Pyrethrins Permethrin

Type I pyrethroids include pyrethrins, permethrin, resmethrin, tetramethrin, allethrin, bifenthrin, and metofluthrin. The structures of these type I pyrethroids are as follows ... 

The debilitating Chagas disease of humans, transmitted by a triatomid bug Rhodwus proiixus endemic throughout much of South America, can be controlled by premise sprays of pyrethrins, permethrin, and PBO. New formulations with synergized deltamethrin show considerable promise in the battle to eradicate (his disease. 

Permethrin, a pyrethrin pesticide, decreased the inhibition of brain cholinesterase activity by methyl parathion, but methyl parathion decreased the LD50 of permethrin when the two pesticides were simultaneously administered to rats (Ortiz et al. 1995). The potentiation of permethrin lethality may be due to the inhibition by methyl parathion of carboxylesterase, which metabolizes permethrin. 

The structures of some pyrethroid insecticides are shown in Figure 12.1. They are all lipophilic esters showing some structural resemblance to the natural pyrethrins. They can all exist in a number of different enantiomeric forms. Permethrin, cypermethrin, and deltamethrin, for example, all have three asymmetric carbon atoms... 

These synthetic pyrethroids mimic natural counterparts, of which the most important is pyrethrin 1 (10.265). Unfortunately, the natural products lack the photochemical and hydrolytic stability necessary for use as wool insect-resist agents. The synthetic products have the required stability, yet retain the low mammalian toxicity and low environmental retention of the natural products. Permethrin, however, is toxic to aquatic life and is therefore subject to increasingly severe discharge limits. There is some evidence that permethrin is less effective against larvae of a certain beetle. This can be compensated for by using a combination of permethrin with the hexahydropyrimidine derivative 10.264. Some possible alternative pyrethroids have been mentioned [517] as development products (10.266-10.269). 

The R/S ratios for permethrin and phenothrin in Table 3 are 204 and 283, respectively. The Hiroyama strain showed high cross-resistance to permethrin and phenothrin, whereas the R/S ratio for natural pyrethrins was only 7. Moreover, the R/S ratio for racemic allethrin was 97, indicating the slow development of resistance compared with permethrin and phenothrin. These results agree well with the findings of Sawicki et al. [49], who reported that pyrethroids with the cyclopentenolone ring showed only a slight resistance at the International Congress of Pesticide Chemistry in Ottawa in 1986. 

In the example of M. domestica shown in Table 3, the development of resistance to permethrin (21) and phenothrin (16) was 204 and 283 times, respectively, but the resistance to natural pyrethrins was only 7 times and that to d-allethrin was very low, 33 times. Moreover, the development of resistance to natural pyrethrins and prallethrin (ETOC ) was found to be low in A. aegypti as described previously. 

Bioallethrin is a synthetic pyrethrin insecticide. Piperonylbutoxide, a weak insecticide itself, has synergistic activity. The same holds true for the combination of pyrethrum extract with piperonylbutoxide. These combinations have the same efficacy as permethrin. Local irritation occurs frequently effect and contact with mucous membranes and the eyes must be avoided. 

Pyrethrin and pyrethroid insecticides Pyrethrin I, IP, fenvalerate, permethrin... 

Pyrethrins are naturally occurring pesticides derived from chrysanthemum plants. They are active against many insects and mites. Over-the-counter liquid and gel preparations of pyrethrins with piperonyl butoxide are available for the treatment of pediculosis (piperonyl butoxide inhibits the hydrolytic enzymes that metabolize the pyrethrins in the arthropod). A synthetic pyre-throid, permethrin (Elimite), is available by prescription. A lower concentration of permethrin (Nix) is available without prescription. Pyrethrins and permethrin are quite safe. 

It is recommended that permethrin 1% cream rinse (Nix) be applied undiluted to affected areas of pediculosis for 10 minutes and then rinsed off with warm water. For the treatment of scabies, a single application of 5% cream (Elimite) is applied to the body from the neck down, left on for 8-14 hours, and then washed off. Adverse reactions to permethrin include transient burning, stinging, and pruritus. Cross-sensitization to pyrethrins or chrysanthemums has been alleged but inadequately documented. 

Pediculicides (head lice) Permethrin 1% Nix Instructions for use vary consult product labeling. Avoid contact with eyes. Comb out nits. Linens, pajamas, combs, and brushes should be washed daily until the infestation is eliminated. For pyrethrin products, retreat in 7-10 days to kill any newly hatched nits. Permethrin products have residual effects for up to 10 days therefore, reapplication is not required unless live nits are visible 7 days or more after the initial treatment. 

Figure 15.5 Structures of the natural pyrethrins and the synthetic analogs permethrin and deltamethrin. Data from [136, 141].

Aircraft disinection Pyrethrins, resmethrin, D-phenothrin, permethrin 13... 

Some pyrethroids have as many as eight different isomers, and there are several different types. Acute toxicity of a mixture of two isomers depends on the ratio of the amounts of isomers in the formulation. For example, the female rat s acute oral LD50 of permethrin increases from 224 mg/kg to 6,000 mg/kg as the proportion of transisomer increases from 20% to 80%. The route of exposure also is critical in assessing the acute toxicity of a synthetic pyrethroid. Like DDT and many other registered insecticides, naturally occurring pyrethrins and the synthetic pyrethroids are nerve poisons. 

Representative Chemicals Pyrethrin I Cyperme-thrin Deltamethrin Permethrin... 

Chemical Abstracts Service Registry Numbers CAS 121-21-1 (Pyrethrin I) CAS 52315-07-8 (Cypermethrin) CAS 52918-63-5 (Deltamethrin) CAS 52645-53-1 (Permethrin)... 

Chemical Formulas Pyrethrin I, C21H28O3 Cypermethrin, C22H19O3NCI2 Deltamethrin, C22Hi9Br2N03 Permethrin, C21H20O3CI2... 

The acceptable daily intakes set by the Joint Meeting on Pesticide Residues (JMPR) for cypermethrin, deltamethrin, and permethrin are 0-0.01 mg kg body weight, with acute oral reference doses for deltamethrin or permethrin of 0.05 mg kg bw. The National Institute for Occupational Safety and Health maximum allowable concentration (MAC) for pyrethrins at an 8 h time-weighted average is 5 mg m ... 

Pyrethrins and synthetic pyrethroids are among the safest of the topically applied ectoparasiticides, because of their selective toxicity for insects (mam-malian-to-insect toxic dose ratio is greater than 1000, compared with 33 for organophosphates and 16 for carbamate insecticides). In contrast to the very wide margin of safety for mammalian species, pyrethroids are toxic to fish. The synergistic action of pyrethrins and piperonyl butoxide (in combination preparations) is due to the inhibition by piperonyl butoxide of the microsomal enzyme system of some arthropods. Preparations of synthetic pyrethroids (permethrin, cypermethrin) often contain a mixture of drug isomers in varying proportions. 

Synthetic pyrethroids (cypermethrin, permethrin) and natural pyrethrins (pyrethrum extract) combined with piperonyl butoxide constitute the most important class of insecticide applied topically to horses (Table 5.7). The liquid concentrate containing cypermethrin which, after dilution, is applied to horses by spray contains a mixture of the isomers in equal proportion (cis trans 50 50). 

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