Wow - thanks for all the great photos and the really helpful answers to my questions! :-)
What you are describing sounds like Miliary Dermatitis.
This is an allergic skin condition in which the itchiness and scabs are typically in the tail head area, and the back legs. The skin is very itchy and the cat chews, licks, and scratches. Kitty may also "ripple" the skin often.
The skin has many small scabs and bumps (just like your photos) - supposedly they look like millet seed which is where the name comes from.
Here are some links to further information about this condition:
Miliary Dermatitis is caused by an underlying allergic condition.
The most common allergy is a flea bite hypersensitivity (flea allergy dermatitis, FAD).
With FAD, all it takes is ONE flea bite to cause this problem. It's kind of the same as how some people are allergic to bee stings. For most of us, a bee sting hurts a bit but isn't really a big deal. For others, however, a single bee sting can make their whole arm swell up or even cause an anaphylactic reaction that can be life -threatening.
In the same way, animals who have FAD get insanely itchy over a single flea bite.
In your situation, Pumpkin may have 1 flea bite a month when he goes outside. This would be enough to make him itchy! So, even though you may not see any fleas on your cat if a single flea hops on, takes a bite and then hops off, this cat could be insanely itchy for weeks!
This is an allergic problem that tends to get worse as the animal ages. Although I have not read any reports of it being triggered by placement of a microchip, if that stimulated his immune system it is possible.
Here is more about flea control: http://www.peteducation.com/article.cfm?cls=1&cat=1333&articleid=590
For more information about flea allergy dermatitis, you can check out these links:
Since this may be what has triggered your itchy cat's Miliary Dermatitis, I would strongly agree that you should treat Pumpkin with Advantage or Frontline. In Canada, you can only get Advantage through a veterinarian, but in the USA you can get Frontline at large pet supply stores (Pet'SMart, etc), or even on-line. Frontline is SAFE and effective.
Miliary dermatitis is generally treated with corticosteroids, or in a mild case with antihistamines and essential fatty acids.
You could start Pumpkin on essential fatty acids as they are helpful at decreasing inflammation and improving skin health.
Here's a link to one example:
Antihistamines are much less helpful in cats than they are in humans and will significantly help only 25 to 30 percent of cases. You mentioned that you have tried them with little effect. They are often prescribed since they are relatively safe drugs when compared to corticosteroids. Some cats respond better to one antihistamine than another, so veterinarians usually try two or three different types before giving up on them. I usually start with chlorpheniramine in cats as you mentioned.
Here is more about it:
Another choice would be atarax:
So, in summary, what you are describing sounds like Miliary Dermatitis. This is an allergic skin condition. Often it is due to an underlying sensitivity to a flea bite. However, there may be another cause of the allergic dermatitis.
If Pumpkin remains itchy despite prednisone, I would add in DermCaps, and even consider injections of DepoMedrol. It is a long acting steroid, and somteimes really allergic cats need something that strong to control their symptoms.
Your veterinarian would need to examine Pumpkin in order to be able to prescribe corticosteroids but if you wanted to try something else before seeing the vet, you could treat start him on Derm Caps, and could even consider antihistamines (doses in link above). Unfortunately, the best way to resolve this compeltely and without drugs would be to keep ALL the animals strictly indoors to avoid 100% of flea contact.
I hope that helps!
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.