PRINCIPLES OF TOXICOLOGY - Biology East Borneo

354 PROPERTIES AND EFFECTS OF PESTICIDESthe insecticidal component of the extract. The pyrethrins jasmolins I and II, cinerins I and II, andpyrethrins I and II are extracted from the powder for formulation into commercial aerosols and sprayproducts. These compounds are often formulated with a synergist such as piperonylbutoxide or n-octylbicycloheptene dicarboximide. These synergists are incorporated in order to slow down the degradationof the pyrethrin compounds. This class of compounds are commonly used in household insecticidesand in pet products (e.g., flea and tick dips and sprays).Pyrethrins and pyrethrum are very rapidly metabolized and excreted from humans and have verylow mammalian toxicity. Crude pyrethrums have been associated with allergic responses in individuals,although this action is most likely due to the noninsecticidal components of this compound. A studyof 59 workers who had been employed in a pyrethrum factory ranging from 1 to 25 years showedessentially no adverse health effects, with the exception of inflammatory pleural lesions in someindividuals exposed to high air levels of pyrethrums. Treatment of pyrethrin and pyrethrum exposureis primarily symptomatic.Synthetic PyrethroidsPyrethrins and pyrethrum insecticides are unstable in light and heat. Because of this instability,synthetic pyrethroids, which have better stability to light and heat, have been developed and are usedin agricultural settings as well as for home pest control. Over 1000 synthetic pyrethroids have beendeveloped over the years and include compounds such as cyfluthrin, cypermethrin (Cymbush),deltamethrin, fenpropathrin (Danitol), fluvalinate (Mavrik), permethrin (Ambush), resmethrin(Chryson), and tralomethrin (Scout) (Figure 15.3).The action sites of the pyrethroids are the voltage-dependent sodium channels in nerves. The generalbasis for nerve impulse generation and conduction is the ionic permeability of the membrane combinedwith the sodium (high levels outside the cell) and potassium (high levels inside in the cell) concentrationgradients. The resting cell membrane is maintained by the sodium–potassium pump, and the inside ofthe cell is negatively charged with respect to the outside of the cell. A normal nerve impulse is causedby a quick transient increase in the permeability of the membrane to sodium ions, causing an inwardClOCNCCHCOCHClH 3 CCH 3OCypermethrinCH 2OClOCOCCHClCH 3CH 3PermethrinFigure 15.3 Synthetic pyrethroids.