MANAGING MYCOTOXIN RISKS IN AQUACULTURE
by Radka Borutova, DVM, PhD and Peter Coutteau, PhD Nutriad International, Belgium
eed represents over 60 percent of operating costs in aquaculture (FAO, 2014). Efforts to reduce the cost of feed have led to a reduction in the level of costly fishmeal and increased levels of plant ingredients in the feed of most fish species (Tacon et al., 2009). As a result, all herbivorous and omnivorous fish have a high risk of exposure to feed that might contain significant levels of mycotoxins and this may potentially lead to significant economic losses (Pietsch et al., 2013). Mycotoxins are secondary toxic metabolites, produced by filamentous fungi, such as Aspergillus, Penicillium and Fusarium fungi. Several mycotoxins commonly occur in feed and feedstuffs and some of the toxins such as aflatoxin B1 (AFB1), zearalenone (ZEN), deoxynivalenol (DON), fumonisin B1 (FB1) and ochratoxin A (OTA), draw the most scientific attention due to their toxic potentially adverse impacts on animal health (Placinta et al., 1999).
Discovery of trichothecenes
One of the most important groups of mycotoxins are the trichothecenes. This group (A type e.g. T2 toxin and B type e.g. deoxynivalenol [DON]) of structurally related mycotoxins has a strong impact on the health of animals and humans. Trichothecenes are powerful inhibitors of protein synthesis. More than a century ago, plant pathologists in Europe and the United States associated wheat head blight with infections by Fusarium graminearum (F. graminearum), which produces DON and Nivalenol (NIV). F. graminearum is a plant pathogen which causes fusarium head blight, a devastating disease on wheat and barley responsible for worldwide economic losses worth billions of dollars each year. F. graminearum infection causes shifts in the amino acid composition of wheat which results in shriveled kernels. In addition, the remaining grain is contaminated with mycotoxins, mainly DON, which inhibits protein biosynthesis, and ZEN. Consumption of over-wintered grain contaminated by F. sporotrichioides and related species during world war II caused alimentary toxic aleukia and the deaths of hundreds of thousands of people in the former Soviet Union. F. graminearum caused severe epidemics of akakabi-byo (red mold disease) on green wheat and other grains during the 1970s in Japan. People
who ate products containing the contaminated grains typically developed nausea, vomiting, diarrhea, hemorrhageg, anemia and other symptoms of trichothecene toxicosis. Japanese scientists were successful in identifying DON and NIV in grain infected with F. graminearum in 1972 (Desjardins, 2003). The Japanese researchers named it â€œRd-toxinâ€? (Moorooka et al., 1972). Shortly afterwards, the same mycotoxin was isolated from maize associated with emesis in pigs and given the name vomitoxin (Vesonder et al., 1973).
Effects of trichothecenes in aquaculture
Data showing the negative effects of trichothecenes in aquatic species is very limited. It is known that the effects of DON on animals vary depending on the nutritional and health status of the animals prior to exposure. Environmental factors, forms of DON, as well as its dose and duration of exposure also affect animals (Hooft et al., 2011). Swine are considered a sensitive animal species to DON with concentrations as low as 1 to 2 mg/ kg following oral exposure reducing feed intake and growth. Feed refusal and vomiting are typical clinical signs in swine fed on diets containing 12 and 20 mg/kg DON, respectively (Young et al., 1983). The sensitivity to DON contaminated feed appears speciesspecific in fish. Only a few studies have reported adverse impacts of dietary DON on aquatic species; e.g., rainbow trout (Oncorhynchus mykiss) and channel catfish (Ictalurus punctatus) (Hooft et al., 2011; Woodward et al, 1983). Rainbow trout are extremely sensitive. Feed intake, weight gain, and feed efficiency were observed to decrease significantly with increasing levels of DON in diets with starting levels of 0.5 mg/kg (Hooft et al., 2010). Significant decreases in weight gain, feed intake, feed efficiency, recovered energy, energy retention efficiency and nitrogen retention efficiency of rainbow trout fed diets containing low levels of DON ranging from 0.3 to 2.6 mg/kg from naturally contaminated corn were observed in a follow-up study (Hooft et al., 2011). Moreover, livers of rainbow trout fed dietary DON showed fat infiltration. Furthermore, pyknosis and karyolysis was observed in hepatocytes (Hooft et al., 2011). Significant reduction in growth, feed efficiency and feed intake was observed in rainbow trout fed diets containing corn artificially contaminated with graded levels of DON ranging from 1 to 12.9 mg/kg for eight weeks (Woodward et al., 1983). In contrast, channel catfish
18 | April 2017 - International Aquafeed