The Meaning of Sludge

Urinary sludge is a common finding in rabbits, either visibly when we observe the passing of “mucussy chalk” or radiographically when we are investigating other clinical signs. It is dramatic and definitely looks abnormal. Yet it may not be- a lot of these rabbits are passing sludge yet not showing signs of disease (and often not signs of urinary tract disease), and re-radiograph some of these rabbits a few days later and their bladders may be empty of sludge. So, what does it mean?

Posted: 25 August 2020

The Meaning of Sludge


Learning objectives:

  • Crystalluria is normal in rabbits and other herbivores
  • Persistent sludging is a clinical/ radiographic sign; it is not a distinct disease.
  • Where seen, underlying causes should be investigated and treated- these may be linked to pain and lack of movement; husbandry; or reduced drinking.
  • Dietary management is useful but is not the single answer, and is rarely successful on its own.
  • Affected rabbits need to drink more.

Formation of sludge

Sludge is the largescale accumulation of calcium (usually in the form of the oxalate and carbonate salts) in the urine and urinary bladder.

In this situation it should be remembered that crystalluria is normal in rabbits as it is in other herbivores. This represents the rabbit’s means of controlling body calcium (bone and blood) by excreting excess intake rather than simply not absorbing excess levels from the gut, as in dogs, cats and humans.

Normally this will shown by the passing of a cloudy urine and the finding of calcium oxalate or carbonate crystals in rabbit urine is entirely normal.

However, sludge formation is clearly not right- it is a massive excess of mineral, and as such merits investigation.

It is proposed that sludge may result from:

  1. Excessive dietary calcium. This seems simple- rabbits control excess dietary calcium by excretion via the urine. Too much coming out must represent too much going in?  As with so many simple theories, this is just too simple! Rabbits are designed to control calcium in this way so, unless they are being fed simply massive amounts of calcium, there has to be a failure in the excretion mechanism. We are also aware that over-restriction of dietary calcium can be a causal factor in dental disease so any restriction much be done with very great care.
  2. Reduced drinking. Studies in Zurich have firmly linked sludge build-up with reduced drinking, and very high calcium-containing diets could be fed as long as the rabbits were able to drink more. In other words, rabbits are perfectly capable of passing very large amounts of calcium salts as long as they can take in enough water to flush it through- the higher the calcium content of the diet, the more fluid is required. In the pet situation, reduced drinking may be seen with:
    1. Failure to provide potable water
    2. Providing water in a form that is less accessible to the rabbit. The Zurich studies also showed that rabbits prefer drinking from bowls rather than drinkers and even those that had been used to drinkers drank more when given a bowl.
    3. Position and number of bowls/ drinkers- just as in other species, this can play a part with reduced drinking being possible if the drinker/ bowl is in a position where the rabbit simply doesn’t want to go (how thirsty must a nervous rabbit get before it goes to the most exposed part of its hutch/ run to drink?) or if another rabbit “dominates” the food/ water.
    4. Water content of foo This is an area we often forget- extra water can be given by feeding large amounts of fresh wet food (ie. Grass!). However, the Zurich studies showed that this may not always be a huge factor as dry diets were absolutely fine in that they made rabbits more thirsty so they drank more!
    5. Pain – rabbits in pain are reluctant to move and so will be much less likely to move toward a drinker and so drink less.
  3. Reduced urination
    1. Restricted movement is associated with a lower frequency of urination and, hence, urine retention.
    2. Obesity also results in reduced movement and reduced micturition.
    3. To urinate the rabbit will lift its hind quarters and then eject urine. Therefore any painful lesion in the spine, abdomen or hindlimbs will make the rabbit less inclined to urinate and reduce urination frequency.
    4. Bladder atony- neurological diseases, especially spinal, may reduce the bladder’s ability to empty and result in a large flaccid atonic bladder.

Consequences of sludge

Most importantly, sludge is a sign of other problems- problems that may be causing pain and/or stress and so must be addressed.

However, sludge can also cause some issues. The crystals are present in large number and can cause bladder irritation. The resultant cystitis may manifest as discomfort and increased micturition frequency (just as in other species). There will also be increased levels of inflammatory proteins in the presence of the crystals and this will predispose to urolith formation.

In other words, sludging issues will also need addressing as they can progress to more serious disease. Certainly, if a rabbit is showing signs of cystitis it is worth radiographing to look for the presence of sludge or uroliths rather than using antibiotics (bacterial cystitis is unusual in rabbits).

Investigation and treatment of sludge

The simplistic approach to bladder sludging is restriction of dietary calcium and to wash out the bladder. Unsurprisingly, most of these cases recur!

Instead a much more holistic approach is needed:

  • History, including:
    • Diet – importantly the relative amounts of foods given and what the rabbit actually eats.
    • Water provision- type; how provided; and how many drinkers/ bowls and where they are positioned.
    • Amount drunk each day.
    • Photographs/ videos of the rabbit’s hutch and run showing bedding, hiding places, size, etc.
    • Urination- frequency; amount seen (eg. wetness of the toileting area); vocalisations when urinating.
    • Disease history:
      • How long the sludging has been seen for.
      • Other clinical signs.
      • Previous illnesses.
      • Current medications and dose rates.
  • Clinical examination, including:
    • Abdominal palpation and gentle assessment of bladder size and whether urine is easily expressed.
    • Weight and body condition
    • Dental assessment
    • Assessment of muscling over spine and hindquarters.
    • Urine staining and/or scalding on hindquarters.
    • Presence or not of pododermatitis.
  • Urinalysis
    • Physical appearance.
    • Culture
    • Cytology (this should always be done to enable assessment of an inflammatory response and the significance of any bacterial isolates)
    • Specific gravity.
    • Protein levels (though it is important to remember that small amounts of protein and glucose can be normal). Stick tests should be interpreted with care as the normal rabbit urine alkalinity can affect the protein strip.
  • Bloods
    • Renal function
  • Radiography. This is vital in all cases. The rabbit should be sedated or anaesthetized to enable proper positioning. Lateral and dorso-ventral views should be taken of:
    • Spine
    • Skull- dental assessment and middle ears)
    • Hips/ pelvis/ hindlimbs
    • Abdomen

Depending on results of these tests, other investigations (eg ultrasound or CT) may be indicated.


The first principle is to treat and control underlying disease, thinking especially of analgesia where arthritic or degenerative lesions are found. Once these are being managed, many rabbits will void the urinary sludge by themselves.

Similarly, where bladder sludging is seen as an “incidental” finding in investigating another syndrome, eg. gut stasis, it is generally sufficient to manage the original problem as the sludging is usually another clinical sign rather than a causal issue.

Otherwise, the following should be considered as a general approach where the rabbit is showing urinary signs, eg. cystitis with frequent small amounts of urine being passed:

  • Fluids
  • Anxiolytics- eg. midazolam
  • Analgesia- especially meloxicam. However, opiates may also be required in severe cases, and maropitant can be an excellent bladder anti-inflammatory.
  • Support feeding if anorexic or thin- it is worth making such feeds more liquid than usual to increase urine production.

In many cases this will be sufficient.

If there is evidence of excessive pain on micturition or if the bladder/ urethra appear blocked, phenoxybenzamine (at normal cat doses) can be effective. Otherwise, the radiograph should be reviewed to see if there is urolith formation within the sludge. Where this is hard to assess, ultrasound may assist in distinguishing sludge from stone.

In genuinely blocked cases, bladder pressure may be relieved by cystocentesis and urinary catheterization performed.

Washing out the bladder

This is frequently advocated as part of the management of sludging. However, it is rarely needed- I have never performed this technique! Most rabbits will void the sludge spontaneously once underlying issues and pain are resolved. Certainly, given that sludging is very much a clinical sign of underlying disease rather than a primary problem, washing out the bladder should be viewed as relieving a symptom rather than a primary treatment option.

It should therefore be reserved for rabbits where there is genuine urine blockage or where pain associated with passing the sludge cannot be resolved to allow spontaneous voiding.

Longer-term management

The above relates to the acute situation. Not addressing the underlying issues nor putting in place longer term treatments and husbandry changes will result in recurrence of the problem.

The following should therefore be considered:

  • Control of underlying disease- especially long term analgesia for arthritic conditions.
  • Bladder anti-inflammatories
    • Maropitant may be used in cases where it is hard to control bladder irritation
    • Nutraceuticals. I very often use a combination of nutraceutical agents aimed at stress-relief and bladder inflammation. There are many on the market especially aimed at cat cystitis and I have seen good success with many as part of overall control.
    • In two cases of atonic bladder I have had some success using deslorelin implants that had presented with urinary incontinence and large sludge deposits in the bladder.
  • Husbandry. The following should be provided
    • Ample space to move
    • Hiding places throughout the hutch and run
    • Companions (if none already) and suitable numbers of feeders and drinkers for this number.
    • Water bowls and drinkers to allow choice. Many should be provided throughout the hutch and run especially near bedding/ hiding places. This is hard work for the owner, but means the rabbit never has to go far to drink.
  • Diet.
    • Plenty of fibre – hay and fresh grass. If the rabbit is drinking well then alfalfa hay is fine, but in general should be avoided once there has been an issue.
    • Fresh grass and greens may be wetted before feeding
    • Pellets – maximum 25g/kg/day as required
    • Some particular dietary components may assist with the problem (and be a more natural way of providing the nutraceutical agents mentioned above). As an example, dandelions provide a natural diuretic and so assist in promoting urine flow (in spite of their high calcium level).
    • If calcium levels are extreme then some moderate dietary restriction may be advisable to reduce the amount of drinking required- eg. Moving away from alfalfa hay. However, this is often not needed. Similarly, giving hard water to drink should also be fine and there is generally no advantage moving to softened water. If water type is changed to bottled water, the mineral content should be checked as many have very high levels of mineral! Overall, just bear in mind that the more calcium in the diet, the more the rabbit has to drink. Also, if mobility is restricted (eg resting an injury) , especially, in older animals, then extreme restriction of calcium intake may also affect skeletal calcification.

Prevention of sludge

Prevention is always better than cure!

In this case it is basically quite simple with the need for:

  • A good quality high-fibre diet
  • Wet fresh food prior to feeding just to make sure!
  • Plenty of water bowls that are easy to access
  • Good husbandry allowing rabbits to exercise, get privacy, hide when frightened, etc
  • Sympathetic handling from an early age (soon after emergence from the nest) to reduce handling injuries to the spine that often lad to formation of arthritic lesions.

Fortunately many of these will also prevent a lot of other rabbit problems – good husbandry is vital!


1. Urolith formation in the bladder and renal pelvis. Arthritic lesions are seen along the lumbar spine likely causing reduced urination.

2. A & b . Lateral and DV radiographs of a rabbit with an atonic sludge-filled bladder. Note the size, position and lack of distension of the bladder. 2a shows some disc space compression between the T10-12 vertebrae which is likely significant. Clinically the rabbit showed smelly urine and periodic wetness of the hindquarters and sludging. The cause of the soft tissue abdominal calcification was not determined and blood calcium levels were not significantly high.

3. A&b. DV and lateral views of a rabbit with significant damage and chronic change to the thoracic spine. Presenting signs were hindlimb paresis along with sludge production and urinary incontinence. Bladder opacity is only mildly increased and bladder size/ position is normal. Use of meloxicam at 1mg/kg bid enabled more normal movement and positioning for urination. Leakage reduced and sludge was passed normally.

John Chitty BVetMed CertZooMed MRCVS

Anton Vets, Anton Mill Road, Andover, SP10 2NJ