Are ‘Safe’ Flea Medications A Threat to Our Health?
A large variety of insects and other pests bother us in our daily lives. Mosquitoes bite us, ticks latch onto us, cockroaches invade our homes, and fleas irritate our pets. Naturally, we want to keep these insects out of our lives as much as possible, so we use bug sprays, traps, and medicines to keep common pests at bay. In 2005, it was estimated that over 80% of U.S. households use pest-control substances once a year or more (Chambers et al., 2007). However, many of these countermeasures endanger not only the health of undesirable insects, but our own as well.
Today, we will be focusing on the health impacts of flea medications, particularly the newer and safer varieties. Earlier types of flea medicines, such as flea collars, flea shampoos, dips, powders, and sprays, which generally used a group of pesticides called organophosphates, which are known to degrade quickly but significantly increase the chances of humans and pets of developing various health problems (even today, these chemicals are used in many lice treatments for children). While these forms of flea treatments have declined in use, understanding their dangers is key to understanding the newer treatments that have replaced the older ones.
The two primary organophosphate chemicals used in older flea treatments are chlorpyrifos and tetrachlorviniphos (Chambers et al., 2007) (Davis et al., 2008), and the dangers of both are well-documented. When applied to small rodents such as rats, chlorpyrifos can change their behavior and neurochemistry, effectively acting as an antidepressant (Savy et al., 2015). For humans, chlorpyrifos has been shown to make its way into the umbilical cord blood of unborn children whose mothers are exposed (Liao et al., 2011). Afterwards, babies whose mothers were exposed to chlorpyrifos during pregnancy were found to have lower birth weights than babies whose mothers were not exposed (Whyatt et al., 2003).
These are just a small sample of the many studies on the health effects of chlorpyrifos, which has since been heavily restricted and is no longer used for flea treatments in the U.S. (Chambers et al., 2007). The other organophosphate, tetrachlorvinphos, has similarly been found to easily enter the bloodstream through skin exposure (Davis et al., 2008), and animal studies suggest that it can increase the risk of cancer in humans (Guyton et al., 2015). However, tetrachlorvinphos has not been restricted to the same extent as chlorpyrifos (Davis et al., 2008). Pets treated with either chemical can leave residues throughout the houses they live in and outdoors, which thus allows their owners to be easily exposed (Davis et al., 2008). Flea collars are especially dangerous, as they are worn for long-periods of time, while the other treatments are intended to be used in the short-term.
Of course, organophosphate pesticides are not used in newer flea treatments, such as topical medicines places between the shoulders of a flea-ridden pet or pills administered orally. Any of several classes of chemicals are used in current treatments, and are generally thought to be safer than organophosphate chemicals. However, though these chemicals may be safe relative to organophosphates, they may not necessarily be safe in the sense that being exposed to them while treating your pets will have no negative health effects whatsoever. Next, we will go over what these newer and ‘safer’ chemicals are and what negative health effects they may be causing. After that, we will go over a few simple ways to minimize your exposure to these chemicals while using them.
Our first class of chemicals is the pyrethroid group (and the nearly-identical pyrethrin group), a collection of chemicals commonly used in a variety of pest control applications due to their reputation for safety and low-toxicity to mammals, as well as their ability to quickly degrade (Atmaca and Aksoy, 2013). However, their effectiveness in some of these applications, such as flea and lice treatments, are declining as some pests are becoming more resistant to these pesticides (Bass et al., 2004). Moreover, some studies have indicated that pyrethroids can, in fact, have some negative health effects on humans. One study that examined liver and kidney DNA in rats found that exposure to phenothrin, a particular variety of pyrethroid, can mutate and otherwise damage the liver and kidney DNA of rats (Atmaca and Aksoy, 2013). While this study only proves that rat livers and kidneys are affected by pyrethroids, it still shows that there is a possibility that pyrethroids can affect human livers and kidneys, potentially causing cancer or other forms of damage.
In addition, another study looked at the effects of pyrethroids on both humans and rats, and found that pyrethroids are neurotoxic (Soderlund, 2002). Although pyrethroids have a hard time entering the bloodstream through skin contact, they are still capable of inducing pins-and-needles feelings in this manner, and can more strongly affect neurological functions through oral exposure, which allows the chemicals to spread rapidly through the bloodstream (Soderlund, 2002). Oral exposure can result in heightened blood pressure and increased brain glucose levels (Soderlund, 2002), thoroughly demonstrating the toxic potential of pyrethroids.
The second class of chemicals used in current flea treatments is the neonicotinoid group, specifically the chemical imidacloprid. Imidacloprid is generally considered to be more dangerous than pyrethroids, as both the U.S. Environmental Protection Agency and the World Health Organization have labeled imidacloprid as being moderately hazardous (Fahim et al., 2009), though this greater risk likely comes with greater effectiveness than pyrethroids. On the other hand, the studies that have indicated imidacloprid’s danger have largely been animal studies (Fahim et al., 2009), and studies regarding imidacloprid’s effects on humans have been plagued with confounding variables (Cimino et al., 2017). In addition, even in cases where humans have been suddenly exposed to large amounts of imidacloprid on their skin, the people exposed did not exhibit much in the way of adverse symptoms, though chronic exposures may have different effects. Either way, evidence of imidacloprid’s dangers is limited to animal studies, and its effects on humans are currently unclear.
Next on the list of chemicals used in flea medication are avermectins, which are typically found in oral flea medications (often in the form of selamectin)and are often used in agriculture to treat livestock suffering from parasite infestations (Zhang et al., 2016) (Butters et al., 2012). These chemicals are generally considered to be more dangerous than pyrethroids or imidacloprid, and the Environmental Protection Agency even went as far as to label avermectins as being extremely toxic to humans (Zhang et al., 2016). Researchers have found that avermectins can have some disturbing effects; they can outright kill cells by damaging their DNA and impairing the function of the mitochondria (Zhang et al., 2016), which normally provide a cell’s energy.
Avermectins can also acts like hormones in mice (and potentially within the human body), binding to receptors in the brain that normally accept the hormone GABA (Dawson et al., 2000), an important neurotransmitter that, among other functions, plays a role in neural development, making avermectin exposure a potential concern for pregnant women. In addition, high doses of avermectins have been shown to outright kill mice (Dawson et al., 2000). Overall, these findings are very concerning for both humans and their pets who may be treated with avermectins. However, skin exposure may be less of a concern given that avermectin-based flea medications are administered orally.
Our last group of chemicals used in flea treatments are phenylpyrazole insecticides, specifically the insecticide fipronil. Similar to avermectins, fipronil is used for both agricultural and household pest control, and its residues have been found in many American homes (Lee et al., 2010), indicating that the use of fipronil-based flea medications may lead to long-term, chronic exposure to fipronil. This leaves the question of whether or not chronic exposure to fipronil can be dangerous to humans, and research indicates that there are some possible health risks.
First, short-term exposures, such as when pet owners treat their pets with fipronil-based topical flea medication and subsequently play with them, can have a myriad of mild but unpleasant effects, including dizziness, headaches, fatigue, eye problems, digestive issues (nausea and vomiting), respiratory issues (coughing, irritation, wheezing, exacerbating asthma), and skin rashes (Lee et al., 2010). These short-term effects alone are enough to show that you should be very careful when using fipronil-based flea medications on your pets, but we have yet to look at possible long-term effects from exposure to fipronil.
Fipronil is an endocrine-disruptor (Mnif et al., 2011); this means that when someone is exposed to it and it enters their bloodstream, it acts like a hormone. In particular, animal studies have suggested that fipronil inhibits the production of thyroid hormones (Mnif et al., 2011). My mother has thyroid issues, and as a result, I know that thyroid hormones primarily regulate metabolism, so, among other symptoms, thyroid issues can alter blood pressure, induce fatigue, and affect the heart. In addition, hormones in general can have a strong effect on early body development, meaning that chronic exposure to fipronil could affect the development of unborn children, thus making it especially important for pregnant women to avoid exposure to fipronil.
Finally, fipronil has been to shown to act as a mutagen in mice at high exposure levels (Oliveira et al., 2012). This means that it change and damage the DNA of mice cells that it contacts, which would suggest that fipronil could be carcinogenic, and DNA damage and mutation is generally what causes cancer. This would also bode poorly for an unborn child, as DNA mutations and damage at such an early stage of development could very well cause long-term damage and health problems. In short, fipronil is something you should stay away from in general, and doubly so if you are pregnant.
So, put together, what does all of this mean? First, the danger of older flea medications is fairly clear, so you should avoid any flea collars, shampoos, powders, dips, and sprays. Second, each of the main pesticides used for newer flea treatments, oral and topical medications, come with their own dangers. Pyrethroids are neurotoxic, avermectins are cytotoxic and act like hormones, while fipronil can have a variety of negative health effects.
Only imidacloprid’s effects appear to be entirely unclear, as many mammalian mammals are negatively affected by it, but there is little evidence to suggest that it negatively impacts humans, and even a little evidence to the contrary. Of course, this is not to say that you should use imidacloprid flea medications with absolutely no caution, as their safety has not been proven, and even if they do not negatively impact you, they could negatively impact your pet. Moreover, the research on imidacloprid’s negative effects have focused on grown humans, and there is no telling if it could affect unborn children, so pregnant women should be especially cautious with imidacloprid medication.
The best option is unclear: on the one hand, imidacloprid’s impacts on human health have not surfaced. On the other hand, Avermectin-based oral medications are unlikely to expose you to significant amounts of the chemicals, as avermectin pills will not leave avermectin residues on your pet (who can then potentially spread residue throughout the house). However, given the high toxicity of avermectins, they could negatively impact your pet who would be ingesting the medicine.
Of course, at this moment, you may be wondering about a third option: natural alternatives. “Surely,” you may be thinking, “there is some form of herbal or other plant-based medicine I can use on my pets?” And you would be correct: there are essential-oil based flea medications available by the market. Unfortunately, these unregulated natural alternatives are far from a good solution to the issue of flea medication toxicity, as these substances have their own adverse effects on pets, including agitation, seizures, vomiting, and a variety of other side effects (Genovese et al., 2012).
Fortunately, whichever flea medication you use on your pets, there are some simple ways to minimize your exposure to the pesticides in them. First, before treating your pet, you can take them to a particular room, treat them there, and confine them in that room for a short time afterwards in order to prevent the medicine’s residue from being spread in your house. Second, make sure that you and anyone else in your home minimize contact with the treated pet. Third, you may consider wearing gloves while applying the medicine.
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