MYCOTOXINS, DIAGNOSIS & ANTIFUNGAL AGENTS --- SECTION 7: MYCOTOXINS Definition - Mycotoxin — toxic secondary metabolites produced by fungi - Low molecular weigh
MYCOTOXINS, DIAGNOSIS & ANTIFUNGAL AGENTS --- SECTION 7: MYCOTOXINS Definition - Mycotoxin — toxic secondary metabolites produced by fungi - Low molecular weight compounds with diverse chemical structures and biological activities - Mycotoxicosis — disease caused by mycotoxin action; mediated through liver, kidney, lungs, nervous, endocrine and immune systems - At least 25% of the world's agricultural products are contaminated with mycotoxins - Effects: acute and chronic toxicity, carcinogenic, mutagenic, genotoxic, immunotoxic Historical Perspective - Modern mycotoxicology began with discovery of aflatoxins in early 1960s as the cause of "Turkey X disease" — killed 10,000 turkeys fed contaminated peanut meal --- Toxicogenic Fungi — Main Genera Most mycotoxin-producing fungi belong to three genera: - Aspergillus - Fusarium - Penicillium Field fungi (contaminate crops before harvest): A. flavus , Alternaria longipes , Claviceps purpurea , Fusarium verticillioides , F. graminearum Harvest fungi : F. sporotrichioides , Stachybotrys atra , Myrothecium verrucaria , Trichothecium roseum Storage fungi : Penicillium citrinum , P. cyclopium , P. expansum , A. parasiticus , A. flavus , A. ochraceus , Fusarium verticillioides --- Factors Affecting Mycotoxin Production Environmental: weather conditions, plant stress, invertebrate vectors, microbial competition, geographic and seasonal factors Physical: temperature (optimal 24–28°C ; T-2 toxin produced at 15°C), humidity, water activity, insect damage, time of exposure Chemical: nutritional status of crops, fungicides used in crop management Storage factors: water activity (aw), temperature, aeration (O2/CO2 levels), pH, presence/absence of specific nutrients and inhibitors. Ecological succession of different fungi occurs as temperature and water activity of stored grain changes. --- Characteristics of Mycotoxin-Induced Disease - Not always transmitted between animals - Pharmaceutical treatment does not alter the course of disease - Most often presents as uncertain, sub-acute or chronic condition - No specific treatment for most mycotoxins - Supportive treatment: vitamins and selenium supplementation --- THE MAJOR MYCOTOXINS 1. Aflatoxins Produced by: Aspergillus flavus (most common), A. parasiticus , A. pseudotamarii , A. nomius Affected crops: Corn, peanuts, tobacco, oilseeds, dried fruits, cotton seeds, rice, pepper Types: - B1, B2 — blue fluorescence under UV light (cyclopentane E-ring) - G1, G2 — green fluorescence under UV light (xanthone ring) - M1 — hydroxylated metabolite of AFB1; found in milk, urine and tissues of animals fed contaminated feed; ~10 times less toxic than AFB1 - AFB1 is the most toxic and one of the most potent naturally occurring carcinogens ; main target organ = liver Factors favouring production: - Warm temperature: 28–33°C - High moisture content Toxicity: - Young animals more susceptible than adults - Nutritional deficiency increases susceptibility - Teratogenic — disturbs foetal development - Hepatocellular carcinoma (chronic exposure) - Liver damage in poultry, fish, rodents, non-human primates - AFB1 also affects lungs and respiratory system - Clinical signs: decreased growth rate, reduced feed efficiency, mild anaemia, increased susceptibility to infectious disease Treatment: - Detoxification: Hydrated sodium calcium aluminosilicate (HSCAS) absorbs aflatoxins - Supportive: Vitamin E and selenium - Prevention: mould inhibitor — treatment of grain with anhydrous ammonia for 10–14 days --- 2. Zearalenone (RAL / F-2 Mycotoxin) Produced by: Fusarium roseum ( F. graminearum/Gibberella zeae ), F. equisetti , F. cerealis , F. verticilloides , F. incarnatum Affected crops: Corn, wheat, barley, oats, rice, sorghum Factors favouring production: - High moisture level: 22–25% - Alternating high and low temperatures: 7–21°C Mechanism: Potent estrogenic metabolite — binds to estrogen receptors → infertility, abortion, breeding problems. Functions as a weak oestrogen Most susceptible: Swine Clinical signs: Infertility, abortion, vulval swelling/prolapse, mammary enlargement Treatment: - Remove contaminated feed; provide clean feed - Animals return to normal within 1–4 weeks - Surgical intervention may be needed for prolapsed vulvae/rectums --- 3. Ergot Alkaloids Produced by: Claviceps purpurea (most prominent ergot fungus) Crops affected: Rye, barley, wheat, oats (pathogens of grass species) Conditions favouring: Warm and humid conditions Disease: Ergotism (St. Anthony's Fire) Two types: - Gangrenous ergotism — vasoconstriction → reduced blood supply to extremities → necrosis of feet, ears, tail; abortion - Convulsive ergotism — CNS effects → suppressed lactation, hypersensitivity, ataxia (abnormal movement), increased temperature/pulse/respiration, hyper-excitability, tremors, heat intolerance Mechanism: - Potent initiators of smooth muscle contraction - Mimic the action of dopamine (neurotransmitter) Treatment: - Warm, clean, stress-free environment - Control secondary bacterial infection - Milk supplement - IV/intra-arterial infusion of vasodilators (sodium nitroprusside or nitroglycerin) Medical applications: Used in abortion induction and headache/migraine treatment --- 4. Ochratoxin and Citrinin Ochratoxin sources: Aspergillus ochraceus , A. niger , Penicillium verrucosum Citrinin sources: Penicillium viridicatum , P. citrinum , A. terreus , A. oryzae Crops: Wheat, oats, corn, barley, rye Mechanism of toxicity: - Target the renal proximal tubule → porcine nephropathy; nephrotoxin in ducks and chickens - Disrupt carbohydrate metabolism - Increase generation of free radicals Clinical signs: - Acute: vomiting, diarrhoea, dehydration, depression - Sub-acute to chronic: weight loss, reduced feed efficiency, dehydration, immunosuppression, teratogenicity, carcinogenesis, haemorrhage Treatment: - Oral cholestyramine, charcoal, phenylalanine — prevent toxin absorption - Antioxidants: vitamins A, E, C and liposomal glutathione — mitigate oxidative effects --- 5. Patulin Sources: Penicillium patulum , P. griseofulvum , P. expansum Found in: Rotting apples and apple products; mouldy fruits, grains and foods Toxicity: - Acute: liver, spleen and kidney damage; immunotoxicity - Humans: nausea, GI disturbances, vomiting - Immunotoxic and genotoxic (damages DNA) Paradox: Previously used as nose/throat spray for common cold and fungal skin infections Management: Good agricultural practices — remove mould, wash produce, do not use rotten/damaged apples for baking, canning or juice production --- 6. Fumonisins Sources: Fusarium moniliforme , F. proliferatum , F. verticillioides Crop: Corn — fumonisins are the most common mycotoxins in maize Most important form: Fumonisin B1 (most common and economically significant); A, B, C, P series also exist Symptoms: - Leucoencephalomalacia (hole in the brain) — in horses - Pulmonary oedema — in rabbits - Hepatotoxic and carcinogenic effects - Abdominal pain and diarrhoea - Oesophageal cancer in humans Treatment: No specific treatment; remove contaminated grain source --- 7. Trichothecenes (T-2 Toxin) Sources: Trichoderma trichothecium , Fusarium spp., Stachybotrys , Myrothecium Mechanism: Slows down production of protein and nucleic acid in the body (inhibits protein synthesis) Symptoms: - Haemorrhage (vagina, nose, mouth) - Vomiting - Dermatitis - Necrosis of oral cavity Important note: T-2 can be aerosolized and released into the air — potential use as a biological weapon --- SECTION 8: DIAGNOSIS OF FUNGAL DISEASES Specimen Collection by Type of Mycosis Superficial Mycosis: - Clean infected part with 70% alcohol - Collect in sterile paper or petri dish (avoid drying, reduce bacterial contamination, maintain viability) - Ringworm: scrape outwards from edge of lesion with scalpel - Scalp lesion: scrape with blunt scalpel including hair, scales and contents of plugged follicles - Onychomycosis: stop antifungals one week before collection Subcutaneous Mycosis: - Scrap