I have had cats around for decades, and until the past couple years
never had a cat with an allergy. Now I have two, unrelated cats,
different ages, with allergies.

About 1 1/2 years ago one cat, Speedy, began loosing hair. Two different
vets said the same thing, flea allergy. Speedy got a shot and some pills
to take for a week or so.  His hair grew back, I kept the Frontline on,
etc., but a few months after the hair filled in, it began thinning
again. Both vets again said it was a flea allergy and treatment began
again, and it worked.

Now the hair is beginning to thin again.

My question about this issue is, if the Frontline has been applied on a
regular monthly basis, why the flea allergy problem? Seem if there are
no fleas, there should be no allergy.

Now on to the 2nd cat. I noticed a small scratch at the edge of one eye
so took her to the vet. He took one quick look and said it was a bad
allergy. He also looked in the front of the mouth and said there was
some swelling, so was positive it was an allergy. Got a shot and some
eye drops.

Where do allergies come from? Why is it over 30+ years of cat ownership,
not one cat until now has had an allergy?

Before the allergy showed up in Speedy, all the cats ate was dry food. I
began splitting one can among the three each day. Is there anything in
canned food more likely to cause allergies than dry food?