Influencia de los ácidos grasos altamente insaturados (HUFA) sobre el sistema de defensa y la susceptibilidad al estrés en camarón blanco Litopenaeus vannamei

Diseases are considered nowadays one of the main problems affecting the shrimp industry. Many diseases are associated to a decrease in the immune defense system, which can be produced by stress. One alternative to reduce these diseases is the use of highly unsaturated fatty acids (HUFA) because they modulate the immune response. The aim of this work was to evaluate the metabolic and immune responses in the Pacific whiteleg shrimp Litopenaeus vannamei subjected to a handling chronic stress and determine if HUFA influence these responses. Three experiments were performed: in the first one, shrimp raised in outdoor concrete tanks or indoor plastic tanks were submitted to a repeated stress induced by daily handling for four weeks. Metabolic responses (glucose, lactate, total protein, cholesterol, triacylglyceride, total lipid, hemocyanin, and carotenoid content) and immune responses (total hemocyte count, superoxide anion production, and superoxide dismutase activity) were measured at the end of the experimental period and compared to control shrimp. Stressed shrimp from concrete tanks had significantly lower concentrations of total proteins, triacylglycerides, and total lipids in hemolymph and a significantly lower concentration of carbohydrates in muscle. Stressed shrimp from plastic tanks showed higher concentrations of glucose in hemolymph and higher concentrations of total lipids, but lower concentrations of carbohydrates in hepatopancreas. Immune responses were not significantly different between stressed and control shrimp raised in either tank system, suggesting that chronic stress does not depress immune response capacity, with the exception of lower levels of total proteins in hemolymph. Lower amounts of disturbance occurring in outdoor control shrimp, as well as natural food availability in outdoor tanks could partly explain differences in the metabolic responses. In a second experiment, shrimp were fed a low, medium, or high-HUFA diet (0.1, 1.0, and 1.8%) for 24 days. At the end of the experiment, immune variables and the composition of fatty acids in muscle were determined. A significant difference between initial and final weight occurred in shrimp fed either the medium-HUFA or high-HUFA diet. The number of total molts was significantly higher in shrimp fed the medium-HUFA diet. The high density lipoprotein-beta glucan binding protein (HDL-BGBP) concentration was significantly higher in shrimp fed the high-HUFA diet, but no other significant differences were observed in the immune variables. The proportion of HUFA in muscle increased with the increased content of HUFA in diets. In the third experiment, two diets containing a low or high level of HUFA (0.1% and 2.1%) were administered for 38 days to two groups of shrimp. In one group, handling stress was applied daily for 30 days and metabolic and immune variables were measured at the end. In the other group, handling stress was applied only once, and metabolic and immune variables were determined 1 and 24 hours after the stressor event. In both groups, controls were unstressed shrimp fed the same diets. Shrimp that were stressed daily for 30 days had significant reduced survival, final weight, and consumption of feed. Among the immune variables measured, only the concentration of the HDL-BGBP was significantly affected, with higher levels in shrimp fed the high-HUFA diet. General fatty acid composition of polar and neutral lipids in hemocytes was influenced by the diet. The proportion of EPA (20:5n-3) and DHA (22:6n-3) in polar lipids in hemocytes was higher in stressed shrimp fed the low-HUFA diet compared to controls. A significant increase in glucose concentration in hemolymph occurred in stressed shrimp, regardless of diet. Lactate level in hemolymph was significantly lower in shrimp fed the high-HUFA diet. Shrimp submitted to the acute stress event showed significant differences in their immune response. Total hemocyte count rose one hour after the stressor event, especially for shrimp fed the low-HUFA diet. In contrast, superoxide anion production increased one hour after applying the stressor, but only in shrimp fed the high-HUFA diet. Lactate and glucose concentrations in hemolymph increased one hour after the stressor and returned to control levels after 24 hours. Thus, a negative effect of repeated handling stress was mainly observed in biological performance, whereas acute stress had a more pronounced effect on shrimp metabolic and immune responses. A beneficial role of enrichment with HUFA on tolerance to handling stress was partly observed on survival during culture and possibly on the immune response capacity.