Both cats have bald spots near tail, same spot, same time — what’s going on?

Pictures in the comments

Both of my cats have developed bald spots in roughly the same areas, and around the same time. One has hair loss on the underside of the belly and tail base, the other at the tail base and back knees (outer joints). They’re licking these areas, but not creating wounds. The skin looks mostly fine — not red or crusty.

Some background:

• They had fleas about 2 months ago. We treated both with a topical flea pipette, and the fleas seemed to go away.

• We haven’t retreated them since.

• Now I’m wondering if this is a return of fleas, or a delayed reaction (flea allergy?).

• They are otherwise healthy and acting normal — eating, grooming, playing.

Last week I took one of them to the vet because her symptoms started just a bit earlier. The vet suspected either a food/environmental allergy or possibly bladder crystals (though she’s showing no pain or urinary symptoms). We are trying to get an urine sample, so they can research this. The other recommendation is prednisolone, but I’m hesitant to start steroids without a clearer diagnosis.

Other info:

• The cats are indoor-only, yet they are allowed in the backyard.

• No recent changes in food, litter, detergent, or routines.

• The licking is focused in classic flea allergy zones: tail base, belly, knees.

• The fact that they both have the exact same issue at the same time makes me think this is external or environmental, not behavioral or stress-related.

• No visible fleas or flea dirt, but I know that doesn’t mean much. The vet did check for fleas and said she seems clear.

So…

Could this still be flea allergy even if I don’t see fleas?

Is the bladder issue a red herring?

Would you try another round of flea treatment first before considering steroids?

Any experience with this kind of symmetrical hair loss?

Really appreciate any input — thank you!