The occurrence of mycotoxins and their impact on rainbow trout (Oncorhynchus mykiss)

Concern about mycotoxins in aquaculture has been growing, partly due to the gradual replacement of animal-derived proteins, such as fishmeal, by plant sources. Plant-based ingredients already represent the major dietary protein source used within feeds for lower trophic level fish species and the second major source of dietary protein and lipids after fishmeal and fish oil for shrimp and high trophic level fish species. The tendency to use plant-based ingredients in aquafeeds is set to increase due to sustainability issues and the price of fishmeal. Plant ingredients used in aquaculture are of varying origin and quality and recent reports show the risk of mycotoxin contamination in aquafeeds.

Mycotoxin occurrence: more than just aflatoxins

Generally, in SE Asia, it has been shown that raw materials such as: soy bean meal, wheat, wheat bran (WB), corn, corn gluten meal (CGM), rapeseeds/canola meal and rice bran are mostly contaminated with Fusarium mycotoxins (zearalenone (ZEA), deoxynivalenol (DON) and fumonisins (FB). Only cotton seed meal was observed to be contaminated primarily by aflatoxins (AF) and Fusarium toxins (ZEA and DON) at lower concentrations (data from BIOMIN mycotoxin survey 2015/16). European aqua feedstuffs are mainly contaminated by Fusarium mycotoxins. Moreover, the majority of mycotoxin contamination found in finished feeds are Fusarium mycotoxins, i.e., they come mainly from the raw materials used to produce feeds (so, from crops) and not aflatoxin contamination as is commonly believed within the aquaculture industry. An important factor is also the co-occurrence of mycotoxins which is high for all commodities, raising the probability of co-occurrence in finished feeds. An accumulation of mycotoxins on processed plant-based ingredients, (e.g., CGM and WB) has been observed when compared to the respective whole grains (C and WH, respectively). Regarding finished feeds, contamination detected in recent years poses a risk for several important aquaculture species, assuming single mycotoxin contamination, i.e. excluding any possible additive and synergetic effects between mycotoxins.

Relevance of mycotoxin occurrence to rainbow trout

To evaluate the consequences of DON contamination in European aquaculture finished feeds, two experiments were performed with rainbow trout (Oncorhynchus mykiss). First, the effect of short term feeding of high levels of DON (50 days; 1,166 µg/kg DON and 2,745 µg/kg DON) was tested. Moreover, the influence of mycotoxins against Yersinia ruckeri susceptibility was also evaluated. A second experiment studied the long term feeding of low levels of DON to rainbow trout (168 days; 367 µg/kg DON).

The experimental design tried to replicate two possible scenarios commonly observed. First, a contamination higher than 1000 ppb due to an inclusion of a highly contaminated raw material in the diets, which normally affects only a few batches of feed (short term). Second, a lower contamination, around 300-500ppb, which is commonly found all year round according to the BIOMIN aquafeeds survey, and can be ingested by animals over long periods of time.

Impact of short term exposure to DON on rainbow trout

As previously described by other authors, rainbow trout growth performance is affected by dietary contamination of DON. At the levels tested (1,166 µg/kg DON and 2,745 µg/kg DON), the thermal growth coefficient decreased 17% (p = 0.001) and the specific growth rate decreased by 13% (p < 0.001). Also, important numerical differences (p > 0.05) were found for protein efficiency rate and feed efficiency rate. Ingestion of DON did not influence the trout’s Yersinia ruckeri susceptibility, however the ingestion of DON resulted in gut and liver tissue destruction, confirmed by blood enzyme values and histology. Experiment results confirmed that the ingestion of DON at levels higher than 1,166 µg/kg, even if during short periods (50 days) can lead to an overall decline in performance which ultimately results in economic losses.

Long term exposure to DON

The second experiment aimed to study the impact of low mycotoxin contamination (367 µg/kg DON) over a longer period. Despite no statistical differences being found for final body weight (FBW) and other performance parameters, after 92 days of DON exposure, a more accentuated difference between the control and DON fed animals was observable. Actually, at day 168, the differences between the two treatments were relatively high, with a p-value at this sampling point of 0.053 (Figure 1). Feed conversion rate (FCR) showed a similar pattern to FBW and after 168 days, animals fed DON presented a FCR 25% higher when compared to the control (Figure 2). Growth performance reductions observed, even if not statistically different in certain cases, can negatively affect the profit of farmers, especially if we think that feed costs account for 60% of total production costs for salmonids. FCR showed an important numerical increase, which results in economic but also environmental consequences for the salmonids industry, especially so because European countries have legal limits for N-compound emissions.

As well as the negative impact described above, mycotoxins caused size dispersion, probably due to the individual history (health/nutritional status) of the trout. This size dispersion is extremely negative for famers, who need to invest resources in size sorting.

Authors believe that the ingestion of low levels of mycotoxins is often a reality in the aquaculture industry, however due to the lack of clinical signs of mycotoxi­coses and the lack of regular analyzes of mycotoxins in feeds, it is difficult to evaluate the possible impact of these mycotoxins in feed.

Lack of clinical signs makes diagnosis difficult

In aquatic organisms, it is difficult to prove that a disease is a mycotoxicosis. Even when mycotoxins are detected, it is not easy to show that they are the etiological agents in a given veterinary health problem. In the short-term period experiment, it was observed that animals fed the higher dosage of DON (2,745 µg/kg) presented haemorrhaging in the abdominal cavity and rectal hemorrhaging and irritation (Figure 2 and 3). Due to the controlled rearing conditions and the known ingested levels of DON, the authors associated the presented clinical signs to DON. However, in complex environmental conditions, such as those found on commercial sites, these clinical signs could easily be attributed to other etiological factors.

In the case of feeding DON chronically, it is even more difficult to detect any signs of mycotoxicoses. This experiment did not detect any evident clinical sign that could be associated to the ingestion of DON. However, interestingly, high levels of individual variability in sizes within the fish fed DON were observed, suggesting that the individual immune/nutritional status of each animal might influence the DON susceptibility.

Possible synergism still to evaluate

In both experiments, we were only considering the effect of a single mycotoxin (effect of DON). It is important to note that there are many different mycotoxins, and in many cases they appear simultaneously in feed. This is known to amplify the negative effects in animals, referred to as a synergistic effect. This essentially means that the sensitivity levels found with the present experiments can decrease in practice. Farmers would do well to regularly test feed materials for mycotoxins and use a proven mycotoxin risk management solution in order to maintain health and profitability.

Science & Solutions No. 46 - Aquaculture

Science & Solutions No. 46 - Aquaculture

This article was published in our Science & Solutions No. 46 - Aquaculture.

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