Toxicology 2 Course 10: Mycotoxins and Associated Pathologies

MYCOTOXINS

CHRONIC MYCOTOXIC NEPHROPATHY

Chronic mycotoxic nephropathy is a chronic mycotoxicosis produced by the synergistic effect of ochratoxin A, citrinin and oxalic acid produced by Aspergillus ochraceous and Penicillium verrucosum which parasitize cereal grains (especially barley, maize, oats) and combined feed. The mycotoxicosis affects pigs and birds.

Toxicodynamics of Chronic Mycotoxic Nephropathy

Ochratoxin A is a mycotoxin with nephrotoxic, hepatotoxic, immunosuppressive, and reproductive toxic actions for mammals and birds. Single oral LD50 is 6-25 mg/kg in mammals and 3-10 mg/kg in birds.

Ochratoxin disrupts carbohydrate and lipid metabolism, alters prothrombin time and reduces haematopoiesis (in birds). The action of ochratoxin in mammals translates histologically into necrosis of distal renal tubules and periportal necrosis of hepatocytes. In birds, it produces acute nephrosis, visceral gouty deposits, diffuse vacuolization of hepatocytes, hepatic focal necrosis and haemorrhages.

Citrinin is a non-protein mycotoxin, a secondary metabolite with nephrotoxic and hepatotoxic action.

Oxalic acid is a secondary toxic mycotic metabolite that precipitates calcium ions, predominantly in the capillaries and renal tubules.

Clinical signs of Chronic Mycotoxic Nephropathy

In pigs, the mycotoxicosis is associated with polyuria, polydipsia (which is maintained even after removal of contaminated feed) and decreased growth rate and feed efficiency whilst feed intake is not affected.

In birds, apathy, prostration, ruffling of feathers, haemorrhagic diarrhoea, tremors, delayed sexual maturity, low egg production and hatching percentage are observed. The growth of hatched chicks is greatly decreased in the first two weeks.

The mycotoxicosis can also affect dogs, in which is causes nephrotoxic and immunosuppressive effects, with anorexia, weight loss, vomiting, haemorrhagic diarrhoea, tenesmus, increased body temperature, tonsillitis, dehydration, and prostration. It also triggers tonsillitis and necrosis of lymphoid tissues.

Anatomopathological changes in Chronic Mycotoxic Nephropathy

In pigs, anatomopathological changes are represented by the enlargement of the kidneys, which are grey or marbled in colour, with abundant perirenal oedema. Additionally, fibrosis or even cystization of the renal cortex, with degeneration of proximal renal tubules, and interstitial conjunctival proliferation are found. In advanced stages, lesions are also observed at the glomerular level, with atrophy and glomerular sclerosis.

In birds, the lesions are more complex and consist of nephrosis, haemorrhagic enteritis, hepatic steatosis or hepatic focal necrotic, visceral gout, and bursa of Fabricius regression.

Diagnosis of Chronic Mycotoxic Nephropathy

The diagnosis is based on the clinical sings and anatomopathological changes. Ochratoxin A can be dosed from urine samples, and the chemical, macro- and microscopic analysis of the feed is used to confirm the diagnosis.

Differential diagnosis includes oedema disease and oxalate intoxication (e.g., amaranth, ethylene glycol).

SLAFRAMINE INTOXICATION

Slaframine intoxication is a mycotoxicosis caused by the mycotoxin slaframine, which is produced by Rhizoctonia leguminicola which parasitizes red clover.

Toxic sources are represented by red clover (both in the form of green mass and in the form of hay) that is affected by the "black spot disease" produced by Rhizoctonia leguminicola. Humidity favours the development of the fungus on the clover and the production of slaframine.

Slaframine intoxication usually occurs in spring or autumn, with low and high humidity, ideal for the development of Rhizoctonia leguminicola.

Toxicodynamics of Slaframine Intoxication

Toxicity is due to slaframine, which is a bicyclic amine which has no physiological activity per se; however, after its biotransformation (bioactivation) into a quaternary amine, it will have a structure and activity similar to that of acetylcholine.

Slaframine is metabolised in the liver by a microsomal flavoprotein oxidase to the active metabolite ketoamine, which consists of a quaternary nitrogen separated from an acetate ester of two carbon atoms.

The mechanism of action is similar to that of acetylcholine; however, the administration of atropine has no curative effect in poisoned animals, probably due to the fact that acetylcholine competes with slaframine for the same cholinergic receptor.

Clinical signs of Slaframine Intoxication

The clinical signs of slaframine toxicity are similar in all species. When animals are fed slaframine-contaminated feed, they salivate profusely.

A single dose of slaframine causes hypersalivation for 6-10 hours, but affected animals can salivate for several days because they are fed the same feed contaminated with slaframine.

In horses, excessive salivation is usually the first sign of exposure to slaframine. Signs usually develop within a few hours of exposure. Other clinical signs include epiphora, refusal of food, tympany, joint stiffness, diarrhoea, and weight loss.

In other animals, such as sheep, pigs and guinea pigs, the intoxication also evolved with cyanosis, dyspnoea, and mouth breathing.

In pigs, vomiting and decubitus with rigid hind limbs were also observed.

Anatomopathological changes in Slaframine Intoxication

Anatomopathological changes are usually missing. Sometimes, pulmonary oedema and centrilobular hepatic necrosis are found.

Diagnosis of Slaframine Intoxication

In general, the diagnosis of slaframine intoxication is made on the basis of clinical signs (excessive salivation) in animals that consume legumes, especially red clover hay.

Samples from crops can be taken to identify Rhizoctonia leguminicola fungus in the hay.

Chemical analysis for slaframine uses chromatographic methods for the detection of the toxicant in hay, plasma, or milk.

Treatment of Slaframine Intoxication

The treatment involves the removal of the feed, the emptying of the digestive contents, by administration of purgatives, hydric and electrolytic rebalancing with bicarbonate isotonic solutions and antihistamines (promethazine). Atropine only works if used preventively.

STAHIBOTRIOTOXICOSIS

Stahibotriotoxicosis is a mycotoxicosis caused by satratoxins produced by Stachybotrys chartarum (syn. S. alternans, S. atra) which parasitizes straws and grasses.

Stachybotrys chartarum develops in black colonies in uneven areas in the feed mass. The optimal conditions for the development of the fungus are represented by temperatures above 5℃ and the humidity of the feed of 40-50%, circumstances met during rainy autumns.

Stachybotrys chartarum is destroyed in 5 minutes by heating at 100℃, in an hour in 2% formaldehyde solution, and it is sensitive to 2-4% caustic soda solution.

On infested straw they look like a layer of soot, especially at the nodes.

Animals that consume or come into contact with these straws or people handling contaminated straws can develop stahibotriotoxicosis.

Toxicodynamics of Stahibotriotoxicosis

Toxicity is due to satratoxins (e.g., satratoxin H) which have leukopenic and irritating effect on the skin. The serious agranulocytosis they produce often confers conditions for the development of potentially pathogenic germs and thus the occurrence of severe secondary infections.

Stahibotriotoxicosis in horses

The disease occurs only in the period of housing (September-April). When animals are released onto pastures, the disease disappears spontaneously.

The character of the disease is enzootic, with spontaneous annual outbreaks, which suddenly cease after a few years, and then reappear. When stahibotriotoxicosis lasts a few years, both animals that have passed through the disease and those newly introduced into the herd are affected.

The occurrence of disease is related to the improper harvesting and preservation of feed, as well as climatic conditions.

Clinical signs of Stahibotriotoxicosis

The symptomatology is expressed by two forms of evolution: the typical form and the atypical form.

The typical form evolves in three phases.

Phase I occurs 2-10 days after consumption of contaminated feed and may pass unnoticed, due to the maintenance of a relatively good general condition. Clinical signs are discrete, consisting of exfoliation of the perioral epithelium (perinasal and peri-buccal), sometimes associated with oedema and the appearance of crevices at this level, catarrhal stomatitis or superficial necrosis of the gums. Perioral lesions may be affected by suprainfections, which include the submaxillary lymph nodes. The duration of this phase is usual 8-12 days, but, when suppurative complications occur, they may last 15-30 days.

Phase II is characterized by the appearance of haematological changes represented by marked thrombocytopenia (35-40,000 platelets/mm3) and leukopenia (2-5,000 leukocytes/mm3), which lead to haemorrhagic diathesis and diarrhoea. The duration of the phase is 5-30 days.

Phase III is characterized by marked hyperthermia (40-41℃) due to the increase of leukopenia (several hundred leukocytes/mm3) and microbial invasion. Other clinical signs include ulcers on the oral and pituitary mucosa with smooth and non-reactive bottom, diarrhoea that continues or worsens (dysentery), cardiac arrhythmias, and hypotension. The third phase lasts 1-5 days, with death occurring by toxic shock or cardiac arrest. Sometimes spontaneous healing can occur, being preceded by a drop in body temperature and improvement of the general condition.

The atypical form has a sporadic incidence and is characterized by overacute or acute evolution, usually with a fatal end, often described as a very rapid form with a sudden onset. Clinical signs include hyperthermia, cyanotic or haemorrhagic mucous membranes, intense dyspnoea, and death within 10-12 hours of the onset of the evolution. A nervous form, which begins with excitation, evolves with generalized hyperesthesia followed by cortical depression ("immobility syndrome"), with death occurring within 1-3 days of the onset of clinical signs.

Anatomopathological changes in Stahibotriotoxicosis

Anatomopathological changes consist of perioral dermatosis, stomatitis, perioral ulcerations, haemorrhages (especially digestive), pulmonary and renal congestion, hepatic dystrophy,