As I am sure you are aware the mystery of what makes a cat have incessantly itchy skin can be a real challenge to get to the bottom of. It is because itchiness and hair loss in cats can be caused by a number of things. We can see it with mites, fungal infections, and bacterial infection, but as you didn't not any skin changes (pustuls, pimples, scaling, crusts, etc), these are less likely here. As well, the most common reasons for feline overgrooming are either stress based or allergy based conditions. Now if you are seeing him itch/scratch, then stress based causes are unlikely. Instead, we 'd be considering the more irritating allergy based differentials that would drive him to target this area. Just to note, with allergies, these can be to literally anything in their kitty world. This includes food, parasites (like fleas), pollens, and anything that they essentially come into contact with.
Now based on his history, life style, and the area he is targeting; I'd be most concerned about a flea allergy dermatitis. As I am sure you can appreciate, this is one of the most common types of allergies of the cat. The problem with wee kitties with sensitivities to fleas that they aren't allergic to the fleas themselves. They are allergic to its saliva. So, all it takes is one flea having a nibble to start things off. The body releases histamines and the allergic response takes over. This is itchy and sore, so our cats may itch, scratch and some will even start pulling out tufts of hair. They scratch which can irritate the skin even more, and its a vicious cycle (they itch b/c its itchy, but its itchier because they itched, if you know what I mean). As well, as they are causing damage to the skin from scratching, they will open the door for bacterial infection (which makes it even itchier). If it is flea saliva induced, the flea that may have bitten him is probably long since dead. Cats caught in this kind of allergic response often respond well to monthly flea treatment but sometimes need additional short term medications to help break the itchiness cycle. This is why I was so keen to know they were up to date on flea meds and I would note that if it has 10 months since his last treatment, it'd be ideal to cover that now. Further to that, I'd just warn that Frontline isn't as good as it used to be and we are seeing flea resistance. Therefore, it'd be better to use a more up to date product (ie Advocate, Activyl, Advantage II, etc) to make sure this is covered for him.
Otherwise, if you see this twice a year, then we'd have to consider a potential environmental allergy. As I mentioned before, kitties can also have allergies to the world around them. I have had patients that have been allergic to grass (which I think is such a cruel joke of the universe). Sometimes these animals will only show signs during particular seasons, so if you have noticed it being worse at different times of the year then this could be a hint that an environmental allergy is a distinct possibility. And then we'd have to consider whether it is pollen, harvest mites, or something present this time of year.
Now with all of this in mind, if his signs are mild, then we can help reduce the allergy trigger for itching via treating with an antihistamine. Often we will use OTC Piriton (chlorphenamine) at a dose of 0.5 tablets twice daily. Since he is a communal cat and likely to roam, you can alternatively use Cetirizine at a dose of 5mg once daily. Either one can be crushed into food and can just settle allergic skin reactions, decrease general itchiness and may just be enough to help give him some relief. We like to keep the dose low in kitties, as they can have drowsiness with this medication (just like people). And of course, this medication shouldn't be used if he has any pre-existing conditions or are on any other medication without speaking to your vet.
Further to all of this, if his signs are very severe or he isn't going to be around enough to ensure you can use the antihistamine; I would note that you may want to consider having him seen by your local vet. They can confirm the nature of his itchiness and administer a long acting steroid injection to soothe the skin, stop the itching, and avoid you needing to medicate him.
Overall, we do see itchiness and overgrooming in the cat for a range of reasons. In his case, I'd be most concerned about flea allergy dermaitis or a reaction to seasonal allergens. Therefore, do keep up on the flea medications, since its an economical way to rule this differential out. And do also consider an anti-histamine trial to block the 'allergic response' and halt his skin irritation. But if you are struggling at all or if he has already caused severe damage to the skin, then he may benefit from a long acting steroid injection from the vet to just halt this for him and stop his overgrooming.
I hope this information is helpful.
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