Pesticides: the necessary evil [Archives:2006/942/Health]

May 1 2006

Dr. Mohammed Alhifi
One of the consequences of technological progress and industrial revolution has been the release of a large number of chemicals into environment. Although, considerable research efforts have been made into a variety of alternative pest control methods in recent years, use of chemical pesticides is still the mainstay in modern agriculture and public health programmers. At the same time anxiety over chemical pollution has made the study of hazardous effects of pesticides as one of the principle areas of research.

In particular organochlorine insecticides have been extensively investigated and today several of them have been suspended from general application in the USA and other countries. Nevertheless, it's felt for the time being that pesticides are necessary evil and in absence of any feasible substitute, their use will continue to combat the vast range of vectors of diseases. With the slow development of civilization, so man has gradually realized the extent to which pests harm his crops, annoy him and transmit diseases to both human and domestic animals.

The use of chemical to kill pests is not a new concept. As early as the late 16th century arsenic could be used to kill insects and the Chinese used arsenic sulphide as an insecticide. The use of arsenical compounds has continued, and during the early part of the 20th century, large quantities of such compounds as lead arsenate were used to control insect pests. Another arsenical compound Paris green (Copper Aceto-arsenite) was extensively applied to pool the standing water in the tropics, in attempts to control malaria-transmitting mosquitoes. It was not realized at the time how persistent arsenical pesticides were, although it is now known that they can persist in soil for 40 years, and many orchard soils still contain large amount of these chemicals. For instance, in a recent survey of arsenic residues in arable soils in Canada, residues of arsenic ranging from 11-121 PPM were reported. Although it was known that organochlorine were very persistent, up till the early 1950s there was little anxiety as to possible long-term ecological hazards caused by their use. There was some evidence that large residues in soil could be pytotoxic, small quantities of some were reported from plant and animal tissues and in cows' milk and there were some instances of fish being killed when water was sprayed in antimalaria and other pest campaigns, but unavoidable hazards and of little concern.

As pesticides pass in most cases directly or indirectly into the foodstuffs. For many pesticides legal limits (tolerance doses) are imposed regarding the residues that may be left in the food stuffs. By tolerance dose is meant the quantity of a substance that may be absorbed by one person from his daily diet in the course of a lifetime without coming to any harm as far as can be judged from present scientific knowledge. The level at which a pesticide dose not make damage to the biological system is expressed as milligrams per kilogram of body weight. In ordinary circumstances, one hundredth part of this dose is then prescribed as the safe limit for human being.

Effect of pesticides on embryonic development

Some of the pesticides used today in the modern farming, are very poisonous. Even if used in low concentrations of pesticides, but their accumulation can reach a level where health is affected. The fetus also can not escape pesticides, a team of American and Canadian Scientists found pesticides and industrial chemicals in the amniotic fluid of fetus. Some pesticides have the potential to cause birth defects if the exposure to a fetus occurs at a critical time in pregnancy. In the recent studies we have carried out a study to test the effect of insecticide called as Dimethoate (commercially marketed in the brand name of Perfekthion) on the embryo development during the early stages of organogenesis using chick embryo as a model in our experiments. This insecticide is classified by the WHO in the class II (organophosphate), and it is used dramatically in the growing of Qat and during the cultivation of fruits and vegetables. So lots of people including pregnant women are getting exposed to Dimethoate through Qat. Results indicated that Dimethoate has potential to induce serious abnormalities during the embryo organogenesis as follow:

1-Affecting the brain shape and size (undefined brain and microcephaly).

2-Opening and wavy shaped neural tube.

3- Diffusing of the somites and

4-Heart miss-position or sometimes missing of the heart.

Abnormalities such as those mentioned above are serious since these are the beginners of the organs. For e.g. somites will contribute to the development of the vertebrae and some parts of the skull. In addition the nephrotomal band (a part of the somites) will contribute to the transformation of the kidney. So any disturbance during this stage that somites might be affected, will alarm with gross abnormalities that may be induced such as missing some parts of the skull, neck scholiosis (carved neck), missing of the eye (exophthalmia) or some parts of the limbs (phocomelia). In the same study it is also proved that Dimethoate inhibits the acetylecholinestrase (the enzyme which is playing important role in transmitting nerve impulse) by 40% of its activity at normal situation. Deformities in brain development suggest the defective cell proliferation. Inhibition of acetylcholine esterase (AChE) suggests disruption of nerve function during the embryo development. Inhibition of AChE might be due to the linkage of Dimethoate with Cholinestrase active group. This inhibition suggests a possible damage to the Central Nervous System (CNS) by Dimethoate during the embryo development. As has been reported the accumulation of acetyle choline in the heart and brain induce the bradycardia and heart arrest. This may explain the sudden death of some youth while chewing the Qat sprayed with insecticides (such as Dimethoate), while they were enjoying a healthy life before. In the sight of the present researches, the concern governmental institutions should implement laws to control entrance of pesticides to the country and spreading of the awareness should be made amongst farmers and consumers in general. The agriculture products specially fruits and vegetables should be monitored for examination the level of pesticides the people are getting exposed to.

Dr. Mohammed Alhifi holds a Ph.D. in Environmental Toxicology. He is a lecturer at Dept. of Biology, Faculty of Education in Arhab, University of Sana'a.