Urolithiasis is the formation of calculi in the urinary tract, also called kidney and bladder calculi or stones, or urinary tract stones. The stones are rock hard crystal aggregations of all shapes and sizes. Sludge is the name given to the thick, almost toothpaste consistency deposit that can build up in the rabbits bladder or kidneys.

Calcium is very important for rabbits; they need it to form strong and healthy bones and teeth (and so to prevent dental problems and osteoporosis). Just as too little can cause problems, so can excess quantities, and although calcium consumption isn’t believed to be the sole factor responsible for the development of calculi (stones), it is thought to play a part, so getting your rabbit’s calcium intake correct may help to prevent future problems.

A rabbit fed a good balanced diet shouldn’t be calcium deficient and wont need a calcium or mineral supplement. It has been suggested that the average sized, adult, non-breeding, pet rabbit needs around 510 mg of calcium daily.

Rabbits have an unusual calcium metabolism; In most mammals, the amount of calcium absorbed from the diet is regulated at gut level by Parathyroid hormone (PTH), but rabbits predominantly absorb calcium in proportion to what is present in their diet, whether or not they require it. Meaning that whatever amount of calcium a rabbit has in their diet, they will absorb it all. Any excess calcium that is absorbed into the bloodstream from the gut is excreted through the urinary tract, where it may be deposited and form calculi/sludge. Rabbits can form solid calcifications in their kidneys, ureters, bladder or urethra, as well as depositing thick calcium sludge in their kidneys or bladder.


One or more stones may develop, with or without sludge being present. Symptoms of calculi include: loss of appetite, loud painful teeth grinding, hunched posture, lethargy, unwillingness to move, pressing abdomen on the ground, loss of litter training, blood in the urine, perineal scalding and straining to urinate. One or more symptoms may appear gradually, or have a sudden onset and be intermittent.

A blood test is useful for determining kidney function and calcium levels in the blood. A urine test should also be taken to see if calcium crystals and/or a urinary infection (cystitis) are present.


Rabbits suffering from sludge may display any of the symptoms described above for calculi, as well as passing thick, white, gritty urine that in severe cases will become semi-solid when passed. White or sludgy urine on their own are common in rabbits and often occur after the rabbit has eaten high calcium foods. They aren’t necessarily indicative of sludge within the urinary tract; this must be confirmed. Diagnosis is the same as that of calculi; as the sludge is made of calcium, it too will show up on an x-ray or ultrasound as a mass.

As some rabbits exhibiting no clinical symptoms, can upon investigation, be found to have some degree of sludge in their bladder or kidneys, treatment to flush the sludge from the bladder, is only necessary if the sludge seems to be causing the rabbit problems. Sludge in the kidneys is more difficult to eliminate, although kidneys can be flushed, and flushings may be sufficient to return the kidneys to an acceptable working capacity, it is unlikely to clear all the sludge.

Treatment depends upon the location of the calcification/s. The deposit of calcium salts (calcifications) show up on x-rays and ultrasound scans, as the calcium content makes them radiopaque (light in colour). Calcifications detected in the bladder, ureters or urethra should be surgically removed once the rabbit’s condition has been stabilised.

Rabbits suffering from calculi are often in a lot of pain and as a result will not eat or drink, subsequently being dehydrated and necessitating fluid therapy, gastrointestinal support and pain relief prior to surgery. Currently there is no known method of breaking up rabbit calculi and it is rare for the rabbit to pass them naturally, due to their often-jagged texture and size to which they can grow. Ureteral calculi may cause enlargement of the kidney (hydronephrosis) due to obstructed urine flow from the kidney to the bladder. With such a situation, once the rabbit is stabilised, prompt surgical intervention to remove the calculi is vital to avoid irreparable damage.

Rabbits presenting with calculi in both kidneys, displaying clinical symptoms have a guarded to poor prognosis. Surgically removing calculi from the kidney is extremely hazardous, and for these cases management may be the only treatment option, with their quality of life paramount. If calculi are only located in one kidney, it may be possible to remove the affected kidney, as the remaining kidney should have adequate function to keep the rabbit alive. However, if the remaining kidney becomes unable to work to near full capacity, the rabbit’s prognosis is extremely grave.

Calcium consumption isn’t believed to be the sole factor responsible for calculi/sludge development, which is why solely reducing the calcium intake often doesnt prevent the problem from reoccurring. Other possible reasons, include:

A low water intake

This is suspected to be one of the most important factors leading to the formation of urinary calculi. Less urine is produced with a low water intake and calcium precipitates out when urine is saturated with calcium. To rectify this problem you must increase the rabbits fluid intake, which can be achieved by:

  • Adding apple juice or other sweet flavouring to their water.
  • Giving the rabbit a plain salt lick or sprinkling a small amount of table salt onto their veg.
  • If neither of the above works, syringe-feeding fluids or fluid administration by your vet may be the only way of increasing the fluid intake.

Overweight rabbits

Overweight rabbits may be physically unable to completely empty their bladder, due to weak muscle tone, or adopt the correct position for urination, which will give any calcium in the urine a chance to settle in the bladder.

Kidney damage

The rabbit’s kidneys may be unable to manage the amount of calcium, which they are required to process. Kidney damage only shows up on blood tests when at least 50% of kidney function is lost.

Litter training

Most rabbits will only urinate in a select amount of places. If the rabbit is unable to reach these places, i.e. no litter tray in pen, it may wait until it can urinate in its desired area, so always ensure your rabbit is able to reach its toilet areas.

Oxalate (a poisonous salt of oxalic acid)

Some green foods are high in oxalate (spinach, parsley, spring greens, etc). Oxalates should be metabolised by bacteria in the gut, but if there aren’t enough oxalate-eating bacteria, the oxalate will be absorbed rather than broken down and excreted via the kidneys, with calcium hitching a ride.

Other illnesses

Illness which causes the rabbit to sit around and not urinate regularly, e.g. osteoarthritis, spinal problems or anything causing discomfort/malaise, etc, can cause problems. The underlying factor needs to be corrected, on top of addressing the sludge/calculi problem.


Even after a low calcium diet has been implemented and all the above explored, if the rabbit still continues to produce calculi/sludge, then a genetic reason should be considered. Some rabbits may fundamentally be “stone/sludge formers” and whilst a genetic reason cant be ruled out, rabbits affected with urolithiasis problems should never be bred from.

Fruit and vegetable can be grouped into good, moderate and poor calcium providers. The approximate calcium mg content per 100 g is shown alongside:

Good calcium providers

  • Kale (130)
  • Spinach (170)
  • Parsley (200)
  • Watercress (170)
  • Mint (210)
  • Spring Greens (210)

Moderate calcium providers

  • Cabbage (49)
  • Broccoli (56)
  • Celery (41)
  • Parsnip (41)
  • Swede (53)
  • Turnip (48)
  • Oranges (47)

Poor calcium providers

  • Brussel Sprouts (26)
  • Carrots (25)
  • Cauliflower (21)
  • Cucumber (18)
  • Peppers: Red/Green (8)
  • Tomatoes (7)
  • Apple (4)
  • Apricot (15)
  • Banana (6)
  • Grapes (13)
  • Kiwi Fruit (25)
  • Nectarines/Peaches (7)
  • Pears (11)
  • Pineapple (18)
  • Strawberries (16)
  • Baby sweetcorns (2)

Alfalfa hay contains approximately 1.5% calcium, compared with 0.4-0.8% calcium in grass hays. Nutritional values for hay and fresh foods vary depending upon the soil the product was grown in, the time of year it was harvested and other conditions that affect nutritional values.

Once the initial condition has been treated, attention must focus on trying to prevent reoccurrence of the condition, but even then there is a possibility of further calculi or sludge forming, particularly if the rabbit still has sludge within their kidneys or bladder.

The most successful approach to preventing reoccurrence of calculi or sludge seems to be by adopting a multi-tiered approach and tackling several possible factors concurrently:

  • Dried rabbit mixes should be excluded from the diet or fed in tiny proportions as an occasional treat. Any alfalfa-based foods and calcium/mineral supplements should be permanently removed from the diet.
  • Feed limited amounts of foods classed as moderate calcium providers, with their main intake from the poor group, avoiding those in the good group. This not only lowers the calcium intake but also increases water consumption, which seems to be critical for rabbits with urolithiasis.
  • Offer grass hays (timothy, fescues, ryegrass, bromegrass or orchard grass) ad lib.
  • Ensure the rabbit isnt overweight and gets plenty of exercise.
  • Always ensure the rabbit can reach its toilet areas.
  • Implement careful monitoring, usually consisting of blood and urine tests, as well as periodic x-rays/ultrasounds to monitor the condition and detect any reformation of calculi/sludge early on.

Changes in diet should be done over at least a couple of weeks to prevent digestive upsets. Introduce one new food at a time and if it seems to upset the rabbit in the next 24-48 h exclude it from the diet. Try to feed smaller quantities of numerous different foods, rather than larger quantities of a few foods. This gives the rabbit a variety, and also helps to achieve a balance of other minerals.

The vast majority of rabbits on a hay and veggie diet manage fine, and get through a mound of fresh food their own body size daily. Hay and water must always be available.

Finally, it is vital to ensure the phosphorus intake doesn’t exceed the calcium. Discuss your rabbit’s diet with your vet, and expertise is also available from nutritionists employed by feed companies in the UK and US.