|Dr. Pedro Encarnação has an extensive background in aquaculture and nutrition, and conducted several research projects focusing on the improvement of feed formulations for aquaculture species. He has an Honors Degree in Marine Biology and Fisheries and an MSc in Aquaculture from the University of Algarve (Portugal), and obtained is PhD in Animal Nutrition from the University of Guelph (Canada).Pedro currently works as aquaculture specialist at Biomin.|
Contamination of feed for aquatic species is common in humid tropical regions, in South East Asia, for instance. The problem can be caused by many factors, such as low quality of feed ingredients and inappropriate methods of feed storage. Mycotoxins are secondary metabolites produced by fungi, commonly referred to as molds. They are produced by these organisms when they grow on agricultural products before or after harvest or during transportation or storage.
Given the trend and the economical need to replace expensive animal-derived proteins, such as fish meal, with less expensive plant proteins sources, the impact of mycotoxin contamination in aquaculture feeds will have the tendency to increase due to the higher susceptibility for mycotoxin contamination in ingredients of plant origin. Most of the mycotoxins that have the potential to reduce growth and health status of shrimp and other farmed animals consuming contaminated feed are produced by Aspergillus, Penicillium and Fusarium sp. (CAST, 2003). These toxic substances are known to be either carcinogenic (e.g. aflatoxin B1, ochratoxin A, fumonisin B1), estrogenic (zearalenone), neurotoxic (fumonisin B1), nephrotoxic (ochratoxin), dermatotoxic (trichothecenes) or immunosuppressive (aflatoxin B1, ochratoxin A and T-2 toxin).
Mould toxins vary in their toxicity toward different animals species and while the effect of mycotoxins is relatively well known in most terrestrial farm animals the effect of mycotoxins on aquaculture species has not been studied extensively. Nevertheless, several studies have reported pathological signs of mycotoxin poisoning in fish and shrimp species which can cause economic losses to the industry. These economic losses can be caused either by unfavorable effects on the animal themselves, by exposure to high contamination levels, or by an increase potential for detrimental health effects when being exposed to low or moderate contamination.
Studies on aflatoxins
The general indifference regarding the consequence of mycotoxin contamination in shrimp feeds is directly related to the lack of information on the impact of the different mycotoxins in crustacean culture. Even though the information is limited, several studies have been conducted on the toxicity of mycotoxins toward aquatic invertebrates. These studies however, have been focusing mainly in aflatoxins. It has been reported that dietary aflatoxin B1 (AFB) adversely affected growth performance, feed conversion, apparent digestibility coefficients, and cause physiological disorders and histological changes, in particular on hepatopancreatic tissue1.
Nevertheless, these reports have shown inconsistent results regarding the sensitivity of shrimp to AFB. According to Bintvihok et al. (2003) AFB levels below 20 ppb (20μg/kg) can already cause reduction in weight gain and slightly increase mortality after only 10 days. Histopathological findings indicated hepatopancreatic damage by AFB with biochemical changes of the haemolymph2. Similar findings were reported by Bautista et al., (1994) which observed histopathological changes in the hepatopancreas of shrimp at levels of 25 ppb AFB. These effects were aggravated with increasing toxin concentration, nevertheless, reduction in weight gain was only observed for AFB concentrations above 75 ppb during the 60 day study with juvenile black tiger shrimp (Penaeus monodon)3.
Conversely, AFB levels between 50–100 ppb showed no effect on growth in juvenile black tiger shrimp (Penaeus monodon)4. However, growth was reduced when AFB concentrations were elevated to 500–2500 ppb. Survival dropped to 26.32% when 2500 ppb AFB was given, whereas concentrations of 50–1000 ppb had no effect on survival5.
|Consumption of diets contaminated with mycotoxins suppresses the immune system and decreases disease resistance of shrimp.|
There were marked histological changes in the hepatopancreas of shrimp fed diet containing AFB at a concentration of 100–2500 ppb for 8 weeks, as noted by atrophic changes, followed by necrosis of the tubular epithelial cells. Severe degeneration of hepatopancreatic tubules was common in shrimp fed high concentrations of AFB6. Abnormal hepatopancreas and antennal gland tissues were also reported by Ostrowski-Meissner, et al., 1995 in shrimp fed 50 ppb AFB/kg after only 2 weeks. Feed conversion efficiency and growth were significantly
affected, but only at AFB levels of 400 ppb. Apparent digestibility coefficients decreased significantly at AFB 900 ppb7. According to Burgos-Hernadez et al. (2005), the effect of AFB toxicity to shrimp results in the modification of digestive processes and abnormal development of the hepatopancreas due to exposure to mycotoxins. These effects might be due to alterations of trypsin and collagenase activities, among other factors, such as the possible adverse effect of these mycotoxins on other digestive enzymes (e.g. lipases and amylases)8. These results show that AFB contamination in shrimp feed may cause economic losses by lowering the production of shrimp.
Effect of other mycotoxins
Information on the effects of other possible harmful mycotoxins on shrimp and other crustacean species are scarce. Only a few studies have been conducted to access the effects of deoxynivalenol (DON), ochratoxin A (OTA), zearelenone (ZON) and T-2 in shrimp. Deoxynivalenol, also known as vomitoxin, and other type B trichothecenes are produced by Fusarium sp. and can be an important contaminant of wheat.
Deoxynivalenol levels of 200, 500, and 1000 ppb in the diet significantly reduced body weight and growth rate in white shrimp Litopenaeus vannamei9. However, the effects of 200 and 500 ppb DON were manifested at later stages of growth, and 200 ppb DON affected only growth rate and not body weight. Feed conversion ratio and survival of shrimp fed diets containing 200, 500, and 1000 ppm DON were not significantly different from those of shrimp fed the control diet (0.0 ppm DON)10. In Penaeus monodon, feed supplemented with DON to levels up to 2000 ppb caused no effect on growth11.
Supamattaya et al. (2006) reported that in white shrimp growth was significantly reduced by T-2 toxin at 0.1 ppm while for black tiger shrimp reduced growth was observed at levels of 2.0 ppm. The presence of T-2 toxin at 1.0-2.0 ppm produced atrophic changes and severe degeneration of hepatopancreas tissue, inflamation and loose contact of hemopoietic tissue and lymphoid organ on black tiger and white shrimp after feeding for 10 weeks and 8 weeks respectively. The same pathology was found in shrimp received 1.0 ppm zearalenone12. It was concluded by the authors that white tiger shrimp are more sensitive to mycotoxins then black tiger shrimp.
Suppressed immune system
Evidence suggests that consumption of diets contaminated with mycotoxins suppresses the immune system and decreases disease resistance. This can occur even when animals are consuming low or moderately contaminated products, as such, its effects pass unnoticed and the economic losses are normally just associated with the disease outbreak causing the damage. Mycotoxins that impair the immune system include AFB, T-2 toxin, OTA, DON and fumonisin.
The effects of mycotoxins on immunological responses of terrestrial animals have been examined extensively. Most of these toxins cause impairment of the immune system by inhibiting the synthesis of key proteins associated with the immune function. Haemocytes, in conjunction with fixed phagocytes form the immunocompetent components of the shrimp immune system, and as such a reduction on their numbers can result in a decreased disease resistance, making the shrimp more susceptible to infections.
Consumption of trichotecene mycotoxins causes suppression of immune response by reducing both phagocytic activity and chemotaxis by macrophages13. Total hemocyte, granulocyte and phenoloxidase activity were reduced in shrimp fed with T-2 toxin and zearalenone. Conversely, no difference in numbers of haemocytes in blood circulation was observed between shrimp fed various concentrations of OTA and DON (0 – 2000 ppb) after eight weeks period14.
The results of phenoloxidase (PO) activity however, showed that feeding with high level of OTA (1000 ppb) caused significant decreasing of PO activity15. The effect of aflatoxins on the immune system is to reduce the production of certain cell components such as C4 complement and lymphokines e.g interleukins, and T lymphocytes16. Aflatoxin, suppresses phagocytosis by macrophages, which alters subsequent processing and
presentation of antigen to B lymphocytes with consequential reductions in disease resistance17.
A negative correlation between the number of haemocytes and dietary concentration of AFB was reported by Boonyaratpalin et al. (2001) when feeding shrimp diets ranging from 0-2500 ppb AFB during an eight week period. A biochemical change of the haemolymph by AFB was also reported by Bintvihok et al. (2003). A decline in the activity of such immuno-competent cells causes a decline in shrimp immune response18.
The contamination of feeds and raw materials by mycotoxins is a reality which is increasing on a global basis, making it increasingly likely that any given feedstuff could contain one or, more likely, several mycotoxins. They are invisible, odorless and tasteless toxins with a major impact on animal health. Although the presence of mycotoxins in feed represents an increased threat to aquaculture operations, there are a number of options available to feed manufacturers and farmers to prevent or reduce the risk of mycotoxicosis associated with mycotoxin contamination.
These range from careful selection of raw materials, maintaining good storage conditions for feeds and raw materials, and using an effective mycotoxin deactivator product to combat the widest possible range of different mycotoxins that may be present.
Binders or adsorbents have been used to neutralize the effects of mycotoxins by preventing their absorption from the animal’s digestive tract. Unfortunately, different mycotoxin groups are different in their chemical structure and therefore it is impossible to equally deactivate all mycotoxins using only one single strategy. Furthermore, special algae extracts can support the immune system and thus overcome the immune-suppressive effect of all mycotoxins.
Mycotoxins can also be detoxified by using microorganisms or enzymes which specifically degrade the toxic structures to non-toxic metabolites. The liver, the main target organ of mycotoxins, is protected by selected plant and algae extracts, which are able to eliminate toxin related effects by supporting the immune system and reducing inflammation.
*References 1-18 are available on request