CHAPTER 2-134: PYRETHRINS AND PYRETHROIDS

INTRODUCTION

Pyrethrins are naturally occurring insecticides derived from the chrysanthemum plant. Pyrethroids (Table II–54) are synthetically derived compounds. Acute human poisoning from exposure to these insecticides is rare; however, they can cause skin and upper airway irritation and hypersensitivity reactions. Piperonyl butoxide is added to these compounds to prolong their activity by inhibiting mixed oxidase enzymes in the liver that metabolize the pyrethrins. Common pyrethrin-containing pediculicides include A-200, Triple X, and RID.

MECHANISM OF TOXICITY

In insects, pyrethrins and pyrethroids rapidly cause death by paralyzing the nervous system through disruption of the membrane ion transport system in nerve axons, and pyrethroids prolong sodium influx and also may block inhibitory pathways. Mammals are generally able to metabolize these compounds rapidly and thereby render them harmless.

TOXIC DOSE

The toxic oral dose in mammals is greater than 100–1,000 mg/kg, and the potentially lethal acute oral dose is 10–100 g. Pyrethrins are not well absorbed across the skin or from the GI tract. They have been used for many years as oral anthelminthic agents with minimum adverse effects other than mild GI upset.

1. Deltamethrin. There is one report of seizures in a young woman who ingested 30 mL of 2.5% deltamethrin (750 mg). Miraculous Insecticide Chalk (illegally imported from China) contains up to 37.6 mg of deltamethrin per stick of chalk. Ingestion of a single stick of chalk is generally considered nontoxic.

2. Cypermethrin. A 45-year-old man died after ingesting beans cooked in 10% cypermethrin.

CLINICAL PRESENTATION

Toxicity to humans is associated primarily with hypersensitivity reactions and direct irritant effects rather than with any pharmacologic property.

1. Anaphylactic reactions including bronchospasm, oropharyngeal edema, and shock may occur in hypersensitive individuals.

2. Inhalation of these compounds may precipitate wheezing in persons with asthma. An 11-year-old girl had a fatal asthma attack after applying a pyrethrin-containing shampoo to her dog. Inhalation or pulmonary aspiration may also cause a hypersensitivity pneumonitis.

3. Skin exposure may cause burning, tingling, numbness, and erythema. The paresthesias are believed to result from a direct effect on cutaneous nerve endings.

4. Eyes. Accidental eye exposure during scalp application of A-200 Pyrinate has caused corneal injury, including keratitis and denudation. The cause is uncertain but may be related to the surfactant (Triton-X) contained in the product.

5. Ingestion. With large ingestions (200–500 mL of concentrated solution), the CNS may be affected, resulting in seizures, coma, or respiratory arrest.

DIAGNOSIS

Is based on a history of exposure. No characteristic clinical symptoms or laboratory tests are specific for identifying these compounds.

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