By Dr. Mohd. Isa Abdul Majid
The Sun, March 15, 1997
INSECTICIDES ARE MEANT TO KILL insects, especially those threatening the growth of vegetables and primary comodity plants. There are many wide-spectrum or all purpose products to eliminate insects. Most of these are made from synthetic chemical compounds which are convenient and effective.
However, farmers came to recognise that such panaceas carry certain penalties - they poison more than the insects they are designed to attack.
There are six types of insecticides - kerosene or borax, botanicals or natural organic compounds (nicotine, pyrethrin and rotenone), chorinated hydrocarbon (DDT, lindane, chlordane), organophosphates (malathion and diazinon), carbamates (carbaryl and propoxur), fumigants (naphthalene) and benzene (mothballs).
Newer insecticides include insect growth regulators and natural predators such as beetle-eating wasps or insect-killing bacteria. While all insecticides are toxic, most fatal poisonings occur only when quantities are accidentally or intentionally ingested or inhaled.
For the purpose of creating awareness about the potential of poisonings by insecticides, the focus groups of chemicals for this issue are natural pyrethrins and synthetic pyrethroids which are widely available in our homes in the form of mosquito aerosol, mosquito vapour mats and mosquito coils.
Originally, natural pyrethrine, which are constituents of flower extracts of Pyrethrum cinerariae folium and other species of flowers, were used to repel insects. It is believed that the natural pyrethroids were discovered by the Chinese in the first century. The first writen accounts of these agents are found in 17th century literature. Commercial preparations made their appearance in the mid-1800s. Japanese woodblock prints from the early 1800s indicated that smouldering insecticide coils of pressed pyrethrum powder had been used even though these are different from those manufactured and used today.
The chrysanthemum varieties grown in Kenya yield highest proportions of active ingredients but the plants are grown commercially many regions of the world's. In 1965, the world's output of pyrethrum was approximately 20,000 tonnes, with Kenya alone producing some 10,000 tonnes.
The rising increase in the use of this preparation lies in the fact that it has a rapid knockdown or paralytic action on flying insects, appearing to be very potent although having a very low toxicity in both insects and mammals due to efficient enzymatic degradation.
The demand for this product has far exceeded the limited world production, leading chemist to focus attention on the synthesis of similar analogues, hope-fully with better stability in light and air, persistence and more selectivity in target species as well as further reduction in mammalian toxicity.
Through chemical isolation and identification, the major active principles in pyrenthrum are pyrethrin I, esters of chrysanthemic acid (pyrethrin I, cinerin I and jasmolin I) and pyrethrin II, which are ester of pyrethric acid (pyrethric II, cinerin II and jasmolin II).
Pyrethrin I is the most active ingredient for lethality whereas pyrethrin II possesses remarkable knockdown properties for a wide range of of household, veterinary and post-harvest storage insects. The natural pyrethrins and early synthetic chrysanthemic acid derivaties are more active as contact than stomach poison, where as the more recent synthetic agents show particular potency when ingested and are less susceptible to biotsndformation by insects and mammals.
As for the synthetic pyrethroids, more than 1,000 pyrethroids to date have been synthesised and made available for use in today's market. These synthetic compounds were also noted to possess greater insecticidal activity and lower mammalian toxicity than other insecticides such as the organochlorine, organophosphate and carbamate insecticides. In addition, due to their excellent cost/benefit ratio for pest control, synthetic pyrethroids at present commonds approximately 30% of the commercial market.
Synthetic pyrethroids are among the most active insecticides in terms of their efective concentrations. The low concentration required in their usage - 0.02% to 0.05% (with estimated application of five to 200g/ha) - make these compounds readily accepted in the agricultural sector. Even some of them (such as fenvalerate, fenpropathrin and cyhalothrin) are acaricides, an additional benefit being used as insecticides. These are used in fields, orchards and greenhouses against domestic insects and animal pests.
Pyrethroids are also active againts many pests resistant to other insecticides. Among their other attributes are the short harvesting period which ranges from zero to 14 days and their easy decomposition in the environment (between one and three weeks by ultraviolet radiation and temperature).
Based on the symtoms observed in test animals, two distinct signs of poisoning have been designated the Type I and Type II poisoning syndromes.
The Type I or "T syndrome" is characterised by restlessness, incoordination, prostration and paralysis in the cockroach. In the rat, such signs as sparring and aggressive behaviour, enhanced startle response, whole body tremor and prostration are seen.
The Type II or "CS syndrome" elicits intense hyperactivity, incoordination and convulsions in cockroaches whereas rats display burrowing behaviour, coarse tremors, clonic seizures, sinuous writing (choreoathesis) and profuse salivation without lacrimation, hence the term CS (choreoathetosis salivation) syndrome. Certain agents, fenprophantrin for example, cause a mixture of Type I and II effects, depending on the species treated and possibly on the route of administration.
Although these insecticides cannot be considered highly toxic to mammals, their use indoors, in enclosed and poorly ventilated areas has resulted in signs and symptoms of toxicity to humans. Exposure to pyrethrum is known to cause contact dermatitis on the skin, the descriptions of effects raging from localised erythema to a severe vesicular eruption. The elergenic nature of the natural product also includes asthma-like attacks and anaphylactic reactions with vascular collapse being observed in some cases.
A lot of scientific and clinical evidence at present has shown that human toxicity associated with natural pyrethrins is due to the allergenic properties rather than direct neurotoxicity as seen in insects or animals.
One notable form of toxicity associated with synthetic pyrethroids has been cutaneous paresthesia observed in workers spraying synthetic pyrethroids such as deltamethrin, cypermethrin and fenvalerate. The parasthesia which develops for several hours following exposure is described as a stinging or burning sensation on the skin, which in some cases, progresses to a tingling sensation and numbness. The effects could last from 12 to 18 hours.
Occupational exposure to these compounds have been reported and symptoms include dizziness plus a burning, itching or tingling sensation of the exposed skin, which is exacerbated by sweating and washing with warm water. The signs and symtoms disapear within 24 hours after exposure.
Spilling these agents on the head, face and eyes results in pain, lacrimation, photophobia, congestion and oedema of the conjunctiva and eyelids. Ingestion of pyrethroid esters causes epigastric pain, nausea and vomitting, headache, dizziness, anorexia, fatigue, tightness in the chest, blurred vision, paresthesia, palpitations, coarse muscular fasciculations in large muscles in extremities and disturbances, seizures and the loss of consciousness.
The frequency of these seizures is usually of the order of 10 to 30 times a day in the first week after exposure, gradually decreasing in incidence, with recovery within two to three weeks. The signs and sympton of acute intoxication appear to be reversible and no chronic toxicity has been reported to date.
The writer is a Pharmacist and head of the Toxicology Laboratory at the National Poison Centre, Universiti Sains Malaysia, Penang.
