Pyrethrins: Fact or Fiction

Christy Michael, BVMS, dispels the myths of pyrethrins and their toxic effects on veterinary patients.

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Pyrethrins are a plague visited upon our planet by cat hating celestial beings in an attempt to eradicate their very existence.
Fiction. Pyrethrin is a naturally occurring insecticide that comes from the chrysanthemum flowers commonly found in Africa and Australia. The Chinese used parts of chrysanthemum plants as an insecticide as early as 1000 BC and the insecticidal properties of the flower were first identified around 1800 in Asia.  We humans went and fiddled with pyrethrins, synthesizing artificial insecticides called pyrethroids (such as permethrin) and then went crazy producing them as quickly as possible. As of January 2012, the US EPA had over 3,500 registered products that contain pyrethrins and pyrethroids.

Pyrethrins do something to the nerves--details are for students.
Fiction. Pyrethrins may do something to the nerves, but details are not just for students, they are also for inquiring minds like ours! Pyrethrins reversibly bind to neuron sodium channels and prolong conductance, therefore increasing depolarization. Pyrethroids antagonize GABA and glutamic acid and are much more effective as insecticides than pyrethrins. Both compounds are frequently used in combination with other enzymes to ensure effectiveness and prolong toxicity.

Pyrethrins are not particularly toxic or dangerous to mammals.
Fact. Pyrethrins are actually one of the safest insecticides for mammals and are even safe to use topically in cats if used at an appropriate concentration. Overdosing and ingestion of pyrethrins can certainly still cause signs of toxicity and even inhalation can cause symptoms consistent with asthma in our pets or in people. However, when used appropriately, pyrethrins are quite safe. Pyrethroids are also generally safe in mammals with the exception of cats. The most commonly used pyrethroid in the world of pet applied insecticides is permethrin, which is metabolized by glucuronide conjugation. Since the feline liver is especially inefficient at glucuronide conjugation, permethrin is highly toxic to our feline companions.

I’ve got this--pyrethrin is safe for use on cats but not permethrin.
Fiction. Why can’t this be easy, you ask?! The truth is that permethrin is also safe to apply to most cats but only at a concentration of 0.05-0.1% as compared to the 45-65% concentration used in topical canine permethrin products. Unfortunately, feline exposure to toxic levels of pyrethroids can occur through multiple potential routes. Inadvertent dosing with another pet’s medication is a common cause of exposure along with intentional dosing with a less expensive product labeled for another size or type of pet. Accidental ingestion of insecticide applied to the pet in question, or to another pet in the household, is another common means of exposure. The sad truth is that sometimes even if everything is done right, some animals will simply be more sensitive to pyrethrins or pyrethroids and may develop toxicity when these medications are used strictly by the book.

Pyrethrins and pyrethroids can be toxic not only to cats but also to dogs and fish.
Fact. Everybody has heard about the toxicity of these products in cats but don’t be fooled, cats are not the only species at risk. Pyrethrin/pyrethroid toxicity is also noted in dogs with greater risk of toxicity in smaller breeds of dogs. Typical symptoms of toxicity include vomiting, hypersalivation, tremors, ataxia, and seizures but not all reactions are neurologic. In addition to this type of toxicity, dermal reactions can be noted. These are characterized by localized or generalized pruritus and occasionally dermal erythema or excoriation from self-mutilation. As mentioned earlier, it is also possible to show respiratory symptoms secondary to inhalation of these products. All forms of pyrethrins and pyrethroids are highly toxic to fish so make sure pet owners clean up well between application of these products and handling anything that is going into a fish tank. Nobody wants to know that their actions resulted in the death of their prized arowana when it could have been avoided!

When faced with a patient suffering tremors and/or seizures secondary to pyrethroid toxicity, the best approach is to throw up ones hands, curl rapidly into the fetal position, and wait for the symptoms to pass.
Fiction. In fact, we have many means of intervention that can save the lives of these patients. Generally the first order of business is controlling tremors or seizures adequately to allow our other treatments. The best control of tremors secondary to pyrethroid toxicity is provided by methocarbamol IV but this is safest administered via an IV catheter that might not be so easy to place in a tremoring/seizuring patient. Most of us that treat these patients on a regular basis are not shy, starting our methocarbamol doses at 50-100 mg/kg IV and repeating as needed up to 330 mg/kg/day. Diazepam is unlikely to provide complete control of tremors and will not last long but a single IV or rectal dose (0.5 mg/kg) can be used to facilitate placement of IV catheter. Once we have a glimmer of control of tremors, we promptly cover up our IV catheter and bathe our patient all over with a mild detergent such as dish soap to remove as much of the toxin as possible. Avoid replacing any collar or harness that has not been laundered to similarly remove any toxin residues. IV fluids are started to protect the kidneys from the myoglobin that will surely be flowing through them shortly. For patients with refractory tremors or seizures, phenobarbital, propofol, or inhalant anesthesia may be necessary to control these symptoms. Treatment length varies depending on the exposure and independent patient sensitivity, anywhere from 12-48 hours.

Dermal reactions to pyrethin/pyrethroid exposure are treated similarly to any other basic allergic reaction.
Fact. As with any allergic reaction, treatment with an antihistamine and steroid are very helpful in treating dermal reactions to these toxins. In addition to those traditional treatments, these patients will also benefit from bathing all over with a mild detergent. Avoid replacing contaminated collars or harnesses on these pets as well.

I now understand and no longer fear the arrival of my next pyrethrin/pyrethroid toxicity case.

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