Current Status of Mycotoxin Contamination of Food Commodities in Zimbabwe

Agricultural products, especially cereal grains, serve as staple foods in sub-Saharan Africa. However, climatic conditions in this region can lead to contamination of these commodities by moulds, with subsequent production of mycotoxins posing health risks to both humans and animals. There is limited documentation on the occurrence of mycotoxins in sub-Saharan African countries, leading to the exposure of their populations to a wide variety of mycotoxins through consumption of contaminated foods. This review aims at highlighting the current status of mycotoxin contamination of food products in Zimbabwe and recommended strategies of reducing this problem. Zimbabwe is one of the African countries with very little information with regards to mycotoxin contamination of its food commodities, both on the market and at household levels. Even though evidence of multitoxin occurrence in some food commodities such as maize and other staple foods exist, available published research focuses only on Aspergillus and Fusarium mycotoxins, namely aflatoxins, deoxynivalenol (DON), trichothecenes, fumonisins, and zearalenone (ZEA). Occurrence of mycotoxins in the food chain has been mainly associated with poor agricultural practices. Analysis of mycotoxins has been done mainly using chromatographic and immunological methods. Zimbabwe has adopted European standards, but the legislation is quite flexible, with testing for mycotoxin contamination in food commodities being done voluntarily or upon request. Therefore, the country needs to tighten its legislation as well as adopt stricter standards that will improve the food safety and security of the masses.


Introduction
Mycotoxins are secondary metabolites produced mainly by moulds belonging to the genera Aspergillus, Fusarium, and Penicillium [1] that contaminate food commodities worldwide, with developing countries being most affected [2]. Different fungal species produce different mycotoxins [3] which are named on the basis of the mould that produces them; for example,

Methods
The review was done under Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [16] in order to gather information on mycotoxin contamination of foods and feeds in Zimbabwe. Key words/phrases used to access information were: mycotoxin, aflatoxin, food, stock feed, feed ration, cereal grains, oil seeds, legumes, legislation, analysis, tolerance limits, maize, sorghum, mealie-meal, groundnuts, silage, milk, livestock, poultry, cattle, Aspergillus, Zimbabwe, and sub-Saharan Africa. Papers with relevant information were downloaded and analysed. A total of 81 papers were identified and 31 of these had information relevant to the review.

Occurrence of Toxigenic Fungi in Foods and Feeds in Zimbabwe
Little information is available on the typical range of mycotoxins in foods that are mainly consumed in Zimbabwe [1,17]. However, considerable work on mycotoxins has been done in Zimbabwe. Research has revealed the presence of mycotoxin-producing moulds on maize, the staple food of Zimbabwe which constitutes about 70% of the diet, and other cereal crops such as wheat, finger millet, millet, and sorghum [6,[17][18][19][20][21][22][23]. The most common genera that were isolated are Aspergillus and Fusarium [22,23], which are known to produce mycotoxins of concern to animal and human health, namely aflatoxins, zearalenone, trichothecenes, fumonisins, and ochratoxins [24]. Colonisation of the grains by the moulds is mainly due to infection of the cobs in the field by insects such as the maize stalk borer (Busseola fusca), inadequate drying of crops, as well as the primitive methods used during the shelling of maize and other grains [1,19,25].
Other researchers have worked on groundnuts, beans, cowpeas, and Bambara nuts, where they also isolated Aspergillus and Fusarium [21,26,27]; whereas in another study [2], Aspergillus flavus (A. flavus) and Aspergillus parasiticus (A. parasiticus) were isolated from peanut butter samples from Zimbabwe and Botswana. This suggests that the temperature of 160 • C used for roasting in commercial peanut butter production [28] is not high enough to destroy the vegetative cells and spores, not to mention the temperatures used in homemade peanut butter, which are not even monitored [2]. This increases the chances of continual aflatoxin production in the peanut butter during storage.
A 120-day storage trial carried out by Titterton [29] to assess the effects of different storage conditions on the nutritional value of maize and sorghum stovers showed a positive effect of storage on the nutritional values. In addition, several potential mycotoxin-producing moulds belonging to the genera Fusarium, Phoma, Altenaria, Penicilium, and Aspergillus were also isolated from the stovers [30]. Aspergillus and Penicillium species were detected in both freshly harvested and stored samples. During the dry season, farmers face the challenge of feed shortage [29], hence the use of supplements such as plant stovers. The presence of mycotoxigenic fungi in the stovers is of concern as this may expose the livestock to mycotoxin poisoning and can also be carried over into foods of animal origin, followed by subsequent transfer along the food chain to humans, causing harm [31].

Aflatoxins
A lot of attention has been given to aflatoxins due to their effect on animal and human health [7], which ranges from gastroenteritis to cancers in acute and chronic aflatoxicosis, respectively. Aflatoxins are produced by moulds belonging to the genus Aspergillus, dominated by A flavus, A parasiticus, and A nomius. There are four naturally occurring aflatoxins in food and feed, namely aflatoxin (AF) B 1 , B 2 , G 1 , and G 2 , depending on the fluorescence colour under ultraviolet light at 365 nm [32][33][34]. AFB 1 is the most toxic of them all and is produced by almost all aflatoxigenic Aspergillus species, whereas there are some species that cannot produce the G series, such as A flavus (produces B 1 and B 2 only) [35][36][37][38]. When consumed, aflatoxins cause liver cancer and stunted growth as well as suppressing immunity. In the body, aflatoxins are biotransformed predominantly in the liver, and the by-products or metabolites are eventually excreted mainly through renal flux [39]. When lactating cows and other mammals ingest aflatoxins in contaminated feed, toxic metabolites can be formed and found in milk due to the biotransformation of AFB 1 and AFB 2 by the hepatic microsomal mixed-function oxidase system to a hydroxylated metabolite known as "milk toxin" or aflatoxin M 1 (AFM 1 ) and AFM 2 , respectively, which is excreted in the milk and urine [40]. It has been shown that AFM 1 retains some carcinogenicity, thus exposing humans to chronic toxicity, and thus making milk a source of aflatoxin contamination in humans [41].

Aflatoxins in Food and Feed
Research on aflatoxins in Zimbabwe was initiated by disease outbreaks on farms, particularly in poultry. One such particular incident was the death of 70 ostriches on a farm. Analysis of the feed showed contamination by aflatoxin of 11 µg/kg. Samples from other feed batches were also tested and gave results as high as 129 µg/kg. However, no action was taken as there are no limits for ostrich feed [42]. The presence of determinants of aflatoxin exposure in the Zimbabwean population has been used as indicators of the people's exposure to aflatoxin poisoning [43]. This has been revealed by the presence of AFM 1 in urine and breast milk of the rural populations [43][44][45].
Analysis of some food commodities and their products showed the presence of aflatoxins [2,27,28]. Foods that have been analysed include maize, groundnuts, peanut butter, beans, Bambara nuts, and cowpeas, among others, as shown in Table 1. Probst et al. [46] analysed maize samples from African countries including Zimbabwe, and all the samples tested positive for aflatoxin contamination, ranging from 9-123 µg/kg, which is above the Codex Alimentarius Commission permissible limit of 5 µg/kg for maize intended for human consumption [47]. Siwela [48] also analysed maize and maize meal samples intended for human consumption, and 35% of the samples were above the Zimbabwean 20 µg/kg limit, with one sample having a concentration of 1391 µg/kg. The presence of both Aspergillus and aflatoxins in peanut butter is of great concern as it is considered a highly nutritious food recommended for infants [53]. Infants are more susceptible to the effects of mycotoxins [54]. Aflatoxin presence in the peanut butter could be attributed to their high melting points, which range from 237 • C to 289 • C [55]. The temperatures used during peanut roasting are not high enough to destroy them, hence their presence in the final product. This was shown by the presence of aflatoxins in peanut butter at levels that were above the 4 µg/kg European Union (EU) limit and some above the Zimbabwean legislation of 15 µg/kg for total aflatoxin [2,48,56].
All four major aflatoxins were detected in groundnuts, Bambara nuts, beans, and cowpeas in research done by Maringe and others [27]. Groundnuts and other legumes are important protein supplements in animal feeds [57][58][59] as well as the human population [48]. Traces of aflatoxins may remain in the seedcake, which is usually used as feed [42]. Therefore, it is of paramount importance to assess the levels of aflatoxin contamination of these seeds so as to avoid or reduce carryover into foods of animal origin such as milk, eggs, and meat, among others.
There is only one published study with regards to aflatoxin contamination of milk (AFM 1 ) in Zimbabwe, carried out by Choga et al. [50]. Milk and dairy products, as important constituents in the human diet, may be the major way for entrance of aflatoxins into humans if not monitored [60]. Crop residues have been used as alternative animal feeds [61], and research has indicated the presence of aflatoxigenic moulds with potential to produce mycotoxins which can be carried over into foods of animal origin such as milk. Findings by Morton and coworkers [52] on the effects of post-harvest practices on the production and nutritive value of maize residues in Zimbabwe indicated the presence of AFB 1 in the maize stovers used to feed dairy cows in the Silozwi Communal Area area south of Bulawayo, although the concentrations were low. Milk is consumed mainly by infants, especially during the weaning period, thus the growth of infants and young children could be impaired by the presence of AFM 1 in milk [43]. Nevertheless, there are no reports on mycotoxin contamination of milk products.

Fusarium Mycotoxins
The major mycotoxins produced by Fusarium species are trichothecenes, zearalenone (ZEA), and fumonisins [8,62], and can cause several acute and chronic mycotoxicosis in humans [63]. Fusarium species are classified under field fungi as they tend to contaminate food crops before harvest [64,65].
Fumonisins are toxic compounds produced predominantly by F. verticillioides, previously known as F moniliforme and F. proliferatum, and other toxigenic Fusarium species [17,21,64]. They have been associated with oesophageal cancer in South Africa [66]. Fumonisin B 1 (FB 1 ), the most toxic of the fumonisins, has been shown to interfere with folate uptake, resulting in congenital disorders in human babies such as neural tube defects [4].
Work done on Fusarium mycotoxins on Zimbabwean foods is shown in Table 2. Other researchers [17] reported on fumonisin contamination on all the 388 samples of maize grain collected from households in Shamva and Makoni rural areas. Even though the concentrations were below the 1000 µg/kg European Community (EC) limit, there is still the risk of the development of chronic ailments such as cancer in these populations. Research by Mubatanhema and others [19] revealed coproduction of zearalenone, moniliformin, and fumonisin B 1 by Fusarium isolates from maize samples collected from nine grain marketing board (GMB) centres in Zimbabwe. There has been limited research in Zimbabwe on other Fusarium toxins such as trichothecenes and zearalenone, which has also been the case with other countries [8]. Multitoxin production by mycotoxigenic fungi in Zimbabwean food samples was also demonstrated by Gehesquière and collegues [20] and Murashiki and coworkers [17]. Fumonisins were also shown to be the prevalent mycotoxin in the maize stovers used as feed during the dry season [52].

Mycotoxin Analysis
Mycotoxin poisoning can occur even after consumption of very small quantities of the toxin, therefore there is a need to use sensitive methods for their analysis and quantification [69,70]. The most common methods for mycotoxin analysis are chromatographic, such as thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), and immunoassays (radioimmunoassays, RDIs, and enzyme immunoassays, EIAs) [69]. Chromatographic methods combined with mass spectrometry have also been recently used in the detection and quantification of mycotoxins [71], and are capable of multitoxin detection [69,72] and even that of masked mycotoxins. Analysis of food and feed matrices from Zimbabwe has been carried out using some of these methods as shown in Table 3.

Legislation
Many countries are recognising the importance of aflatoxins and have generated regulations governing permissible concentrations of aflatoxins in food depending on the intended use of the particular food involved. These limits are, however, guided by the food regulating bodies such as the Food and Agriculture Organization (FAO), World Health Organization (WHO), and EU. The limits for individual countries range from 4-20 µg/kg [7] for foods intended for human consumption; whereas for milk and milk products, the range is 0.05-1.0 µg/kg. The strictest standards are those for the EU, which are 2 µg/kg for AFB 1 for human consumption and 5 µg/kg for lactating cows' feed, 4 µg/kg for total aflatoxin, and 0.05 µg/kg for AFM 1 . There are no regulatory limits set for mycotoxin level in food and feed in Zimbabwe, except for aflatoxins [74], which are not even considered when farmers come to deliver their produce as well as when food producers buy the raw materials [64]. This is the same for most developing countries in Southern Africa, mainly due to the shortage of resources for mycotoxin testing making it difficult to enforce laws on mycotoxin regulation [7]. Zimbabwe initially adopted the 5-25 µg/kg limit for AFB 1 in food intended for human consumption in 1971; however, it was revised in 1990 to 20 µg/kg [42]. Politics and economic crisis have made it tough to come up with an up-to-date and effective food control system [75]. When the GMB was responsible for handling agricultural commodities, it was easy to control the movement of produces. No grain would be released before being tested for aflatoxins, either for export or local consumption. However, with the liberalisation and indigenisation of the economy, farmers and manufacturers trade without government interference [42]. This has made the country the only one in the region with a twofold system of food standards, including the voluntary one and the controlled one [76]; one where the processor makes a choice on whether they want the raw commodity tested for contaminants or not, and one where the analysis of produce is mandatory. Zimbabwe has been affected by droughts which caused food shortages, and coupled with economic hardships, its citizens now worry about having food on the table, without being concerned too much about quality. In the case of livestock poisoning, there are no rules regarding aflatoxin limits in feed, and this has seen these cases settled out of court between the feed manufacturer and the farmer [42].

Discussion
This review has shown that the Zimbabwean population and livestock are exposed to a wide range of mycotoxins, as evidenced by multitoxin occurrence in some of the foods and feeds [19,20,52], which exposes the populations to more severe toxicity and immune suppression as compared to single mycotoxin contamination [63]. Work done by Siwela [48] showed that the aflatoxin contamination in 69% of the samples were within the limit of 20 µg/kg, with the remaining 31% above the limit. These results concur with those that were reported in Kenya, where 96% of the groundnut samples were contaminated with <4 µg/kg of AFB 1 , while 4% were above the national regulatory limit of 20 µg/kg [77]. Although most of the samples are within the limits, the population is still at risk of chronic aflatoxicosis, which could lead to cancer development later in life and immunosuppression, as well as stunted growth in children and animals [78]. However, a study in the Democratic Republic of Congo showed that a greater percentage of the groundnut samples analysed (70%) had aflatoxin contamination of >5 µg/kg [79]. Foods such as groundnuts and legumes are used as alternative protein sources [53] and their contamination by aflatoxins is of great concern, especially to children and vegetarians. Research on maize showed that the most common mycotoxins are aflatoxins and fumonisin B 1 [67], as the maize crop is known to be susceptible to fumonisin B-producing moulds. The EU maximum limit for fumonisins in food intended for human consumption is 1000 µg/kg [80], and most of the samples that were analysed had figures above this limit (Table 2). Gehesquière and others [20] showed that the levels of mycotoxins increase mainly during storage compared to pre-harvest. This was supported by Mubatanhema and coworkers [19], who attributed mould infection of maize as being a result of primitive methods used when shelling the grain which damage the kernels, paving the way for fungal infection. Therefore, there is need for the use of storage facilities that result in minimal growth of mycotoxigenic species. However, for groundnuts, contamination was mainly before harvest, with no significant change during storage [28]. Very little relevant research has been done on milk, even though it is an important component of the diet, especially used for weaning of children. The limited information on contamination levels of AFM 1 in milk and milk products means that the country is still very far from dealing with stunted growth in children, as mycotoxins have been associated with growth suppression [43,54,81].
The bulk of the research on mycotoxins in Zimbabwe was conducted between 1980 and the early 1990s, and most of the samples analysed were taken from the GMB. This might not have given a true reflection on the extent of food contamination, as farmers have a tendency of sending the best produce to the GMB so that they can earn more money, whereas what would remain for their consumption will be of compromised quality. The majority of research done after 2000 has been conducted outside the country, due to limited funds and equipment making it difficult for the authorities involved in policy formulation to access this information. Also, most journals with high impact factors would not publish research done in most developing countries, as they lag behind in technological advancement in terms of analytical methods. In addition, most researchers from developing countries cannot afford the exorbitant publication fees charged by most journals. This means that most of the research findings remain unpublished or get published after several years.

Conclusions
Most of the work done in Zimbabwe has been focused on Aspergillus and Fusarium mycotoxins and has shown the presence of these mycotoxins in foods and feeds. Therefore, there is a need to come up with strategies that will help reduce and manage levels of the mycotoxins in foods. Zimbabwe does not have legislation on the maximum levels of mycotoxins, but adopts European legislation [20]. This is a cause of concern, especially with regards to maize as it is used as an animal feed in Europe, whereas in Zimbabwe it is the staple food. Zimbabwe should come up with its own standards which cater for the nation's own diet. For legumes, there are no limits on the level of mycotoxins, making it difficult to enforce law [27]. There is also a need for researchers to share their findings with the general population. A study by Loreen and Moses [82] revealed that most of the people of Dora village in Mutare did not know anything about aflatoxins, except for the agricultural extension officers, meaning that these officers are not passing on information to the farmers. Research has also indicated that mycotoxin contamination of smaller grains is lower compared to that of the large grains, with some having no mycotoxins at all [21][22][23], so growing smaller grains could be one of the options in trying to reduce the mycotoxin burden; a replacing grains that are highly susceptible to mycotoxin contamination (such as maize) with grains that are less susceptible (such as sorghum or finger millet) could be another option. This will in turn reduce the rate of exposure of consumers to mycotoxins in their diet, especially for children. Improvement on the methods of harvest as well as sorting of the grains before storage will also help in reducing and managing mycotoxins in the granaries [20]. Dehulling, although not nutritionally recommended [83], has been proven as one of the ways of reducing mycotoxin contamination in grains [73,84].

Conflicts of Interest:
The authors declare no conflict of interest.