1. Introduction
Aflatoxins, a group of toxic mold metabolites, are significant to nutritionists because Ž
. they affect human and animal health Bullerman, 1986 . Aflatoxin contamination occurs
over large geographic regions and in many potential feedstuffs, such as cottonseed, peanut, corn, milo, rice, dried fish, shrimp, and meat meals. The principle aflatoxin,
AFB , is one of the most toxic of all naturally occurring carcinogens. A major epidemic
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Ž .
of liver tumors hepatomas , which struck US trout hatcheries in the early 1960s, was Ž
. traced to contaminated cottonseed meal in the feeds Wolf and Jackson, 1963 . Liver
tumors are still a problem for many hatcheries producing 2–3-year-old trout for breeding Ž
. purposes Charlie Smith, 1992, personal communication . Aflatoxin also continues to be
a problem outside the United States, with noted outbreaks of fish aflatoxicosis reported Ž
. Ž
. in Germany Wunder and Korn, 1982 , Mexico RuizPerez et al., 1984 , Denmark
Ž .
Ž .
Rasmussen et al., 1986 , and Chile Tim Phillips, 1990, personal communication . Ž
In farm animals, low level mycotoxicoses from aflatoxin and other mold metabo- .
lites are correlated with feed refusal, reduced feed conversion ratios, anemia, reproduc- Ž
. tive failure, impaired immune response, and renal damage Hamilton, 1990 . Similar
symptoms have been reported in hatchery reared fish. Under FDA regulations, most feed Ž
ingredients and nonruminant feeds are restricted to F 20 mgrkg total aflatoxin FDA, .
1989a,b . Heavily contaminated feeds are usually destroyed or are sold at a steep discount for non-feed uses. Farmers in Texas recently suffered several consecutive years
of economic hardship resulting from contaminated corn crops, after which Texas relaxed quality controls by permitting heavily contaminated corn to be blended into cattle feeds
Ž
. Anon, 1991 . When feed producers carry contaminated ingredients for use in livestock
feeds, the potential for cross-contamination of fish feeds is increased. Ž
Of all domesticated animals, trout are among the most sensitive to AFB CAST,
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. 1989 . Tumors can take a year or more to develop, but 20 mgrkg is well above safe
Ž .
limits for trout, even for temporary feeding Lee et al., 1971 . Once ingested, AFB is
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metabolized to other forms. Carcinogenesis occurs when epoxide derivatives penetrate Ž
. the nucleus of a cell and bind to genetic material DNA . However, aflatoxicosis is not
limited to fish and farm animals. The same basic process is believed to occur both in Ž
. Ž
. trout Nakatsuru et al., 1990 and in humans Groopman et al., 1985 . New methods for
Ž controlling fish aflatoxicosis may therefore have human applications Anon, 1976;
. Bullerman, 1986 .
Aflatoxin extraction or detoxification has proven practical for only a few feed ingredients, e.g., ammoniation of cottonseed meal has been reported to effectively
Ž .
detoxify the aflatoxin contaminant Ahmad et al., 1996 , while most existing aflatoxin detoxification technologies are not applicable to most fish feed ingredients. A cost-effec-
tive feed ingredient, capable of detoxifying low level aflatoxin contamination without deleterious side effects, would represent a significant advancement in animal and human
Ž .
health. Smith 1980 documented unexpected improvements in growth and feed conver- sion ratio among rainbow trout when clay minerals were added to their diet. Smith was
unable to provide a suitable explanation until several years later, when researchers Ž
working with farm animals presented evidence of clay–aflatoxin interactions Harvey et
. al., 1988; Phillips et al., 1995; Lindemann et al., 1997 . Examination of Smith’s data
supported his hypothesis, and initiated this study. Ž
. Winfree and Allred 1992 developed a method to detect and measure clay–aflatoxin
Ž .
or substance–aflatoxin interactions, and calculated a numerical absorption coefficient based on a substance’s capacity to remove aflatoxin from a mixture of feed and water.
Sodium bentonite exhibited a high affinity for AFB , and preliminary feeding trials
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indicated that dietary bentonite could reduce the percentage of fish developing tumors Ž
. after consuming AFB
unpublished data . Bentonite is commonly employed as a feed
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Ž .
Ž .
binder, and previous work by Smith 1980 and Reinitz 1984 have established the mineral to be safe for trout at dietary inclusion levels up to 10.
The current study was initiated to trace the absorption, metabolism, and elimination of dietary aflatoxin in fish consuming AFB contaminated diets, with or without 2
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dietary bentonite added as an adsorbent. Our purpose was to determine whether Ž
. bentonite’s demonstrated ability to adsorb AFB in vitro Winfree and Allred, 1992
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would block absorption of the toxin in vivo.
2. Materials and methods
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