The most common reason for an itchy spot and secondary fur loss is flea allergy dermatitis given the location of his lesion and the time of year his symptoms started. Fleas are at their highest numbers in areas with distinct seasons. Flea allergy dermatitis is horrible. If you don't see any fleas you might not have a huge population. And in allergic cats there doesn't need to be many, it only takes one bite a month for an allergic cat to itch like crazy. Even if he doesn't go out fleas can make their way in via screened windows and doors. Ideally I recommend topical flea products be applied every 3 weeks during the spring/summer/fall months in an allergic cat when fleas are at their peak, or with an active problem and then monthly as a prevention. Many over the counter products have a problem with fleas being resistant and some can be down right dangerous. I never recommend any products that Hartz or Sargent's make. I recommend Advantage II or Frontline Plus, or the new Seresto Flea collar. I don't normally recommend flea collars, but this one really works and lasts for at least 6-8 months.
Inhaled allergies (to pollens, dust mites or mold spores) are another possibility.
To control allergic symptoms you can try a combination of antihistamines and an omega 3 fatty acids. Ideally they should be started about a week before his symptoms usually start, as they work much better when they can stop the allergic pathway before it starts rather than trying to regulate an already ramped up allergy.
Options for antihistamines include:
1) Benadryl (diphenhydramine only, the combination products with decongestants and acetaminophen are toxic to cats) at a dose of 1mg to 2mg per pound of body weight every 8 hours. That's a half of 25mg tablet every 8 hours for a cat that is 8 to 15 pounds. Benadryl is very bitter and some cats will drool excessively or may even vomit because of that. That doesn't mean it is making him ill, he just hates the taste. If that's the case with him try a different antihistamine.
2) Chlorpheniramine at 4mg twice daily.
3) Zyrtec (Cetirizine hydrochloride) at a dose of 5 mg per cat given orally every 24 hours. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because cats cannot tolerate decongestants.
Combined with high doses of omega-3 fatty acids antihistamines work to relieve his itchiness. I like Welactin as the company that makes it is highly reputable, producing high quality products. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give him 20mg of EPA per pound of body weight per day. For example an 8 pound cat could take 160mg of EPA per day. Antihistamines and omega-3 fatty acids work synergistically, better together than either one alone. These should help reduce the itch. Be aware that antihistamines can cause drowsiness or hyperactivity which should resolve with continued use.
Food allergy is another reason for excess grooming, but should not have a seasonal pattern, so watch for that.
If he won't leave the spot alone you may need to place an elizabethan (cone of shame) collar on him to give the medications time to work and reduce the itch.
If there are other cats that he doesn't get along with outdoors that upset him, and he has more awareness of them with doors and windows being open, perhaps some of this is stress related. If so a product called Feliway, which is a synthetic version of a calming feline pheromone, may help. You can purchase this from petstores or online.
If he went outside other parasites like Cheyletiella or sarcoptic mange that need close contact to pass would be possible causes as well, although since he doesn't go out those are less likely.
For now I would start with flea control, an antihistamine and omega 3 fatty acids. If he's not significantly better in 2 weeks then it's time for a veterinary examination to look further for problems. If he has a secondary skin infection from excessive grooming that too is itchy and will need prescription antibiotics to help get under control.
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