Categories: Cat Flea

Flea Treatments Can Kill

Photo by Meneer Zjeroen (opens in a new window)

“Flea treatments can kill” sounds a bit frightening but it is true. The spot-on flea treatments can kill your cat. And if they don’t kill your cat they can make your cat ill. That is not to say that they are inherently dangerous – they are not. In fact they are very effective and a good last resort if flea combing can’t contain the problem. The products may even be used as a preventative measure. Frontline, for instance, protects against fleas and ticks…..

The Instructions on the packet should be read and understood! It will save lives.

….But applying spot-on flea treatments is not a super convenient method to control fleas while ignoring the whole picture. What I mean is there is little point in routinely dropping this rather toxic chemical onto the skin of your cat on a regular basis to control fleas that are on your cat whilst letting the fleas breed and live in the carpet or on the furniture. Just treating the cat is convenient but the cat takes all the burden when it should be spread around a bit by us doing some work on the home to control fleas there.

There are various ways of controlling fleas in the home. One is simply keeping it clean and hovering it regularly, on a basic level. In addition, an environmental insecticide can be used such as Acclaim and Indores. A non-insecticidal spray is Skoosh. I don’t like the idea of spraying the home with chemicals myself but there again fleas aren’t great either. In the right place some success can be achieved with food grade diatomaceous earth (try and type that!) but this would not suit your typical household. See Cat Flea Life Cycle (new window).

A report by ASPCA ( The American Society for the Prevention of Cruelty to Animals®) highlights the dangers of spot-on flea treatments. Actually you don’t really need to see the horror stories of cats becoming ill from this sort of treatment because the packet that contains the product is pretty scary!

If you read the small print: “ONLY USE ON CAT” – first problem don’t use cat flea treatments on dogs and vice-versa. Using dog flea treatments on cats can cause serious illness and death. I bet a few owners have done this and found out the hard way. It is something that is very easy to do when pressed etc. More instructions: “Do not use on sick or convalescing cats, nursing queens or kittens under 12 weeks old”. The last bit seems particularly important.

At the base of the packet it says: “IRRITANT” – in other words we shouldn’t get it on our hands but it is OK to put it on our cat’s skin. See what I am getting at?

The spot-on flea treatment that I have contains an active ingredient called: d-limonene. This is a chemical extracted from citrus rind. The chemical has a wide range of uses including, paint solids, cooling liquid, solvent. (src:

Neem oil is another ingredient:

Neem oil is a vegetable oil pressed from the fruits and seeds of Neem (Azadirachta indica), an evergreen tree which is endemic to the Indian subcontinent

It can be used for a variety of purposes including:

preparing cosmetics (soap, hair products, body hygiene creams, hand creams) and in Ayurvedic, Unani and folklore traditional medicine, in the treatment of a wide range of afflictions

(the quotes are from Wikipedia). There are other ingredients.

The active ingredient in Frontline the best known topical flea treatment is Fipronil:

Fipronil is used as the active ingredient in Frontline Top Spot at about 9.8% concentration, and is used together with (S)-methoprene (8.8%) in Frontline Plus, a topical flea and tick control commonly used on dogs and cats. It kills adult fleas before they lay eggs. It is also the active ingredient in GardenTech’s Over N Out season long fire ant control product. After a local application of Frontline, fipronil is slightly absorbed (approx. 15%) through the skin. Low levels of fipronil may be detected in the plasma, with a very high variability between dogs. (source: Wikipedia verbatim quote as allowed under their license)

The point is this. When used inappropriately these chemicals can sometimes harm or, on rare occasions, kill your cat and even when used as instructed they can kill (albeit very rarely) or cause illness (less rarely). Here are the figures from ASPCA on how flea treatments can kill or upset cats. The table compares percentage of cats who were affected negatively when the product was misapplied and applied correctly from all calls to ASPCA – note: these figures do not represent percentages from all cats receiving this treatment. They represent a breakdown of cats that became ill from the treatment. I don’t know the percentage of cats that become ill after being treated:

Not Applied per directions on packetApplied per directions on packet
no illness despite calling ASPCA18%7%
mild illness17%69%
moderate illness45%22%
major illness19%2%

A rather odd finding is that of all cats that became ill, more cats became mildly ill when being treated correctly than when being treated incorrectly. This may just be an anomaly.

Bottom line: when applying spot-on flea treatments in defiance of the instructions there were a lot more cases of adverse reactions and sometimes flea treatments can kill – 2% killed. If ASPCA got 1000 calls from anxious people after applying this product, 20 of those calls related to cats that had been killed or were dying.

Flea Treatments can Kill to Home Page

To Cat Health Problems

Cat and dog Parasite Pictures (new window).

Can Cat Fleas Bite Humans (new window)

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Michael Broad

Hi, I'm a 71-year-old retired solicitor (attorney in the US). Before qualifying I worked in a many jobs including professional photography. I have a girlfriend, Michelle. I love nature, cats and all animals. I am concerned about their welfare.

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