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What is cat flea allergy dermatitis (FAD)?

Flea allergy dermatitis refers to an allergic reaction that cats can have to the bites of fleas. Flea saliva contains at least 15 substances that can trigger an allergy so it’s not surprising then that flea bite allergies are among the most common allergies in our cats.


By far not all cats with fleas will develop an allergy to them. Most, in fact, will not show other signs than the occasional scratching and you might discover they have fleas only when you see the little parasites or get bitten yourself.


There is no specific age at which cats can get FAD. Both males and females are equally affected.


How to recognise flea allergy symptoms?


Fleas are not that small and can be spotted with the naked eye, so it makes sense that the first sign to look for is the parasites themselves.


But flea-allergic cats are usually grooming themselves so thoroughly and their blood contains so much anti-flea antibodies that fleas don’t like spending time on them and the parasite numbers are usually very low. Therefore, you may not see fleas or flea dirt on your cat.

The symptoms may get worse during the warmer months when the flea population grows and get better in the winter, when there are less fleas around. But the seasonality of the symptoms is not as pronounced as in the case of the atopy.


An allergic flare-up in cats with FAD usually starts with an itch that can become very severe as the time passes. The itchiness can often go unobserved though because cats are fastidious groomers anyway and they often prefer to do this when alone.


Later you might see:


  • skin redness, spots, weepy or crusty areas,

  • patches of thinning coat or even complete baldness,

  • thickening and darkening of the skin.

The affected areas are lower back and tail base, inside of thighs, but allergic cats also very obviously groom their lower belly and flanks to almost complete baldness.

Some cats also get sores on their lips or body.


When to take your cat to the vet?


If you suspect your cat may have a flea allergy, it’s recommended to speak to a vet as soon as possible.


The anti-flea antibodies concentrations in your pet's blood will vary depending on how many flea bites they get, giving milder or worse symptoms (allergic flare-ups). But unfortunately, allergies of any kind do not go away by themselves and the flare-ups get worse if measures are not taken to control them.


You should also take your cat to the vet if you see fleas on them despite using flea treatment as this means that, for whatever reason, the current flea treatment is not working.


How is flea allergy diagnosed and treated?


If your cat has bad skin symptoms, your vet will start with medication to treat these first, for example:

  • antipruritics to reduce the itching (possibly, but not always steroids),

  • antibiotics to control secondary infections,

  • anti-inflammatories to help with the skin irritation.

  • A thorough medical history of your cat will also be taken and a complete physical exam performed to determine if a flea allergy is the most likely cause of their symptoms.

An allergy to flea bites can be measured by getting a blood sample from the cat and sending it to the lab for testing.


But often it is simpler and cheaper to prescribe an effective and continuous flea prevention regime for the allergic cat and all other pets in close contact. Many cats with FAD have an irritated skin and a spot-on product or a flea shampoo might make it worse. For these cats a flea prevention tablet is a better option.


Fleas have several life stages (adult, egg, larvae, pupae) and spend big chunks of some of these in the environment, so it is essential to treat your home as well. The products for this can be obtained at your vet or over the counter. See our article on how to deal with fleas in your cat for detailed information about fleas and how to get rid of them.


If the symptoms resolve completely, this confirms the diagnosis. (If they resolve only partially or not at all, other types of allergy, like food allergies or atopy should be investigated).


What can you do to help your cat with flea allergy?


If you have a cat with FAD, the best thing you can do is to treat all your pets against fleas all year round. Preventing your cat from getting flea bites is the absolute best way of stopping FAD symptoms from developing.


Use vet-prescribed products. Many over-the-counter flea products don’t work very well, either because the substances included in them weren’t very effective to start with or have been around for decades and fleas developed resistance to them.


Monitor your allergic cat closely and get in touch with a vet as soon as you see the beginning of a flare-up.

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