Aflatoxin is one of several extremely toxic, mutagenic and carcinogenic compounds produced by Aspergillus (A.) flavus and A. parasiticus [6]. Research studies have revealed four major aflatoxins; B1, B2, G1 and G2 as well as two additional metabolic products, M1 and M2, that are direct contaminants of foods and livestock feed. Of these, aflatoxin B1 (AFB1) is the most prevalent and toxic for both animals and humans [17].

Aflatoxin interferes with disease resistance and vaccine-induced immunity in livestock [7], exemplified by immunity suppression by AFB1 observed in turkeys, chickens, pigs, mice, guinea pigs, and rabbits [22]. Symptoms of acute aflatoxicosis in mammals include inappetence, lethargy, ataxia, rough hair coat, and enlarged pale fatty livers. In contrast, chronic aflatoxicosis exhibits symptoms including reduced feed efficiency and milk production, icterus, and decreased appetite [18]. Reduced growth rate is possibly the most obvious indication for chronic aflatoxicosis and other mycotoxicoses [20] and is related to disturbances in protein, carbohydrate and lipid metabolism [3].

Aflatoxins have been detected in numerous agricultural commodities such as cereal grains, oilseeds, cotton seeds, wheat, corn, peanuts and dried fruits as well as in animal feed and various dairy products [19]. The toxin becomes stable once formed in grain, resistant to degradation during normal milling and storage [2]. This presents the toxicity of contaminated feed stuffs as a significant, potential health hazard to animals and human beings.

Several strategies for the decontamination/detoxification of grains contaminated by mycotoxins have been reported using physical, chemical and biological methods specific to the commodity. However, previous treatments have exhibited limitations due to considerations for safety, which require not only the treated products be unaffected by the chemicals used, but also that their essential nutritive values be maintained [16]. A study by Wang et al. [23] found adsorbents like activated charcoal and hydrated sodium aluminum silicates at low percentage were ineffective when used to treat moldy feed. It was observed a high percentage of adsorbents bind essential nutrients, causing negative effects.

The application of enzymes or microorganisms capable of biotransforming mycotoxins into nontoxic metabolites has emerged as an alternative strategy in controlling mycotoxicoses in animals. Microbes transform mycotoxins in the intestinal tract of animals prior to absorption. Biotransformation, the cleavage and detoxification of mycotoxin molecules by microbes or enzymes, is an effective and safer method for mycotoxin control [21].

Several mycotoxins and plant toxins have been shown previously to be detoxified by rumen microbes, ochratoxin A (OTA) [9] and AFB1 [1] among the first. Jones et al. [10] reported the disappearance of AFB1 within several weeks of incubation with broiler and turkey faeces. Karlovsky [13] reported a 42% degradation of aflatoxin when incubated in vitro with rumen fluid. The ability of ruminants to metabolize selected mycotoxins has also been investigated [14]. It was found the mycotoxins zearalenone (ZON), trichothecenes mycotoxin (T-2 toxin), diacetoxyscirpenol and deoxynivalenol were well-metabolized by whole rumen fluid, whereas AFB1 and OTA were not. Westlake et al. [24] investigated the effects of these mycotoxins along with Verrucarin A on the growth rate of Butyrivibrio (B.) fibrisolvens specifically. They found this organism degraded all tested mycotoxins except AFB1, and that growth of B. fibrisolvens was not inhibited. Kurmanov [15] previously reported ruminants are more resistant to mycotoxin poisoning than monogastrics, which implies livestock are not equally affected by other toxins as well. In vitro rumen fermentation studies on the plant toxin pyrrolizidine alkaloid (PA) showed a higher degradation ability in sheep and goat than cattle [4,8]. Likewise, animals fed tansy ragwort containing PA demonstrated vastly different quantities of plant material required to manifest clinical symptoms. The consumption at the rate of more than 200% of the body weight of sheep and goat, 4-10% of the body weight of cattle and horse and 5% of the body weight of chicken was required to show clinical signs [5,8].

In this study, aflatoxin degradation in Holstein steers and Korean native goats was examined using rumen fluid as a microbial source. Our intent is to use the findings for the future selection of potential ruminant species containing bacteria having aflatoxin degradation ability.

Goats demonstrated higher (about 20-25%) AFB1 degradability than steers (10-14%) when the donor animals were fed a roughage:concentrate mixture (60 : 40). Kiessling et al. [14] has previously suggested mycotoxins are not completely degraded and furthermore, the extent of degradation tends to vary between different species, age, sex and breed. This could be attributable to the types of microbes inhabiting the rumen. Some studies have observed goats native to places where Leucaena grew were able to eat the plant, whereas domestic animals introduced to the same areas became ill and in some cases died [11]. Since it is commonly accepted bacteria in the rumen are responsible for the metabolism of whatever plant matter is consumed by an animal, investigations were performed to study if the observed difference between native and domestic livestock was due to rumen microbes. In a study by Jones and Megarrity [11], resistance to Leucaena toxicosis was successfully conveyed from Hawaiian goats to Australian cattle by transferring whole rumen fluid.

Rumen fluid also displayed different levels of degradation when applied to various treatments, though not large enough to be significant. The reason why rumen fluid filtered by a Millipore (0.45 µm) filter showed a numerically lesser degradation when compared to cheese cloth-filtered rumen fluid might be due to the Millipore filter pore size and the types of microbes limited by this treatment. Indeed microbe type may be a factor as Kiessling et al. [14] reported protozoa were more active than bacteria in the degradation of OTA, ZON and T-2 toxin .

No degradation was observed in autoclaved rumen fluid in the present study. This is most likely attributed to the destruction of live microbes and enzymes in autoclaved rumen fluid, therefore indicating the role of microbes in toxin degradation.

The time required for the biotransformation of toxins which enter the body through the digestive tract is also important, as some toxins are degraded within a short time and others need longer. Zearalenone toxin was degraded by bovine rumen fluid by an average of 37.5% after 48 h of incubation [12]. In our study, degradation of aflatoxin could be seen after incubation of 3 h or more, in both species. Several studies suggest feeding time influences the biotransformation of mycotoxins entering the digestive tract. In fact, the microbial population as well as the metabolic activity of the microbes increases at specific times after feeding, leading to higher mycotoxin degradation. Indeed, Keisseling et al. [14] observed the capacity to degrade Ochratoxin A decreased after feeding yet was restored by the next feeding time. However, our study showed rumen fluid collection time did not significantly affect AFB1 degradation in the two species.

Some reports indicate mycotoxin effects were moderated by different environmental factors, stress, animal's physiological ability and their preference for food [25]. Though the experimental steers and goats in our study were provided identical feed and environmental conditions, the possibility for differing food preferences might have influenced the rumen fluid components and bacterial population, thereby leading to differences in aflatoxin degradability. In addition, not only the feeding time but also the type of feed influenced degradation. With high-concentrate diets, ability to degrade OTA falls by 20% [14]. Similar observations in our study showed AFB1 degradation was about 25% when goats were fed a roughage:concentrate mixture of 60 : 40, compared to an average of about 50% when fed a 80 : 20 mixture. Possibly responsible is the influence feed could have on the number and types of microbes residing in the rumen ecosystem. Our comparative study on aflatoxin degradation, by feeding rice straw or timothy hay as roughage sources to three goats, showed rumen fluid obtained from timothy fed animals demonstrated better aflatoxin degradation, although not significantly higher. Not surprising, the nutritional quality of timothy is better than that of rice straw. Obviously microbes can use the feed source from the host animal for their own survival easiest with quality feed. Moreover, a higher number of microbes will often increase metabolic activity, leading to higher degradation of aflatoxin. However, individual differences in aflatoxin degradation existed among the three goats. This may be because individual animals have unique physical abilities, organ sizes, functions, sensory abilities and microbial populations.

In conclusion, our experimental findings show rumen microbes from Korean native goats demonstrated higher AFB1 degradability compared to Holstein steers. We observed AFB1 degradation in rumen fluid was influenced by animal species and type of feed fed to the animals. Individual animals and to a certain extent, the feeding and incubation time also contributed. The findings from this study furthers our research in selecting species as potential rumen fluid donors for the isolation of bacteria having aflatoxin degrading ability.