups than in the control.

Ulvan is particular interest because it is a biodegradable and biocompatible sulphated polysaccharide obtained from seaweeds of the genus Ulva. These compounds are being investigated as alternatives or supplements to mineral – and oil-based adjuvants, which can represent viscosity problems and cause local reactions. However, the study does not yet demonstrate that ulvan could replace existing commercial formulations.

Although survival was numerically higher with the seaweed extract, the paper does not clearly establish that the difference between 73.3% and 86.7% was statistically significant in a direct comparison.

Furthermore, the challenge involved injecting the bacterium, the follow-up period was limited, and neither the cost and stability of the formulation nor its performance over a full production cycle was assessed.

The next step, therefore, will be to determine whether this advantage is maintained following natural exposure and under commercial farming conditions.