Maize is a staple food crop in Kenya for both urban and rural areas with an estimated 1.6 million hectares under cultivation. It is grown by both small- and large-scale farmers. Small-scale farmers contribute 75% of the total maize produced in the country. The quantity of maize consumed in Kenya per person per year is approximately 98 kg translating to about 30–34 million 90 kg bags per year [1
], and in some families maize may be consumed twice daily. Each family in Kenya has a garden if not a farm where they grow maize that is mostly for their own consumption and sometimes for sale signifying the importance of this crop in the country. Several cases of aflatoxicosis have been reported in Kenya yearly since 1981 following consumption of maize contaminated with Aspergillus flavus
and aflatoxins. In 1981 the outbreak was as a result of drought followed by heavy rains during harvest of home grown maize [2
]. The worst outbreak was in 2004 where 317 cases and 125 deaths were reported [3
], and ever since cases have been reported yearly. In 2010 Kenya had about 2.3 million bags (estimated at $69 million) of maize contaminated with aflatoxins making it unfit for both human and livestock consumption and also for trade, which was a loss to the small-scale farmers who depend on the crop for food and income [4
and A. parasiticus
are ubiquitous and cosmopolitan fungi producing aflatoxins on a wide variety of substrates such as maize, peanut and cotton. Aspergillus flavus
is a very important toxigenic fungus that produces aflatoxins that are toxic and of great concern because of their health effects on humans and animals [5
can attack crops at different times, in the field, during harvest, transport and storage. High moisture and temperatures are favorable for the growth of this fungus and toxin production.
In this study two agro-ecological zones were selected to compare the distribution in maize of Aspergillus spp. and their toxigenicity. The Nandi district in the Rift Valley Province was chosen as it is the main maize growing zone in the country, but no aflatoxicosis has been reported in this region. The other zone is the Makueni district in the Eastern Province where most aflatoxicosis cases have been reported.
The results obtained in this study have provided, for the first time, an important comparison of fungal contamination between maize sampled in Makueni County, the main region that has experienced repeated lethal human aflatoxicosis outbreaks and Nandi County, the main maize growing region in Kenya. The study shows that Aspergillus
, specifically Section Flavi
, are the main contaminants of maize in household storage in the two regions and A. flavus
was the most common species. The incidence of occurrence of A. flavus
in Nandi and Makueni was the same regardless of the differences in mean temperatures (20 °C and 24 °C, respectively) and rainfall (900–1800 and 950–1500 mm). High temperatures and drier conditions are known to favor infection by A. flavus
but this was not the case in this study. Similar results were observed in Nigeria [25
Toxigenic strains of A. flavus
were more prevalent than non-toxigenic strains across five out of the six locations. This indicates the risk of aflatoxin poisoning in the event that favorable conditions occur in both regions. The widespread occurrence of the fungus indicates the extent of pre-harvest infection; thus field management strategies stand out as an indispensible intervention strategy towards the fight against aflatoxin contamination of maize in Kenya. In this regard, Abbas et al.
] observed a higher incidence and greater numbers of A. flavus
infection and toxin production when there was no crop rotation. Maize is planted in the same fields every season in the two regions. The importance of field management is further stressed by Zablotowicz et al.
] who report that the history of maize cultivation in terms of soil fertility factors correlates with the occurrence of A. flavus
and toxin production.
All the A. flavus
toxigenic strains from Makueni maize were of the S-type while those from Nandi belonged to the L type. Quantitative and qualitative differences in aflatoxin production in vitro
between isolates and between these strains were detected. The S strains were confirmed to produce relative larger amounts of total aflatoxins, AFB1 and AFB2 and lower values for AFG1 and AFG2 in vitro
compared with the L strains. AFB1 is known to be more toxic than the other aflatoxins, and this explains why the S strain has been associated with acute aflatoxin poisoning in Makueni [28
]. This is accentuated by high temperatures in Makueni (range 20–28 °C) compared to Nandi (range 18–24 °C), which promotes toxin production [6
]. However, some L strain isolates from Nandi produced large amounts of AFB1 and AFB2 contrary to the findings of Cotty [13
], and Egel [30
]. It is important that in vivo
tests for toxin production with the L strains from Nandi are done to confirm the capability of these isolates to produce large amounts of toxin. These L strains pose a threat of endemic chronic exposure to humans if the maize is exposed to conditions suitable for toxin production given that they are distributed widely in the region. Further, Nandi is the major maize production zone in the country implying that the maize is distributed to most parts of the country. Control of moisture and temperature during transportation and storage is important since the maize is already contaminated with the toxigenic A. flavus.
The most toxic AFB1 were produced in larger quantities compared to the AFB2. AFG1 were produced in low quantities and only eight out of 78 samples tested produced AFG2.
This is the first report of A. flavus
S and L strains isolated from Kenya producing both B and G aflatoxins on YES agar. Probst et al.
] isolated in Kenya both S and L strains, which produced only B aflatoxins on maize. Ehrlich et al.
] reported that 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 agreeing with Cotty and Cardwell [32
] and Egel [29
]. However, S strains from Benin produced both AFB1 and AFG1 aflatoxins [33
]. Davis et al.
] also observed production of aflatoxin AFB1 and AFG1 by A. flavus
in a synthetic medium. Cotty and Cardwell [32
] reported that SBG strains from the United States and West Africa produced both B and the G aflatoxins. It is not clear if the difference in media is the reason for the variation in the production of B and G toxins or if the isolates that produced both toxins in this study are similar to the SBG strains referred to above. However Abbas et al.
] demonstrated that cultural methods are suitable and effective in screening aflatoxin production by Aspergillus
isolates. Aspergillus parasiticus
was rare in both regions. The number of isolates was too small to allow a conclusion on their toxin production ability. The main source of aflatoxin contamination in maize in eastern and western Kenya is A. flavus
Incidences of sclerotia formation were greater among the S-type than the L-type on V8-juice agar. Sclerotia are important survival structures in the life cycle of many fungi. When conditions are favorable, they germinate into hyphae, which then form conidia. The conidia are blown away by the wind and reinfect maize kernels through the silk. Studies on the conditions responsible for sclerotium initiation might be important to develop methods for suppressing the formation of sclerotia, resulting in reduced survival of the fungus and better disease management [36
]. Sclerotia have been associated with aflatoxin production [37
]. However Cotty [14
] explains that failure of some isolates of A. flavus
to produce sclerotia on culture media can be due to one of the following: an attenuation of sclerotial production in culture, an unfavorable medium, unfavorable temperature, the differential sensitivity of isolates to light, or other environmental constraints in culture and that strain L isolates require more precise conditions to produce sclerotia than strain S isolates. However, disagreements between studies correlating the sclerotial production of isolates with aflatoxin production exist [37
Apart from the risk of aflatoxin contamination of maize, bio-deterioration is another problem associated with high fungal contamination of kernels [38
]. The internal mycoflora of maize in Nandi and Makueni is similar and is dominated by the species of Aspergillus
which predisposes the kernels to bio-deterioration. The resulting physiological and biochemical changes in the maize kernels eventually render grains unsuitable for human consumption. In addition, species of the genera Fusarium
were isolated and some species are capable of producing a wide spectrum of compounds shown to be toxic to man and animals [39
] and so increase the risk of multi-mycotoxin contamination and exposure.
About 40% of maize kernel contamination was caused by Fusarium
and less frequently Penicillium
were also part of the internal mycoflora of maize. Fusarium
species are the most important mycotoxin producers in northern temperate regions and their presence as part of the internal mycoflora of maize raises a concern. Occurrence of Fusarium
spp. in maize in Kenya and fumonisin production has been reported previously [40
]. Muthomi, et al.
] reported Fusarium
as the most predominant species in maize from eastern Kenya and not Aspergillus,
but this observation may have been influenced by the fact that a Fusarium
selective media was used for isolation. The frequency of isolation of Aspergillus
, Penicillium and Alternaria
was the same in the two regions. However, location variation was significant and this may be attributed to differences in pre- and post-harvest management practices, at location and household level. There was a high incidence of Aspergillus
contamination in Nguumo and Kaptumo. Fusarium
were dominant in Kilibwoni and Ukia. Laboret, a location constituted mostly of medium scale maize producers, recorded the highest incidence of clean maize. Medium-scale farmers are better placed to manage farming and storage of maize than small-scale farmers resulting in reduced aflatoxin contamination.