Mycotoxins: an overlooked threat in shrimps // 09 Apr 2008

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.

 

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.

 

 

 

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.

 

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.