At wits end with cat - advice?

[RosieMapleLeaf]

We have two cats, both female, both spayed, aged about 14 years. We have had them since they were kittens. One is healthy and always has been, the other has been hyperthyroid three different times, has been treated with radiation for it twice and is now treated with something we apply to her ear daily. Her thyroid levels were checked a few months ago and were fine, and she isn't demonstrating any of her typical behaviour that would say it's come back again.

The problem: one cat (or both?) has decided not to use the litter box. I believe this all started years ago when the one cat was first hyperthyroid. After she was first treated she seemed better, but since then there have (many) been occasions of not using the litter box. We assume it is the unhealthy cat that does it, but no cat has ever been caught! Last autumn we had to clean out the entire playroom and clean carpets etc because of this, now she/they have found a new spot in the basement. It stinks downstairs. :(

Same cat litter that we have always used, we have two litter boxes and they are in different spots in the basement. No new stresses, etc. Have really had enough of this now.

Any suggestions for me? They are indoor cats and always have been.

It's possible that one cat isn't happy with the other at present, and is making a 'statement' about who is dominant.

What would happen if you were to separate the cats into a room each with a litter tray overnight, say. If they both use their trays/ make no mess I'd say it was between the cats. If the poorly cat won't use its' tray, I'd say it isn't well again. It has been known for cats to attack another because it is ill, even ones who have lived together happily for so long. Unless you catch one of them in the act, you won't know who's soiling; the poorly one who is defending it's territory because it's threatened, or the well one who's asserting dominance because it knows the other is sick.

The other thing you could try, is putting extra litter trays in other locations, not just in the basement. It could be that one is preventing the other from using the trays, not letting the other past without attacking it, or even just acting aggressive, hissing and so on. Cats feel very vulnerable while toileting, as they aren't in a very good position to fight back.

Clean soiling using a biological washing detergent solution in water, or an enzymatic specialist pet accident spray. Don't use anything containing ammonia which will attract them to the same spot each time.

Try a feliway diffuser