ISSUE FOCUS 28 FEED & ADDITIVE MAGAZINE September 2022 COMMON AQUATIC BACTERIAL PATHOGENS In one challenge trial conducted in Vietnam, Pacific white shrimp (Litopenaeus vannamei) were fed 0, 2, or 6 g/kg of MCFA for 14 days and then challenged with WSV (oral, 108 copies/g, 4 meals/day, at 5% BW). The survival rates (SR) were recorded for the next 10 days. The negative control (no virus, no MCFA) had a 86% SR, the positive control (with virus, no MCFA) has 2% SR, and the MCFA groups (with virus and MCFA) have 12 and 17% SR (Figure 3). Reducing pathogen virulence Bacteria secrete two enzymes, caseinase and haemolysin, that are specifically designed to enter and damage the host’s tissues and cells. The activity of these enzymes is therefore directly linked to virulence/damage to the host. In a trial, MCFAs at concentrations of 0.025 and 0.05% lowered the caseinase activity of V. harveyi BB120, but there was no effect on haemolysin activity. Therefore, by lowering the caseinase activity, MCFAs are shown to reduce pathogen virulence and specifically can render vibrio less infectious. Improving intestinal and hepatopancreatic morphology Pacific white shrimp (L. vannamei) were fed 0, 4 and 4.5 g/kg of MCFA for two weeks before the health of the intestine morphology was evaluated by measuring the height of the brush border. The MCFAs increased the brush border height by 23% and 32% comparing to the control (Figure 4). It is Figure 4. Comparison of the height of the brush border after shrimp were fed with 0 (T0), 4 (T1) and 4.5 g/kg (T2) of medium-chain fatty acids Figure 2. Medium-chain fatty acids C6 and C8 inhibit gram-negative bacterial growth, while C10 and C12 tend to interact with gram-positive bacteria Figure 3. Survival rate at the termination day, 10 days post-challenge. Values are presented at mean ± SD; n=4; p-value < 0.05
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