WHO Acute Hazard classification: Class II, moderately hazardous.

Mechanism of action of Cypermethrin

Synthetic pyrethroids, including cypermethrin, have a similar mode of action as organochlorines. They act on the membrane of nerve cells blocking the closure of the ion gates of the sodium channel during re-polarization. This strongly disrupts the transmission of nervous impulses, causing spontaneous depolarization of the membranes or repetitive discharges. At low concentrations insects and other arthropods suffer from hyperactivity. At high concentrations they are paralyzed and die. Sensory and nervous cells are particularly sensitive.

Acute Toxicity and Tolerance of Cypermethrin

• Toxicity of cypermethrin depends strongly on the cis:trans isomer ratios (usually 80:20; 40:60; 25:75), whereby the cis isomers are biologically more active but also more toxic than the trans isomers. 
• LD50 acute, rats, p.o. 250 (oily vehicle) to 5150 mg/kg (aqueous vehicle). The vehicle-dependent and isomer-dependent differences in the acute oral toxicity are typical for synthetic pyrethroids and some other active ingredients.
• LD50 acute, rats, dermal >4920 mg/kg
• As a general rule, dogs, livestock (cattle, sheep, goats, swine), horses, and poultry tolerate cypermethrin and most synthetic pyrethroids very well, since toxicity is about 1000x higher to insects and other arthropods than to mammals. But in case of sustained skin or inhalation exposure, or after direct contact with open wounds toxicity to mammals can be higher.
• Besides neurotoxic effects, cypermethrin has also hepatotoxic effects and induces microsomal enzymes in the liver.
• WARNING: cypermethrin is toxic to cats! Cats do not tolerate therapeutic doses for dogs. This is associated with glucuronidase deficiency in cats, the enzyme responsible for breaking down cypermethrin and other synthetic pyrethroids in the organism in a process called glucuronidation. As a consequence, cypermethrin remains much longer in the cat's organism than in dogs or other mammals. 

Toxic Symptoms caused by Cypermethrin Poisoning

• The primary symptoms of intoxication with cypermethrin and other synthetic pyrethroids affect mainly the nervous and muscular systems.
• Most frequent symptoms are:
• Ataxia (uncoordinated movements)
• Hyperreactivity (exaggerated reaction to stimuli)
• Tremor (uncoordinated trembling or shaking movements)
• Paresthesia (skin sensation of tingling, tickling, prickling)
• Exhaustion (lethargy, fatigue)
• Hypersalivation (drooling)
• Vomit
• Diarrhea
• Urinary incontinence

• Other symptoms after severe poisoning include: hyperthermia (fever) or hypothermia (too low body temperature), dyspnea (difficult breathing), disorientation, cramps or spasms (sudden, involuntary contractions of muscles or hollow organs).
• Symptoms appear a few hours after exposure, but depend strongly on the formulation, the dose and the kind of contact (skin, inhalation, ingestion etc).
• Sustained skin exposure can cause local dermatitis (skin irritation) with pruritus (itching) and erythema (red skin).
• Mucous membranes are particularly sensitive to synthetic pyrethroids, e.g. in the nose and the respiratory system (coughing), in the eyes (conjunctivitis), genital organs, etc.
• After excessive inhalation of synthetic pyrethroids patients can develop allergic sensitization with asthmatic symptoms. In extreme cases, sustained inhalation of high doses can cause respiratory paralysis and death.
• As a general rule, young animals are more sensitive to overdosing and react stronger. 
• A frequent cause of intoxications in pets is off-label use of products for livestock, crop protection or vector control on dogs and cats. Since such products do not include use instructions for pets, massive overdosing is possible, and inert ingredients in such products may also be toxic for pets.

Cypermethrin Side Effects, Adverse Drug Reactions (ADRs) and Warnings

• Do not administer cypermethrin topically (pour-ons, spraying, spot-on, shampoos, soaps, sprays, etc.) in case of extended skin lesions: this can lead to an excessive absorption through the damaged skin.
• Pour-ons containing cypermethrin and other synthetic pyrethroids can be irritant for cattle. This can be particularly annoying when handling dairy cows for milking.
• In small dogs paresthesia (skin sensation of tingling, tickling, prickling) can happen at the therapeutic dose, which usually disappears in 12 to 24 hours.
• Toxic effects can be potentiated after simultaneous exposure to organophosphates or other synthetic pyrethroids.
• Cypermethrin is rarely used in spot-ons (= pipettes, squeeze-ons) for dogs: other synthetic pyrethroids are preferred (e.g. permethrin, cyphenothrin, phenothrin, etofenprox, etc.).

• Unless prescribed by a veterinary doctor, never use on dogs or cats products for livestock that are not explicitly approved for such use. There is a high risk of overdosing or of adverse drug reactions due to ingredients that are not tolerated by pets or are even toxic to them.

Antidote and Treatment of Cypermethrin Intoxication

• There is no antidote for cypermethrin poisoning.
  
• Treatment consists in preventing further exposure together with supportive and symptomatic measures.
• In case of dermal exposure rinse the skin with abundant water and soft detergents.
• After accidental ingestion administer activated charcoal (2 g/kg), magnesium sulphate or sodium sulphate (0.5 mg/kg in a 10% aqueous solution)
• Spasms can be treated with anticonvulsants (e.g. diazepam). If ineffective, fenobarbital or pentobarbital can be tried.
• Hypersalivation can be treated with atropine.
• In case of strong vomit and/or diarrhea rehydration measures should be considered.
• Calcium gluconate and vitamins of the B complex can be used to protect the liver.

Pharmacokinetics of Cypermethrin

• Topically administered cypermethrin remains mostly on the hair-coat of the treated animals and is very poorly absorbed through the skin. In contrast with natural pyrethrins and older synthetic pyrethroids, cypermethrin is quite resistant to UV-light, which allows a residual effect between 5 and 10 days after topical administration.
• Treated animals can ingest cypermethrin through licking or grooming. Absorption to blood is low. The absorbed cypermethrin is quickly metabolized in the liver to non-toxic metabolites that are excreted through urine. This is done by a specific enzyme called glucuronidase. However, cats lack this enzyme and cannot metabolize cypermethrin and other synthetic pyrethroids. This is why cypermethrin and most other synthetic pyrethroids are toxic to cats.
• Cypermethrin products are approved for use on dairy animals and laying hens in many countries.

Environmental Toxicity of Cypermethrin

• Cypermethrin, as all synthetic pyrethroids is extremely toxic to fish and aquatic invertebrates. For this reason disposal of cypermethrin residues (e.g. in empty containers) in watercourses must be absolutely avoided. Disposal of old dip wash charged with cypermethrin (or other synthetic pyrethroids) into watercourses is strictly forbidden worldwide because it would have catastrophic consequences for fish and other aquatic animals. There are countries where products for livestock dipping containing synthetic pyrethroids have been withdrawn by the regulatory authorities for this reason.
• In contrast with organophosphates cypermethrin (as most synthetic pyrethroids) is not toxic to birds.
• Correct use on dogs and livestock is unlikely to result in any significant environmental pollution.
• There is a certain environmental risk of water pollution from run-off after pour-on administration to large cattle herds. However this risk is substantially lower than the one associated with the use of cypermethrin (or other synthetic pyrethroids) in plunge dips or as a crop pesticide.
• Cypermethrin is quite resistant to photodegradation, i.e. exposed to sunlight it breaks down rather slowly. Half-life is 8-16 days.
• Cypermethrin is almost insoluble in water and tends to bind to soil particles. Therefore groundwater contamination is unlikely to occur. Persistence in water depends on pH and temperature. Under usual conditions half-life in water can be 50 days and longer.
• Persistence in soils is moderate but depends strongly on their structure. It breaks down faster in sandy soils with scarce organic material than in clayey soils or those rich in organic material. Half-life in soil under aerobic conditions (i.e. with sufficient oxygen is 4 days to 8 weeks. Under anaerobic (no oxygen) conditions substantially longer.
• Soil bacteria contribute to the biodegradation of cypermethrin.
• Cypermethrin does not bioaccumulate.

Additional information

Click here for a list and overview of all safety summaries of antiparasitic active ingredients in this site.

• Cypermethrin belongs to the chemical class of the synthetic pyrethroids and to the so-called type-II pyrethroids (with a cyano group in their molecular structure).
• Commercial products with cypermethrin may use different qualities regarding the cis-trans isomer ratios, usually 40:60, 80:20 (high-cis) or 100:0 (= alphamethrin=alfamethrin=alphacypermethrin).
• Cypermethrin is not used in human medicines.
  
• Cypermethrin is used in crop pesticides.
• Cypermethrin is used in public or domestic hygiene as a biocide.
• Click here for General safety of antiparasitics for domestic animals.
• Click here for General safety of antiparasitics for humans.
• Click here for General safety of antiparasitics for the environment.
• Click here for technical and commercial information on cypermethrin.