Mycotoxins are biotoxins produced by several molds such as Aspergillus and Penicillium. About two hundred mycotoxins have been identified, most of which are known to be severely harmful to animals. Terrestrial animal husbandry manufacturers are well aware of the hazards of this toxin, but the impact of these toxins on the types of aquaculture has not been studied in depth. Considering that the current source of feed is increasingly inclined to use cheaper, such as the use of vegetable protein instead of high-priced animal protein such as fish meal, the seriousness of contamination of mycotoxins caused by feeding plant-derived feeds, including by-products, is greatly increased. There will be more cases of mycotoxin contamination. Therefore, the research on the toxicity of mycotoxins in aquatic animals is extremely urgent and important. This paper reviews the hazards of mycotoxins to plants and animals, and describes their toxicity in aquafeeds, as well as methods and solutions for the determination of mycotoxins.
1 Mycotoxin harm
Mycotoxins mainly include aflatoxin, CPA, ochratoxin, deoxycubiferol, Fusarium toxin, and the like.
Aflatoxins are very susceptible to contaminating feeds such as corn, peanuts and cottonseed kernels. A large part of the world's use of corn is used as a major component of fish feed. It may also contain more mycotoxins, especially aflatoxins, which are reported to contain up to 6,000 μg/kg of aflatoxin. In the southeastern United States, 27% of corn samples contained more than 400 μg/kg mycotoxin, and more than 50% of samples exceeded 100 μg/kg. Peanut kernels and cottonseed kernels are most susceptible to aflatoxin contamination, so feed production should be avoided as much as possible. If it must be used, make sure the feed source is not contaminated with aflatoxin and check all feeds. Cottonseed and corn are common ingredients in crabs and squids and account for 25% to 30% of the feed formulation, so the possibility of aflatoxin being transmitted to fish is very high.
More and more studies have shown that mycotoxins are similar to terrestrial species in terms of aquaculture species. A lot of work is to study the damage of aflatoxin to fish, and only a small amount is to study the damage of mycotoxins to other species such as shrimp.
It is well known that rainbow trout is one of the most susceptible species of aquaculture species to be infected by aquaculture species. Experiments have shown that the LD50 (maximum semi-lethal dose) of aflatoxin is between 500 and 1 000 μg/kg. The serious hazards of aflatoxin in rainbow trout include whitening of the fish, reducing the number of red blood cells, destroying the liver and so on. Dr. Richard Lovell of Auburn University in the United States found that salmon and other warm-water fish are less susceptible to aflatoxins than rainbow trout. When feeding the semi-purified feed of squid, if the aflatoxin content reaches 10 mg/kg, the aflatoxin will show obvious symptoms after 10 weeks of feeding, such as significantly reducing the growth rate and the number of red blood cells and hemoglobin. There are liver necrosis, renal enlargement, and gastric gland necrosis. Oral administration of 12 mg/kg (weight) of aflatoxin can cause reflux of gastric contents, and aflatoxin entering the peritoneal cavity can cause hemoglobin to fall to 90% of normal levels if it exceeds 12 mg/kg. It can cause necrosis of the intestinal mucosa and blood cells, which can produce pancreas, stomach glands, and make some organs lighter. These side effects, in turn, cause the fish to grow slowly and the feed effect to deteriorate.
Although research on shrimp is still rare, some studies have shown that levels of mycotoxins in feed may be problematic. Studies in Thailand and the Philippines have been conducted to investigate the consequences of detecting mycotoxin levels. Histopathological studies have found that mycotoxin B1 damage to the (crustacean) hepatopancreas suggests that mycotoxins in shrimp feed can affect yield. Filipino scholars have found that the shrimp toxin has a mycotoxin concentration of 73.8 μg/kg, which is slow to grow, and it is easier to get a skin disease. (Crustacean) damage to the liver and pancreas can cause other conditions. Liang Mengqing et al. investigated the effects of aflatoxin on the growth of Chinese shrimp, and found that the content of aflatoxin B1 in feed was 472.0 μg/kg and 78.7 μg/kg, respectively, if the control group was 100, China The survival rate of shrimp was 55%, the weight gain rate was 44% and 45%, respectively, and the digestibility was 79.4% and 83.2% of the control group. Shrimp swims slowly, and individual prawns swim on the surface of the water, seldom hug, and die after leaving the water.
2 other mycotoxins
Other mycotoxins can also cause production problems in farmed fish. Cyclopiasonic acid (CPA) is a toxin produced by Aspergillus and Penicillium, which is often found in the same sample as aflatoxin in warm climates, and is even more common than aflatoxins. It is the most frequently found in mycotoxins in the detection of 1,500 feed and food samples. CPA is more toxic in salmon than aflatoxin. In one study, aflatoxin and CPA levels were 0, 0.1, 0.5, 2.0, and 10.0 mg/kg, respectively. Aflatoxin only slowed fish growth and decreased at the highest concentration. The number of red blood cells is the same; CPA can produce the same hazard only at 0.1 mg/kg, and it will also cause necrosis of the digestive tract at higher concentrations. Another study showed that if the catfish were injected with 7.0 mg/kg body weight of CPA, the fish would experience severe convulsions and die within 30 min. Similar hazards are found in other warm-water fish, such as in tilapia and squid. This problem is exacerbated by the fact that the concentration of CPA may be larger than other mycotoxins, which is particularly acute in hot climates.
Ochratoxin is also a toxin produced mainly by Aspergillus and Penicillium. It often harms the kidneys of fish and, when it appears with other toxins in the feed, it strengthens the toxins. Ochratoxin contains seven structurally similar compounds, of which ochratoxin A is the most toxic. Ochratoxin A has kidney toxicity and liver toxicity, and it mainly invades the kidneys to cause kidney damage when present in low concentrations in nature. When humans and animals ingest foods and feeds contaminated with this toxin, acute or chronic poisoning may occur, or teratogenic, carcinogenic, and mutagenic. The semi-lethal dose of oral ochratoxin to the rainbow trout in the growing season was 4.7 mg/kg. The damage of ochratoxin to rainbow trout is liver necrosis, darkening of color, enlargement of the kidneys, and high mortality.
Deoxymelanol, abbreviated as DON, also known as vomiting toxin, is produced by the metabolism of Fusarium. When the growth period is in wet weather, DON is an important toxin in wheat. Feeding rainbow trout DON at 0, 2.0, 5.0 and 1.0 mg/kg will slow the growth of fish. When the feed concentration of the rainbow trout is 20 mg/kg, antifeeding occurs.
Fusarium toxins, especially trichothecene, are another highly harmful mycotoxins. Zearalenone is the most common Fusarium toxin and is a metabolite of estrogen. In terrestrial livestock, such as pigs, even if the concentration is only between 0.6 and 5.0 mg/kg, it will cause breeding problems. Other symptoms in the land animal also significantly reduce the growth rate, destroy the regeneration of red blood cells, trigger hemorrhage and tissue damage, and reduce immunity.
A contaminated feed or ingredient thereof may contain more than one mycotoxins. Many studies have reported that mycotoxins have a synergistic effect, and the combined harm of the two toxins is much greater than the harm of the individual effects. Squeezing during heating and granulation does not remove enough toxins. In addition, the practice of mixing contaminated feed with clean feed to reduce its toxin content has not been promoted. If the feed contains active mold or spores, the amount of toxin in the feed will increase greatly when the conditions are right.
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3 causes of mildew
Toxin-producing molds tend to survive in warm, humid environments, which is the environment for aquaculture operations dominated by tropical and subtropical climates. Most feed mills are very careful when checking whether raw materials are infected with aflatoxins. However, the presence of other mycotoxins, as well as the unsafe exposure of the feed, can cause the mycotoxins to increase significantly after leaving the plant, especially if the storage period is exceeded.
Many companies store feed conditions that are less standard or even rudimentary. In reality, the feed may become damp during transportation. It may rain when it is stored, and even mice and insects may cause the feed to be mildewed and contaminated with mycotoxins. Studies have shown that poor storage conditions can lead to an increase in aflatoxins. Aflatoxin was detected in 59 of the 62 shrimp feed samples, and 22% of the stored aflatoxin content exceeded 50 μg/kg. Another survey showed aflatoxins and fumagillin in samples from all tested areas. The amount of aflatoxin found in some samples actually exceeds the limit of harmful effects of livestock feed on mammals and lower vertebrates. Aflatoxin B1 reached 0.651 μg/kg in the shrimp feed sample test in Thailand. These studies have shown that the contamination of feed by mycotoxins, especially aflatoxins, is beyond the level previously thought, and is already a common phenomenon.
4 Detection of mycotoxins
In view of the fact that mycotoxins are harmful to animals, have a wide range of pollution, and are difficult to detoxify, the detection of toxins in feeds is very important. Since the early 1960s, more than 30 detection methods have been established, which can be classified into three categories: biological methods, chemical methods and immunological methods.
4.1 Biological methods
There are more than ten such methods, the most commonly used for the detection of aflatoxin fluorescence reactions. The principle is to use AF to emit fluorescence under ultraviolet light irradiation, and after the sample to be tested is processed, it can be detected by a fluorometer. The method is characterized by rapid and qualitative detection of the presence or absence of AF in the feed, the sample does not need to be very pure, and a small amount of impurities mixed has little effect on the result; the disadvantage is that the application range is small and can only be used for the toxin which emits fluorescence under ultraviolet light. Identification.
4.2 Chemical methods
Mainly used for quantitative analysis of toxins, commonly used thin layer chromatography and high pressure liquid chromatography. Thin layer chromatography (TCL) has the advantages of high sensitivity, convenient color development, and simultaneous detection of several toxins. The disadvantage is that the purification of the sample is cumbersome and requires the use of standard poison, which is easy to cause environmental pollution. High performance liquid chromatography (HPLC) is fast, sensitive, accurate and automated, and is used for microanalysis of toxins, especially for the determination of residual amounts of toxins in tissues. However, due to the high price of the instrument, it is difficult to promote the application at the grassroots level.
4.3 Immunological methods
Compared with chemical methods, it has the advantages of high specificity, high sensitivity, fast and convenient, and no need for expensive equipment. Compared with the thin layer chromatography method, the sensitivity is improved by 500 times, and the sample pretreatment is also simplified to some extent, and it is easy to popularize. In short, after 20 years of hard work, immunochemical technology has opened up a new way for mycotoxin detection.
5 Mycotoxin detoxification
The problem of contamination of mycotoxins is divided into the following methods: 1 physical methods, such as color difference selection technology, density separation technology and washing, heat treatment, microwave treatment, daylight degradation, etc.; 2 chemical methods, such as heat treatment with reducing sugar, alkali treatment Hydrolysis, acid sulfite treatment, ammoniation treatment, hydrogen peroxide or sodium bicarbonate treatment, activated carbon treatment; 3 microbial methods: ethanol fermentation treatment, beneficial bacterial mixture preparation. But the best way is to pick the uncontaminated feed and ingredients, which requires a good relationship and strict standards with the supplier, and can ask the supplier to ensure that it can inhibit the growth of mold. This is an extremely serious challenge in warm and humid aquaculture areas, especially where storage conditions are extremely crude.
One of the effective ways to avoid Fusarium toxin poisoning is to develop a feed binder that can be effectively combined with mycotoxins in the gut by adding a small amount. This method can reduce the virulence of contaminated feed and minimize the impact of the feed on production performance. However, it is difficult to find an adhesive that can specifically bind to common mycotoxins. If the binder lacks specificity, it will bind some micronutrients or drugs, which will weaken the effectiveness of the adhesive. It is equally important to achieve a sufficiently high binding capacity with low dose addition. This is because the binder is usually non-nutritive and is also a diluent that dilutes the nutrient concentration of the feed. In a recirculating system, clay can hinder or damage the filtration system. Tiny clay particles can also damage fish and shrimp. . It is important to note that some clays have been found to contain dioxins (a strong carcinogen) that can accumulate in the fat portion of the fish, causing food safety problems. Other alternatives to clays have improved glucomannan extracted from the inner walls of yeast cells, especially some glucomannans which strongly absorb large amounts of very low levels of mycotoxins. Since it is extracted from yeast, it is naturally soluble and safe for animals and humans. It performs very well in the inhibition of mycotoxin by terrestrial livestock, and its prospects for aquaculture applications are expected to be very broad.
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