Mycotoxins in Aquaculture

What are mycotoxins?

Mycotoxins are secondary metabolites produced by a wide variety of filamentous fungi, including species from the genera Aspergillus, Fusarium and Penicillium. The fungi typically grow on various feedstuffs, such as grains and cereals. Mycotoxins are invisible, tasteless, chemically stable, and survive high temperatures and many environmental conditions. Most animal feedstuffs are likely to be contaminated with multiple mycotoxins. The growth of moulds and mycotoxin production occurs worldwide, especially in climates with high temperatures and humidity and where grain is harvested with high water content. The Food and Agriculture Organization (FAO) estimates that as much as 25% of the world’s agricultural commodities are contaminated with mycotoxins, leading to significant economic losses.

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Types of mycotoxins 

The most common source of food and feed contamination are mycotoxins produced by the fungi Aspergillus, Penicillium and Fusarium genera. Other mycotoxin-producing fungi include Alternaria, Chaetomium, Cladosporium, Claviceps, Diplodia, Myrothecium, Monascus, Phoma, Phomopsis, Pithomyces, Trichoderma and Stachybotrys.
While Aspergillus and Penicillium species are generally found as contaminants in feed during storage, Fusarium and Alternaria species can produce mycotoxins before harvesting or immediately after. Every plant can be contaminated by more than one fungus, and each fungus can produce more than one mycotoxin. Up until now, approximately 400 secondary metabolites with toxigenic potential produced by more than 100 moulds have been reported.

Aspergillus, Fusarium, Penicillium and Claviceps produce the most extensively studied mycotoxins. Some of the mycotoxins produced by these moulds include aflatoxins, ochratoxins, deoxynivalenol (DON), T-2 toxins, zearalenone, fumonisins, Citrinin and ergot alkaloids. Mycotoxins cause diverse effects on animals, such as carcinogenesis, hepatotoxicity, and neurotoxicity, as well as impaired reproduction, digestive disorders, immunomodulation, and decreased performance.

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Mycotoxin occurrence

Multiple factors determine the contamination of agricultural commodities with mycotoxins. Mycotoxin occurrence varies between crops, as fungal species and strains differ in their ability to infest a particular host. It also varies between varieties of the same plant species, as varieties show different levels of susceptibility or resistance to fungal infestation. Furthermore, environmental conditions, such as temperature and humidity, affect the infestation of crop plants with mycotoxigenic fungi and mycotoxin; therefore, climate and weather are strong determinants of mycotoxin contamination. Moreover, agricultural practices, the timing of harvest, and post-harvest handling of crops affect mycotoxin formation. 

Crops may be infested with multiple strains of fungi, and most fungal strains produce more than one type of mycotoxin. Therefore, co-contamination of agricultural commodities with multiple mycotoxins is frequently observed. When feed raw materials are mixed, mycotoxin co-contamination becomes even more likely. If mycotoxins co-occur, their combined toxic effect may be much greater than the summed effects of the individual mycotoxins.

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Effects of mycotoxins on Aquaculture

Plant-based ingredients have been successfully replacing fishmeal in finished aquaculture feeds. However, using crops in feeds results in an increased risk of contamination by fungi and mycotoxins and a higher incidence of mycotoxicosis in fish. This might decrease aquaculture's productivity as mycotoxicosis generally results in reduced body weight, growth impairment and higher rates of disease and mortality in fish. Additionally, some mycotoxins might accumulate in the fish's musculature. As such, fish consumption might become another way for mycotoxins to enter the human food chain, threatening food security and public health as mycotoxins are important genotoxins, carcinogens and immunosuppressors to humans.

Mycotoxins can result in a broad range of damage depending on the type of toxin, its concentration in the feed, and the length of exposure, as well as the susceptibility of the animal species. Therefore, the risk levels associated with mycotoxin exposure for aquatic species are divided into three categories:
Marine species and salmonids: Trout, sea bream and sea bass
Fresh warm-water fish: Tilapia, carp, channel catfish, African catfish, pangasius.
Shrimp: Based on studies in white shrimp and tiger shrimp.

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Symptoms of mycotoxin contamination in aquaculture

• Immune suppression
• Liver damage
• Poor growth rates
• Pale gills

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Signs of mycotoxin infection

• Chronic inflammation
• Liver and kidney damage
• Reduced weight gain
• Tumours
• Lesions

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Aflatoxins in fish

Aflatoxins are produced by Aspergillus fungi, which can infect many potential feedstuffs such as corn, rice, fish meal, shrimp and meat meals. Aflatoxin B1 (AFB1) is one of the most potent cancer-causing agents in animals. Initial findings associated with aflatoxicosis in fish include pale gills, impaired blood clotting, anemia, poor growth rates or lack of weight gain. The extent of disease caused by the consumption of aflatoxins depends upon the age and species of the fish. Fry are more susceptible to aflatoxicosis than adults. Trout is reported to be one of the most sensitive animals to aflatoxin poisoning. The carcinogenic or toxic effects of aflatoxins in fish seem to be species-specific. While trout are extremely sensitive to AFB1, warm water fish such as channel catfish are reportedly less susceptible to aflatoxins. Although less sensitive, warm water species are still affected by aflatoxin contamination. Feeding a diet containing 10 ppm AFB1/kg to catfish caused reduced growth rate and moderate internal lesions over a 10-week trial period. Studies on the effect of aflatoxin in Nile tilapia showed reduced growth rates and tissue abnormality or lesions, liver cancer development and 17% mortality. In the case of sea bream,  AFB1 shows adverse effects on the hepatocytes after 24 of exposure.

Aflatoxins in shrimp

In marine shrimp, several studies showed that AFB1 could cause abnormalities such as poor growth, low apparent digestibility, physiological disorders and histological changes, principally in the hepatopancreatic tissue. Consumption of diets with AFB1 results in a higher mortality rate. AFB1 also affects the growth of juvenile shrimps. Histopathological findings indicated hepatopancreatic damage by AFB1 with biochemical changes in the hemolymph. There were marked histological changes in the hepatopancreas of shrimp fed diet containing AFB1 as noted by atrophic changes, followed by necrosis of the tubular epithelial cells. Studies show severe degeneration of hepatopancreatic tubules was common in shrimp fed high concentrations of AFB1. 

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Ochratoxins

There is very little research into the toxicity of Ochratoxin A (OTA) in aquatic animals. Based on the few studies available, the kidneys and liver suffer degeneration and necrosis, resulting in decreased weight gain, decreased FCR, lower survival rates, and lower hematocrit levels. In one study on catfish, OTA also has immunosuppressive effects, making animals more vulnerable to pathogenic infections.

Studies on the effects of Ochratoxin A on shrimp have reported hepatopancreatic atrophy, necrosis, and degeneration, as well as disruption of hematopoietic tissue and lymphoid organs.  

Fumonisins 

Aquaculture species generally exhibit reduced growth rates, feed consumption, feed efficiency ratios, and impaired sphingolipid metabolism when exposed to fumonisin B1 (FB1). At levels of FB1 below 100 ppb, rainbow trout were shown to experience changes in their sphingolipid metabolism and to develop liver cancer.  

At concentrations as low as 500 parts per billion, the consumption of FB1 by carp has been shown to cause liver and pancreatic lesions. Intake of low doses of FB1 reduced performance parameters such as average body weight and average weight gain. In addition to affecting Nile tilapia performance, FB1 has been shown to result in reduced weight gain and an increased sphinganine/ sphingosine ratio. 

In shrimp, there is no extensive research on the effects of FUM. Still, some evidence suggests that low doses of these mycotoxins can affect the texture of the meat and negatively affect the quality of the product.  

Deoxynivalenol (DON)

Salmonids, particularly rainbow trout and Atlantic salmon, are sensitive to deoxynivalenol (DON). Both species exhibit sensitivity to low concentrations (300-500 ppb) of DON. Several studies observed significant reductions in growth, protein, energy utilization, feed intake, and feed efficiency.

In terms of the impact of DON on shrimp, studies have shown that concentrations as low as 200 parts per billion are associated with reduced body weight and a slow growth rate. 

Zearalenone 

Zearalenone (ZEN) studies have mainly focused on reproductive dysfunction or structural disorders in farm animals. ZEN modulates estrogen receptor-dependent gene expression, thus affecting fish reproduction, according to several studies. ZEN exposure reduced spawning frequency or changed relative fecundity in zebrafish. ZEN has also been shown to cause genotoxic effects, including defects in zebrafish heart and eye development and an upward curvature of the body axis.  

Observed effects of ZEN on shrimp include abnormalities in the development of hepatopancreas, with consequences for the immune system and growth.

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In order to effectively manage mycotoxins, we need to see the whole picture: from the feed mill to the farm and from risk assessment to feed management. Biofeed's Mycotoxin Risk Management Program provides a number of solutions to help you mitigate the threat you could face from mycotoxins.

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