1. Introduction
Public health, food security and safety, and economic stability are negatively affected by aflatoxin contamination of crops. Aflatoxins are a group of secondary metabolites produced by several species of the ubiquitous
Aspergillus section Flavi fungi [
1]. High aflatoxin content makes foods and feeds unsafe, chronic dietary exposure to aflatoxins causes morbidity, and acute dietary exposure can result in mortality [
2,
3,
4]. Consequently, aflatoxin levels in foods and feeds are highly regulated at very low levels, in µg/kg. There are four major aflatoxins: B1, B2, G1, and G2. Aflatoxin B1 (AFB1) is the most toxic of the four. Naturally occurring mixes of aflatoxins are categorized as a Group 1 carcinogen, the highest category given by the International Association for Research on Cancer [
5]. Since AFB1 is a genotoxic carcinogen, there is no tolerable daily intake for aflatoxin. However, exposure to 1–2 ng/kg bw/day is estimated to be of little to no risk for populations in regions where the hepatitis B virus (HBV) is not endemic [
6,
7], but significant threats exist in regions where daily exposure is acute (e.g., 20–120 µg/kg bw/day) and where HBV is endemic [
8]. Aflatoxin exposure, including sub-acute levels, are directly linked to hepatocellular carcinoma (HCC) and are associated with other cancers, immunosuppression, and stunting in children [
9,
10]. In animals exposed to aflatoxin contaminated feeds, intestinal, kidney, and/or liver disorders, reduced productivity, and mortality can occur depending on aflatoxin concentration and age of the specimen, among other factors [
11,
12,
13]. The toxins commonly accumulate in susceptible crops and unfortunately many people, particularly in developing countries, are exposed to the toxins beginning in utero and then throughout their life [
10].
For most foods, regulatory limits for total aflatoxins are set at below 4 µg/kg and 20 µg/kg in the European Union and the United States, respectively. Several African countries have also adopted regulatory limits between 4 and 20 µg/kg (
www.aflatoxinpartnership.org (accessed on 20 April 2022)), although this is enforced more for crops to be exported than for locally consumed crops, including home-grown crops. Crops exceeding regulatory limits have reduced access to domestic and international premium markets, and this contributes to decreased economic empowerment [
14]. Crop products not meeting standards typically enter local markets where regulations are difficult to be enforced [
15]. Consequently, there is increased risk of higher aflatoxin dietary exposure in local food systems. Unfortunately, this tends to be the norm in several low- and medium-income countries (LMICs), including within sub-Saharan Africa (SSA).
Aflatoxins contaminate a wide range of foods, including staples of many LMICs, such as maize, groundnut, sorghum, and traditional foods [
16,
17,
18,
19]. Sorghum, a drought-tolerant crop, previously reported to be less susceptible than other cereals [
20], has been reported in recent years to contain high aflatoxin levels [
18]. Maize and groundnut are continuously reported to have high aflatoxin levels, including in the Sahel, a semi-arid region immediately south of the Sahara that cuts across Sudan to Senegal along the East-West axis [
16]. Although introduced from the Americas, maize and groundnut have become dietary staples in the Sahel and in SSA in general. Consequently, as aflatoxin-susceptible staples, a significant proportion of populations in the Sahel are continuously exposed to aflatoxins. Furthermore, climatic shifts are increasing crop stress in SSA [
21], which is already within the zone of high risk of perennial exposure to mycotoxins, thus, further increasing aflatoxin biosynthesis by the causative fungi [
22].
There are several
Aspergillus spp. capable of producing aflatoxins, but
A. flavus and
A. parasiticus are the most common causal agents of contamination. The former produces only B aflatoxins at variable levels, while the latter produces consistently high levels of both B and G aflatoxins
1.
A. flavus is composed of the L and S morphotypes. These morphotypes differ in several characters, but the most obvious is that the L produces few large (>400 µm) sclerotia, while the S produces numerous small (<400 µm) sclerotia [
23]. The S morphotype consistently produces high B aflatoxin concentrations, while members of the L morphotype produce variable B aflatoxin levels with some completely lacking abilities to produce aflatoxins due to defects in genes responsible for aflatoxin biosynthesis [
24]. Non-aflatoxin-producing members of the L morphotype are being used in aflatoxin biocontrol programs in several countries, including in SSA [
25]. Across the globe, fungi resembling the S morphotype of
A. flavus have been recovered from a variety of substrates. Using phylogenetic analyses, those fungi have been assigned to diverse species, including
A. aflatoxiformans,
A. austwickii,
A. cerealis,
A. minisclerotigenes,
A. pipericola, and
A. mottae [
1]. Many of these spp. occur in SSA, and some of them produce both B and G aflatoxins. Their correct assignment to the species level is still expensive (particularly if thousands of isolates are examined) and therefore, are sometimes referred to as S morphotypes, S strains, or S
BG strains if producing both B and G aflatoxins.
In the current study, aflatoxin contamination in the staple crops sorghum, groundnut, and maize was investigated in major production zones of Burkina Faso, Mali, and Niger. The objectives of this work were to identify aflatoxin hotspot areas and the risk that aflatoxin contaminated crops may pose to populations in the three Sahelian countries. We found elevated aflatoxin levels in some samples which were mostly collected within 1–2 weeks of harvesting. This is indicative of pre-harvest aflatoxin contamination, which can worsen under sub-optimal storage conditions. The results indicate that populations in those countries are at high risk of aflatoxin-associated diseases. Prompt, effective technical, institutional, and policy actions are needed to reduce threats that aflatoxins pose to food security and safety, public health, and trade in the Sahel.
6. Niger
Aflatoxin analysis. Aflatoxins were detected in all crops in all regions within Niger (
Table 5). There were detectable aflatoxins in 41% of the samples. All contaminated samples had over 10 µg/kg TAF, and 39% had more than 20 µg/kg TAF. Aflatoxin concentrations in sorghum, maize, and groundnut, reached 1988 µg/kg, 5886 µg/kg, and 8593 µg/kg, respectively. Generally, aflatoxin concentrations in sorghum were lower than concentrations in maize and groundnut (
Table 5). Mean aflatoxin levels were significantly (
p < 0.05) higher in Dosso for maize (659 µg/kg) compared to other regions. For groundnut, aflatoxin content was statistically similar in all regions except Tillabéri where levels were significantly (
p < 0.05) lower. Nevertheless, the average aflatoxin content in Tillabéri was still high (90 µg/kg;
Table 5). The aflatoxin content in sorghum was lowest (
p < 0.05) in Maradi. Aflatoxin concentrations in sorghum were lower (
p < 0.05) than those in maize and groundnut in Zinder, Maradi, and Tillabéri (
Figure 3). Maize and groundnut from Tillabéri contained safer aflatoxin levels than the same crops in other regions (
Figure 3).
Assessment of exposure. The dietary exposure to aflatoxins was very high in Niger, ranging from a PDI of 310 ng/kg bw/day in Maradi to 2100 ng/kg bw/day in Dosso (
Table 6) from maize consumption. Consequently, low MOE were recorded, ranging from 0.1 to 0.5. HCC risks from maize consumption were high ranging from 17.7 to 119.7 CPY. The PDI from sorghum consumption was also high and ranged from 253 ng/kg bw/day in Maradi to 2221 ng/kg bw/day in Niamey. Consequently, the MOE was also very low (range = 0.1 to 0.7), and HCC risks due to sorghum consumption ranged from 14.4 CPY to 126.6 CPY across regions (
Table 6).
7. Discussion
The current study evaluated aflatoxin concentrations in sorghum, groundnut, and maize grown in Burkina Faso, Mali, and Niger; the samples were collected in different years. The results discussed reflect the prevalence of aflatoxins in the different years collected during the dry season. Aflatoxin contamination would vary across years, seasons, and with variations in environmental and management conditions. Concentrations within and among crops and countries varied (
Table 1,
Table 3 and
Table 5). Over 44% of sorghum, groundnut, and maize samples were contaminated with aflatoxins, and 30.6% of those contained levels above 20 µg/kg, the regulatory limits in the U.S. (
Table 2,
Table 4 and
Table 6). There were some cases in which extremely high aflatoxin levels were recorded and that put the population at high exposure, particularly in Niger. Sorghum is generally regarded to be less susceptible to aflatoxin contamination compared to other crops [
20]. Due to its tolerance to drought, it is also an important crop for food security. However, results from the current study, although revealing that it was the less susceptible to contamination compared to maize and groundnut, indicate that it requires integrated strategies to manage aflatoxins.
The high levels of aflatoxins in many of the examined samples continue to demonstrate that farmers in the three Sahelian countries need aflatoxin management interventions at both the pre- and post-harvest stages (
Table 1,
Table 3 and
Table 5). Aflatoxin contamination occurs when toxigenic members of
Aspergillus section
Flavi infect crops, and the right conditions for contamination occur. Aflatoxin-producing fungi reach crops at the pre-harvest stage from propagules that are present in organic material on the fields as debris or other crop materials. During storage, high levels of aflatoxins occur when conducive conditions of temperature, humidity, and sub-optimal storage converge [
42]. Moreover, populations in these countries get most of their dietary needs (over 60%) from low diverse diets that include mostly cereals, roots, and tubers [
43,
44], and many of those staples are prone to aflatoxin contamination. This suggests that there is a high exposure to aflatoxins, as demonstrated in the current study. Other studies have reported high prevalence of aflatoxin contamination and/or exposure in Burkina Faso, Mali, and Niger. In Burkina Faso, it has been reported that up to 50% of maize samples were contaminated with aflatoxins [
19], up to 135 µg/kg of aflatoxin were found in infant formula made from locally sourced grains, and up to 258 µg/kg in maize and rice [
45]. Milk in Burkina Faso, on the other hand, appears not to be an important source of exposure to aflatoxin (aflatoxin M1; found in milk produced by livestock that ingested aflatoxin contaminated feeds) based on preliminary data [
46], and this can be related to cattle being mostly grass-fed with little supplementation with cereal brans and crop residues [
47]. In a study in Mali, aflatoxins were prevalent in 100% of the samples collected during the rainy season [
48]. Other studies reported high contamination of grain samples at harvest from Mali (about 60%) with levels exceeding 4 µg/kg (the EU regulatory limit) that increased during storage [
16]. In Niger, maize production is not sufficient, and therefore, maize has to be imported from neighboring West African countries (e.g., Benin, Burkina Faso, and Nigeria); a recent study reported that some maize offered in Nigerien markets contain high aflatoxin levels, and this was associated with poor post-harvest management, including high insect infestation [
49]. Also in Niger, local production of groundnut has been reported to be affected by pre-harvest aflatoxin contamination attributed to stress conditions and agronomic practices [
50].
Safety of staples must be improved in the three Sahelian countries. In addition, improvement of the economies to enable citizens to have sufficient economic power to diversify their diets is needed. Managing aflatoxins can help to address both needs. Food safety is improved if crop quality is protected, including successfully reducing aflatoxin contamination. Also, with improved food safety, household income may improve as health burdens caused by aflatoxin exposure DALYs are reduced [
51]. Furthermore, capacity to engage in international trade is enhanced and income is improved when crops meet regulatory requirements of importing countries [
14,
52]. Of course, access to premium markets to producers of aflatoxin-safe crops is critical for this to be realized.
Aflatoxin levels in some samples were very high across regions in all the crops in the three countries (
Table 1,
Table 3 and
Table 5). There was a high proportion of samples exceeding tolerance thresholds (
Figure 1 and
Figure 2). The samples were collected immediately after harvest or within 1–2 weeks of harvesting, which suggests that aflatoxins accumulated at the pre-harvest stage. Several samples contained aflatoxin levels extremely unsafe for human and animal consumption. In countries where food and feed grade systems exist and are operational, breeding and finishing cattle can be fed with maize and groundnut containing less than 100 µg/kg and 300 µg/kg, respectively [
53]. Several samples in the current study greatly exceeded those levels. In the EU, aflatoxins are regulated at less than 4 µg/kg, but levels in some crops averaged hundreds of times more than that level. There were some samples with well over 900 µg/kg aflatoxin in the three countries (
Table 1,
Table 3 and
Table 5) and up to 8500 µg/kg aflatoxin in Niger. Either highly toxigenic fungi contaminated those crops at alarming levels in the field or the short storage period (1–2 weeks) and most likely in sub-optimal conditions was sufficient to allow toxigenic fungi to produce such dangerous concentrations. Although these grains are seldom consumed raw and would undergo processing, these levels of exposure pose a risk. Processes, such as boiling and roasting, would mildly reduce aflatoxin levels as the toxins are heat stable.
Aflatoxins do not have a tolerable limit due to their genotoxic properties, and no consensus has been reached on a tolerable daily intake. In EU countries, aflatoxin levels are required to be as low as reasonably achievable [
33]. In many African countries there are regulatory limits set but hardly enforced for domestic markets. In the current study, the detected high aflatoxin levels in staple crops are a serious public health concern since these crops constitute a major source of energy. Regressive child development has been associated with high dietary exposure to aflatoxins in weaning foods, breastmilk, and pre-birth through transplacental exposure [
54,
55,
56,
57,
58,
59]. Dietary exposure to aflatoxins has also been associated with disorders in spermatogenesis [
60]. There is an established causal relationship between chronic dietary exposure to aflatoxins and HCC, particularly in regions where the levels of exposure to HBV is high, resulting in 30 times higher HCC risk [
61,
62]. The high exposure to unsafe aflatoxin levels (
Table 2,
Table 4 and
Table 6) requires urgent attention by all relevant stakeholders. Up to 95% of the contaminated samples contained AFB1 proportions that were above 50% of the total aflatoxins. This is a typical pattern for samples contaminated by
A. flavus and poses a high risk of HCC, especially as AFB1 is the most carcinogenic of the four major aflatoxins [
5].
In all three countries, the aflatoxin exposure threshold (0.017 ng/kg/day) was surpassed more than 14 times (
Table 2,
Table 4 and
Table 6). Among all types of cancer, HCC is the fourth most common in SSA, with aflatoxins contributing to 10% of these cancers [
63]. This estimate is possibly conservative as HBV is considered to contribute to 70% of HCC and may not have been combined with aflatoxin exposure but considered independently. It is imperative that aflatoxin exposure is considered as a high priority for intervention in SSA countries for public health, food, nutrition, and income security in the sub-region. There are several technologies and practices available for aflatoxin management in these crops [
64,
65,
66]. Effective technical, institutional, and policy options need to be converged to reduce the incidence of aflatoxins in these countries to protect populations and enhance international trade. Grain samples in this study were collected in three different years—2010, 2017, and 2019. Whereas there would be variations in the Sahelian environmental conditions across these years in these countries, the data collected also presents a persistent aflatoxin contamination problem in these crops regardless of the sampling year. Although not reported in the current study, the samples were also used to characterize the aflatoxin-producing fungal communities associated with these crops. There were a large number of atoxigenic isolates of
A. flavus identified, and these could be used as biocontrol agents to limit aflatoxin contamination (unpublished). Some atoxigenic isolates from Burkina Faso have been characterized, and the type of lesions in the aflatoxin biosynthesis gene cluster causing loss of aflatoxin-production ability have been described [
67]. Currently, atoxigenic isolates of
A. flavus used as active ingredients of aflatoxin biocontrol products in SSA successfully reduce aflatoxin contamination when used at the pre-harvest stage [
25,
68]. Such strategy used at scale could help reduce aflatoxin levels and exposure in Burkina Faso, Mali, and Niger and would contribute to economic growth through trade in domestic and international aflatoxin-conscious markets. The results presented in this study reflect the prevalence of aflatoxins in raw samples collected at those locations and times. There is often variability in aflatoxin contamination across seasons and locations. Regular up-to-date monitoring is important for current risk assessments and for guidance to policy makers towards the institution of risk management systems pre-harvest and post-harvest.