Special Features Associated with use of Pesticides

Some of the special features associated with the use of pesticides and the problems which arise as a result of their unrestricted use include:

(1) Synergistic Action:

Synergism is the cumulative action of two or more than two foreign chemicals introduced simultaneously or consecutively into a biological system. Even small doses which have a nominal effect, or none at all, may cause acute toxicity if another foreign chemical is present in the system – the toxic response being often stronger than the cumulative action of the toxicants.

Malathion which is usually considered to be safest of all organophosphate insecticide (oral LD50 in male rats being 1375 mg per kg) in presence of small amounts of EPN, (ethyl-p- nitrophenyl phenylphosphonothionate) produces strong toxic reaction in vertebrates. It has been shown that malathion is detoxified by carboxyl-esterases and amidases which are inhibited by EPN (Frawley el al 1957). Similar reaction is obtained if malathion samples are contaminated with phenthoate another organophosphate insecticide (Baker et al 1978).

(2) Antagonistic Action:

Antagonistic action of two or more than two chemicals introduced simultaneously or one after the other results in production of a milder toxic response than would normally be expected. The toxicant nullify the effect of each other. Several organochlorine insecticides are known to exercise a protective action against the acute toxicity of organophosphate insecticides (Du Bois 1959, Murphy 1969). Malathion administered with DDT results in quick disposal of the insecticide mixture.

Accumulation of DDT in adipose tissues is considerably reduced when it is fed with Dieldrin (Street et al 1969). It is suggested that presence of some other foreign chemical induces the activity of bio-transformation enzymes which cause quick metabolism and excretion of other toxicants and hence the reduced toxicity.

(3) Inter-Action with Clinically Administered Drugs:

The capacity of some pesticides, for example organochlorine derivatives, to induce the activity of liver microsomal cytochrome P- 450 enzyme system may cause rapid inactivation or elimination of many drugs administered for benefit of the organism. Persons who are frequently exposed to DDT, drugs like hydroxycortison and phenylbutazone have little effect. Similarly acute toxicities may be caused by many clinically administered drugs if some bio-accumulated pesticide is already present in the system which causes the inactivation of the mechanism of bio-degradation (Hodgson et al 1980).

(4) Effect of Pesticide Exposures on Fertility:

There has been an increased concern about the effect of pesticides on fertility. Post natal exposures to low doses of many pesticides have been shown to depress fertility in mouse, rats, dogs, bull and even human beings (Mc Lachlan et al 1981). If the reproductive system is susceptible to such small doses at which no obvious toxic response occurs it may have a subtle effect on reproductive capacity of organisms. This in turn could adversely affect population densities and thereby the biotic spectrum present in the habitat concerned.

(5) Immunotoxicity of Pesticide and Allied Chemicals:

The effect of various pesticides on immune system has become on area of great concern. A number of pesticides which include atrazine, captan, dinitro-ortho-cresol, hexachlorobenzene, 2-4-5-T have been shown to have marked effect on immune system of laboratory rats (Vas et al 1983). About forty different pesticides have been cited to influence the host defence mechanism by Street (1981).

With an impaired immune system, the organism becomes susceptible to any type of infection howsoever mild arid symptoms like AIDS syndrome could follow. The mechanism of this depression is not clear nor the degree or extent of damage done is well documented. The fact that many pesticides affect the defence mechanism of such animals which show a remarkable degree of resemblance to human system should be enough to caution us and trigger further studies.

(6) Mutagenic, Carcinogenic and Teratogenic Action:

A number of pesticides like DDT, Aldrin, heptachlor, Hexachlorobenzene, Toxaphene, Captan, Mirex etc. all of which are in common use these days have been shown to induce cancerous growth and tumours in mammalian systems (IARC 1983). Likewise Carbaryl, Captan, Folpet, Difolatan, Organo-mercury compounds, 2-4-5-T, Pentachloronitrobenzene and Paraquat has been demonstrated to cause teratogenic effects in experimental animals (Durham and Williums 1972). Similarly many pesticides have been shown to cause mutations in a series of tests by Epstein and coworkers (1972).

Mutagenic, carcinogenic and teratogenic action of foreign chemicals in biological systems stem from the interference which they cause in function, synthesis and structure of DNA and RNA molecules. Most of these observations conducted on test animals in laboratory do suggest that exposure to these chemicals may bring about irreversible carcinogenic, embryo-toxic and even mutagenic affects in higher animals including man.