TOXICOLOGY OF PESTICIDES

Pesticides are preparations for the eradication of plant and animal pests, for the protection of plants, animals and man.

About 800 compounds of active ingredients of pesticides have been registered world-wide.

Historical background:

• alkaloids nicotine and anabasine contained in tobacco – insecticides
• pyrethrines contained in plants of the genus Pyrethrum – insecticides

Pyrethrines later became the prototype for synthetic pyrethroids

• rotenon – is highly toxic to all forms of life, is from roots of lianas. Rotenon was used as a piscicide and insecticide
• an important milestone was introduction of phenyl mercury in 1913 for the protection of seed – fungicide
• insecticidal effect of DDT was discovered in 1939 by the Swiss Paul Müller

Classification of pesticides

• fungicides
• zoocides (insecticides, rodenticides, molluscocides)
• herbicides – including desiccants
• selective
• non-selective
• plant growth regulators (to shorten the straw of cereals)

Desiccation – drying

desiccant – drying agent

Desiccants are used for alfalfa (lucern) or clover for seed: plants dry up and can be harvested.

Pesticide degradation

• in abiotic environment

the most important factors are light, temperature, photolysis, free radicals produced in photochemical reactions, hydrolysis

• in biotic environment

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1. phase 2. phase

XH X – OH X – O – conjugate

The final products are inactive and are

excreted.

Pesticide transformation

• mostly detoxicative nature
• result may even be a more toxic substances (desulphuration of organophosphates)

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parathion paraoxon – a powerful ACHE inhibitor

trichlorfon dichlorvos

diazinon diazooxon

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DDT DDE (extremely persistent and

xenoestrogenic)

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Pesticides:

• Organochlorine pesticides
• Organophosphates
• Carbamate pesticides
• Pyrethroids
• Phenoxyacetic acid – based pesticides
• Urea – based pesticides
• Diazine and triazine pesticides
• Bipyridil – based pesticides
• Phenylpyrazoles
• Metal – based pesticides

Organophosphates�- insecticides�- antiparasitics

Mechanism of toxic action – irreversible

inhibition of enzymes, particularly of

acetylcholinesterase on nerve synapses (by

phosphorylation of hydroxyl group of serine

bound in the active centre of ACHE).

Carbamate pesticides�- insecticides�- herbicides�- fungicides

Mechanism of the toxic action – reversible inhibition of acetylcholinesterase (by carboxylation of hydroxyl group of serine bound in the active centre of ACHE).

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Carbofuran is very up-to-date substance in toxicology. It is used to control vermin (foxes) and is used in baits. Birds are 10 times more sensitive to carbofuran than mammals (LD50 for mammals 3 – 19 mg/kg body weight). Frequent carbofuran poisoning cases among predatory birds.

Pyrethroids�- insecticides�- antiparasitics

Mechanism of the toxic action -

• pyrethroids T (tremor) – contain no α-cyano group

cause reversible block of sodium channels (e.g. permethrin)

• pyrethroids CS (choreoatetosis, salivation) – contain α-cyano group

cause reversible block of sodium channels and inhibition of GABA (e.g. deltamethrin)

Pyrethroids are��- highly toxic for fish (LC₅₀ below 0,1 mg/l)�- toxic for bees (LD₅₀ 2 – 11 µg/bee)�- not very toxic for mammals��Cats are most sensitive mammals to pyrethroids.�Why?� - Pyrethroids’ detoxification, similarly to other organic toxicants, takes place in two phases. Activity of conjugation enzyme, especially of glucuronyl transferase, is very low in cats.

Phenoxyacetic acid – based pesticides (MCPA)�- herbicides

Mechanism of the toxic action –

disruption of oxidation and phosphorylation processes (drop in the ATP production and disruption of energy metabolism).

They are little toxic for mammals, fish, bees.

Symptoms of poisoning: hypotermia, hypodynamia,

paresis, paralysis, tympania in ruminants.

But: In the production and use of those herbicides (2,

4-D; 2,4,5-T) dioxin was produced.

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Urea – based pesticides �- herbicides

They cause damage of the thyroid gland and diuron may cause methaemoglobinemia.

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In mammals linuron reduces haematopoiesis

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In dogs triasulfuron causes cystic hyperplasia of the prostate, vacuolisation of liver cells, anaemia and accumulation of pigment in the liver

Diazine and triazine pesticides�- herbicides

Diazine pesticides are less toxic than triazine ones

Mechanism of the toxic action –

• triazines are antimetabolites of pirimidine bases - components of nucleic acids and folic acid
• atrazine damages the liver detoxication functions
• simazine, prometryne, terbutryne – inhibit haematopoiesis

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Toxicity: toxic for fish

relatively harmless for bees

LD₅₀ for mammals exceeds 1000 mg.kg^-1 live weight

Serious risk of triazine – based pesticides

1. very low biodegradability (risk for drinking water)
2. triazines are secondary amines (secondary amines + nitrosation agents

nitrosamines)

3. atrazine has xenoestrogenic effects (causes abnormal development of gonads, turns amphibians into hermaphrodites)

Bipyridil – based pesticides�- herbicides�- desiccant

They are very rapidly deactivated in soil, but leave

residues in plants: diquat for 3 – 5 days, paraquat for 21 days.

Diquat (Reglone)

LD₅₀ for cattle 30 – 50 mg.kg^-1 l.w.

for rabbit 280 mg.kg^-1 l.w.

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Symptoms of poisoning – pulmonary oedema, damage of liver and kidneys, arthritis, periarthritis

Paraquat (Gramoxone)�LD₅₀ for man 40 mg.kg^-1 l.w.� for cattle and pigs 30 – 70 mg.kg^-1 l.w.� for dogs and cats 25 – 50 mg.kg^-1 l.w.��Mechanism of toxic action – is mediated by� - free oxygen radicals� - proteolytic enzymes formed by active neutrophilic leucocytes��Symptoms of poisoning – pulmonary oedema, fibrotic pneumonia fetotoxicity,retardation of ossification

Metal – based pesticides

arsenic compounds – insecticides, rodenticides

phenylmercury – fungicide for treatment of seed (1913 – 1993)

tributyltin – fungicide (xenoestrogenic effect)

thalium compounds – rodenticides

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Today

Copper compounds – copper sulphate

- copper oxichloride

fungicides, algicides, molluscocides

Toxicity for fish – LC₅₀ 1 – 10 mg.l^-1 depending on water quality

Phenylpyrazoles �- insecticides�- antiparasitics

Mechanism of the toxic action – inhibition of GABA

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Fipronil is very toxic for bees (LD₅₀ is 5 ng/bee)

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It causes secondary toxicity in bees.

Its residua persists for 21 days.

It produces the „knock down“ effect.