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VetBooks.ir Chapter 39

Pyrethrins and Pyrethroids

Steve M. Ensley

INTRODUCTION to the 3-phenoxybenzyl alcohol group in the second-
generation pyrethroids has increased the insecticidal
Pyrethrins are the insecticidal compounds obtained from
potency and stability of these compounds.
the flowers of the plant Tanacetum cinerariaefolium, also
Pyrethrins cause hyperexcitability with very little
called Chrysanthemum cinerariaefolium or Pyrethrum
cytotoxicity. The molecular targets of the pyrethrins
cinerariaefolium. Pyrethrum comes from extracts from the
and pyrethroids are similar in mammals and insects and
flowers that contain the active pyrethrin compounds
include voltage-gated sodium, chloride and calcium
(Proudfoot, 2005). The use of pyrethrum in insecticide
channels, GABA-gated chloride channels, nicotinic
preparations dates back to Persia, about 400 BC.
receptors, membrane depolarization and intercellular
Pyrethroids are synthetic analogs of pyrethrins. Because of
gap junctions (Forshaw and Ray, 1990; Song and
stability problems with the natural pyrethrins, these insec-
Narahashi, 1996a). Mammals are less susceptible to
ticides were replaced by the more stable organophosphate
pyrethrin and pyrethroid toxicoses than insects primarily
and organochlorine insecticides developed after World
because they have a faster metabolic clearance, higher
War II (Valentine, 1990). As a result of the toxicity and
body temperatures and a lower affinity for the pyrethrins/
environmental contamination associated with the organo-
pyrethroids (Song and Narahashi, 1996b; Gammon et al.,
phosphate and organochlorine insecticides, interest in the
2012).
use of pyrethrins and pyrethroids reemerged in the 1970s.
Pyrethrin and pyrethroid insecticides are effective against
a variety of insect pests on companion animals and live- PHARMACOKINETICS/TOXICOKINETICS
stock, and are used on farms, in the home and garden and
Determination of the toxicity of pyrethroids in vivo is dif-
have many public health applications because of the safety
ficult because they have low water solubility, easily parti-
associated with these compounds. The pyrethroids are con-
tion into lipids and will bind to plastics and glass. The
siderably safer than the organochlorines. The neonicoti-
reported toxicity of this class of insecticides has high
noids were developed to replace the pyrethrins.
variability.
Most pyrethrin and pyrethroid products are applied
BACKGROUND dermally in animals, but because of grooming, there can
be oral and inhalation exposures, too (Anado ´n et al.,
There are six compounds that comprise the natural pyre- 2013). Less than 2% of topically applied pyrethrin and
thrins: pyrethrin I and II, jasmolin I and II, and cinerin I pyrethroid insecticides are absorbed dermally (Wollen
and II. Synthetic pyrethroids have been developed et al., 1992). One study confirmed that absorption of
because the natural pyrethrins tend to break down quickly cypermethrin across human skin is minimal and peak
when exposed to air, light and heat. The synthetic pyre- excretion rates after dermal application were not
throids can be classified as first and second generation. observed until 12 36 h after dosing. Pyrethrins may be
First-generation pyrethroids are esters of chrysanthemic sequesteredinthe skinandslowlyreleased intothe
acid and an alcohol, having a furan ring and terminal side systemic circulation (He et al., 1989). Oral or inhalation
chain moieties. Second-generation pyrethrins have 3- exposure results in faster systemic exposure (Anadon
phenoxybenzyl alcohols derivatives in the alcohol moiety et al., 1996, 2013).
and have had some of the terminal side chain moieties Approximately 40% 60% of an orally ingested dose
replaced with a dichlorovinyl or dibromovinyl substitute is absorbed. When cypermethrin was administered orally
and aromatic rings. The addition of the alpha-cyano group to six adult male volunteers, oral absorption ranged from

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