The synthesized derivatives of pyrethrins which occur naturally, taken from pyrethrum, are known as synthetic pyrethroids. They are the ole-resin extracts of dried chrysanthemum flowers. The keto-alcoholic esters of pyrethroid acids and chrysanthemic acids are the ones responsible for the insecticidal properties of pyrethrins. The strong lipophilic nature of these acids aids them in rapidly penetrating the insect and paralyzing their nervous system.
Both pyrethrins and synthetic pyrethroids are sold as commercial pesticides for the control of pest in homes, agriculture, restaurants, schools, communities, and as well as for topical head lice treatment. There are various formulations of these pesticides which are a result of their combination with other chemicals, called synergists, which lead to an increase in potency and also their persistence in the environment.
Despite being toxicologically and chemically similar to pyrethroids, pyrethrins have an extreme sensitivity towards sunlight, moisture, and heat. Their half-life is of a mere number of hours in direct sunlight. On the other hand, synthetic pyrethroids were created to have the same toxicological effect of pyrethrins, while also having increased persistence in light and for yielding significantly longer residence times.
The health effects of pyrethroids
The effect of pyrethroids is that of an irritant having sensitizing properties. Their absorption through the skin is not easy, but they can be easily absorbed through the pulmonary membrane and the gut. Tests have been conducted on laboratory animals which have revealed their striking neurotoxicity when introduced orally or by injection. The possibility of toxicity resulting from dermal absorption or inhalation is low, which is attributed to the following reasons:
- Some of these have limited absorption
- Mammalian liver enzymes rapidly disintegrate pyrethroids by the processes of ester hydrolysis and oxidation. Insects which do not have the liver functionality are thus exponentially more prone to these chemicals.
Pyrethroids function through their interference with the ionic conductance of nerve membranes by extending the duration of the sodium current. This results in the stimulation of nerves leading them to discharge repeatedly and finally lead to hyperexcitability in poisoned animals. According to the WHO, synthetic pyrethroids are neuro-poisons which act on the axons in the central nervous system and the peripheral nervous system by their interaction with the sodium channels in insects and/or mammals.
The symptoms and signs of poisoning caused by pyrethroids can occur in many forms. Pyrethroids being similar to crude pyrethrum can also act as respiratory and dermal allergens. Exposure to these chemicals has resulted in the formation of asthma-like conditions and contact dermatitis. People, specifically children having a history of asthma or allergies are significantly more sensitive and their cross-reactivity with ragweed pollen has also been recognized. The most severe reactions to these chemicals include respiratory difficulty and also peripheral vascular collapse. The basic symptoms of toxicity caused by inhalation include sneezing, nausea, tremors, nasal stiffness, incoordination, convulsions, facial swelling/flushing, and burning and itching sensations. Infants are reported to suffer from the most extreme cases of toxicity as they are not able to efficiently break down pyrethroids. The oral ingestion of these chemicals leads to nervous symptoms such as excitation and convulsions leading to paralysis, which is accompanied by diarrhea and muscular fibrillation. Death in these cases is primarily due to the failure of the respiratory system.
Apart from the negative health effects of these chemicals, they have an extremely toxic impact on aquatic organisms, mosquitos, blackfly, among others. The non-lethal effects of pyrethroids on aquatic organisms such as fish include behavioural changes and damage to gills. Pyrethroid toxicity towards birds is moderate, but they are more indirectly affected by these chemicals as the predator-prey relationship balance is severely affected by pyrethroids.
As mentioned earlier, these have been designed to be more resilient than the naturally occurring pyrethrins which aid them in persisting in the environment for longer periods of time. In areas getting low amounts of sunlight, they can also last for months. The amalgamated use of pyrethroids with synergists is further aiding the cause of increasing the toxicity of pyrethroids and further increasing their persistence in the environment. However, the application comes with its associated costs which are not only related to the environment but also its impact on the entire food-chain as well as human beings.