none of my 10 cats wanted anything to do with the cases of friskies turkey & giblets i bought and i was told it was the same formula too!! but, afterwards when i saw others complaining about their cats suddendly not wanting to eat their friskies' pates, i checked the cans from the variety packs i usually buy to the cans from the cases and sure enough formula ingredients themselves didn't change but, the order/amounts of the ingredients in the case cans sure did, making it NOT the same formula in the end to me..
Try PetSilver on your kitty, a little pricey but, it really works wonders on rashes of all types. after all else failed, it was what got rid of my two long hair cats bumpy itchy allergy rash (called military ?? something).
Also a past cat i had had a rash that look like your kitty's, loss of fur and all, he got it around his back legs too. Vet though food or flea bite/poop allergies but, change of foods/flea stuff didn't help and for about 2 1/2 yrs it would start getting better on its own and than come back full force again. i finally decided it was perhaps something on my husband's work jeans since my Oreos would bombard hubby's legs when he got home from work all the time. i started insisting my hubby start going straight upstairs and changing his work clothes and poof my cat's fur grew back in and he never got the rash/loss of hair again for the rest of his life. if not food or flea/mite allergy, perhaps than your cat is coming into contact with a chemical/work dust or something brought into your house like ours was??