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After reading this article you will learn about the metabolism of organophosphate esters in animals.
Most of the phosphate insecticides are decomposed and excreted rapidly by mammals. Water soluble excretion products are excreted mainly through urine while excretion with the feces is very less.
In mammal body toxically active cholinesterase (ChE) inhibitors are formed by oxidation to the 0-analog of thiophosphates, di-thiophosphates and di-methyl-phosphate amides. Hence the toxicity depends upon the accumulation or degradation rate of 0-analog in the tissue. It has been found that maximum degradation occurs in liver, therefore, it contains 30 to 50 times more residue than muscle tissue.
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The quantity of substance and/or its metabolites in tissue, however, cannot be indicated by the degree of ChE inhibitors. Some degradation products are formed by hydrolysis, which generally go very fast. These products do not exhibit specific enzyme inhibition.
Hydrolysis attacks the bond between the phosphorus atom and the acid or alkyl group. With the cleavage of the alkyl bond little or no toxic hydrogen phosphates are formed and are excreted through urine within a few hours.
The organochlorine pesticides derivatives have tendency to accumulate in the body specially fatty tissue, therefore, its use as a ectoparasite control in domestic animals is prohibited while the organophosphate esters leave no residues, or in negligible amount which are excreted from the animal body. On the basis of toxicity organophosphate esters can be divided in three groups.
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1. Highly toxic:
LD50 < 50 mg/
2. Moderately toxic:
LD50 – 500 mg/
3. Slightly toxic:
LD50 > 500 mg/
The quantity of effective substance which comes to the human being as a consumer of meat from chemical exposed meat animals is only a fraction of total dosage applied to the animal. There is no binding regulation for the judgement of meat and meat products containing organophosphate esters for the consumer’s safety. There are no fixed tolerance value or waiting times before slaughtering or medically aimed application of organophosphate esters to the animals.
B. Halogenated hydrocarbons (DDT, Aldrin, Dieldrin etc.):
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Halogenated hydrocarbons are of great, importance as agricultural chemicals. Their only use of any significance is as pesticides. Health problem arises because these are easily inhaled due to its low vapor pressure. When they are applied as a dust or a liquid spray the finer particles disperse in the air and are readily breathed.
In lungs they may be absorbed. The coarser particles are trapped in the upper respiratory tract and swallowed and shows toxic effect. The chronic absorption in man and animals occurs as residue in food and feed sprayed with these pesticides and are deposited in the fatty tissues of the body.
DDT:
DDT can be absorbed through skin or can enter body through contaminated feed, food and water. Lactating animals may excrete 2-3 ppm in milk, it may increase to 65 ppm m butter made from it.
It is a very widely used insecticides in agricultural sector and as a control of mosquito in domestic use. It has not been a serious hazard as spray and dusting in agricultural use and can be avoided by reasonable cleanliness. Man and animals are intoxicated by inhalation during spraying and by consuming food and feed sprayed/ dusted with this pesticide.
The chief effect of DDT is on the nervous system. Common manifestations are apprehension, excitement, tremors and finally convulsion followed by paralysis and death. The acute exposure usually does not occur.
The long term chronic exposure are most common even at fairly low concentration noticeable as tremors. Some changes in liver and very little changes in kidney may be seen. Animal experiments have shown that 2.5 to 5 mg/hr/day for several months produce mild illness.
Aldrin:
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Aldrin is widely used insecticide for agricultural purpose. Aldrin can be absorbed when it is breathed in as a dust. Therefore, it is advisable to take proper protection during dusting. The main toxic effect of aldrin is seen on central nervous system. Hyperirritability, convulsion, and /or coma are observed. In acute poisoning nausea and vomiting are typical. The chronic intoxication led to anorexia, loss of weight, headache and nervousness.
The threshold limit of aldrin was established by the American Conference of Governmental Industrial Hygienist, in April 1959,at 0.25 mg/Cu meter of air. The residue tolerances of specified agricultural commodities are 0, 0.1, 0.25 and 0.75 ppm by weight. Dieldrin is a epoxide of aldrin. Aldrin is converted to dieldrin in the body. The toxic effect of dieldrin is similar to that of aldrin but it is slightly more toxic than aldrin.
Carbamates:
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It is widely used insecticide for agriculture as well as domestic purpose. It is a reversible inhibitor of cholinesterase. Food, vegetables and food animals sprayed with this insecticide or food accidently contaminated causes poisoning.
The symptoms like epigastric pain, vomiting, abnormal salivation, sweating, contraction of pupils and muscular incoordination occurs with in 1/2 hour after consumption of contaminated food. Their symptoms resemble those produced by organio-phosphorus compounds. Atropine is the antidote.
Carbamates (for example carbaryl) are immunosuppressive, cause reduction in antibodies titres, gammoglobulin contents, counts of lymphocytes and monocyte, cell mediated immunity, weight of bursa fabricious spleen, depletion of lymphoid cells in spleen, thymus and bursa fabricious. Thus chronic toxicity in human beings and animals increase their susceptibility to infections.
Pyrethroides are usually not accumulated in environment and are less toxic. However, these (for example Cypermethrine) are immunosuppressive and cause chromosomal aberration, leucopenia and decreased phagocytosis. The susceptibility to infections and even neoplasm in man and animals is increased due to chronic exposure.
