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Fipronil is classified by the World Health Organization (WHO) as a Class II moderately hazardous toxin when applied orally or by inhalation, such as may happen when using the spray. It works by disrupting the central nervous system (CNS) via the gamma-aminobutyric acid (GABA) regulated chloride channel, resulting in uncontrolled CNS activity and ultimately death. Symptoms of fipronil poisoning include excitability, lack of co-ordination and tremors. It is considered slightly irritating to the skin and moderately irritating to the eyes. Fipronil has been shown to be carcinogenic to rats and has therefore been classified as a Group C (Possible Human) carcinogen. Organs that can be affected by repeated exposure include the liver, thyroid and kidney.
The GABA channel is important in nerve transmission in both insects and humans. It has been claimed that fipronil binds ‘less tightly’ to the GABA receptors in vertebrates, like humans and ferrets, than it does to those same receptors in insects, and so gives a degree of selectivity. Perhaps more likely is that the different modes of absorption of insects and mammals determine whatever differential in toxicity exists. It has also been claimed that fipronil cannot interfere with a mammal’s CNS because it is not absorbed through the skin, being stored in the sebaceous glands of the animal, from which it is slowly released via follicular ducts. But studies carried out on rats have cast doubt on this claim. Simple logic would also indicate that if it were not absorbed into the skin, then it could hardly cause skin irritation.
Even if the manufacturer’s claim that fipronil is not absorbed into the body but remains on the skin surface were true, it would only serve to make fipronil even more dangerous than it is already known to be. In the presence of sunlight fipronil is now known to break down into an extremely stable chemical approximately 10 times as toxic as fipronil itself, fipronil-desulfinyl, which has a much greater tendency to bind to the GABA receptors than fipronil.
Frontline is specifically contra-indicated for use on rabbits and comes with a warning that recently treated animals should not be handled until dry and should not be allowed to sleep with humans, especially children (The Veterinary Formulary, 5th Edition, 2001). It is difficult not to draw the conclusion that since there are safer treatments for fleas, ticks and more especially ear mites available, then Frontline should never be used on any animal, even on those for which it is licensed.
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