Effects of two delivery matrix components, alginate and polyethylene glycol, on the intestinal tract and inflammatory response of juvenile rainbow trout, Oncorhynchus mykiss, Walbaum

A critical step in the administration of oral vaccines in aquaculture is the effective protection of the vaccine antigens through the delivery route of water to stomach to the mucosal immune system in the gut. Bioactive substances can be incorporated into protective matrices to ensure safe ingestion and controlled release into the posterior digestive tract. Polyethylene glycol and alginate are promising and frequently applied substances in the oral administration of pharmaceutical substances; however, their direct influence on the intestinal tract or inflammatory response is not well investigated in aquaculture fish. In the current study, these two substances were evaluated separately and in combination. They were included in a commercial diet for juvenile rainbow trout (18.7 ± 0.1 g) and applied over 22 days in controlled feeding to determine possible gut alterations and inflammatory responses. Three different sections of the intestine, the anterior pyloric caeca, the mid-intestine, and the posterior intestine, were histologically examined, and expression of inflammatory genes was measured in each. The inclusion of polyethylene glycol in the diet led to a significant reduction of villus height in the pyloric caeca and thus a significant reduction in active surface area. In the mid-intestine, alginate inclusion resulted in a significant reduction of the mucus-secreting goblet cells and a significant increase in villus width. In the posterior intestine, polyethylene glycol inclusion caused a significant increase in number of large vacuoles present; the addition of alginate buffered this effect. The results of the histological evaluation indicated symptoms that are usually evident in intestinal inflammation and can reduce functions of the respective intestinal segments. No significant increases in pro-inflammatory gene expression tumor necrosis factor-α, interleukin (IL)-8, or IL-1β were present, indicating no severe inflammatory response. Based on the observed histological impairments, negative consequences such as reduced efficiency of an oral vaccine or productivity of aquaculture rearing due to hindered nutrient digestion can be considered unlikely.