INTRODUCTION
Aflatoxins are mycotoxins produced by two species of Aspergillus , a fungus which is especially found in areas with hot and humid climates. Since aflatoxins are known to be genotoxic and carcinogenic, exposure through food should be kept as low as possible.
Aflatoxins can occur in foods, like maize, treenuts and groundnuts, as a result of fungal contamination before and after harvest.
Several types of aflatoxins are produced in nature. Aflatoxin B1 is the most common in food and amongst the most potent genotoxic and carcinogenic aflatoxins. It is produced both by Aspergillus flavus and Aspergillus parasiticus . Aflatoxin M1 is a major metabolite of aflatoxin B1 in humans and animals, which may be present in milk from animals fed with aflatoxin B1 contaminated feed. Aflatoxin B1 has been classified as a known human carcinogen and is known to cause liver cancer in animals.
Aflatoxins may also be associated with liver cell cancer in humans in Africa and Asia where peanuts are a dietary mainstay. Recent research has shown a strong association between long-term dietary aflatoxin exposure in conjunction with hepatitis B (a viral infectious disease of the liver) and increased rates of liver cancer.
Aflatoxins in food
ASPERGILLUS FLAVUS: THE MAJOR PRODUCER OF AFLATOXIN
The Aspergillus flavus is a fungus. It grows by producing thread known as hyphae. Filamentous fungi such as A. flavus are sometimes called molds. A network of hyphae is known as the mycelium. The unaided eye cannot see individual hyphae, but dense mats of mycelium conidia (asexual spores) often can be seen.
The maize above shows the growth of the fungus
In nature, A. flavus is capable of growing on many nutrient sources. It is predominately a saprophyte and grows on dead plant and animal tissue in the soil.
Aspergillus flavus can also be pathogenic on several plant and animal species, including humans and domestic animals. The fungus can infect seeds of corn, peanuts, cotton, and nut trees. The fungus can often be seen sporulating on injured seeds such a maize kernels as shown above. Often, only a few kernels will be visibly infected.
Growth of the fungus on a food source often leads to contamination with aflatoxin, a toxic and carcinogenic compound. Aspergillus flavus is also the second leading cause of aspergillosis in humans. Patients infected with A. flavus often have reduced or compromised immune systems.
At least 13 different types of aflatoxin are produced in nature, but four are the main types: B1, B2, G1, and G2, with aflatoxin B1 considered as the most toxic. While the presence of Aspergillus flavus does not always indicate harmful levels of aflatoxin it does mean that the for aflatoxin production is present.
Aflatoxins in Your Food
Aspergillus flavus
METABOLISM
Because of the aflatoxin’s common occurrence in feedstuffs, feeds and milk products, these mycotoxins pose a serious threat to human and animal species. Although the oral route is the main contamination means, inhalation may also occur as a result of people or animals being exposed to the grain’s dust. After respiratory exposure, AfB1 may appear in the blood more quickly than after oral exposure. Nevertheless, after 4 hours the plasmatic concentration does not differ between the two routes of contamination. Following injestion, AfB1 is efficiently absorbed by duodenum (due diffusion as the particle’s low molecular weight shows). The main metabolizing organ is the liver, but this can also occur in the place of absorption , in the blood or in several extra-ephatic organs; the metabolism of AfB1 can be divided in 3 phases:
1) Bioactivation
2) Conjugation
3) Deconjugation
1- In bioactivation aflatoxins exert their toxic effects. AfB1 is oxidized in several hydroxilated metabolites. The metabolic pathways for AfB1 includes o-demethylation to AfP1, reduction to aflatoxicol and hydroxylation to AfB1-8,9 epoxide (acutely toxic, mutagenic and carcinogenic), AfM1 (acutely toxic), AfQ1 and AfB2 ( both relatively non-toxic).
2- AfB1-8,9 epoxide is highly unstable, thus several reactions may occur: biological nucleophils (such as nucleic acids)- stable links to DNA or RNA are formed, inducing point mutations and DNA strand breaks. These reactions and the formation of AfB1-DNA adducts are highly correlated to carcinogenic effects of AfB1. Also, in presence of water molecules it can link with serum proteins. This mechanism may explain the toxic effect of aflatoxins.
With the conjugation to glutathione S-transferase beta-glucuronidase and/or sulphate transferase the major detoxification pathways of this molecule can be activated. The resulting conjugated are exctreted via the bile in the gastrointestinal tract.
3- Deconjugation can also occur in the intestine as a result of bacterial activity. Also urine excretion is contemplated.
Aflatoxin B1 pathway
CONTAMINATION
Aflatoxins may be present in a wide range of food commodities, particularly cereals, oilseeds, spices and tree nuts.Here is a list of some of the most exposed food commodities:
*Maize
*Peanuts
*Pistachios
*Brazils
*Chillies
*Black pepper
*Dried fruit
*Figs
The highest levels are usually found in commodities from warmer regions of the world where there is a great deal of climatic variation.
It is important to recognise that, although it is primary food commodities that usually become contaminated with aflatoxins by mould growth, these toxins are very stable and may pass through quite severe processes. For this reason they can be a problem in processed foods, such as peanut butter.
Milk, cheese and other dairy products are at risk of contamination by aflatoxin M.
ACUTE TOXICITY
At high enough exposure levels, aflatoxins can cause acute toxicity, and potentially death, in mammals, birds and fish, as well as in humans. The liver is the principal organ affected, but high levels of aflatoxin have also been found in the lungs, kidneys, brains and hearts of individuals dying of acute aflatoxicosis.
Acute necrosis and cirrhosis of the liver is typical, along with haemorrhaging and oedema. LD50 (lethal dose) values for animals vary between 0.5 and 10 mg/kg body weight.
CHRONIC TOXICITY
Chronic toxicity is probably more important from a food safety point of view, certainly in more developed regions of the world. The liver is again the main target organ. Ingestion of low levels over a long period has been implicated in primary liver cancer, chronic hepatitis, jaundice, cirrhosis and impaired nutrient conversion.
Less is known about the chronic toxicity of aflatoxin G1 and M1, but these are also thought to be carcinogens, though probably a little less potent than B1.
Little is known about the level of dietary exposure to aflatoxins necessary to affect health, especially in humans, and diagnosis of chronic toxicity is very difficult. It is generally agreed that the best approach is to minimise the levels in all foods as far as is technically possible and to assume that any dietary exposure is undesirable.
Food Safety Watch
SYMPTOMS
Here are two main ways people are usually exposed to aflatoxins. The first is when someone takes in a high amount of aflatoxins in a very short time. This can cause:
*Mental impairment
*Abdominal Pain
*Vomiting
*Convulsions
*Edema
*Pulmonary Edema
*Hemorrhaging
*Disruption of food digestion
*Coma
*Death
The other way people suffer aflatoxin poisoning is by taking in small amounts of aflatoxins at a time, but over a long period. This might happen if a person's diet has a small amount of aflatoxins, for example. When this happens it can cause:
• Growth and development impairment
• Liver cancer due to DNA mutation caused by aflatoxins
Aflatoxins: Symptoms, Poisoning, Limits
HUMAN HEPATOCELLULAR CARCINOMA
The role of aflatoxin in human hepatocellular carcinoma (HCC) has been studied in China and sub-Saharan Africa. In certain regions of these countries, at least 250,000 deaths from HCC occur annually.
A specific missense mutation at codon 249 in P53, AGG to AGT, leading to a substitution of arginine for serine, was frequently observed in liver tumors of these particular areas.
Therefore biomarkers of exposure were elaborated to study its carcinogenic effects. These include measurement of aflatoxin metabolites in urine, DNA and protein adducts in blood and tissues and excised guanine adduct in urine.
In addition to the formation of adducts, it is believed that AFB1 acts as a carcinogen by mechanisms that include the formation of reactive oxygen species (ROS) leading to increased hepatic oxidative damage.
In previous studies, we found that AFB1 exposure was positively associated with level of oxidative DNA damage in humans as measured by urinary 8-oxo-7, 8-dihydro-2′-guanine. Increased HCC risk was also associated with higher levels of these oxidative stress markers in prospective cohort.
EPIDEMIOLOGICAL STUDIES
A study in the early 1970s measured AFB1 levels in major diet components from different parts of Uganda over one year and found that the frequency of AFB1 contamination was particularly high in provinces with high HCC incidence. Similar associations were reported in Swaziland and Thailand.
Pooling these observations confirmed higher HCC incidence with higher levels of AFB1 consumption; Later studies monitored AFB1 exposure by measuring urinary excretion of AFB1 metabolites and released DNA adducts. Further studies confirming an etiological role for AFB1 have mainly been conducted in Asia comparing levels of AFB1 exposure between HCC cases and controls.
Early case-controls studies estimated individual levels of dietary AFB1 consumption retrospectively from dietary questionnaires data. A study conducted in the Philippines compared dietary intakes of HCC cases with age- and sex-matched controls. By using dietary recall, the frequency and amounts of food items consumed were used to calculate units of AFB1 load per day. These calculations revealed that the mean AFB1 load of HCC cases was 4.5 times higher than that of controls.
Overall, prospective studies have shown a strong association between biological markers of AFB1 exposure in serum or urine and risk of subsequent HCC. The interaction between AFB1 exposure and HBV infection on HCC risk was replicated in different cohorts.
The Role of Aflatoxins in Hepatocellular Carcinoma, Hui-Chen Wu and Regina Santella, 2012
TREATMENT
There is no specific antidote for toxicity of aflatoxins. Timely administration of l-methionine 200 mg/kg) and sodium thiosulfate (50 mg/kg), at eight-hour intervals, is proven to be of therapeutic value.
Supplementation with increased levels of protein, vitamins, and antioxidants can also be rewarding. Immediate removal of contaminated feed is the single most important step in avoiding any further loss to animal productivity and/or death.
ADDITIONAL PROTECTION
Biological exposure of aflatoxins can be minimized by chemoprotection and/or enterosorption.
Chemoprevention against aflatoxins has been demonstrated with the use of a number of compounds (such as esterified glucomanoses and other yeast extracts) that either increase detoxification of aflatoxins or prevent the production of the aflatoxin epoxide, thereby reducing or blocking AfB1- induced hepatocarcinogenesis.
Compounds such as oltipraz and chlorophyll are available to decrease the biologically effective dose. Enterosorptive food additives are recommended because they bind aflatoxins and render them biologically unavailable to humans and animals.
Dietary supplementation with a feed anti-caking agent or adsorbents, such as 0.5% hydrated sodium calcium aluminosilicate (HSCaS or NovaSil), has been demonstrated to minimize the aflatoxicosis problem in a number of species. Selective calcium montmorillonites have proven to be the most selective and effective of these enterosorbents. Following absorption, some zeolites can be effective reactive oxygen species (ROS) scavengers.
The efficiency of mycotoxin binders differs considerably depending mainly on the chemical structure of both the adsorbent and the toxin. It is important to mention that by no means should these binders be considered mycotoxin eliminators. One disadvantage with these binders is that there can be some interference in the absorption of essential nutrients. This factor should definitely be taken into consideration, especially during pregnancy and the fetal developmental period.
Symptoms, Diagnosis, and Pathophysiology of Mycotoxin Exposure, Ramesh Gupta, 2012
PREVENTION
Accumulation of aflatoxins is dependent on weather conditions. A dry growing environment or drought stress tends to favor the development of aflatoxins in corn. When soil moisture is below normal and temperatures are high, the number of Aspergillus spores in the air increases.
During pollination, these spores infect corn kernels either through silks (pollination tubes) or through areas of damage caused by insects, birds and weather events. Once infected, plant stress such as nutrient deficiency, continued dry weather or kernel damage during harvest may increase aflatoxin levels.
Aflatoxin consumption by livestock and poultry results in a disease called aflatoxicosis.
Aflatoxins are metabolized in the liver of all living organisms. High concentrations can lead to acute liver disease or death within 72 hours. Lower concentrations have produced various symptoms, such as feed refusal, decreased feed efficiency, impaired reproduction, hemorrhaging in muscles and suppression of the immune system.
Feeding grain contaminated with any level of aflatoxin carries a considerable amount of risk. Testing for aflatoxin concentrations should be the first step in proper feeding management. The use of a black light to detect the presence of aflatoxins at the elevator is common.
REGULATORY LEVELS
Recommended aflatoxin levels in feed is 0 parts per billion (ppb), but this is not always possible. If feeding contaminated grain to lactating dairy cattle, immature poultry or immature livestock, do not exceed 20 ppb aflatoxin in the total diet. Calves should not receive milk from cows fed more than 20 ppb aflatoxin. Breeding cattle, swine and mature poultry should not exceed 100 ppb in their total ration.
Finishing beef cattle and swine can tolerate grain up to 300 ppb aflatoxin. Animals should not consume any level of aflatoxin in their diet for at least three weeks prior to slaughter. Any grain with levels exceeding 1000 ppb should be destroyed and not be salvaged by blending with grain of lower concentrations.
Around 100 countries around the world have regulations governing aflatoxins in food and most include maximum permitted, or recommended levels for specific commodities. The EU sets limits for aflatoxin B1 and for total aflatoxins (B1, B2, G1 and G2) in nuts, dried fruits, cereals and spices. Limits vary according to the commodity, but range from 2-12 μg/kg for B1 and from 4-15 μg/kg for total aflatoxins. There is also a limit of 0.050 μg/kg for aflatoxin M1 in milk and milk products. Sampling and analytical methods are also specified. Limits of 0.10 μg/kg for B1 and 0.025 μg/kg for M1 have been set for infant foods.
Understanding and Preventing Aflatoxin Poisoning, Jeff Ball, 1998
