The outbreak in 2009 in the world’s largest shrimp producing countries, like China, Thailand, Malaysia and Vietnam, shifted the market share to “second-tier” shrimp producing regions like India, Indonesia, Ecuador and Myanmar. The following high shrimp prices caused a drop in US and EU shrimp consumption.
This long lasting effect on the shrimp trade and it’s still doubtful recovery strongly increased the awareness of producers on the general health status of their farmed animals:
In recent years, there has been growing awareness of mycotoxin-related issues within the aqua industry, supported by increasing scientific evidence of the negative impact of mycotoxins in aquatic species and by frequent reports on the prevalence of mycotoxins in many raw materials. However, not much information is available on the risk levels of different mycotoxins for different aquatic species.
To compile such information, it must be understood that mycotoxins are invisible, odorless and tasteless toxins with a major impact on the growth and health of fish and shrimp. This can result in significant economic losses. Mycotoxins also appear to be very heat stable and the pelleting and extrusion process of fish and shrimp feeds do not seem to reduce appreciable amounts of mycotoxins in the feed.
Mycotoxins have different toxic effects in aquatic species such as carcinogenic (e.g. aflotoxin B1, ochratoxin A, fumonosin B1), estrogenic (zearalenone), neurotoxic (fumonosin B1), nephrotoxic (ochratoxin), dermatoxic (trichothecenes) or immunosuppressive (aflatoxin B1, ochratoxin A and T-2 toxin).
The extent of the damage caused by the consumption of mycotoxins depends mainly on the type of toxin, its concentration in the feed and time period of exposure, as well as animal species susceptibility. Other factors including age and stress levels can also affect mycotoxin toxicity. Fry are more susceptible than adult fish and some fish species are more sensitive to mycotoxins than others. Marine and cold water species seem to be more sensitive to mycotoxins than warm-water fish. Although less sensitive, warm-water species are still affected by mycotoxin contamination. Thus, mycotoxin risk levels for aquatic species have been considered in terms of three categories: marine species and salmonids; fresh warm-water fish; and shrimp.
Economic losses from mycotoxin contamination can be caused by exposure to high contamination levels, or by detrimental health effects in animals consuming feed with low or moderate levels of mycotoxin contamination. These subtle effects often go unnoticed and profits are lost due to decreased efficiency in production, such as slow growth, reduced weight, increase in FCR and increased medical costs.
Figure 1: Aflatoxin B1 (60 – 230 ppb) has a strong effect on the growth of tilapia
Three pathological forms of mycotoxicosis may be described: acute, subacute and chronic. Acute mycotoxicosis occurs when moderate to high doses of mycotoxins are ingested. Signs of acute mycotoxicosis in aquatic species depend on the species and mycotoxins. Among all known mycotoxins, aflatoxins are the best characterized and most investigated due to their acute and chronic toxicity on aquatic species. Aflatoxin B1 (AFB1) is the most biologically active toxin known. Low levels of this toxin (<10 ppb) are enough to produce negative effects when compared to other toxins like Fumonisin (FUM) (> 5000 ppb). Among aquatic species, trout seem to be the most sensitive species to AFB1 (5 ppb), while channel catfish seem to be the most resistant (>500 ppb).
Acute mycotoxicosis is normally marked by a significant reduction in performance (growth and feed efficiency) and can result in high mortality levels, depending on the type of toxin and contamination level. Aquatic animals affected by subacute mycotoxicosis display several symptoms that commonly include moderate to severe liver damage, reduced feed conversion ratios, and impaired immune responses. Chronic mycotoxicosis occurs when low-to-moderate doses of mycotoxins are ingested over a long period of time. Generally, it is difficult to recognize or diagnose this condition because of its slow, subclinical trend. The majority of clinical signs is related to chronic, impaired liver function, such as reduced feed efficiency and increased susceptibility to secondary infectious diseases, which are due to immunosuppression.
The low, medium and high risk levels (Table 1) for the different categories of farmed aquatic animals take into account the possible economic effects of mycotoxin exposure, the sensitivity of each species to the different type of mycotoxins according to published scientific studies, and the possible period of exposure during their grow-out phases.
Table 1: Aflatoxin B1 risk levels for aquatic species
|Risk level of aflatoxin B1 [ppb = μg/kg]||Low||Medium||High|
|Salmonids and marine fish||< 5||5-20||> 20|
|Fresh warm-water fish||< 20||20-60||> 60|
|Shrimp||< 10||10-40||> 40|
Marine fish: based on studies conducted with trout, sea bass and sea bream.
Fresh warm-water species: based on studies conducted with channel catfish, African catfish, pangasius, tilapia, yellow catfish, carp and giebel carp.
Shrimp: based on studies conducted with white shrimp and tiger shrimp.