Extensive surveys of commercial maize crops performed in South Africa have consistently demonstrated a very low incidence of aflatoxin contamination (Shephard, 2003). However, analysis of samples of peanut butter being used in a school feeding scheme showed total aflatoxin contamination of up to 271 µg/kg (Shephard, 2003; PROMEC Unit, 2001) and AFM1 levels (range: 0.3-1.54 µg/L) above the South African legislation levels have been detected in raw milk (Dutton et al., 2011). Kamika (2012) reported total aflatoxin levels above 5 ppb in 60% (12/20, range: 2.1-73.83 ppb) and 85% (17/20, range: 4.02-825.67 ppb) of peanut samples collected from Pretoria, South Africa and Kinshasa, Democratic Republic of Congo respectively.
Total aflatoxin contamination levels above the acceptable limit set by the US Food and Drug Administration (FDA) were reported in peanut butter from Sudan with a range of 26.6-853 µg/kg, and a mean of 287 µg/kg (Elzupir et al., 2011). In Tanzania 18% maize samples were contaminated with total aflatoxins (range: 1-158 µg/kg) and 12% had AFB1 levels ranging from 5-90 µg/kg (Kimanya et al., 2008). AFB1 was also detected in beer samples (range: 10- 50 µg/kg) in Dar es Salaam, Tanzania (Nikander et al., 1991). High levels of AFB1 were also detected in Ethiopia in red pepper (range: 250-525 µg/kg) and in a processed mixture of legumes (range: 100-500 µg/kg) (Shephard, 2003).In Nigeria AFB1 was detected in 63% of stored sorghum sample, 52% of marketed sorghum samples and 31% of field sorghum samples throughout the year (Hussaini et al., 2009). The highest incidence was during the rainy season.
21 2.2.4.1 Botswana
Peanut, peanut butter, sorghum, chicken feed and mophane worms (an edible larval stage of the emperor moth Imbrasia belina Westwood) samples from Botswana showed high levels of aflatoxin contamination (Siame et al., 1998). All mophane worms were purchased from harvesters in the field and from retail outlets while all the other samples were collected from storage depots or bought from retail outlets. Peanuts, peanut butter and mophane worms had aflatoxin levels ranging from 3.2-48 µg/kg, 1.6-64 µg/kg and 0.1-10 µg/kg respectively. Sorghum, sorghum meal and chicken feed showed minor aflatoxin contamination ranging from 0.1-0.7 µg/kg while maize showed no aflatoxin contamination (Siame et al., 1998). It should be noted that only 4 chicken feed samples were tested hence the results might not be statistically representative.
Mphande et al., (2004), reported 78% of raw peanuts tested in Botswana to be positive for aflatoxin contamination (range: 12-329 µg/kg, mean: 118 µg/kg). AFB1 was the most prevalent being found in 65% of the samples tested. The samples were bought from different retail outlets in the country. However, it should be noted that the investigators collected samples into plastic bags and this could have provided a perfect environment for aflatoxin proliferation post collection. The investigators also noted that most of the peanuts sold in Botswana are imported, yet they did not capture information on the original source of the samples as well as storage and shipping conditions.
No aflatoxins were detected in sorghum malt although Aspergillus species were isolated in 44% of the samples and A. flavus was isolated from 37% of the samples (Nkwe et al., 2005). This confirms the understanding that the presence of fungal isolates does not necessarily mean presence of toxin. There are no official aflatoxin regulatory levels in Botswana (Mphande et al., 2004; Siame et al., 1998), but the maximum allowable levels of total aflatoxins in food meant for human consumption in Codex Alimentarius countries is 15 µg/kg (Codex, 2001).
2.2.4.2 Zimbabwe
Routine monitoring of groundnuts by the Zimbabwe Government Analyst Laboratory noted seasonal variation in aflatoxin contamination. About 46% of the samples analysed during the 1995 season and 8% of the samples analysed during the 1996 season were contaminated with levels above 10 µg/kg (Shephard, 2003; Henry et al., 1998). Studies in the rural villages
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indicated presence of AFM1 in human breast milk at levels up to 0.05 ng/mL, thus raising concerns about post-natal exposure to aflatoxins (Shephard, 2003; Wild et al., 1987). Recently Siwela et al., (2011), carried out a study to monitor aflatoxin carryover during large scale peanut butter production, their findings reveal that peanuts from local farmers have high levels of aflatoxin contamination (>80 ng/g in raw peanuts).
Nyathi et al., (1987), carried out a survey of urinary aflatoxin biomarkers in Zimbabwe and detected aflatoxins in 4.3% and 4.4% of samples from rural areas and urban areas respectively. There was no association between aflatoxin contamination and altitude or rainfall, but there was a significant reduction in aflatoxins in the low temperature province of Manicaland. Also the degree of mobility of the population in Zimbabwe between rural and urban areas is quite significant hence it was difficult for the investigators to draw conclusions about any differences based on whether donors belonged to the urban or rural community. However the highest percentages of contaminated samples for individual centres came from rural areas. The investigators also noted that since most of the samples came from hospitals and clinics, a selection bias towards the sick as opposed to a normal cross-section of the community could have risen.
Zimbabwe is one of the African countries that have aflatoxin regulations in place and 20µg/kg is the maximum allowable level of total aflatoxins in human food. There are no recent mycotoxin studies in Zimbabwe and to compound this situation, the food security situation has deteriorated extensively during the last decade due to a political and economic crisis (Mutisi, 2009). There is therefore an urgent need for continuing surveillance studies in the country.
2.3 Fumonisins
Fumonisins were first discovered in South Africa by Gelderblom et al., (1988) and were chemically characterised by Bezuidenhout et al., (1988). They are a group of non-flourescent mycotoxins thought to be synthesized by condensation of the amino acid alanine into an acetate derived precursor (Bennet and Klich, 2003; Sweeney and Dobson, 1999). Branched chain methyl groups are added at C-12 and C-16 by an S-adenosyl methionine . Much of the biosynthetic pathway is yet unknown and none of the enzymes involved in the fumonisin biosynthetic pathway have been isolated (Sweeney and Dobson, 1999). The fumonisin chemical structure, which is a C-20 diester of propane-1,2,3-tricarboxylic acid and a
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pentahydroxyicosane containing a primary amino acid, resembles sphingosine which forms the back bone of sphingolipids (IARC, 2002; Merrill et al., 2001; Sweeney and Dobson; 1999). Fig 2.5 shows the structural formulas of the major fumonisins.
Figure 2.5: Structural formula of fumonisin B1- B3 (Scott, 2012)
More than 28 fumonisin analogues have now been described and classified as A, B, C, and P series (Rheeder et al., 2002). The major fumonisins are fumonisin B1 (FB1), FB2 and FB3 (Wild and Gong, 2010; Richard, 2007). FB1 is the most commonly found and the most toxic (Scott, 2011; Bennet and Klich, 2003). The other fumonisin analogues differ slightly from the FB series. The FA series is acetylated on the amino group at the C-2 position whereas FB series have a free amine, the FC series lack the methyl group at the C-1 position and the FP series have a 3-hydroxypyridinium functional group at the C-2 position (van der Westhuizen, 2011; Bezuidenhout et al., 1988). Fumonisins are difficult to study because they are hydrophilic, but they are usually extracted with aqueous methanol or aqueous acetonitrile and analysed by high performance liquid chromatography (HPLC) with fluorescence detection (IARC, 2002; Gelderblom et al., 1988).