October 2026
If you've ever cleaned a litter box and found a thick, chalky smear that looks more like toothpaste than urine, you've probably met bladder sludge. Rabbit urine can look a little strange even when it's normal: cloudy, and anywhere from pale yellow to orange or reddish from plant pigments. Sludge is different. It's thick, gritty and heavy, and it's one of the more common problems I see in middle-aged and older rabbits.
The medical term is hypercalciuria, which simply means too much calcium in the urine.
Rabbits handle calcium differently from most mammals. They absorb much of the calcium in their food whether they need it or not, then get rid of the extra through their urine. That's normal rabbit biology. Trouble starts when the urine carries more calcium than it can hold, and the excess settles in the bladder.
Several things usually work together to push a rabbit in that direction. A high-calcium diet, especially alfalfa hay or alfalfa-based pellets, is a big one. So is low water intake, and many rabbits drink more from a bowl than a bottle. Extra weight, arthritis or back pain make it hard to posture and fully empty the bladder, and a small enclosure means less movement. Sore hocks (pressure sores on the heels) often show up alongside sludge for the same reasons.
Movement matters more than people expect. A rabbit that hops around keeps the sludge stirred up so it can be passed. A rabbit that sits still lets it settle. This is biology, not a failing on anyone's part.
The early signs are often about the litter box: peeing outside it, dribbling, straining, or visiting often with little result. You may see thick, beige or gritty urine. A wet, stained bottom is an important clue, because urine-soaked skin quickly becomes red, sore and infected, and it's very painful.
Red urine can be harmless plant pigment or real blood, and only a test can tell the difference. Some rabbits with bladder problems first look like they have a gut problem, eating less and leaving smaller droppings.
I'll feel the bladder, which is often large, check the spine, hips and knees for pain, and look at the feet. X-rays confirm sludge and also show arthritis or bladder stones. Bloodwork and a urine test with culture look for infection and kidney disease. I prefer to collect urine directly from the bladder with ultrasound guidance, which gives a cleaner sample.
There's no single fix. The best results come from working on several things at once.
Diet comes first: switch from alfalfa to grass hays such as timothy, measure the pellets, and offer leafy greens, and we'll go over which greens make sense. Encourage drinking with a heavy, fresh water bowl. Weight loss and more floor time help keep sludge moving and ease the strain on sore joints.
Pain control is a big part of treatment, because most rabbits with sludge also have arthritis, and a comfortable rabbit moves, postures and empties the bladder better. A gentle clip of soiled fur, sometimes under sedation, plus a soothing medicated cream heals urine scald.
Fluids under the skin can help in the short term, but in my experience sludge often returns once they stop, especially when the bladder has been stretched for a long time. For rabbits whose sludge keeps coming back, I sometimes add long-term medication to reduce calcium in the urine, with regular bloodwork and urine checks.
Sludge is usually managed rather than cured. With steady changes, many rabbits become much more comfortable.
Call the same day if your rabbit is straining and producing little or no urine, has stopped eating or is producing few droppings, or seems painful, hunched or very quiet.
Book a prompt visit for peeing outside the box, dribbling, a wet bottom, thick or gritty urine, or red-tinged urine you can't explain.
Dr. Logan Kopp, Nashville's rabbit specialist and a Diplomate in Exotic Companion Mammal Practice, diagnoses and manages bladder sludge and other urinary problems in rabbits at Priest Lake Veterinary Hospital. Call (615) 361-4646.
This article is general information, not medical advice for your pet. Reading it doesn't create a veterinarian-client-patient relationship (VCPR). Please talk with your own veterinarian before making changes to your pet's care.