Perhaps no area of veterinary medicine has grown as rapidly as the field of flea and tick control. Fleas have become resistant to many products and with the explosion in flea numbers, there are huge efforts aimed at controlling them. Ticks can transmit a number of diseases, so tick control is also receiving more attention. Today's insecticides for pets have made great advancements and a wide array of different compounds have been formulated. The goal of this article is to explain some of the common, active ingredients listed on the labels of various shampoos, dips, sprays, powders, foggers, etc. We have included a table at the end of the article to help summarize the information.

Pyrethrins

Pyrethrins are one of the most widely used insecticides in today's flea and tick products and have been used as insecticides for over 100 years. Pyrethrins are natural extracts made from flowers of chrysanthemum plants. These plants grow naturally in the Middle East, Europe, Japan, and most importantly, Kenya. There are six different pyrethrins: pyrethrin I and II, cinevin I and II, and jasmolin I and II. All six are found in flea and tick products, but generally, the label only reads 'Pyrethrin,' regardless of which of the six types is actually present.

Mode of Action: Pyrethrins affect the nervous system of insects and result in repeated and extended firings of the nerves. They do this by affecting the flow of sodium out of nerve cells.

Use: Pyrethrins are used to control ticks, fleas, lice, Cheyletiella mites, and mosquitoes. They are mainly found in products applied directly on the pet. Household products generally contain either pyrethrins, a combination of pyrethrin and permethrin (see below), or pyrethrins plus a synergist. Synergists are chemicals that enhance the performance of other drugs. The synergist most widely used with pyrethrins is piperonyl butoxide.

Safety: All pyrethrins are easily hydrolyzed and degraded by stomach acids, so toxicity following ingestion by pets is very low. Toxicities, although rare, do occur. A cat or dog with pyrethrin toxicosis generally will salivate, tremor, vomit, and may seizure. Generally, signs of toxicosis will be gone after 24 hours.

Pyrethroids (e.g.; Permethrin, Phenothrin, Etofenprox)

Pyrethroids are synthetic pyrethrin compounds. That means they are made in a laboratory and are not natural plant extracts. Common synthetic pyrethrins are allethrin, resmethrin, phenothrin, etofenprox, and permethrin. Allethrin and resmethrin are commonly used as flying insect killers, while phenothrin, etofenprox, and permethrin are used to control fleas and ticks. These may be slower in action than the natural pyrethrins, but have a longer effect.

Mode of Action: Pyrethroids, like pyrethrin, affect the nervous system of the insect, causing repetitive nerve firings.

Use: Pyrethroids are used for the control of ticks, fleas, lice, Cheyletiella mites, and mosquitoes. In addition to killing these parasites, they also repel them. Because permethrins last longer than natural pyrethrins, they are commonly found in premise sprays and in products intended for slower, but sustained action. Permethrins are soluble in oils, but not in water. This is used to advantage in products that use an oil carrier to enhance distribution of the pyrethroid over the animal's body and prolong his activity, e.g., once-a-month BioSpot Spot On for Dogs, Defend, and K9 Advantix. Permethrin should NOT be used on kittens or cats. Bio Spot-Spot On for Cats, which contains etofenprox, is labeled for use in cats.

A pyrethroid that is often used in the environment is fenvalerate.

Safety: Pyrethroids are less easily broken down than pyrethrin, so this makes their toxicity, though low, higher than that of pyrethrin. As with any pesticide, some animals may show a temporary sensitivity where the product is applied. A patient with pyrethroid toxicity will salivate, tremor, vomit, and may seizure. If these signs occur, consult your veterinarian.

When pyrethroids are used in the outdoor environment, there is virtually no leaching into the soil, and when used properly, pyrethroids are very safe. The World Health Organization states fenvalerate 'will only cause a problem if spilled.' Permethrin and most other pyrethroids should NOT be used on kittens or cats.

Organophosphates (and organocarbamates)

Do NOT use pyrethroids, organophosphates, organocarbamates, or amitraz in cats.

Organophosphates and organocarbamates are another class of drugs that have had wide usage as insecticides both in agricultural settings and for pet animals. Commonly used organophosphates and organocarbamates include dichlorvos, cythioate (Proban tablet), diazinon, malathion, carbaryl (Sevin), fenthione, methylcarbamate (Adams Flea and Tick Collar), and prolate.

Mode of Action: Organophosphates function by blocking nerve inhibition, i.e., when organophosphates are present, a nerve that is firing will continue to fire. They do this by inhibiting acetylcholinesterase, an enzyme necessary for normal nerve function.

Use: Organophosphates are used in termite control and have many agricultural uses including the control of corn rootworms and cattle parasites. They are also present in various ant and roach control products. Organophosphates, should NOT be used on cats.

Safety: Organophosphates and organocarbamates are the class of insecticides most likely to cause toxic reactions in pets. If toxic levels are applied to the pet, (or the pet ingests them), the pet will experience nerve abnormalities. A pet suffering with organophosphate poisoning will salivate, tremor, stagger, and may seizure. An antidote is available to counteract its effect. At our hospital, we have seen two cases of organophosphate poisoning in pets. Both were accidental ingestion of large quantities of the chemical, and fortunately neither patient died. NOTE: Greyhounds and Whippets can be overly sensitive to organophosphates so use a different product in these breeds.

Imidacloprid (Advantage)

Imidacloprid is another new insecticide, which is in yet a different class of chemicals. The product Advantage contains imidacloprid.

Mode of Action: Like most insecticides, imidacloprid interferes with the nerve conduction system of insects. Imidacloprid acts by blocking the nerve receptors. It kills fleas, but does not have activity against ticks.

Use: Imidacloprid is most commonly used as a once-a-month topical insecticide on cats and dogs to kill fleas. Imidacloprid is mixed with an oil carrier and the drug collects in the hair follicles from which it is slowly released.

Safety: Imidacloprid has a wide margin of safety, but as with all other pesticides, some pets may develop sensitivity to the product.

Arylheterocycles (e.g.; Frontline)

Fipronil is the most commonly used product in this relatively new group of synthetic insecticides, such as Frontline Top Spot.

Mode of Action: Arylheterocycles block the passage of chlorine through cells in the insect's nervous system and this results in paralysis.

Use: Fipronil is most commonly used as a once-a-month topical insecticide on cats and dogs to kill fleas and ticks. Fipronil is generally mixed with an oil carrier and the drug collects in the hair follicles from which it is slowly released.

Safety: As with any pesticide, some animals may show a temporary sensitivity where the product is applied. Some animals may also develop more severe sensitivities, and if so, a veterinarian should be consulted.

Metaflumizone (e.g.; Promeris)

Metaflumizone is the ingredient in Fort Dodge Animal Health product Promeris for Cats.

Mode of Action: Metaflumizone attacks the nervous system of the flea by blocking movement of sodium atoms. This results in paralysis and death of the flea.

Use: In Promeris, the metaflumizone is mixed with a carrier and applied as a once-a-month topical insecticide on cats. It does not have activity against ticks.

Safety: As with any pesticide, some animals may show a temporary sensitivity where the product is applied. Some animals may also develop more severe sensitivities, and if so, a veterinarian should be consulted.

Insect growth regulators & development inhibitors

Insect growth regulators (IGRs) and insect development inhibitors (IDIs) are relatively new components of flea and tick products. Insect growth regulators include methoprene (Precor), fenoxycarb, and pyriproxyfen (Nylar). Insect growth inhibitors include lufenuron and diflubenzuron. Products containing IGRs and IDIs include Program, Sentinel, Bio Spot, Adams Flea and Tick Spray, Preventic Plus Flea and Tick Collar, and Frontline Plus.

Mode of Action: IGRs and IDIs differ from traditional flea product ingredients in that their main activity is against the immature forms of the flea. The IGRs mimic the juvenile growth hormone of fleas. The juvenile growth hormone is what keeps the fleas from developing into more mature forms. When the levels of juvenile growth hormone decrease, the larva form matures. The IGRs keep this development from occurring and the immature forms of the flea fail to molt and death occurs.

The IDIs inhibit the synthesis of a substance called chitin. Chitin is necessary for the formation of the hard outside skin (cuticle) of the flea. No chitin, no adult flea.

Use: Note that the IGRs and IDIs do not kill the adult fleas, so to be most effective, they should be used along with a product that does kill the adults (adulticide). If there is little risk of flea infestation, the IGRs and IDIs may be enough to prevent a flea infestation. However, if flea problems already exist, or the risk is high, it is best to also use an adulticide.

Many IGRs and IDIs are used in the environment as ingredients in foggers and sprays. They are also applied topically to cats and dogs, given orally, or by injection.

Remember, at this point there are no effective IGRs or IDIs for ticks.

Safety: Because IGRs and IDIs mimic insect hormones or alter a unique insect process (the making of chitin, which mammals do not make), they are extremely safe.

Amitraz

Amitraz is an ingredient that is used as a dip to treat demodectic mange. Amitraz has also been shown to be highly effective when used as an ingredient in tick collars for dogs. It has little or no effect on fleas, so is used in control of ticks only. Do NOT use Amitraz on cats.

Mode of Action: Amitraz belongs to a group of drugs called formamidines and is an ingredient in Preventic Collars. Formamidines kill ticks by inhibiting their nerves.

Use: Amitraz has been proven to kill ticks, which have become resistant to more traditional compounds such as organophosphates. Amitraz is a lipophilic drug, meaning it distributes well over the entire skin, even in large dogs. Most ticks are killed by Amitraz prior to attachment or if they do attach, they are killed in less than 24 hours, thus preventing the transfer of Lyme disease. Tick collars containing Amitraz can be used at the same time with many other flea and tick products such as Bio Spot-Spot On for Dogs (check the labels, or with your veterinarian, first). The concentration of Amitraz in the collars may not be high enough to kill demodectic mange. Do NOT use Amitraz on cats.

Safety: Collars containing Amitraz are very safe in dogs, if used correctly. The collar needs to be placed so you can get two fingers between the collar and the dog's neck – no more, no less. If the collar is placed too tightly, irritation can occur. Be sure to cut off any excess portion of the collar so the dog (or other pets) can not chew the end of it. If ingested by a pet, contact your veterinarian.

Selamectin (Revolution)

Mode of Action: Selamectin kills parasites by blocking nerve signal transmissions. Revolution enters the bloodstream through the skin. It then stays in the bloodstream protecting against heartworm disease, passes into the gastrointestinal tract where it can kill certain intestinal parasites, and passes into the sebaceous glands and then onto the hair and skin providing protection against fleas and certain mites and ticks.

Use: Selamectin is a topical insecticide used for the treatment and prevention of fleas, ear mites, some internal parasites, and some types of mites and ticks; and the prevention of heartworm disease. Over 98% of fleas on the pet are killed within 36 hours of application. Once in contact with Revolution, fleas will not lay viable eggs.

Safety: Selamectin is safe to use on collies, breeding males and females, and pregnant and nursing females. Do not use it on kittens or puppies less than 6 weeks of age. Use caution in using it on sick, weak, or underweight animals, or animals with broken or irritated skin.

Nitenpyram (Capstar)

Nitenpyram (Capstar) is approved in cats and dogs to kill fleas.

Mode of Action: Nitenpyram interferes with the nerve conduction system of insects, blocking the nerve receptors.

Use: Nitenpyram in Capstar, an oral flea control product, is approved for use in cats and dogs over 4 weeks of age and 2 pounds of body weight. Nitenpyram starts killing adult fleas that are on the pet within 30 minutes. The peak effect after administering the medication is about 3 hours for the dog and 4 hours for the cat. It will be very useful in certain circumstances such as prior to surgery, boarding, or grooming. It will also help prevent pets from bringing fleas home from shows, trials, or trips to the park. The pet could be given the pill before leaving for the park or before heading home from a distant show. Any fleas on the pet will be dead within several hours. It will not work for on-going control as the medication is out of the pet's system within 24 hours.

Safety: Capstar appears to be very safe. It is safe for kittens and puppies 4 weeks and older, weighing 2 pounds or more, as well as for pregnant or nursing cats and dogs.

Citrus extracts

Citrus extracts are now being termed 'the all natural way' to control fleas and ticks. The best known of the citrus products generally contain d-Limonene or linalool. These are extracted from the peelings of citrus fruits. We basically have found these all natural citrus products to offer no advantage over traditional products, except perhaps in the pleasant citrus fragrance they impart to the coat.

Mode of Action: Citrus extracts affect the flea's nerves.

Use: d-Limonene and linalool are used in dips, sprays, and shampoos. In heavily infected flea regions, citrus extracts are probably not effective enough as an insecticide unless used in conjunction with more potent products.

Safety: Even though all natural, citrus extracts can be toxic to the pet and should be used in strict accordance to the label. Poisoned pets may salivate, stagger, and lose body heat. Some animals may have hypersensitivity and develop skin rashes, especially on delicate tissue such as the scrotum or vulva. Some citrus-extract products have been fatal to cats. Remember, 'Natural' does NOT necessarily mean 'Safe.'

Herbs & vitamins

Brewer's yeast, thiamine, garlic, citronella, rosemary, and other herbs have been advocated as effective flea control substances. There have been no clinical trials to prove their effectiveness. This does not mean they may not work in some situations, but their reliability is questionable. As with citrus extracts, these 'natural' products are not without potential harm. Use them carefully.

Borax (borate)

Borax, in the form of sodium polyborate has become available to kill adult fleas and inhibit the development of immature forms. It is used as a carpet powder, but is more expensive and toxic than insect growth regulators and development inhibitors. The long-term toxic effects of borate are unknown.

Organochlorines

Organochlorines are seldom seen on today's labels, but are included here for completeness. The best known of the organochlorines is Lindane. Because of its side effects, Lindane is seldom used in over the counter products and would NOT be our choice for safe flea and tick control.

Dioctyl sodium sulfosuccinate (docusate) and undecyclenic acid

These two ingredients act by softening the waxy covering on fleas, ticks, lice, mites, and other insects, eventually affecting the internal organs and causing death of the insect.

Ingredients and Activity of Common Flea and Tick Control Products