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PakAgriFarming

Thursday, September 05, 2013

Organophosphates - their History, Mode of Action and Toxicity

Posted by Unknown On Thursday, September 05, 2013

Organophosphates also known as the OPs. The word organophosphate is derived from the combination of two words; ‘organic compounds’ and ‘phosphate group’. There is no known clinical use of these chemicals. However, the main uses are as agricultural insecticides and fungicides; used in household and garden sprays for flies and other insects.
Organophosphates (Cover)
Organophosphates (Cover)

Usage

Organophosphate insecticides are commonly used for small animals as flea and tick powders, sprays, foggers, shampoos and dips, and formerly, as systemic insecticides. They are also frequently used as household, garden, and farm insecticides. OPs, in general, have short term persistence and limited residual activity. They are more toxic to vertebrates than other insects. OPs have replaced the banned organochlorines (OC) and are a major cause of animal poisoning.

History

OPs were first synthesized in the 1850’s but the modern products were developed in Germany in the 1930’s. Their insecticidal qualities were first observed in Germany during World War II in the study of the extremely toxic OP nerve gases sarin, soman, and tabun. These were poisonous to mammals, birds, reptiles and fish. OPs are the components in chemical weapons called Nerve Gases.
Organophosphates have less residual effect than Organochlorines and their break down in the environment is faster. Organophosphates damage the nervous system and are highly toxic at small doses.
Structure of Organophosphates
Structure of Organophosphates

MODE OF TOXICITY

The common mode of toxicity is ‘acute toxicity’ through skin, inhalation and ingestion. OPs affect the acetylcholine receptors in the nerve endings.

Normal Transmission of acetylcholine (ACh)

ACh is released in the axon terminal and crosses the synaptic cleft. ACh binds to a receptor in the post-synaptic membrane. Acetylcholinesterase (AChE) stops the action of ACh.

Transmission with Nerve Agent (NA)

ACh is released from the axon terminal and crosses the synaptic cleft. ACh then binds with a receptor on the post-synaptic membrane. Nerve agents (NA) blocks the ability of AChE to stop the action of ACh; as a consequence, ACh continues to work and more ACh builds up in the synapse.

Symptoms of Poisoning

Neuromuscular Effects
Autonomic Nervous
System Effects
Central Nervous
System Effects
Twitching
Weakness
Paralysis
Respiratory failure
Reduced Vision
Sweating
Diarrhea
Nausea
Abdominal pain
Vomiting
Headache
Coma
Respiratory arrest
Confusion
Depression
Respiratory

Derivatives of OPs

1. Aliphatic derivatives
2. Phenyl derivatives
3. Heterocyclic derivative

1. ALIPHATIC DERIVATIVES

Aliphatic are simple phosphoric acid derivatives bearing short carbon chains. Some of these are malathion, trichlorfon, dicrotophos, oxydemetonmethyl, disulfoton, dichlorvos, methamidophos and acephate.

2. PHENYL DERIVATIVES

Phenyl (or benzene) ring with one of the ring’s hydrogen displaced by attachment to the phosphorus moiety and other hydrogen atoms frequently displaced by -Cl, -NO2, -CH3, -CN, or S. These are generally more stable than the aliphatic OPs so their residues last longer. Examples include methyl parathion and profenofos (Curacron®).

3. HETEROCYCLIC DERIVATIVES

These are also ring structures but differ in having one or more carbon atoms displaced by O, N, or S. These are complex molecules and generally have much long lasting residues than many of the aliphatic or phenyl OPs. They also have many breakdown products which makes it difficult to measure their residues in the laboratory. Examples  include diazinon, azinphosmethyl and methidathion.

ADVANTAGES AND DISADVANTAGES

Advantages

1. Used as insecticides, fungicides and herbicides.
2. Broad spectrum of activity against a number of pests.
3. Poisons stomach and has contact action.
4. Many also possess trans-laminar or systemic action.
5. Biodegradable and converted to non-toxic metabolites.
6. Less possibility of pollution.
7. Poses low chronic toxicity.
8. Very economical and used in smaller dosages.

Disadvantages

1. Some pose bad order. For example, Malathion.
2. Some pose high acute mammalian toxicity.
3. Require special training for application, for example phorate and disulfotan.

TOXICITY TO HUMANS

1. Respiratory difficulty
2. Muscle Weakness
3. Hypertension

TOXICITY TO ANIMALS

1. Restlessness
2. Hyperexicitability
3. Convulsion
4. Paralysis
5. Death

BIOMAGNIFICATION

Organophosphates break down quickly in the environment and their residues on crops are less. They are also not stored in animal tissue, so biomagnification has not been a problem for either of these reasons, their use has greatly reduced the hazard to non-target species.