Cats/Strange litter box behavior
I adopted a 7 month old kitten in November (He's 9 months now) and his litter box routine is strange. When I first had him, it turned out he had giardia, causing diarrhea on a regular basis. After he's done his business, he scratches all around the box, but then he moves on to scratching the walls around the box (it's in a corner). When he's done with the walls he scratches, or paws really, at the washing machine and dryer which share the same space. It seems to be very obsessive, because if he's interrupted he will return to finish the job. It goes on for several minutes sometimes lasting as long as 10.
This is a problem for me for two reasons: One, because of the giardia and diarrhea, when he'd turn to scrape at the walls and so on he'd move throughout the litter box. This effectively covered his paws in his (awful smelling) waste. From there, he would track it through my kitchen. I'd have to chase him with a baby wipe every time. Second, it's incredibly annoying to listen to his scraping for 10 minutes. I have a small apartment and the sound resonates.
His giardia has cleared up, but it seems as though he's passed it to my older cat (vet appointment this week). Unfortunately they share a litter box and the diarrhea issue has returned. My older cat is much more attentive to his cleanliness and doesn't leave the area until he's clean.
I placed a cover over the litter box which has kept him from scraping at the walls and washer/dryer, thankfully. Any idea why he would be so obsessive?
the simple answer is just because he is. The reason i say this is because my cat is exactly the same. When she uses the box she covers it over and then starts to paw at anything near by for ages. It;s as if she is very ashamed of her mess and wants to make sure she does not leave a trace. I have got used to it over the years as i cannot put a cover on the tray as she is to scared of the cover. I think your solution of cover the tray was the best solution. I would say though that cats usually prefer to have their own litter trays because the smell of the other cats urine is often used as marker and could put them off using it, and as you have seen it may help to pass on illness. I know this can be difficult in a small home but it could save you problems in the future as sometimes out of the blue one cat will decide that the litter tray nolonger belongs to them and will urine etc elsewhere, i come across this problem on this question site regularly.