š¦Plant Toxins Flashcards
Describe the mode of action of pyrrolizidine alkaloids (PA).
Absorbed in the intestine & transported to the liver where they are metabolised to pyrroles. Pyrroles are toxic metabolites that are very reactive chemically and cross link with double stranded DNA. They bind both proteins and nucleic acids within hepatocytes. Upon binding DNA, the toxin has an antimitotic effect on hepatocytes resulting in megalocyte formation. As the megalocytes die, these cells are replaced with fibrous tissue instead of normal hepatocytes. Ultimately, the liver fails due to hepatocellular death and fibrosis. While the liver is the primary organ affected, reactive metabolites may also damage the lung, leading to cor pulmonale or right heart failure. Renal damage may result in megalocytosis of renal tubular epithelium and glomerulosclerosis.
Ingestion of large doses of PAs results in acute toxicosis and most commonly produces centrilobular necrosis with haemorrhage. Chronic ingestion usually produces hepatocellular necrosis in the portal areas, megalocytosis, fibrosis, biliary hyperplasia and obstruction of hepatic veins.
Large species differences exist in susceptibility to PA toxicosis. Which animals are most/least susceptible to PA toxicosis? Why?
Small herbivores such as sheep, goats, rabbits, guinea pigs and other herbivorous laboratory animals are highly resistant to PA toxicity, associated with a low rate of hepatic production of reactive metabolites (pyrroles) and(or) a high rate of activity of detoxifying enzymes
The most susceptible are monogastric species such as pigs, poultry, horses and humans. Poisoning in these species is usually caused by the consumption of cereal grains contaminated by heliotrope seed. Ruminants including sheep and cattle are less susceptible because some of the alkaloids are destroyed by microbes in the rumen.
Heliotrope poisoning problems are more common in British breeds and crossbreeds than Merinos. Merinos tend to avoid the plant when other feed is available, whereas British breeds and crossbreeds will eat it more readily and sometimes preferentially. In general, symptoms are not seen until exposure to heliotrope has occurred over more than one season. The death rate increases with age.
Cattle are more susceptible to the toxic effects of heliotrope than are sheep and deaths can occur within 10 days of commencing to eat heliotrope. Usually however, deaths are spread over several months
What are the typical clinical signs associated with PA toxicosis?
Heliotrope: depression, reduced appetite, loss of condition, diarrhoea, restlessness and persistent aimless wandering
Discuss the major factors contributing to the contamination of feedstuffs with mycotoxins.
Ā§ Weather conditions during growing & harvesting of crops affect incidence & degree of mouldy feeds & mycotoxin problems. Mycotoxins are low molecular weight, non-antigenic substances produced by toxigenic fungi on various substrates including animal feeds under suitable conditions of temperature & moisture. > 300 identified from ~ 350 fungal species
Ā§ Fusarium toxins more likely to occur under cool, wet conditions during growth, harvesting & storage.
Ā§ Hot, humid conditions favour development of aflatoxins
Ā§ Delaying harvest to ā maturity & ā moisture levels may ā mould growth & mycotoxins
Ā§ Storing grains, feedstuffs & forages above recommended moisture levels or in poor storage units may also ā mould-related problems
Define mycotoxicoses, how they are classified and the impact on the health and production of animals.
Primary acute mycotoxicoses occur when toxins consumed in sufficient amounts to cause overt disease. Include hepatitis, haemorrhagic disease, epithelial necrosis or acute death depending on specific toxin and dose.
Primary chronic mycotoxicoses occur when lower levels of toxins consumed over longer period. Results in slower GR, decreased reproductive efficiency, decreased market quality of final product (meat, milk, eggs).
Secondary mycotoxic disease occurs when low levels of toxin intake results in increased susceptibility to intercurrent diseases and infections (not disease). Impairment of immune processes - immunosuppression caused by aflatoxin B1 demonstrated in turkeys, chickens, pigs
What are the main effects of aflatoxins on animal health and production?
Gastrointestinal dysfunction Liver damage Decreased feed utilisation and efficiency Decreased productivity Decreased reproductive performance Decreased milk or egg production Embryonic death Teratogenicity (birth defects) Tumours Suppressed immune system function Liver damage
What effect does zearalenone have on animals?
Mimics effect of oestrogen and, at low doses, increases size or early maturity of mammary glands and reproductive organs. At higher doses interferes with conception, ovulation, implantation, foetal development and the viability of newborn animals
What effect does fumonisin have on animals?
Impaired immune function Liver and kidney damage Decreased weight gains Increased mortality rates Leukoencephalomalacia in horses Respiratory difficulties in pigs
Which (animal) species are most susceptible to pyrrolizidine alkaloid toxicity and why?
Resistance to PA toxicity associated with rate of hepatic prodn of pyrroles &/or rate of activity of detoxifying enzymes
Monogastrics most susceptible. Of domestic animals, pigs most susceptible to PA toxicosis with a resistance rating (RR) = 1
Ā§ Poultry: RR = 5
Ā§ Cattle & horses: RR = 15
Ā§ Sheep & goats: RR = 150
What are the most common causes (i.e., circumstances) that result in the poisoning of either
livestock and/or companion animals?
Livestock: hunger, age (more curious and more likely to eat unfamiliar plants), contaminated feeds including highly contaminated pastures.
Companion animals: being fed human foods that are toxic to them (nuts, onions, etc.), chewing (toxic) garden and indoor plants.
What are the potential beneficial effects of condensed tannins in the diet of ruminants?
Ā§ Increase dietary bypass protein by forming tannin-protein complexes
o Increased milk production, wool growth, ovulation rate and lambing percentages
o Reduced bloat risk (CT interfere with the formation of a stable foam)
Ā§ Increased resistance to gastrointestinal nematodes ā CT linked to the regulation of activation of T and B lymphocytes, natural killer cells (NKC) and macrophages, gene expression and lymphocyte proliferation, production of antibodies, cytokines and other cytotoxic substances.
Why are pigs and poultry more likely than ruminants to suffer from the adverse consequences of phytates?
Unlike ruminants, pigs and poultry do not possess adequate digestive phytase activity to utilise phytate.
For ruminants, why are hydrolysable tannins more toxic than condensed tannins?
Hydrolysable tannins are readily hydrolysed by esterases in the GIT to their constituent phenolics (e.g. gallic acid from gallotannins and ellagic acid from ellagotannins) and sugars. In the rumen gallic acid is further degraded to pyrogallol and other phenolics, which are then conjugated in the liver. Pyrogallol is both hepatotoxic and nephrotoxic.
Condensed tannins are not inherently toxic to ruminants. Multiple phenolic hydroxyl groups of CT lead to the formation of complexes, primarily with proteins and to a lesser extent with metal ions, amino acids and polysaccharides. The pH dependency of the CT-protein complex causes a shift in the digestion of protein from the rumen to the small intestine.
List three strategies for controlling mycotoxins in animal feeds.
Screening: Screening of infested grains/seeds by sorting by colour, density, size or fluorescence. Broken grain particulates can have higher levels of mycotoxins as compared to intact grain.
Propionic acid: Added to stored grain to prevent mould growth. Chemically denatures mycotoxins.
UV irradiation: Irradiation of grain prior to storage to kill fungi. Physically deactivates mycotoxins.
Heat treatment: Denaturation of mycotoxins by heating during cooking, extrusion, and/or pelleting. Some mycotoxins resist denaturation even at temperatures approaching 260oC.
Aluminosilicate clays Ca hydroxide, Na bisulphate, H peroxide, Na hypochlorite: Binds mycotoxins in the GIT to prevent adsorption.
Chemical inactivation of mycotoxin.
Ammoniation: Uses ammonium hydroxide or gaseous ammonia to denature mycotoxins under alkaline conditions.
Ozonation: Gaseous or aqueous ozone is passed through grains to cause oxidative deactivation of mycotoxins.
List three strategies for controlling bacterial contaminants in animal feeds.
Rapid drying: Reduces amount of available water
Shorter storage time: Reduces amount of caking, mustiness and browning of feed
Zinc bacitracin: Control Gram-positive growth and hinder Clostridia spp. sporalation
Mineral acids, SCFA, isopropyl alcohol, aldehydes, trisodium phosphate: Added to feeds as disinfectants
Bacteriophages: Kills actively growing bacteria in feed
Propionic acid producing bacteria: Added to silage to aid in acid production
Bacteriocin producing bacteria: Added to silage to control pathogens
Corynetoxins, mycotoxins and ergot alkaloids are produced as a consequence of microbial contamination of plant materials. Provide an explanation of how these three differ, both in terms of the contaminating agent and the effects (on animals) that result from consumption of contaminated plants (including grains).
Corynetoxins ā generated by bacteria infected with bacteriophages. The bacteria grow in plant galls produced in seeds of certain grasses by invasion of nematode worms. The toxins inhibit production of glycoproteins and thus interfere with the structure of small blood vessels, decreasing oxygen delivery to tissues. The brain is the most affected organ.
Mycotoxins ā metabolites produced by fungi. Because they are non-antigenic substances they do not trigger an antibody response. There is a large variability in the susceptibility (by animals) to mycotoxins. Depending on specific toxin and dose, primary acute mycotoxicoses results in hepatitis, haemorrhagic disease, epithelial necrosis and death. Primary chronic mycotoxicoses are characterised by slower growth rates, immune suppression, decreased reproductive efficiency and decreased product (meat, milk, eggs) quality.
Ergot alkaloids ā secondary metabolites (indole alkaloids) produced by certain fungi. Different ergot alkaloids have different biological activities, some being psychoactive and others have vasoconstricting activities.
Explain why blue-green algae poses a significant threat to animals.
Cyanobacterial toxins in blooms that form floating masses or scums on fresh or brackish water bodies. The toxicity of a particular bloom is determined by the mixture of species involved and their strain composition of toxic and non-toxic genotypes
Animals are poisoned by drinking water containing bloom material; wind may concentrate floating blooms. Some animals appear to be attracted to contaminated water including dried crusts of algae on the top of water. Animals will appear unwell very quickly and acutely affected animals often die within 24 h of ingesting the toxin(s).
