Pyrethrin For Fleas

Pyrethrins are insecticides derived from the chrysanthemum plant. They have good “knockdown” and are relatively safe when properly applied for flea treatment. Pyrethrins are found with enhancers, typically a product called piperonyl butoxide, to reduce the chance of resistance by the flea. This product is available in powders, shampoos, sprays, foggers, etc. It is widely used and only moderately toxic. Pyrethrins are neurotoxic at high levels, and may cause excessive salivation, vomiting, difficulty breathing, muscle tremors, depression, and ataxia (wobbling). Cats may develop excessive salivation, contractions of superficial muscle (ear flicking, paw shaking), or the signs mentioned above. The products are considered fairly safe when applied properly and sparingly. Pyrethrins are alcohol soluble, which explains why most pyrethrin containing insecticide smell like alcohol. The alcohol also helps penetrate the waterproof skeleton of the flea increasing the rate of flea death. Alcohol can be painful if applied to the excoriated, irritated, and oozing skin of a pet with severe flea allergies. Pyrethrins are inactivated by sunlight, therefore, they do not provide much residual activity for outdoor use. The most common instances of toxicity are seen when dips are too concentrated for use in a cat. Heat drying of a pet will increase the absorption of any toxic substance and blow dryers should be used with caution. Anemia also will increase the sensitivity of an animal to toxicity. Longhair coats will increase the total amount of flea product on the coat and can predispose to self-grooming related toxicity.

Pyrethroids (synthetic pyrethrins)

Pyrethroids are synthetic products, derived from the pyrethrins. Pyrethroids generally have better residual action and are less toxic than pyrethrins. Permethrin is the most commonly used pyrethroid. Pyrethroids are found in shampoos, rinses, foggers, pour-ons, and an assortment of household sprays. The synthetic pyrethrins also have several advantages over the natural form of the products. They are water-soluble which allows for less skin irritation, as well as the ability to be mixed with other soothing substances such as aloe vera or lanolin for application to irritated skin. The synthetic pyrethrins are also resistant to sunlight degradation and therefore have residual activity. These products do not have the “knockdown” of the natural pyrethrins and will not give the initial impression of being as effective as the alcohol-based products.


Rotenone is an insecticide derived from the root of Derris ellipta. It is used in shampoos, sprays, and rinses. Rotenone is quite toxic to fish and small mammals (e.g., guinea pigs and rabbits), therefore caution should be exercised when applying this agent around those animals. The compound rapidly decomposes upon exposure to light and air. Ingestion by dogs or cats may result in vomiting. This product is very toxic to wildlife in the streams and lakes affected by runoff from the home that is dipping the pet.

Borate (Carpet Treatments)

The borates include sodium polyborate and boric acid. The borates are available as powders 1) for application by commercial companies, who will come into your home to apply the material, and 2) as raw borates sold by the same companies or, in some cases, over the counter at hardware stores, for application by the home owner. The borates are applied to carpeted areas, where they are worked into the carpet. Residual powder is then removed by vacuuming.

The borates work by two mechanisms. First, they are known to be intestinal poisons upon ingestion by flea larvae. Second, they may act as desiccants, drying up the microenvironment of the flea larvae in the carpet, making it an unfavorable environment for survival. The efficacy of these products appears to be quite high when properly applied to flea infested environments. Anecdotal reports and some studies have supported these observations. From the standpoint of effectiveness, this form of home treatment appears to be quite successful. The toxicity of the borates is unclear. The products are definitely not the “totally safe” forms of flea control advertised by some companies. Kidney and liver damage can result from acute toxicosis from the borates. Clinical signs of toxicosis in pets may include vomiting, diarrhea, depression, and loss of appetite. Overall, the risk of danger to pets appears to be low, unless the animal directly ingests the powder. However, the long-term health effects (to pets) of chronic exposure to low levels of borates are unknown. Risks to humans, including infants, is also unknown, but appears to be low.

Diatomaceous Earth

Diatomaceous earth is a desiccant (drying agent) and also works as a chaffing agent to fleas. It may be spread in an environment to help reduce the humidity in carpet. Diatomaceous earth is available at stores catering to health foods or natural products. Its effectiveness is variable. The product may be toxic to humans, since it contains a significant percentage of silica, and silica can cause lung disease in humans if inhaled. An alternative to the application of diatomaceous earth is the application of the borates.

Citrus Derivatives

D-limonene is the most commonly used derivative of citrus fruits. This substance is a volatile oil that has moderately good “knockdown” properties, but is fairly mild. The main advantage of this product is a high margin of safety, making it a good product for application on kittens and puppies, as well as in households with infants. Citrus derivatives are available as shampoos and rinses.


Frontline® Top Spot™ and spray treatment is a topically applied, stand-alone product that rapidly kills fleas and ticks on dogs and cats with a single application, controlling fleas for up to three months and ticks for a month. Fipronil, the active ingredient in Frontline®, collects in the hair follicle and oil glands of the pet. From the hair follicle, it continually releases onto the animal’s skin and hair. By killing adult fleas and ticks within hours after application, Frontline® breaks the flea life cycle at the adult stage and quickly gives animals relief from the symptoms of flea infestation. Frontline® also controls flea larvae by remaining active on shed hair, thus helping to control emerging flea populations in bedding and other areas. Frontline® kills ticks usually before they attach, reducing the risk of transmitting disease. A single application of Frontline® can control fleas for up to ninety days on dogs and thirty days on cats and controls ticks for thirty days on both species. Monthly applications are recommended to provide continuous protection against pests. Fipronil’s unique chemical structure acts on the gamma amino butyric acid (GABA) receptor of an insect’s central nervous system by blocking the flow of chloride ions.


Advantage® utilizes revolutionary new chemistry to produce unprecedented results on cats and dogs. Advantage® contains imidacloprid, a chloronicotinyl nitroguanidine synthesized from the nitromethylene class of compounds. This new active ingredient has a mode of action unlike any other flea control product on the market. Advantage® works by binding to the insect’s nicotinyl receptor sites on the postsynaptic neuron. This causes impairment of the nervous system and death to the insect. Thanks to this new chemistry, Advantage® begins working almost immediately, killing 98-100% of existing fleas on cats and dogs in just twenty-four hours. Since Advantage® works on contact, fleas don’t have to bite the pet in order for it to work – which is advantageous for pets with flea allergy dermatis. Advantage® is supplied as a topical solution that is easily applied on the back of the neck on cats, and between the shoulder blades on dogs. Once applied, Advantage® spreads naturally on the skin surface at the hair root level, aided by body movement, providing whole body coverage. More than 90% efficacy is observed at twelve hours, and full efficacy is attained within twenty-four hours.

Insect Growth Regulators and Development Inhibitors

Insect growth regulators (IGRs) and insect development inhibitors (IDIs) are key agents for long-term flea control. However, the agents do not kill adult fleas and therefore, other traditional flea control products will be necessary in an appropriate flea control program. The IGRs and IDIs appear to be very safe. Insect growth regulators are juvenile hormone analogs (resemble the natural growth factor found in the flea) that work by interfering with egg development and molting from various life stages of the flea. The two most commonly available IGRs are methoprene (Precor®) and fenoxycarb (just removed from the market). IGRs are found in sprays, foggers, and flea collars. They may be used on the pet or applied to the environment. An advantage of the IGRs is their high margin of safety. These are products that would be among the most safe for application in a household with infants or other people intolerant to insecticides. Methoprene is broken down by ultraviolet exposure. These products do NOT kill adult fleas, and therefore, are generally combined with an insecticide in most products. Insect development inhibitors work by interfering with a particular aspect of development. Most of these products interfere with the synthesis of chitin, a protein necessary for maturation and function of the flea exoskeleton. Chitin inhibitors include lufenuron, pyriproxyfen, and cyromazine. Cyromazine is not commercially available in the United States.

Pyriproxyfen (Nylar®, McLauglin Gromley King Co.)

Pyriproxyfen is an insect juvenile hormone analog, similar to methoprene and fenoxycarb. Pyriproxyfen is available in premise sprays/foggers (e.g.Adams®, Biospot® EctoKyl®, DVM; Knockout®, Virbac), rinses for application on dogs (EctoKyl®, DVM), and sprays for topical application on dogs in products (e.g., Knockout®, Allerderm/Virbac) combined with adulticides. This insect growth regulator is stable in sunlight and is extremely safe. Pyriproxyfen binds to hair and skin, which allows it to remain when an animal becomes wet (e.g., bathing or swimming). Knockout™; is currently only labeled for use on adult dogs.

Program® (Lufenuron)

The flea bites the pet and ingests lufenuron and lufenuron is deposited in eggs. Most eggs cannot hatch. Larvae that are able to hatch soon die and pre-existing larvae that ingest lufenuron from feces of treated fleas also die. It is an oral tablet for dogs and a liquid suspension for cats. It must be given with food to endure complete absorption. This is a common reason for failure. All pets in the household must also be treated. Lufenuron is safe for nursing and breeding animals, and for kittens and puppies as young as six weeks. Program® has no contraindications, no warnings, and no side effects. The cat dose is 30mg/kg and is three times the dog dose at 10mg/kg. To hasten control, you may recommend temporary use of conventional flea products in addition to lufenuron. Once the pre-existing population is destroyed, discontinue additional measures. PROGRAM® (lufenuron) used alone will maintain effective control.


Sentinel™ is a new product by Novartis Animal Health US, Inc. which combines lufenuron with milbemycin oxime. Milbemycin oxime (Interceptor®) is a once monthly, oral medication to control heartworm, roundworm, hookworm, and whipworm infestations. The combination of these two compounds provides a convenient preventive measure for fleas as well as heartworm and internal parasites except tapeworms.

Thiamin (Vitamin B1) Brewers Yeast

Brewers yeast has been often used for flea control. These tablets can be purchased in most pet stores. It has not been shown to be an effective flea control agent when administered orally.

Avon’s Skin-So-Soft®

Skin-So-Soft® (SSS) has been shown to partially repel fleas when topically applied at a concentration of 1.5 ounces of SSS per gallon of water. The repellent effect is not complete, but significantly fewer fleas were found on dogs treated with SSS vs water in one controlled study. The effects seem to last at least eight days. No toxic effects of the treatment were observed in the study, however, long-term effects have not been studied.


Garlic has not been shown to consistently repel fleas (despite the evidence that garlic has many beneficial effects in humans).


Cedar chips or wood may have some repellent properties, however, my personal experiences suggest it is nether complete nor consistent.

Pennyroyal Oil

This oil is derived from the leaves and flowers of the pennyroyal, squaw mint, or mosquito plants. Pennyroyal oil contains a volatile compound called pulegone, which is responsible for the toxic effects of the plants. The product is used for flea control and is available in flea shampoos, powders, and as pennyroyal oil. Exposure to pennyroyal oil may induce depression, vomiting, hepatic necrosis, diarrhea, epistaxis (nose bleeds), seizures, and death. Toxicity is dose-related and the possibility of severe signs is more likely if the pure oil is applied to the pet.


Biologic control of fleas using the nematode Steinernema carpocapsa. These worms live in moistened soils and parasitize the flea and developing larva. They are effective in killing a flea colony within thirty days. They have a very short shelf life and live worms need to be demonstrated by microscopic examination prior to the sale of the product to assure viability. The area of application needs to be shaded and free of previously applied organophosphate pesticides. Beneficial nematodes are a good alternative for the client that wants a purely natural flea control solution.

Chlorinated Hydrocarbons

Organochlorine pesticides are not used as much today because of the effects they have on the environment. They do not degrade naturally and tend to concentrate in the food chain causing damage to certain animal species. For example, DDT was banned because it was shown to cause eggshell thinning in bald eagles. The chlorinated hydrocarbons are neurotoxins that act on the central nervous system. Kidney and liver functions can also be impaired causing myoclonic jerking, convulsions and violent seizures. Convulsions may recur over several days. Death is often due to respiratory difficulty secondary to the metabolic acidosis caused by continual muscle activity. Various disturbances of sensation, coordination, and mental function are also characteristic of acute organochlorine poisoning. High tissue concentrations of organochlorines increase myocardial irritability, predisposing to cardiac arrhythmias. When tissue organochlorine concentrations drop below threshold levels, recovery from the poisoning occurs. Organochlorines are not cholinesterase inhibitors. Symptoms of chlorinated hydrocarbon poisoning in animals are: vomiting, restlessness or excitability, tremors, abnormal postures, convulsions, respiratory failure and comas. There are no specific antidotes, but therapy is directed towards removing the poison and treatment of the symptoms. If exposure is by dermal contamination, skin and hair should be washed with soap and water. If exposure is by ingestion, emetics or gastric lavage is recommended. (Note: Do not induce emesis if the ingested poison is principally a hydrocarbon solvent.). Barbiturates may be administered if necessary for restlessness or convulsions. Watch breathing closely, and provide artificial respiration or resuscitation if needed. Listed are examples of organochlorine products so that you might be able to recognize them on the labels of products given to animals that are suspected of this type of toxicity. Endrin (Hexadrin), aldrin (Aldrite, Drinox), endosulfan (Thiodan), dieldrin (Dieldrite), toxaphene (Toxakil, Strobane-T), lindane (gamma BHC or HCH, Isotox), hexachlorocyclohexane (BHC), DDT (chlorophenothane), heptachlor (Heptagran), chlordecone (Kepone), terpene polychlorinates (Strobane), chlordane (Chlordan), dicofol (Kelthane), mirex (Dechlorane), methoxychlor (Marlate), dienochlor (Pentac), TDE (DDD, Rhothane), ethylan (Perthane).