Underwater intrigue: exploring mycotoxins’ impact on fish

Beneath the surface: subclinical impact of dietary imbalance

Typically, a producer’s key management concern centers around disease, environment, and cost of feed. Diseases are widespread in aquaculture and pose a high risk to profitability in fish. Environmental fluctuations and stressors add additional pressure. Feed price fluctuate based on global market supply and demand and are one of the biggest costs.

Like all living animals, fish and shrimp have a requirement for certain nutrients. This includes amino-acids, cholesterol, vitamins, and minerals. In commercial fish production, this feed is usually grain and oilseed based. This brings in a sometimes-overlooked challenge: mycotoxins, common contaminants in animal feed.

Clinical symptoms due to mycotoxin contamination levels are rarely observed in aquatic species. More frequently, animals are exposed to moderate levels of mycotoxins over a long time. This chronic exposure leads to unspecific effects typically observed as decreased performance.

Poor performance typically manifests as slow growth, increase in feed conversion ratio (FCR), higher susceptibility to disease, and increased medical costs. Nevertheless, these subtle effects often pass unnoticed or are not traced back to the presence of mycotoxins.

Balancing act: nutritional requirement and key feed ingredients

Dietary formulations vary, depending on raw material availability, price, and developmental stage. Production of salmon, trout, Tilapia, seabream and seabass through different developmental stages should include crude protein at 20 – 60% and crude lipid at 4-30% (1-5). For vitamin recommendations, dsm-firmenich has developed OVN Optimum Vitamin Nutrition® guidelines.

From a macro-ingredient perspective, fish feeds are following the global trend to reduce reliance on marine ingredients. This is typically delivered through increased usage of plant-based raw materials.

Plant proteins may be the largest protein contributor, but their increased inclusion is not without risk. Even when diets are nutritionally balanced to limit amino acids, any increase in plant ingredients means a greater chance of introducing unwanted contaminants. These include anti-nutritional factors (ANFs), including mycotoxins. Ultimately, this will impact health and performance.

Deep dive: impact of mycotoxins on salmonid performance

There has been quite some research done on effects of mycotoxins in salmonids. Most studies however are limited to rainbow trout (Oncorhynchus mykiss). Aflatoxins have been shown to lead to lesions in liver tissue at a high dosage of 80 ppm (6). Dose-dependent increase in liver tumor incidence was also observed at 4-64 ppb (7).

Although Aflatoxins are the most toxic mycotoxins, focus of research has been on the more frequently occurring Fusarium mycotoxins DON and ZEN, with less investigations for FUM (see table below).

Research shows that negative effects on the liver can not only be the result of a contamination with Aflatoxin, but also with DON, ZEN and FUM. These mycotoxins show a negative effect on performance of salmonids, as reduced final body weight, reduced feed intake and increased FCR. First studies highlight the negative impact of the estrogenic mycotoxin ZEN on the reproductive tract in rainbow trout with a concentration at EU guidance value of 2,000 ppb.

An impact on the immune system and intestine of salmonids is indicated as well. The immune system of fish is quite complex, and similar to the immune systems of mammals, including innate and adaptive immunity (35). Important for the fish immune system are also specialized organs and cell types as the thyme, head kidney and spleen, as well as the mucosa-associated lymphoid tissue; skin, gill, and gut-associated (35).

Several detrimental effects were described on the intestine and immune system. Studies found histopathological lesions in the intestine, a negative effect on gut mucosa, hematological parameters, oxidative stress biomarkers in the liver, kidneys, and gill. Conversely, some studies describe no increased mortality in the presence of mycotoxins and challenge to Yersinia ruckeri and Flavobacterium psychrophilum (10, 19, 36).

Decreased antibody response to vaccination to Aeromonas salmonicidae was described in Salmo salar. Fish are constantly challenged by pathogens due to their environment, so a negative impact of mycotoxins on different parts of the immune system would add further challenge.

For OTA research results are limited. A high dose intra-peritoneal study at 2,000 ppb resulted in degeneration and necrosis of liver and kidney, swollen liver as well as pale kidney and increased mortality (26).

Closer look: mycotoxin impact on tilapia performance

Fewer studies are available for research on the effects of mycotoxins in Tilapia. High focus rests on aflatoxins, confirming their toxic effects in animals especially on the liver. Contamination with Aflatoxin amongst others, also increased mortality of Tilapia in several studies. Aflatoxin also impacted kidney and spleen as well as fish carcass. Although residues in the liver have been described at high contamination levels, one study indicated residues in the whole body. Negative effects on liver health have been also observed in the presence of DON, and one study suggests a co-contamination of Aflatoxin with FUM might promote liver tumors. For all toxins described in the table below, an impairment of performance can be observed. A study with co-contamination of DON and ZEN showed a dose-dependent decrease of performance and an increase in mortality.

What’s next?

The risk of mycotoxin contamination is never zero. Even with the best quality control procedures, mycotoxins are a constant threat. So how do producers manage this, alongside the demands of disease challenge? A robust mycotoxin risk management program is essential. Part of the toolbox is using mycotoxin deactivator in the feed formulation. Mycofix® is a state-of-the-art mycotoxin deactivator comprising three modes of action for the ultimate insurance policy against a wide range of known, and emerging mycotoxins. Adsorption to bind adsorbable mycotoxins, biotransformation to detoxify non-adsorbable mycotoxins and bioprotection, natural ingredients supporting liver and hepatopancreas, immune system and intestinal barrier.