CHAPTER 2 Literature Review
2.6 Contamination of chicken feed resources
2.6.1 Aflatoxins
Aflatoxins (AF) are mycotoxins produced as secondary metabolites by Aspergillus flavus and Aspergillus parasiticus fungi. The fungi responsible are ubiquitous and can affect many dietary staples such as rice, maize, cassava, nuts, peanuts, chilies, and spices (Fufa and Urga, 2010;
Adebesin et al., 2001; Kaaya and Eboku, 2010; Hell and Mutegi, 2011). Aflatoxin contamination is influenced by high humidity, high temperatures, insect and rodent activity, and inadequate drying of the crops. Contamination occurs frequently in countries of Africa (Kaaya and Eboku, 2010; Hell and Mutegi, 2011). This contamination can occur at any stage of food production from pre-harvest to storage.
2.6.1.1 Aflatoxin metabolism
Six groups of aflatoxins have been identified based on their fluorescent properties under ultraviolet light, and their chromatographic mobility (Singh, 1995). Aflatoxin B1 and B2 produce a blue fluorescence while G1 and G2 produce a green fluorescence under ultraviolet light (Figure 2.3). The other two metabolic products of aflatoxins are M1 and M2 which occur in milk of lactating mammals that have consumed AF contaminated feed (Murphy et al., 2006).
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Figure 2.3. Chemical structure of the different aflatoxins
Source: Grace (2013)
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The effects of aflatoxins on animals depend on various factors such as genetics, physiological phase, nutritional status, and environmental factors. Fetuses are very susceptible to even low levels of aflatoxin, and young and fast-growing animals are more affected than adults (Grace, 2013).
Aflatoxin B1 (AFB1), the most toxic of the AFs, is a potent liver carcinogen, causing hepatocellular carcinoma (HCC) in humans and a variety of animal species (Wu, 2013).
Chronic exposure to aflatoxins has also been reported to cause stunted growth (Leroy, 2013).
Aflatoxin B1 is metabolized by the liver through the cytochrome P450 enzyme system to the major carcinogenic metabolite AFB1 – 8, 9- epoxide (AFBO) or to the less mutagenic hydroxylated metabolites AFM1, Q1, or P1 (Murphy et al., 2006).
There are several pathways that AFBO can take: 1) it can be conjugated to glutathione and excreted; 2) it can be hydrolyzed to AFB1-8, 9-dihydrodiol which can be further metabolized to AFB1-dialcohol which can then be excreted; or 3), it can readily bind with the N7 position of guanine in DNA forming AFB-DNA adducts, that are responsible for the mutagenic and carcinogenic effects of aflatoxin. Figure 2.4 shows the biotransformation pathways for aflatoxin B1.
Aflatoxin (AF) causes cell membrane damage through increased lipid peroxidation in laboratory animals (Souza et al., 1999; Rastogi et al., 2001). Galvano et al., (2001) reported that, antioxidant substances such as selenium and some vitamins (A, C and E) can protect against mycotoxin-induced damage by their potential capacity to actas superoxide anion scavengers.
2.6.1.2 Aflatoxins in chickens
Aflatoxicoses have caused economic losses to the poultry industry, affecting ducklings, chickens, and turkeys. The effects in animals vary with dose, length of exposure, species, breed, and diet or nutritional status with younger animals more susceptible than older ones to the toxic effects of AF (CAST, 2003; Quezada et al., 2000). Aflatoxicosis in chickens is characterized by reduced feed consumption, reduced body weight gain and poor feed utilization. Bryden et al.
(1979) reported that chickens fed greater than 1mg AF/kg diet had reduced body weight gain and feed consumption, and poor feed conversion efficiency.
22 Figure 2.4. Aflatoxin pathways
Source: Bammler et al. (2000)
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Similar results were found by Huff et al. (1986) and Santurio et al. (1999) who observed that chickens fed 2.5 mg/kg and 3 mg/kg of dietary aflatoxin had lower weight gains than controls.
Quezada et al. (2000) also observed that body weight gain of chickens fed 2 mg AF/kg diet showed a marked decrease, compared with those fed an aflatoxin–free diet.
The liver is the target organ of aflatoxin in chickens; however kidney, gizzard and spleen can also significantly increase in relative weight (Dwyer et al., 1997; Miazzo et al., 2005). Gross pathological lesions in birds fed AF included pallor, discoloration of liver, enlargement of liver and kidneys (Hussain et al., 2008), and gall bladder distension (Tessari et al., 2006). Kubena et al. (1989) similar to Smith and Hamilton (1970) indicated that chicks fed 5 mg AF/ kg diet had significant increases in the relative weights of liver, kidney, gizzard, and proventriculus.
Enlargement of the liver and other organs could be the result of oedema and/or the increase in lipid content (Smith and Hamilton, 1970). These findings are in agreement with Bryden et al.
(1979) who reported an increase in relative liver weight of chickens fed 1 mg AF/kg diet for more than 2 weeks.
Microscopic changes in the livers of chickens fed AF were also observed. Ortatatli et al. (2005) reported slight to moderate hydropic degeneration, small fatty vacuoles in hepatocytes in centrilobular, a small amount of bile-duct proliferation located in portal areas, periportal fibrosis located in portal areas, and periportal fibrosis in livers of chickens fed a diet containing 100 ppb AF. Ergün et al. (2006) indicated that histological changes in hepatocytes of chicks fed aflatoxin included increased lipid droplets, high glycogen content, and mild mononuclear cell infiltration in the portal area. The histopathologic changes observed in the liver of birds fed AFB1 (1.0 mg/kg diet) are similar to those reported by Gowda et al. (2008). Gowda et al. (2008) also reported decreased antioxidant function in terms of level of peroxides, superoxide dismutase activity, and total antioxidant concentration in liver due to 1.0 mg/kg diet AFB1. Similar results were found by Che et al. (2011) who reported that liver superoxide dismutase activity was reduced, and myeloperoxidase activity increased by the fungal contaminated diet.
Aflatoxin has been reported to have an effect on nutrient metabolism and activity of enzymes that are important for feed digestion and absorption of nutrients. C hicks fed 1.0 mg AFB1/kg diet had significantly reduced serum total protein, albumin, cholesterol, and calcium levels (Gowda et al., 2008). Rosa et al. (2001) fed a 5 mg AF/kg diet to chicks from day one to 22 days and observed that the level of serum total protein, albumin, and globulin decreased.
Decreases in the concentration of total plasma proteins, albumin, and globulins have been proposed as indicators of the alteration in protein synthesis observed in aflatoxicosis (Solcan et
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al., 2013). Pimpukdee et al. (2004) also reported that broiler chicks fed aflatoxin had reduced serum vitamin A levels. Yarru et al. (2009) concluded that chicks fed 2 mg AFB1/kg diet had physiological responses associated with altered gene expression in livers of broiler chicks.