Pyrethrins are naturally occurring insecticides derived from the chrysanthemum plant. Pyrethroids (Table II-52) 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.

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.

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 antihelminthic 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 is generally considered nontoxic.
2. Cypermethrin. A 45-year-old man died after ingesting beans cooked in 10% cypermethrin.

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 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.

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

1. Specific levels. These compounds are metabolized rapidly in the body, and methods for determining the parent compound are not routinely available.
2. Other useful laboratory studies include electrolytes, glucose, and blood gases or oximetry.

1. Emergency and supportive measures
   1. Treat bronchospasm and anaphylaxis if they occur.
   2. Observe patients with a history of large ingestions for at least 4-6 hours for any signs of CNS depression or seizures.
2. Specific drugs and antidotes. There is no specific antidote.
3. Decontamination
   1. Inhalation. Remove victims from exposure and give supplemental oxygen if needed.
   2. Skin. Wash with copious soap and water. Topical application of vitamin E in vegetable oil was reported anecdotally to relieve paresthesias.
   3. Eyes. Irrigate with copious water. After irrigation, perform a fluorescein examination and consider ophthalmologist referral if there is evidence of corneal injury.
   4. Ingestion. In the majority of cases, a subtoxic dose has been ingested and no decontamination is necessary. However, after a large ingestion of Chinese chalk or a concentrated solution, administer activated charcoal orally if conditions are appropriate (see Table I-37,). Gastric lavage is not necessary after small-to-moderate ingestions if activated charcoal can be given promptly.
4. Enhanced elimination. These compounds are metabolized rapidly by the body, and extracorporeal methods of elimination would not be expected to enhance their elimination.