Are Topical Flea Killers Safe?

Absolutely not, says our author, despite the decades-long debate.


[Updated September 27, 2018]

Tempting as it may be to simplistically consider fleas as horrible insects, the bane of dogs everywhere, poisoning your dog in a vain attempt to wipe fleas out of existence doesn’t really make sense. Even though more than half a billion dollars annually are spent on products that kill fleas in that vain pursuit.

Of course fleas can make dogs (and everyone else in the household) perfectly miserable. But it’s not as if using toxic flea-killing chemicals is the only way to control fleas. When we attempt to get rid of our dogs’ fleas by utilizing chemicals that are toxic to the brain and nervous system, that may disrupt hormone (endocrine) systems, and that cause cancer, it’s sort of like burning the house down to get rid of ants – effective, sure, but what are you left with?

In the next issue of WDJ, we will describe effective, nontoxic methods of flea control. No dogs (or any other members of the household) will get sick from these methods, and no dogs (or any other members of the household) will die from them. In contrast, dogs do get sick and die from the toxic chemicals we will describe in this article.

spot on flea killers

Spot-On Products Aren’t Safer

All pesticides pose some degree of health risk to humans and animals. Despite advertising claims to the contrary, both over-the-counter and veterinarian-prescribed flea-killing topical treatments are pesticides that enter our dogs’ internal organs (livers, kidneys), move into their intestinal tracts, and are eventually eliminated in their feces and urine. Not only that, but the humans and other household animals who closely interact with dogs who have been treated with these chemicals can be affected by the toxins. What happens to the health of all exposed individuals during this systemic absorption and filtration process varies from animal to animal, but the laboratory and field trial results clearly indicate toxicity on the chronic and acute levels.

Until recently, foggers, flea collars, powders, sprays, shampoos, and dips containing organophosphates (chlorpyrifos, malathion, diazinon), pyrethrins, synthetic pyrethroids, and carbamates, were the cutting-edge solutions to our flea problems. They were effective, but unfortunately, they also caused disease and sometimes death. Given enough time, most pesticides eventually cause enough human and animal injuries that they are identified as hazards and are removed from the market.

While the newest flea products – so-called “spot-on” liquids that are applied monthly to a dog’s skin – are being marketed aggressively by the manufacturers and veterinarians and represented as safe alternatives to their predecessors, the fact is, they are simply newer. All the “active” ingredients in these spot-on preparations – imidacloprid, fipronil, permethrin, methoprene, and pyriproxyfen – have been linked to serious health effects in laboratory animals (see chart at end of story).

“The public must recognize that any decision to use a pesticide, or to otherwise be exposed to pesticides, is a decision made in ignorance,” says Eliot Spitzer, Attorney General of the New York Environmental Protection Bureau. “We do not know the identity of the chemicals to which we are exposed. We cannot make informed individual decisions on the acceptability of those exposures, a basic element in the maintenance and protection of our own health.” Spitzer adds, “The requirements for marketing a new product fall considerably short of providing safety for our animal and human families.”

Active and Inert Ingredients in Insecticides

To fully understand the risks associated with any of these products, it is important to understand the various components in a flea product, or any chemical product that you may buy, for that matter.

Like other chemical products, all flea products are made up of “active” and “inert” ingredients; strangely, the actual definitions of those phrases are very different from what they seem to connote. In the case of flea-killing chemicals, the “active” ingredient does, in fact, target and kill fleas – but some of the “inert” ingredients are poisons, too.

While the word “inert” suggests benign activity and even connotes safety in the minds of many consumers, legally, it simply means added substances that are not the registered “active” ingredient. This is important because most people assume that only the “active” ingredient in a chemical product is of concern. Many people feel comforted by the idea that a product contains only a minuscule amount of an “active” ingredient and up to 99.9 percent “inert” ingredients – a typical formula in many pesticide products. Actually, this makeup should frighten consumers.

Why? Because the Environmental Protection Agency (EPA, the government agency that oversees the pesticide industry) requires a higher (if not high enough) standard of scrutiny for “active” ingredients; these must undergo a battery of tests to determine their toxicological profiles, be registered with the EPA, and be listed on the product inserts and packaging. In contrast, “inert” ingredients need not be listed on the product inserts and packaging and are subject to much less testing than the “active” ingredients; “inerts” are generally tested in short-term studies for acute toxicity only.

The word “inert” implies chemicals that are somehow inactive. In actuality, many “inert” ingredients used in pesticides are as toxic, or more toxic, than the registered “active” ingredients. For example, naphthalene, one of the “inerts” in an imidacloprid product, showed clear evidence of cancer activity through inhalation (nasal cancers), as well as anemia, liver damage, cataracts, and skin allergies. An unidentified “inert” ingredient in the flea product Advantage was implicated in the death of kittens who received doses within laboratory tolerances.

Why don’t pesticide manufacturers have to disclose all the ingredients in their products? This kettle began brewing in 1949, when the U.S. Congress passed the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), allowing manufacturers confidentiality on issues they claimed would otherwise make them vulnerable to market competition. “Inert” ingredients, in other words, became protected by industry as “trade secrets.” While protecting industry, this act supersedes the public’s right to know to what we are being exposed and the health hazards resulting from these exposures. And without full disclosure, we are unable to make educated decisions as to which chemicals we want to avoid.

Topical Flea Killer Laboratory Studies

Obviously, products undergo testing in order to qualify for EPA registration, and presumably, most of the overt dangers a product can exert are ameliorated before the product can be marketed. Scientists use healthy, adult, genetically identical mammals to test pesticides, and then extrapolate health information regarding the safety of the product to domestic animals and human beings. In the case of flea products, the laboratory tests are performed on live mice, rats, cats, and dogs.

These toxicological (poison) studies are performed to establish the LD 50 – the oral dose at which the product would kill 50 percent of a test population – and to determine the acute and chronic effects. Throughout and following the test, subjects are killed in order to study the specific system damage (lungs, kidney, etc.). Acute disease tests, such as nervous system and skin reactions, can be performed over a relatively short time period. Most studies are conducted for 3-, 13-, or 52-week intervals, and use exaggerated dosages to compensate for the short testing periods.

“Because of the short period under which the studies are conducted, the health effects resulting from the higher doses of the chemicals are relevant,” says Dr. Virginia Dobozy of the EPA’s Pesticide Division. These effects can include head-nodding; facial twitching; exaggerated blinking; gag responses; weight increase of the spleen, thymus, and adrenal glands; and/or atrophy of the thymus.

Long-term studies, needed to understand the chronic effects of the pesticides, are few by comparison. Chronic disease such as cancer, immune suppression, developmental or reproductive damage, and DNA damage can take months or years to manifest.

However, the cumulative effect – potential damage from continued use of one specific pesticide product or multiple products over a dog’s lifetime – is unknown. Also unknown is the potential for synergistic effects – combined impacts of chemical exposures from their home and outdoor environments. Neither the cumulative nor the synergistic effects of chemicals in products are required to be tested by the EPA before a product is made commercially available. So, our dogs may be more vulnerable to unknown chemical-related dangers than the happy commercials would have you believe.

Critics of the pesticide industry claim that the EPA registers pesticides not on safety, but on a cost-benefit basis, balancing health and environmental concerns against the economic gain to the manufacturer and the end user of the product. But even if the pesticide manufacturers and the EPA are not overly concerned about our safety, we as consumers and guardians should be very concerned.

Spot-On Flea Treatments: Too Good to be True

Today, spot-on flea preparations are considered by many as the Rolls Royce of flea products, and sell swiftly in veterinary clinics and pet stores. Each of the makers of these products claim that they are safe – safer than ever – and that only the targeted insects will be affected by the products’ neurotoxic impacts. The products are frequently advertised as safe for small children and adults as well as puppies (over eight weeks) and geriatric dogs. Do they sound too good to be true? Well, perhaps they are.

The spot-on flea products fall into four general categories of insecticides. All have neurotoxic effects. The first three – imidacloprid (a chloro-nicotinyl insecticide), fipronil (a phenylprazole insecticide), and permethrin (a synthetic broad spectrum pyrethroid insecticide) – all work by disrupting the nervous system of insects, killing by contact or ingestion. The fourth type contains insect growth regulators (IGR), which don’t kill, but interrupt the flea’s life cycle.

Imidacloprid is the first of its class of insecticides, and is relatively new on the block; it was introduced in 1994. Laboratory testing on mice, dogs, and rats, indicates that this insecticide can be neurotoxic to laboratory animals, causing incoordination, labored breathing, thyroid lesions, reduced birth weights, and increased frequency of birth defects.

Fipronil was introduced in the United States in 1996. It is a neurotoxin and suspected human carcinogen. Fipronil can cause liver toxicity, thyroid lesions (cancer), damage to the kidneys, increased cholesterol levels, alterations in thyroid hormones, incoordination, labored breathing, increased miscarriages, and smaller offspring.

In a review of the fipronil pet formulations, Dr. Virginia Dobozy of the EPA’s Pesticide Division states that “this is a persistent chemical that has the potential for nervous system and thyroid toxicity after long term exposure at low dosages.”

Permethrin, a synthetic broad spectrum pyrethroid insecticide, is suspected to be an endocrine disrupter and a carcinogenic insecticide (causing lung cancer and liver tumors in laboratory animals). Some permethrin products have additional “active” ingredients in lesser percentages, and include methoprene, and pyriproxyfen (described below).

Methoprene and pyriproxyfen are both insect growth regulators (IGR), which limit the development of juvenile fleas so they cannot reproduce. Test results indicate that methoprene causes enlarged livers and degeneration of parts of the kidneys.

All of the above active ingredients have induced responses in laboratory animals that give cause for alarm. While these new products are suggested as safer than their predecessors, they indicate high levels of acute and chronic poisoning from short-term use.

Go-To Methods of Action

Whether or not it is purposeful, manufacturers of these spot-on flea products have managed to convince many veterinarians and animal guardians that these products are not absorbed into our dogs’ systems. The companies’ literature describes in vague and contradictory detail how the chemicals don’t go beyond the hair follicles and fat layers of the dogs’ skin.

When the EPA’s Dr. Dobozy reviewed the results of a fipronil metabolism study (fipronil is the active ingredient in Frontline), she reported that “significant amounts of radio-labeled fipronil were found [not only] in various organs and fat . . . [but they were also] excreted in the urine and feces, and were present in other parts of the body…which demonstrated that the chemical is absorbed systemically.”

Veterinarians and pet owners who pay close attention can witness evidence that these products are indeed systemically absorbed. Dr. Stephen Blake, a San Diego veterinarian, relates a client’s experience: “We put Advantage on the backs of our dogs and could smell it on their breath in a matter of minutes following the application.” Blake stated that this indication of immediate absorption did not tally with what he had been led to believe by reading Bayer’s literature. He continues to question its safety for his clients’ animals.

Neurological Health Effects of Topical Insecticides

Logic tells us that a topical chemical that is not absorbed into the skin has no chance of causing neurotoxic effects. Then why do the Material Data Safety Sheets (MSDSs) for all the permethrin-containing pesticides recommend preventing their products from having prolonged contact with the skin? And why do they all state that skin sensations, such as “numbness and tingling,” can occur? Schering-Plough’s MSDS makes an additional statement about its Defend EXspot Treatment: “can be harmful if absorbed through the skin and harmful following inhalation,” causing headaches, dizziness, and nausea.

Bayer does not reveal more than 90 percent of the ingredients in Advantage, but its MSDS does warn us to “use a respirator for organic vapors” in order to avoid “respiratory tract irritation and other symptoms such as headache or dizziness” (symptoms of nervous system exposure). Bayer’s promotional literature for Advantage, however, states that “studies prove that using 20-24 times the dosage on dogs and cats does not cause any internal or external side effects,” and that “. . . switching to Advantage from another flea control product poses virtually no risk to your pet.”

Dr. Graham Hines, a veterinarian from the United Kingdom, treated a four-year-old female German Shepherd who had two Advantage Top Spot treatments. He reported that “both times she became unusually clingy, and would not leave her guardian’s side, yet paced up and down all day, very restlessly. These symptoms persisted for 48 hours before a gradual return to her normal state.” The neurotoxic effects were clear to Dr. Hines.

Dr. Blake also finds different results than the Bayer literature. “We are told that the product affects only insects’ nervous systems, not mammals’. Several of my clients told me that they accidentally got some Advantage on their hands and when they touched their mouths, their lips became immediately numb for several hours. So much for not having an effect on the nervous system of mammals.”

Acute symptoms of headache, nausea, and abdominal and lumbar pain are associated with carbitol, one of the “inert” ingredients in Frontline. According to the MSDS, carbitol induced these symptoms in laboratory settings.

Curiously, these potential side effects are not published in the literature accompanying the products, nor do many veterinarians know the dangers. But there are numerous anecdotal reports from veterinarians in the U.S. and the U.K. of dogs who were treated with spot-on products who have displayed signs of neurological damage, such as depression, lethargy, convulsions, underactivity, tremors, overactivity, stiffened limbs, and lameness.

Adverse Skin Effects

Topical skin irritation is listed on all the MSDSs of the products reviewed in this article; however, product literature inserts fail to emphasize the extreme nature of the problems. They all instruct the users that their products are for “external use only,” and to “avoid contact with the skin,” but only Merial’s product insert appears to suggest there is some possibility of adverse skin contact reactions.

Dr. Dee Blanco, a holistic veterinarian practicing in New Mexico, treated 20 dogs for adverse reactions to Farnam’s flea product. In a letter to the Farnam regarding a client who had used one of Farnam’s permethrin-based insecticides, Dr. Blanco stated, “All the dogs (20 out of her 24 dogs treated with BioSpot ) had pruritus (severe itching of the skin) with bleeding and cracking of the skin, various degrees of erythema (intense redness of the skin), many fluid vesicles (blisters), severe hair loss, and elephantiasis (thickening of the skin) with chronic itching. Many also showed severe mental depression, lethargy, and symptoms concomitant with aggravated liver toxicity. All symptoms appeared within two weeks after applications of your (BioSpot) product, also a consistent time-frame for liver toxicity after absorption through the skin. . . To date, most of the dogs have dramatically improved but a few still remain symptomatic.”

Dr. Blanco also stated that one dog died of liver cancer within three months of this BioSpot application, which she says “could have been exacerbated by the application of BioSpot.” Permethrin is indicated as a possible carcinogen by the EPA, causing liver enlargement and cancers in laboratory mammals.

When Dr. Dobozy reviewed the reports from fipronil product studies, she found that Frontline “does not adequately describe the severe reactions” reported by veterinarians – sloughing, “chemical burn” conditions, and extensively affected areas well beyond the application site. When these incidents were reported, Merial recommended bathing the dogs. That’s strange, because their literature indicates the product remains effective after bathing.

The MSDS for Bayer’s Advantage tell us that “prolonged contact with the skin can cause defatting of the skin due to solvent component in the products,” to “avoid skin contact,” “to wear appropriate gloves when handling the product,” and to “wash off any contamination.”

Chronic Disease Linked to Flea Killers

Based upon toxicological studies, a dog suffering from liver, kidney, thyroid, adrenal, spleen, lung, brain or gonadal conditions could experience heightened states of chronic diseases, with the potential for development of cancer, when spot-on flea preparations are used. Permethrin is linked to malignant liver and lung tumors and autoimmune system disease, and at very low levels suppresses the immune system. Thyroid lesions have developed in laboratory studies in dogs during imidacloprid tests. Further studies are necessary to understand the possibilities of malignancy. Thyroid cancer has been linked to fipronil, according to the EPA. The data from the metabolism and chronic toxicity studies for fipronil indicate that ” . . . this is a persistent chemical and has the potential for nervous system and thyroid toxicity after long-term exposure at low levels,” according to Dr. Dobozy.

In the Journal of Pesticide Reform, author Caroline Cox cites studies that show thyroid sensitivity to imidacloprid can result in thyroid lesions, as well as increased incidences of miscarriages, mutagenic (DNA damage) abnormalities, and abnormal skeletons in animal studies. In addition, one metabolite (breakdown of the chemical into new chemical compounds during the metabolism process in the body) of imidacloprid appears to be far more toxic to mammals than the imidacloprid itself.

General Risk Factors

Of course, not all dogs exhibit immediately noticeable symptoms when dosed with a commercial spot-on flea product. Adult animals and those in the peak of health are less likely to show immediate signs compared to animals that are young, old, or suffering from chronic disease. Animals with a heightened sensitivity to chemicals or with exposures from multiple sources such as a flea collar; other dips, sprays, dust, or flea bombs; yard pesticides; and house termite extermination, are most likely to react. The cumulative and synergistic impacts of pesticides can take a heavy toll on animals.

Dr. Jerry Blondell, of the US EPA Office of Pesticides, has indicated clearly “not to use pesticides on the old, the sick, or the young.” While some of the literature for the spot-on products does discourage this usage, many dog guardians and veterinarians overlook or disregard these written precautions.

Although the number of dogs reported to react to these products may seem small, this does not suggest the overall impact is small. First, spot-on products are relatively new, and many problems are cumulative.

Second, reactivity to chemicals in a population is similar to other population statistics and is represented by a bell-shaped curve. In other words, at one end of the spectrum are sensitive individuals, and at the opposite end are resistant individuals; these groups are relatively small compared to the vast middle group, who show varying degrees of susceptibility – but who are all susceptible. Thus the sensitive group – dogs who have displayed signs of toxicity – happen to be the sentinels for the younger, healthier ones who will eventually be affected; it’s just a matter of time.

Safer Alternatives to Flea Resistance

Integrated pest management (IPM) is a nontoxic approach used to eradicate any insect infestation. Simply, it is a way of thinking about how to preserve the quality of life on this planet and within the earth’s stratosphere – of understanding not only the damages of the pesticide to all species and the environment, but also understanding the consequences of insect resistance to the constant parade of new, more sophisticated, and perhaps more toxic pesticide formulas. The IPM process was initially designed to safeguard all species, including the environment, from the ravages of pesticides.

In the next issue, we will present a complete indoor and outdoor IPM treatment program for effective, non-poisonous flea control.

Also With This Article
Click here to view “Flea Control: Best, Safest Practices”
Click here to view “Flea Tip #1”
Click here to view “Natural Herbs for Flea Relief”

Kathleen Dudley is a writer and photographer who lives in New Mexico.