Hello I am Dr. Joey. I'm sorry the expert you requested appears to be unavailable. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 15 years of experience. I look forward to working with you.
First, I must confirm you are using effective monthly flea control on these cats every month. Any time we have irritation on the neck, spine to base of tail, this is a flea allergy/infestation until we prove it is not. Right now effective control would mean something like Advantage (Advantage Multi) or Revolution. We are finding Frontline Plus and Frontline Topspot and the fipronil-containing knock-offs you can purchase over-the-counter ate not working well in cats. So, if you are using a fipronil product, then I recommend either purchase Advantage or get some Revolution from your vet. Then, treat ALL the pets in the household. Fleas is by and far the most common reason for multiple cats to be itchy. It is very very unlikely that multiple cats have allergies at once. Usually we only see one cat in a household affected by allergies. But it's not impossible.
For the itching, you can safely give the cats an antihistamine such s chlorpheniramine (Chlor-tabs) which comes as a 4mg tab and you give 1/4 to 1/2 tab every 12 hours. It may cause some level of drowsiness.
If you feel one-hundred percent this is not fleas (remember you may never see a flea because cats are so good at grooming them off) and you have them on effective control monthly, then you really should have your vet take a look because then I worry about a mite infestation. Again, we are looking for a highly itchy problem that might affect multiple cats (the list is very short: fleas or mites or some sort of contact irritant).
Chin acne (pimples) can be due to many possible underlying causes. If mild you can use an over-the-counter acne pad wipe as made for people once daily to cleanse the area. This often helps. If there are obvious sores, then this requires antibiotic therapy to resolve.
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