Why are fleas still alive after treatment?

Why does flea-treating a pet sometimes not seem to work? Is the product not doing what it should, or are fleas ‘immune’ to the product? Our pet experts know exactly what’s going on here and have the answers pet parents need. Spoiler alert: seeing fleas alive after treatment doesn’t mean it’s not working. Phew!

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How quickly do fleas die after treatment?

That depends on the product – take a look at the product packaging or leaflet for specific details. FRONTLINE PLUS® and FRONTLINE® SPOT ON kill fleas within 24 hours of them arriving on the pet, while FRONTLINE TRI-ACT® kills fleas within 6 hours of them jumping on a dog. There are a couple of key things to remember, whichever treatment you’ve used.


Flea products never kill fleas instantly

The flea has to come into contact with the ingredient, absorb it and then start to feel the effects. It’s a little like catching a cold – time elapses between us having contact with a cold virus and then becoming ill.


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No product stops fleas from jumping onto pets

This means that, regardless of which flea treatment is used, we may still see new fleas on our pets before the flea treatment kills them. So, even if the treatment kills all fleas on a pet within 24 hours, for example, the next day new hitchhiker fleas could jump onto the pet. These new arrivals will themselves take up to 24 hours to be killed. And so on, every time a pet goes into an area where fleas are present. The important thing is that these newly arriving fleas are being killed by the treatment.

Why am I still seeing fleas days after treatment?

There are two other reasons for this. 

One is that some flea treatments make fleas more active before they die. For example, the ingredient in FRONTLINE® products kills fleas by making them hyper excited. This makes them move around up to the top of the pet’s hair where they’re easier to spot. So, seeing fleas more after treatment is a sign the product is working, strange as that might sound.

The other reason is, as mentioned above, fleas will still jump on our pets from an infested environment. So, if our pets go somewhere that’s scattered with flea life stages – such as gardens, parks or even our homes if there’s an infestation going on – new fleas will keep jumping on the pet. But don’t worry, these new arrivals will keep being killed as long as the pet is treated.

The fleas on a pet are just 5% of the problem.

95% are in the environment as flea eggs, larvae and pupae.


How long does it take to solve a flea infestation?

  • Even when using the best products, it can take up to three months of diligent treatment of both our pets and our homes before gaining the upper hand over fleas

  • This is because of the flea life cycle and the amount of flea life stages (like eggs, larvae and pupae) in the pet’s environment

  • Female fleas on a pet can lay up to 50 eggs a day each. These eggs fall off wherever our pets go, before developing into flea larvae and then pupae

  • There’s no product that can kill pupae, so while we can use household sprays like FRONTLINE HOMEGARD® around the home to kill adult fleas, flea eggs and flea larvae, we have to wait for all the pupae to develop into new adult fleas before the infestation is truly over

  • This can take a few weeks or up to three months, depending on factors like heat and humidity, and the amount of CO2 and vibrations from us and our pets

  • By treating our pets and taking the right steps to get rid of a flea infestation, we’re doing all we can to gain control of the infestation. So, hang in there!

One last thing to remember

Seeing fleas after treatment isn’t really about how long it takes for flea treatment to work or for fleas to die. It’s about the number of life stages in the pet’s environment that develop into new fleas and jump on a pet. That’s why regular flea prevention for dogs and cats at the correct treatment interval is important.

This helps our pets stay healthy and it helps stop flea infestations from developing – a win-win for us pet parents and our furry family members.

Get answers from the experts

Got a question about fleas, ticks or worms? We can help! Take a look at our FAQs or send us a message.

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