As mentioned in methodology, a total of 385 maize samples were selected from 25 communes within 5 districts in Son La province. Maize samples were randomly collected at the end of the rainy season (from 10 September to 15 September 2016) with recording GPS location. Maize samples in 4 districts (including Mai Son, Moc Chau, Son La city and Thuan Chau) were collected from local retail traders, small farmers, households and maize fields, whereas all maize samples in Yen Chau district were gained from maize fields. All samples were analysed in Lab; on the other hand, only 98% of samples (378/385 samples) (Mai Son: n=76; Moc Chau: n=75; Son La city: n=76; Thuan Chau: n=76 and Yen Chau: n=75) were analysed for AFB1 contamination by ELISA kits due to the lack of kits.
Maize was an extremely good substrate for fungal development which can produce mycotoxins, especially aflatoxins (Wang and Liu, 2006 and Zinedine et al., 2007). Therefore, mold contamination can produce AFB1 in maize in Son La.
This phenomenon is unavoidable. Indeed, maize in Son La was frequently encountered molds through observation by the naked eyes. During the sampling survey, 83.2% of respondents indicated that their maize was contaminated by molds and this was shown in Table 4.1. The frequency is often the largest with 37.3%, followed by 26.2% of people revealing "sometimes", and 7.5% saying
“always”.
Table 4.1. Awareness of moldy contamination in maize in Son La
Characteristic Definition %
Does your maize ever get moldy? Yes 83.2
No 16.8
How often maize is contaminated with mold? Rarely 29.0
Sometimes 26.2
Often 37.3
Always 7.5
Vietnam is located in both tropical and sub-tropical zones. It is featured by both strong sunshine and high rainfall (Trung et al., 2008). Consequently, climate in Son La province is also partly affected by this climate characteristic. This kind of climate offers a favourable condition for mold development, particularly for thermotolerant species such as Aspergillus spp. (Raper and Fenell, 1965).
Previous studies proved that higher temperature supports the growth of Aspergillus species (Chauhan et al., 2008), whereas the average annual temperature and the average humidity in Son La are 23°C and 79%, respectively (GSO, 2017). Besides, according to Thomson and Henke (2000) and Alborch (2012), crops, especially in tropical and subtropical areas with high humidity and temperature were vulnerable to mold growth and aflatoxins contamination. Trung et al. (2008) shared the same view that aflatoxins have a distinct association with maize which requires serious concerns in decontamination of toxicity in many agricultural commodities, particularly maize. Among various mycotoxins produced by fungi, Aspergillus fungi, especially A. flavus were found to be the major contaminant in Vietnam maize, whatever their geographic origin or their intended use in Vietnam (including North, Centre and South) and may cause contamination in maize by AFB1. In addition, another research carried out by Huong et al. (2016) in Northern Vietnam recently reported that maize (82.4%) was commonly contaminated by fungi. Specifically, nearly 30% of maize samples were contaminated with A. flavus or both A. flavus and F. verticillioides.
The results for AFB1 contamination in maize from 5 districts in Son La province were revealed in Table 4.2 that 204 samples (54.0%, 95% CI: 48.8 - 59.1%) and 141 samples (37.3%, 95% CI: 32.4 - 42.4%) had above levels of 5 and 20 àg/kg, respectively (such as range: LOD to 417.0 àg/kg, median: 19.4 àg/kg and mean: 37.3 àg/kg). Specifically, the results found that Thuan Chau district had the highest prevalence (76.3%, 95% CI: 65.2 – 85.3) whereas Yen Chau district had the lowest prevalence (17.3%, 95% CI: 9.6 – 27.8) at the level of above 5 àg/kg. Using the cut-off level > 20 àg/kg, the highest AFB1 was found in Mai Son district with 54.0% (95% CI:42.1 - 65.5) whereas Yen Chau district owned the least of positive samples with 4% (95% CI: 0.8 - 11.3). The highest levels of AFB1 from samples in Mai Son district and Son La city were 417.0 àg/kg and 141.8 àg/kg, respectively.
Table 4.2. Prevalence of AFB1 contamination in maize in Son La
Districts (No.
sample)
AFB1 level > 5 ug/kg AFB1 level > 20 ug/kg
Mean
*
Median
*
Range (<LOD) Prevalence
(%) (No.) 95% CI
Prevalence
(%) (No.) 95% CI Mai Son
(76) 65.8 (50) 54.0 - 75.3 54.0 (41) 42.1 - 65.5 73.7 41.0 <LOD - 417.0 Moc Chau
(75) 45.3 (34) 33.8 - 57.3 25.3 (19) 16.0 - 36.7 16.0 4.5 <LOD - 66.8 Son La city
(76) 64.5 (49) 52.7 - 75.1 50.0 (38) 38.3 - 61.7 45.8 34.3 <LOD - 141.8 Thuan Chau
(76) 76.3 (58) 65.2 - 85.3 52.6 (40) 40.8 - 64.2 26.6 26.7 <LOD - 55.6 Yen Chau
(75) 17.3 (13) 9.6 - 27.8 4.0 (3) 0.8 - 11.3 11.3 4.6 <LOD - 69.4 Total
(378) 54.0 (204) 48.8 - 59.1 37.3 (141) 32.4 - 42.4 37.3 19.4 <LOD - 417.0
* Mean and median (àg/kg) were calculated from the sample above limit of detection (LOD = 1 àg/kg)
Interestingly, the prevalence of AFB1 contamination was the lowest in Yen Chau district compared to the other four districts. The main reason was that all maize samples in Yen Chau were collected from fields with huskcover immediately after harvesting. Therefore, level of contamination was likely to be lower than samples collected from storage, households and traders in other districts. In fact, farmers cut their maize, left huskcover, later laid down it on the soil, while the source of inoculum for A. flavus is often soil (Diener et al., 1987).
Additionally, it is often dried, spread or stored unshelled in heaps on the ground/floor; in some cases, under the verandah after harvesting. This revealed that farmers are unsufficiently aware of handling and storage of maize. They did not follow the standards for the processing of maize samples. Therefore, the prevalence of AFB1 contamination in maize in this province cannot be ruled out.
The findings in this study confirmed that all the samples collected in this research are at risk of AFB1 contamination. Consequently, national prevention and control strategies might be implemented properly such as pre- and post-harvest treatment of contaminated maize and standard storage facilities are required to reduce the risk of AFB1 contamination by fungi.
Additionally, to clarify that the prevalence of AFB1 contamination in Son La varies from season to season, a comparison was reflected between this study
and the data collected from Lee et al. (2017). This study represents the sample collected in rainy season while Lee et al’s in dry season. This thesis showed that mean, median and range in this survey were relatively lower than that of our study with 12.02, 14.3, ranging from LOD to 22.20 àg/kg and 37.25, 19.36, LOD to 417.02 àg/kg, respectively. Specifically, there was no remarkable difference between the AFB1 contamination levels of samples collected from this survey (51.4%, 95% CI: 46.3 - 56.4) and our study’s (54.0%, 95% CI: 48.8 - 59.1) which was above 5 àg/kg and prevalently 50%. However, the presence of AFB1
in this survey (1.01%; 95% CI: 0.28 - 2.57) was dramatically lower than the results obtained in our study (37.3%; 95% CI: 32.41 - 42.39) over 20 àg/kg (as shown in Figure 4.1). Consequently, the findings showed that the prevalence of AFB1 contamination in maize in rainy season was higher than in dry season, noticeably above level of 20 àg/kg.
0 10 20 30 40 50 60
Above 5 ug/kg Above 20 ug/kg
Dry season Rainy season
Prevalence (%)
Figure 4.1. Difference in the prevalence of AFB1 contamination in maize during two seasons in Son La
There are several reasons for the high prevalence of AFB1contamination in maize in the rainy season. Firstly, maize samples were collected at the end of the rainy season. This moment was characterized by moisture and heat so it is conducive to the growth of molds-producing aflatoxins. According to a report conducted by Lillehoj (1983) indicated that temperature and humidity may be the vital factors influencing the infection process and toxin production. Also, the highest level of aflatoxins was found in samples which usually have a hot, humid and wet climate (Gbodi et al., 1986). In fact, the largest amount of rain in Son La
was concentrated from June to September (ranging from 250 to 400 mm) (GSO, 2017). Secondly, maize is severely damaged by pests in this season. During maize collection, farmers complained that pests, particularly maize borer largely destroyed their crop, while harvesting maize in the fields and even in successfully collected maize. Indeed, it could be seen a lot of pain caused by insects. These woundings provide easily entry points for fungal spores develop and produce aflatoxins (Lee and Chuang, 1993; Scott and Zummo, 1994 and Lynch et al., 1991). Therefore, it is essential to implement pest and insect control measures in maize, encourage the use of biological control measures.
Finally, climate changes which have strongly taken place in recent years had affected AFB1 contamination in maize. This was also supported by Bilgrami and Sinha (1987) the fact that the natural incidence of aflatoxins in food and feed was affected by climate, especially in Asian tropical countries where climatic conditions were favorable for aflatoxin production. The variation of the climatic conditions characterized by the greenhouse effect or global warming has been observed in most Asian countries, especially in South Asia (Bandara and Cai, 2014). Global climate change has been listed as an element for emerging food and feed safety issues and its impact on mycotoxins, especially on aflatoxins (United Nations, 2011). This matter draws great concerns. Consequently, the model as a supporting tool should be approached to reinforce aflatoxin management in maize, prevent human and animal exposure and health risks. For instance, the aflatoxin risk maps can be generated as a communication tool for general public, especially for farmers. Additionally, the maps could also be utilized as a way to manage aflatoxin contamination risk areas in order to prioritize aflatoxin control and intervention strategies(Battilani et al., 2016).
Therefore, in order to minimize the development of moldy and aflatoxin production in maize, some practices consisting of pre- and post-harvest practices should be conducted as follows:
The timing of harvest may play an important role in generating aflatoxins.
Consequently, farmers need to identify the appropriate harvesting time. It is the best when maize ripens and its silks dries in black and husks change from green to straw-yellow. Furthermore, farmers should take advantage of the sunny days to harvest rapidly. In case of prolonged rainy days, the farmers are advised to keep maize on the plant, cut silks and bend maize to avoid the infiltration of rain
water inside. This leads to kernel spoilage. After that, they should wait for the sunshine to collect. However, it should not be too late to gather maize because it is more likely for maize to be comtaminted with aflatoxins than that done earlier (Scott and Zummo, 1994) and can be damaged by insects, rats and birds.
After being harvested, maize should be separated from good and bad type.
This aims at avoiding infection from one containing potential pathogens.
Matumba et al. (2015) unveiled that removal of moldy foodstuff was an important practice used to prevent aflatoxin contamination. Another study by Hell and Mutegi (2011) also observed that post-harvest interventions such as sorting and cleaning play an important role to decrease aflatoxins contamination in crops in Sub-Saharan Africa. This is considered as a very helpful practice that should be encouraged.
Besides, to minimize or prevent aflatoxins, drying should take place directly after harvest and as rapidly as feasible (Chulze, 2010), but avoid drying or spreading maize on the ground and farmers should pile maize only when it fully dried. This is because maize is not only susceptible to cross contamination by aflatoxigenic molds but it aslo can absorb moisture from the ground. Maize should be dried and stored properly with 13% of moisture content or less.
The control of aflatoxins contamination in agricultural crops should begin in the field with implementation of good agricultural practices (GAPs).
Additionally, it needs to minimize insect damage and avoid kernel damage during harvest.
In terms of maize storage, this area should be cleansed and disinfected before storing newly-gathered maize and removed previously stored maize to limit aflatoxins contamination. Aslo, maize should be retained in clean storage facilities such as modern silos to replace traditional storage structures. Hell et al.
conducted a study in 2000 so as to find that conventional storage structures in Benin (West Africa) were vulnerable to aflatoxin exposure. Traditional storage structures also increased the deterioration of maize grain by aflatoxigenic molds (Dubale et al., 2014). These places are likely to be caught by heat, high humidity, lack of the aeration inside, and insect and rodent damage. As a result, molds spoilage and aflatoxins contamination are unavoidable.
On the other hand, effective implementation of the above practices by the general public requiressurveillance and awareness of the impacts of aflatoxins on human and animal health. People should be behavioural changes in pre- and post- harvest handling practices and put their knowledge and awareness into action.