*The author expresses deep gratitude to Jeffery L. Rhody, DVM for invaluable assistance in editing this article.

Urinary bladder problems are seen in rabbits of all breeds, of all ages, and of both sexes. Historically, these problems have been frustrating to treat and have a high recurrence rate. In recent years, veterinary scientific investigation into this problem and rabbit physiology has helped us to begin to grasp onto better ways to treat urinary bladder problems. We still are very far from completely understanding why and when stones and sludge form.  However, treatment is now more successful and the recurrence rate has been lowered significantly.

Introduction to the Urinary Tract

The urinary tract of the rabbit is similar to that of other mammals. The upper urinary tract is comprised of the kidneys (one each on the left and right side of the body) and the ureters. The ureters are tubes that connect the kidneys to the urinary bladder. The lower urinary tract is comprised of the urinary bladder and the urethra. The urethra is the tube connecting the bladder to the outside of the body and is the pathway for urine to travel during urination.

Urine is formed as blood goes into the kidneys and is filtered of waste products. Based on the hydration status of the rabbit, the urine is either very dilute or concentrated by allowing more or less water into the urine.  Urine that is formed travels through collecting ducts in the kidney to the ureters. Via the ureters, urine flows to the urinary bladder. The urinary bladder is a simple collection bag for urine. When the rabbit is ready to urinate, the muscles of the bladder contract and the sphincter at the exit hole relaxes and urine is expressed out of  the bladder and through the urethra and to the outside of the body. In males, the urethra travels through the penis and is a long tapering tube. In females, the urethra is short and wide and ends in the vagina.

Definition of Bladder Stones and Bladder Sludge

Bladder stones are hard concretions that are found in the urinary bladder. These concretions have a definite shape and have the consistency of a chalky stone. Most rabbit urinary stones are comprised of some form of calcium carbonate. Stones can be found in the kidney collecting ducts or the ureters as well. In some instances, a stone will pass into the urethra be voided to the outside. (If noted, please consult your veterinarian and schedule an appointment to be sure no other stones are present.) In rare instances, a larger stone may leave the bladder and get stuck in the urethra. This will either completely or partially block urine flow to the outside of the body. Complete obstructions are more common in males due the narrower urethra and are a medical emergency.  Luckily, this emergency scenario is quite rare.

Bladder sludge (also called hypercalcinuria or hypercalciuria) is a thickening of the urine that never forms into stones. It is comprised of any number of calcium salts and does not flow like normal liquid urine. We are not sure why but sludge does not form stones. It remains the consistency of chalk or fine sand and is a white to gray color.  If sludge is very thick, it can have consistency of toothpaste. When felt, this sludge can be smooth, have finely granular texture, or even have gritty texture. Sludge can form in the kidneys or ureters, but most sludge is noted in the bladder and suspected to have formed there.

At this time, it is not clear why some rabbits form stones, why some rabbits form sludge, and why most rabbits develop neither stones nor sludge.  Most likely it is a combination of factors that contribute to this pathology. These factors are listed and discussed below. However, I would first like to comment on the rabbit’s peculiar physiology with regard to calcium metabolism.

Calcium Metabolism in the Rabbit

Digestible calcium is the calcium that can be absorbed by the body and is not bound to another substance preventing its absorption. Calcium is used for a variety of processes, the most common being the maintenance of bone and muscle.  Humans and most other domestic animals tend to absorb calcium from a meal in proportion to their body’s needs at that time. The calcium that is not needed by the body is either left unabsorbed or metabolized by the liver and excreted from the body in the feces.

Calcium absorption and metabolism in the rabbit is still not completely understood. They normally have a much higher blood calcium range than any other mammal we are familiar with as a pet. A veterinarian who is familiar with rabbits will understand what the normal calcium range is in the pet rabbit and not compare it to say, a dog or cat. Current thinking is that rabbits, as opposed to some other mammals, may absorb digestible calcium in excess of the amount their body might need at the moment.  Rabbits then excrete the excess calcium they don't need primarily through the kidneys as well as smaller amounts through the digestive tract. The calcium is excreted in the urine in the form of calcium carbonate.  This substance is what makes normal rabbit urine cloudy in appearance compared to the urine from a human, dog or cat.  The calcium carbonate is the white residue seen on the cage paper after the urine is dry.  If you touch this residue it feels like chalk.  Rabbits may have developed this very efficient way of absorbing calcium out of the diet related to the type of environments in which they evolved. This mechanism may have been necessary to get all the calcium possible out of a meal in case there was less calcium available in subsequent meals. While we do not fully understand calcium metabolism in the rabbit, for the purposes of our discussion, the important facts are:

1. Rabbits may absorb more digestible calcium from their diet then they need at that moment, and
   
   
2. The majority of excess calcium is excreted through the kidneys.

As stated above, the normal blood calcium level of a rabbit is generally higher than most other mammals. A blood test demonstrating a high calcium level could be normal, or could represent a change in calcium level in a rabbit who has recently eaten a calcium-rich meal.  If your rabbit is experiencing a high blood calcium level, it is often best to recheck it after removing high digestible calcium sources from the diet for 24 hours for a more accurate reading.  Traditional blood tests measure the total blood calcium content.  While this can indicate a potential problem, measuring the ‘ionized’ calcium level (the form of calcium that can cause kidney, muscle and heart problems) will help differentiate a normal rabbit from one who is in danger from too much calcium. Luckily, ionized calcium levels in rabbits with high total calcium are usually normal.  At this time there is no evidence that rabbits that form sludge or stones have abnormal ionized calcium levels.

Potential Risk Factors for Development of Sludge or Stones

• Genetic predisposition to stone production by that particular rabbit; this is theorized but scientifically unproven
  
  
• Insufficient water intake by the rabbit. This could be caused by lack of water (water bowl overturned, water frozen, rabbit doesn't know how to drink out of a water bottle, water bottle defective) or water that has a bad taste or smell (contaminated, has medications or vitamins added). This can also be caused by lack of exercise (see next factor). This causes a state of chronic, low-level dehydration and may result in more concentrated urine leading to the possibility of sludge or stone formation. 
  
  
• Inactivity leading to less frequent urination.  The inactive rabbit that sits all day in a cage may not drink as frequently or urinate as frequently leading to more concentrated urine.  An inactive rabbit might sit in one spot and thus the urine also sits and there is more opportunity for calcium crystals to coalesce. An active rabbit will consume more water and urinate more frequently.
  
  
• Lack of appropriate toilet area.  Some rabbits are very fastidious and if they are confined in a small space and the toilet area is not kept clean, they may tend to hold the urine longer and thus urinate less frequently.  This could cause similar problems as mentioned under Inactivity.
  
  
• Kidney disease may cause a change in how calcium is excreted or handled.  There are a variety of diseases that can affect the kidneys including parasitic, infectious and non-infectious.
  
  
• Bladder disease may cause a change in the lining of the bladder causing calcium carbonate to accumulate and form stones or sludge. Infections, tumors and benign growths can cause inflammation of the bladder wall.
  
  
• Diet high in digestible calcium? For many years, veterinarians have believed that dietary calcium content was important in the development of bladder stones and sludge. For this reason and because so much about this condition is not known, we have emphasized dietary changes directed toward reducing calcium intake for affected rabbits. Based on what we now know, it can be stated that dietary changes alone will NOT reduce the formation of stones or sludge in rabbits prone to this condition. There have been several scientific studies performed where rabbits were fed huge amounts of digestible calcium for months (far more than a pet rabbit would ever get even on a completely commercial diet), and these rabbits failed to develop stones or sludge. So I don't believe that diet ALONE is ever the entire problem. 

Signs of Bladder Stones and Sludge

Early in this disease there may be very few signs or the signs may be so subtle as to be undetected by the rabbit's caretaker.  As the disease progresses, any combination of the following may be seen.

• Urinating more frequently than usual and often outside the normal toilet area. Frequent inappropriate urination (especially "spraying" urine on vertical surfaces) can be seen as normal behavior in sexually mature intact male and female rabbits that are marking their territory.  However, a rabbit that is experiencing bladder problems will usually not "spray" the urine on vertical surfaces, nor seem particularly interested in marking out a territorial boundary or certain items in a room.  Occasionally a rabbit will be observed to "dribble urine" without knowing it and the hindquarters may be continually damp with urine.  Please be aware that there are other diseases that can lead to urine staining of the hind quarters including sore hock, spinal disorders (arthritis, injuries, nerve damage), obesity (can't clean the hind quarters) and reproductive disorders.
  
  
• Straining to urinate.  The rabbit takes longer than usual to pass a smaller than usual amount of urine.  This straining behavior is frequently mistaken for constipation.  True constipation is extremely rare in the rabbit. Occasionally the straining can be so painful that the rabbit will cry out or moan when urinating, although this is rare.  You should consider it an emergency if a rabbit is continuously straining without producing urine.  This indicates that there may be a complete blockage of the urethra (the tube the urine flows through out of the bladder) and if this is not corrected it could result in the pet's painful death within 24 hours.
  
  
• Blood in the urine.  Normal rabbit urine can range in color from a light yellow to a deep orange-red (a "rusty" color) due to a variety of plant pigments that may have been eaten or pigments, called porphyrins, produced by the bladder itself.  Blood coming from the urinary tract should cause the urine to be a uniform dark to bright red color.  Often the amount of blood in the urine is so small it can only be detected by testing with a specially treated paper, or by looking at the urine sediment under the microscope.  Unless the rabbit is urinating on a light colored surface, it is easy for blood in the urine to go unnoticed.

*It is important to note that there is another condition that can cause blood to appear during urination and is often confused with bladder or kidney disease.  Uterine disease in the female rabbit can result in bleeding into the vagina.  The blood collects in the vagina and when the rabbit urinates, the blood is pushed out usually at the end of urination.  In these cases, the blood appears as a centrally located pool within the urine puddle.  There may also be clots of blood within the urine pool and finally there may be blood staining the area under the tail and hindquarters of the rabbit, which is usually not seen in bladder disease. Please be aware that these signs represent serious reproductive disease NOT urinary tract disease. The urine will test positive on a chemical analysis for blood if you bring a sample to your veterinarian and it will be impossible to tell where the blood originated.  Therefore, it is critically important that you observe a fresh urine sample for the pattern of coloration.  In addition, a complete physical examination by a veterinarian will also help to determine the diagnosis.  Of course, uterine disease will only occur in unspayed female rabbits.

• Sludge in the urine.  Urine that becomes thick with calcium carbonate crystals is not only difficult to pass, but will tend to stick to the fur around the rabbit’s hindquarters.  The sludge will leave a noticeable amount of grayish-white residue on the fur and in the toilet area as it dries. This material has the consistency of very fine sand or chalk.  Remember that a small amount of this material can be seen in normal rabbit urine as it dries as a consequence of clearing excess calcium from the body. However, in normal rabbits the calcium carbonate should not be accumulating on the fur.

Sometimes larger calcium carbonate particles can be passed which are the consistency of coarse sand.  In rare instances, the rabbit will void a large stone.  If this is observed, the pet should be examined by your veterinarian to determine if additional stones are present in the bladder.  Multiple bladder stones are more common than solitary stones. 

• Urine scald.  Some rabbits with hypercalcinuria will develop a skin rash around the area where they void urine and along the belly and inner thighs.  This is related to prolonged contact of urine with the skin.  Any rabbit with persistently wet hair or rash in these areas should be evaluated for urine sludge or stones.
  
  
• Loss of appetite and depression.  This sign usually occurs because of pain due to the pressure, size or location of the stones, or because there is a complete blockage of the urethra.  If there is blockage of one or both ureters the rabbit can quickly develop kidney failure which will result in severe depression and death within a short time.  Loss of appetite and depression should always be considered an emergency in a rabbit regardless of the suspected cause and you should contact your veterinarian immediately.

It goes without saying that if you see any of the above signs in your rabbit that you should contact a veterinarian immediately.  If you can collect a urine sample prior to your visit, this may be helpful. (Please see section below on urinalysis for directions on how to collect the sample).

Diagnosis of Bladder Stones and Sludge

• History and signs of the disease - As I have mentioned many times in my articles, giving your veterinarian a good history is CRITICAL to the ability to diagnose many disorders.  I suggest that you write down a few notes about the problems your pet is having before you get to the veterinarian's office.  Include in your notes the duration of disease, changes in your pet's behavior, changes in the rabbit's life style or environment, any remedies you may have already tried and any ideas you may have on the problem your pet is exhibiting.
  
  
• Physical examination - A thorough physical examination by your veterinarian is an important part of the diagnosis.  Occasionally, bladder stones can be felt in the bladder, however, if the stones are small or if the rabbit is uncooperative, they can be missed.  Examination of the hindquarters of the rabbit may reveal sludge or small stones accumulated on the fur and skin.  The most important contribution of the physical examination is to determine the overall condition of your pet and the presence of any other contributing disease.  Abnormalities such as anemia, weight loss, skin lesions, dental disease, heart or lung abnormalities, and abdominal pain or masses are just a few of the things that can be detected on a physical examination.
  
  
• Urinalysis - A urinalysis can detect abnormal cells and determine the chemical composition of the urine. Urine can be collected from rabbits in a variety of ways.

1. Free catch - Empty the rabbit's litter box and wipe it out with a paper towel, but do not disinfect it so that there is some odor remaining.  When the rabbit urinates in the box, collect the as soon as possible by pouring it into clean glass or plastic container.  You can also use a syringe or eyedropper to suction up the urine. Take the sample to your veterinarian as soon as possible, but it may be kept covered in the refrigerator for up to 8 hours. The drawback to this method of collection is that fecal contamination is common which may alter the urinalysis results.
   
   
2. Manual expression - Your veterinarian or veterinary technician may be able to gently massage and squeeze the bladder in the awake rabbit and cause the pet to urinate.  The urine is directed into a collection cup.  This method is successful if the rabbit is cooperative and has a full bladder. 
   
   
3. Cystocentesis. - This method can be used in either the awake or anesthetized rabbit. The patient is placed on his back and the bladder is grasped by the handler and pulled gently up against the abdominal wall.  A small needle attached to a syringe is inserted through the abdominal skin and into the bladder.  Suction is applied and the urine sample is collected into the syringe. The procedure is accomplished in a few seconds with minimal discomfort to the rabbit.  If the hair is shaved and the skin cleaned, this method allows collection of sterile urine for bacterial culture.
   
   
4. Catheterization - The rabbit is anesthetized for this procedure.  A small soft catheter is placed in the urethra and then passed into the bladder and the urine sample is drawn out into a syringe.  This procedure also allows a sterile sample collection for bacterial culture. This procedure is also valuable in treating hypercalcinuria.  The catheter is used to add sterile fluid to the bladder and flush it clean of debris.

• Radiography (X-rays) - This is probably the most important diagnostic test to definitively determine if there are stones or sludge in the urinary tract.  Stones are very dense and thus will appear as white spots within the urinary tract.  The veterinarian will examine the bladder,  urethra, ureters and kidneys where stones may form.  In addition, if surgery is to be performed, an x-ray of the chest may be performed to evaluate the health of the heart and lungs. Sludge will look like an amorphous white blob and is usually noted only in the urinary bladder.
  
  
• Ultrasonography - Ultrasound is another method of looking at internal organs and may be indicated if a more detailed examination of the kidneys is required.  Ultrasound will also detect calculi in the urinary tract as well as other disease in the abdomen (or the chest).
  
  
• Serum biochemistries – These are blood tests that examine a number of different chemistries indicating the health of various organs of the body.  These tests are important to determine the condition of the kidneys (which may be damaged by calculi).  The blood calcium level should also be determined as discussed above.  Serum biochemistries also give clues as to the health of the liver, which is very important if surgery is contemplated.  Specialized chemistry tests may also be performed.  Measurement of serum bile acids is a specific liver function test that is more sensitive than the routine biochemical tests performed.
  
  
• Complete blood cell count (CBC) - This is a blood test that looks at the numbers of white and red blood cells and platelets as well as the types and abnormalities of blood cells A CBC may be indicated if the patient is not in good physical condition because it can detect signs of infection and anemia which may be significant in the treatment regimen.

Treatment of Bladder Stones and Sludge

The treatment of urinary tract stones and sludge is based on a number of factors including the overall health of the pet, the location and size of the stones or the amount of sludge and the presence of other disease.

• Surgery - If the rabbit has bladder calculi, it will be necessary to surgically remove them.  There are currently no diets that will dissolve these calculi.  The calculi will increase in size over time if they are not removed and may lodge in the urethra causing a life-threatening bladder obstruction. A bacterial culture of the bladder wall should be performed during surgery to detect any underlying infection.  The length of the hospital stay will depend on the surgery and the patient but may range on average from one to four days.

In cases where the calculi are in the kidneys or ureters, surgery may still be an option, but there is a higher risk of post-surgical complications.  Kidney function should be evaluated prior to any surgery in this area.  Kidney function can be evaluated through blood tests, dye studies and ultrasonography. In cases where there a kidney is damage due to blockage of a ureter ( condition called hydronephrosis), surgery to remove the kidney may be necessary.

In some instances, relatively small stones in female rabbits can be removed non-surgically using a technique called voiding hydropulsion.  This technique involves anesthesia and catheterization of the bladder.  Fluid is added to the bladder and the bladder is manually expressed in hopes of pushing stones out through the urethra.

Lithotripsy (the breaking up of stones using ultrasonic waves) is being investigated in dogs and cats for stone removal.  At this time, to my knowledge, this technology has not been used in rabbits.

Most cases of bladder sludge do not require surgery unless there is a blockage present.

• Catheterization and flushing of the bladder - Bladder sludge is usually treatable by anesthetizing the bunny, placing a catheter into the bladder and flushing the bladder with saline to dilute the sludge material.  The diluted material is then suctioned out into a syringe or the bladder is manually expressed. This process is repeated multiple times until the bladder sludge is significantly reduced.  Additionally, the urine can be cultured during this procedure.
  
  
• Diuresis (increasing water intake) - This is an extremely important part of the treatment of urinary disease.  Diuresis means increasing the water intake so that more urine is produced. This has the effect of diluting the urine and thereby decreasing the chances of further stones or sludge from forming. Fluids can be given intravenously, subcutaneously, intraperitoneally as well as orally.  Diuresis will be performed in the veterinary hospital and should be continued at home (see next section on Long Term Treatment).  Some veterinarians are using medications called diuretics to chemically induce more water to be added to the urine.  This therapy is quite unconventional and does run the risk of both creating dehydration and depleting the body of potassium.  At this time, I do not recommend the use of diuretics.  However, if they are used, they must be combined with aggressive oral or subcutaneous fluid diuresis performed at home.
  
  
• Analgesics – Non-steroidal analgesics (pain relievers) should be used before and after surgery or catheterization.  The length of time that pain relievers should be used depends on the procedure and the patient.  Once a rabbit is urinating normally, moving about without discomfort and eating well, analgesics can be discontinued.
  
  
• Vitamin C or cranberry tablets - Vitamin C can aid in the healing of the damaged urinary tract tissue after surgery or catheterization.  In addition, studies have demonstrated that the levels of ascorbic acid (vitamin C) in rabbits under stress drop dramatically.  Since disease or surgical procedures can cause stress it is likely that vitamin C will be of benefit in these cases.  Studies in humans indicate that cranberry juice contains alpha D-mannopyranoside that helps to prevent bacteria from adhering to the bladder wall.  In addition it is a good source of vitamin C.  Therefore you can supplement rabbits healing from urinary tract disease with cranberry tablets based on their vitamin C content at 25 to 50 mg of vitamin C per pound of body weight one to two times daily. Do not use cranberry juice cocktail, which has very little cranberry juice and in addition are loaded with sugar.  If cranberry tablets are not available, use regular chewable vitamin C tablets for the vitamin C source. (NOTE: I am not sure that cranberry juice does not exert its effect via acidification of the urine… is this a good idea in herbivore?)
  
  
• Antibiotics - Antibiotics may be prescribed after surgery or catheterization particularly if a bladder infection is suspected or a positive culture was obtained from the urine or bladder wall.

Not Useful:

• Urinary acidifiers - Due to the fact that rabbits are herbivores their urine is normally alkaline. Using products designed to make the urine more acidic not only do not work, but in some cases could be harmful.  This is different than the situation in cats which are carnivores with naturally acidic urine. In cats with bladder stones it is common practice to administer urinary acidifiers or to feed special diets designed to create a slightly acid urine to prevent further stone formation.

Treatment and Prevention of Bladder Stones or Sludge

Although there is no guarantee that urinary tract stones or sludge will not return after treatment, you can greatly reduce your pet's chances of dealing with a recurrence by using the following suggestions.

• Increase water consumption -  I consider this to be the most important factor in the prevention of the reoccurrence of bladder stones and sludge. If the urine is kept dilute, the possibility of stone or sludge formation is dramatically decreased. Feeding fresh leafy greens that are still wet from washing will help increase water intake (See note on this below). Adding a natural sweetener, not refined sugar, to the drinking water will encourage your pet to drink considerably more.  Natural fruit juices with no added sugars, or honey, are an excellent flavoring choice. Test various juice flavors by administering them directly to your rabbit undiluted in a syringe. If your pet takes the juice willingly then try adding it to the drinking water.  Record how much water is consumed daily by your pet (this is easy with sipper bottles as they are often marked in ounces on the outside).  Use fresh flavored water daily and gradually decrease the amount of flavoring until you see that the rabbit decreases its water consumption.  At this point, increase the flavoring just enough to increase water consumption again and continue to use that amount of flavoring daily. Some flavorings that people have reported success with include pineapple, grape, apple and cherry juice and apricot, peach or pear nectar and honey.
  
  
• Remove commercial food (pellets)/alfalfa hay from the diet - As discussed previously in this article the level of calcium in the diet is not the primary factor in causing urinary calculi. However, excessive amounts of calcium above and beyond what the pet needs on a daily basis may add to the calcium content of the urine and be a problem in a rabbit that is already experiencing urinary calculi. Commercial rabbit diets that are alfalfa based are packed with digestible calcium and I recommend removing them from the diet. Alfalfa hay also has very high digestible calcium content and should be removed. The recommended diet for a house rabbit is grass hay (e.g. timothy, oat, or orchard grass hay) along with large amounts of fresh leafy greens, and some smaller amounts of fruits and other vegetables. If pellets are to be fed, they should be grass hay based.
  
  
• Feed large amounts of fresh leafy greens - Fresh leafy greens are not only an excellent source of fluid for the rabbit (to keep the urine dilute) but also contain a variety of nutrients (including vitamin C) and fiber (for the digestive tract). Use dark leafy greens that are richer in nutrients than the pale green types. Feed a minimum of three different types of greens daily. Don’t stick to just one type. Examples of greens to feed include kale, mustard greens, dandelion greens, parsley, cilantro, chard, beet tops, carrot tops, raspberry leaves, broccoli leaves, and romaine lettuce

As mentioned previously, although the calcium content of dark leafy greens is higher than some other foods, it is not all digestible and doesn't approach the level of commercial alfalfa-based pellets or alfalfa hay.  NEVER attempt to remove all calcium sources from a rabbit's diet or problems with abnormally low calcium blood levels, such as bone, dental and muscle disease can occur. I do not consider feeding dark leafy greens a part of the problem causing urinary calculi, and, in fact, I consider these foods to be an essential part of a healthy rabbit diet. I do not restrict the type of leafy green based on calcium content.

• Exercise - Exercise is extremely important to cause the rabbit to urinate more frequently, moves the urine around in the bladder and keeps it mixed instead of stagnant, as well as causing the rabbit to consume more water. In addition to helping with the urinary problem, exercise helps to strengthen bones and improve the rabbit's mental attitude.
  
  
• Routine veterinary checkups - It is necessary to follow up your pet's progress with routine rechecks with your veterinarian. A urinalysis should be performed several times during the first few weeks after surgery or catheterization to detect any infection or increase in mineral formation. A physical examination should be performed post-surgically to evaluate healing of the bladder and abdomen. Radiographs should be performed within six months initial treatment to look for the return of stones or sludge in the urinary tract. Please follow your veterinarian's recommendations on recheck visits.

Final Word

Twenty to thirty years ago when we first dealt with rabbits that had bladder stones or sludge, the prognosis for successful treatment and further prevention was dismal. Many rabbits were euthanized because recurrence of the disease was so common. Thankfully we now have a better understanding of this disorder and improved techniques for working with rabbits and thus the prognosis is tremendously improved. The majority of rabbits can recover completely from this problem. Although the prognosis must remain guarded because each patient has his or her own individual response to therapy, a diagnosis of bladder stones or sludge need not be the death sentence it was not so long ago.

This article is reprinted here with authorization from the author.  It was written for the House Rabbit Society. The author and the HRS would like to recommend you visit www.rabbit.org for more informative articles on rabbit care.  This article was reviewed an approved by the HRS Health Committee 9/06.