Maize contaminated with aflatoxins has been implicated in deadly epidemics in Kenya three times since 1981, but the fungi contaminating the maize with aflatoxins have not been characterized. Here we associate the S strain of Aspergillus flavus with lethal aflatoxicoses that took more than 125 lives in 2004.The 2004 outbreak of acute aflatoxicosis in Kenya was one of the most severe episodes of human aflatoxin poisoning in history. A total of 317 cases were reported by 20 July 2004, with a case fatality rate of 39% (1,26). This epidemic resulted from ingestion of contaminated maize (22). However, identities of the fungi causing the contamination remain unclear.Aflatoxins are carcinogenic metabolites produced by several Aspergillus species (4, 28). Aflatoxin-producing fungi vary widely in many characteristics, including virulence for crops and aflatoxin-producing capacity (10). A. flavus and A. parasiticus are most commonly implicated as causal agents of aflatoxin contamination. A. flavus has two morphotypes, the typical or L strain (sclerotia of Ͼ400 m in diameter) and the S strain (sclerotia of Ͻ400 m in diameter) (10, 18). S-strain isolates produce more aflatoxins than L-strain isolates, on average (10). Many L-strain isolates produce no aflatoxins ("atoxigenic") (7). All members of A. flavus lack the ability to synthesize G aflatoxins due to a 0.8-to 1.5-kb deletion in the 28-gene aflatoxin biosynthesis cluster (15). In contrast to cases in the United States, studies conducted in West Africa found that an unnamed taxon (sometimes called strain S BG ) is commonly implicated in contamination events (12). Strain S BG is morphologically similar to the S strain of A. flavus, but DNAbased phylogenies reveal strain S BG to be a distinct species ancestral to both A. flavus and A. parasiticus (14, 16). In order to determine the primary causal agent(s) of the 2004 contamination events in Kenya, we considered both fungal aflatoxinproducing potential and frequency of occurrence in the contaminated crop (7).Representative maize samples were collected from major agricultural markets and storage facilities of the most affected Kenyan districts by the National Public Health Laboratory Services in Nairobi, Kenya, during the 2004 outbreak (24). Samples were screened for aflatoxin content, and only B aflatoxins were detected (22,24). Subsamples (n ϭ 104; average weight ϭ 87.5 g; range of contamination ϭ 0.27 to 4,400 ppb total aflatoxin) were imported to the United States from the National Public Health Laboratory Services for fungal analyses. Fungi were isolated from the maize by using the dilution plate technique on modified rose Bengal agar (8). Isolates were classified into species and strains by observing colony characteristics and sclerotial and conidial morphologies after subculturing on 5/2 agar (5% V8 juice; 2% agar; pH 5.2) (10). Isolations were repeated two to four times to verify results. Isolates from each sample were collected from at least two isolations. Quantities of Aspergillus section Flavi isolates in maize w...

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Humans and animals are exposed to aflatoxins, toxic carcinogenic fungal metabolites, through consumption of contaminated food and feed. Aspergillus flavus, the primary causal agent of crop aflatoxin contamination, is composed of phenotypically and genotypically diverse vegetative compatibility groups (VCGs). Molecular data suggest that VCGs largely behave as clones with certain VCGs exhibiting niche preference. VCGs vary in aflatoxin-producing ability, ranging from highly aflatoxigenic to atoxigenic. The prevalence of individual VCGs is dictated by competition during growth and reproduction under variable biotic and abiotic conditions. Agronomic practices influence structures and average aflatoxin-producing potentials of A. flavus populations and, as a result, incidences and severities of crop contamination. Application of atoxigenic strains has successfully reduced crop aflatoxin contamination across large areas in the United States. This strategy uses components of the endemic diversity to alter structures of A. flavus populations and improve safety of food, feed, and the overall environment.

Aspergillus flavus has two morphotypes, the S strain and the L strain, that differ in aflatoxin-producing ability and other characteristics. Fungal communities on maize dominated by the S strain of A. flavus have repeatedly been associated with acute aflatoxin poisonings in Kenya, where management tools to reduce aflatoxin levels in maize are needed urgently. A. flavus isolates (n = 290) originating from maize produced in Kenya and belonging to the L strain morphotype were tested for aflatoxin-producing potential. A total of 96 atoxigenic isolates was identified from four provinces sampled. The 96 atoxigenic isolates were placed into 53 vegetative compatibility groups (VCGs) through complementation of nitrate non-utilizing mutants. Isolates from each of 11 VCGs were obtained from more than one maize sample, isolates from 10 of the VCGs were detected in multiple districts, and isolates of four VCGs were found in multiple provinces. Atoxigenic isolates were tested for potential to reduce aflatoxin concentrations in viable maize kernels that were co-inoculated with highly toxigenic S strain isolates. The 12 most effective isolates reduced aflatoxin levels by >80%. Reductions in aflatoxin levels caused by the most effective Kenyan isolates were comparable with those achieved with a United States isolate (NRRL-21882) used commercially for aflatoxin management. This study identified atoxigenic isolates of A. flavus with potential value for biological control within highly toxic Aspergillus communities associated with maize production in Kenya. These atoxigenic isolates have potential value in mitigating aflatoxin outbreaks in Kenya, and should be evaluated under field conditions.

Aims:  To evaluate the potential role of fungal community structure in predisposing Kenyan maize to severe aflatoxin contamination by contrasting aflatoxin‐producing fungi resident in the region with repeated outbreaks of lethal aflatoxicosis to those in regions without a history of aflatoxicosis. Methods and Results:  Fungi belonging to Aspergillus section Flavi were isolated from maize samples from three Kenyan provinces between 2004 and 2006. Frequencies of identified strains and aflatoxin‐producing abilities were assessed, and the data were analysed by statistical means. Most aflatoxin‐producing fungi belonged to Aspergillus flavus. The two major morphotypes of A. flavus varied greatly between provinces, with the S strain dominant in both soil and maize within aflatoxicosis outbreak regions and the L strain dominant in nonoutbreak regions. Conclusions:  Aspergillus community structure is an important factor in the development of aflatoxins in maize in Kenya and, as such, is a major contributor to the development of aflatoxicosis in the Eastern Province. Significance and Impact of the Study:  Since 1982, deaths caused by aflatoxin‐contaminated maize have repeatedly occurred in the Eastern Province of Kenya. The current study characterized an unusual fungal community structure associated with the lethal contamination events. The results will be helpful in developing aflatoxin management practices to prevent future outbreaks in Kenya.

Aflatoxins are potent poisons that contaminate crops in warm regions worldwide and reduce health and economic welfare in several portions of Africa. Crops are contaminated in two phases: First, Aspergillus species infect crops during development; and second, after maturation contamination builds during exposure to warm humid conditions. Identification of the exact fungi causing contamination can provide clues to management strategies. Crops usually are infected by complex mixtures of aflatoxin-producing and closely related fungi. Among these are atoxigenic strains that produce no aflatoxins. In the United States atoxigenic strains are used to reduce contamination. Such technologies also may have value in Africa.

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