How Frontline Plus for Dogs Kills Fleas and Ticks

Frontline is a popular medication used monthly on dogs to kill fleas and ticks. It comes in a small plastic squeeze tube. The contents are to be placed on a dog, between its shoulder blades, working the neck of the tube under the fur so that the liquid reaches the dog's skin. It can clear a badly infested animal of fleas in 12-24 hours, and with a bit of help from the vacuum cleaner and washing machine pretty much wipe out an entire flea problem in a house in just a few days. How is this possible?

There are two active ingredients in Frontline, fipronil and (S)-methoprene. They are both broad-spectrum insecticides, meaning that they will kill many kinds of insects. Very little of these chemicals are absorbed through the dog's skin. Rather, the product spreads out over the surface of the skin and settles in the hair follicles and sebaceous (oil) glands. It continues to be released to the surface for about a month, at which time another dose can be applied.

Fipronil's actual chemical name is Fluocyanobenpyrazole, C₁₂H₄Cl₂F₆N₄OS, It is a slow-acting poison which allows insects to return to nest sites and infect other insects. The fipronil disrupts an insect's central nervous system and will kill adult insects. It blocks the passage of chloride ions through an insect's nerve transmission system. The chemical does not have this effect on the nervous system of a mammal because it only works on what are known as glutamate-gated chloride channels. Mammals do not have many of these kind of gates. Think of this as a gate with a lock. The fipronil has a key to open the gates in an insect's body, but not the key to the gates in a mammal's body. Once the fipronil opens the insect nerve gate, it stands in the way and doesn't allow any of the necessary chemicals to pass through. It's as if someone knew how to blockade the delivery entrance to the kitchen. After a while, the groceries in the house would be used up, and no others would be delivered, so everyone would starve. (OK, for that analogy you have to assume that there is no other way to get food into the house, but I think the point is made.)

(S)-methoprene, C₁₉H₃₄O₃, works in a completely different way to control insects. It is a growth regulator. It fools insect larvae into thinking that it is never time to change into the next stage toward becoming an adult insect by acting like a hormone which "tells" the insect to remain a juvenile. Insects have to change through several stages of growth which are very different from each other: pupa, larvae, adult. Often there are several intermediate stages as well. Methoprene keeps insects from ever growing up, and in the case of fleas and ticks it is the adult insect that causes problems for mammals.

As with any strong chemical, there are associated hazards and risks. Fipronil is considered a moderately toxic poison for mammals. If a dog or human that comes into contact with it develops nausea, agitation, vomiting or seizures, medical treatment should be sought. Fipronil poisoning can be treated and this is successful in over half of the cases. Doses large enough to harm humans are possible, but there have been only a few deaths reported in the entire world, and several of these were large self-inflicted doses.

Methoprene is not toxic to mammals when ingested or inhaled. It is actually used in some water systems to control mosquito breeding. It is only slightly toxic when absorbed through the skin. There have been no reported effects of methoprene poisoning in the few cases known where humans were accidentally exposed to high levels of the chemical.

Many people are also concerned about the effects of chemicals on the environment, and other species of animals. Of the two chemicals in Frontline Plus, the fipronil is the one of greatest concern. Fipronil is highly toxic to fish and other aquatic invertebrates. If you have a home aquarium, be very careful when handling Frontline to not transfer any to the aquarium. It is also toxic to bees, so you should avoid getting any of the liquid on outdoor vegetation. Some birds are affected by fipronil, mostly those which are not waterfowl. And there has been some evidence that it is harmful to rabbits. So if you have pet dogs and rabbits or birds, you might want to explore this topic further.

Methoprene has little effect on anything except insects. There is some evidence that it can affect the growth of crustaceans, such as lobsters. It has prevented many cases of malaria and West Nile Virus because of the fact that it can be safely used in water supplies.

While many people, myself included, have reservations about applying pesticides directly on a dog's skin, sometimes it's the lesser of two evils... the other evil being the damage and disease brought by the fleas and ticks. It is always good to be reminded that every single thing is a chemical of some kind. Rejecting the use of chemicals that can save lives, prevent disease, and protect the animals we love may not make much sense. Be sure to read the label on the Frontline Plus to choose the correct dosage for the size of your dog. But in the long run, it's a very safe product.