Calcium Metabolism in Rabbits and Its Possible Role in Bladder Stones and Urinary “Sludge” Syndrome
Calcium is an essential mineral needed in every day bodily functions. Most calcium is found within our bones and teeth, but the small amount circulating in the blood stream and body fluids helps the body perform such vital functions as muscle contractions, heart contraction, intestinal movement and nerve impulse conduction.
In most mammals the body has a system of checks and balances which keeps calcium at a steady, normal level in our blood stream. Vitamin D and the hormones calcitonin and parathyroid hormone (PTH) all play a role. When the calcium level drops low, PTH is released and stimulates the development of an active form of Vitamin D in the kidneys. This Vitamin D then tells the intestines to absorb more dietary calcium into the bloodstream thus raising the blood calcium levels. When calcium levels rise, calcitonin is released promoting calcium withdrawal from the blood and deposition in bone, thus keeping blood levels in a normal range.
Now, rabbits don’t metabolize calcium like other mammals (aren’t we finding out that rabbits do a lot of things different from other mammals!). In the rabbit, the higher the dietary intake of calcium, the higher the level of circulating calcium in the bloodstream. Therefore, rabbits that eat diets high in calcium will have proportionately higher blood calcium levels. So what happens to all this excess calcium? It is excreted in the urine! Yes, rabbits will excrete up to 60 percent of their ingested calcium in the urine (vs. other mammals which will excrete only 2 percent of ingested calcium in the urine). That’s why normal rabbit urine will look “cloudy”-this opacity is due to all the calcium carbonate crystals in the urine.
This syringe shows a rabbit’s urine with a significant amount of calcium-type crystals. If this builds up in excess it can cause a urine sludge syndrome as is seen in this rabbit having his bladder emptied and flushed (below).
Image 1 Image 2
On a radiograph (x-ray – image 1) the high degree of urine calcium crystals will show up light and outline the bladder-the bladder show up as the white, ovoid structure on the above x-ray. Some degree of calcium buildup in the rabbit’s urine is normal but too much may predispose to infection and lead to trouble with urine leakage and soiling of the fur on the back end (image 2).
So what does this mean to a rabbit’s health? Well, that depends a lot on genetics, the individual rabbit, and factors we still don’t know about. The normally alkaline urine of herbivores , such as rabbits, may increase the risk of precipitation of urinary calcium into stones or thick toothpaste-like deposits of calcium crystals known as sludge. When a rabbit develops bladder stones or sludge it will usually show signs of not feeling well. You may notice depression, lack of appetite, weight loss, lethargy, thick creamy urine, straining to urinate, a hunched position, teeth grinding (indicating abdominal pain) and urine scald of the skin around the prepuce or vagina. The rabbit will lose control of its normal urinary habits and dribble urine, which keeps the fur around the perineum moist, thus leading to an inflamed skin or scald.When these signs are seen, a trip to your veterinarian is in order. With knowledge of the above history, a physical exam, urinalysis, bladder x-rays and blood calcium levels your veterinarian will be able to rule out bladder disease as the diagnosis. Due to the high levels of calcium excreted in the urine, many normal rabbits will have a small amount of calcium “sand”/crystals show up on a radiograph. Larger volumes of this crystalline sludge and actual bladder stones are not normal, and will be reflected in the rabbit’s overall disposition and health.
The diagnosis is made . . . now what can be done? Now the focus is on treatment and prevention of recurrence. Treatment involves ruling out underlying bacterial infection and treating with antibiotics if needed. If bladder stones are present, surgery is necessary to remove the offending, painful stone. If sludge alone is present, subcutaneous fluids are given on a frequent basis to increase urine output and “flush” the bladder. Also, manually expressing the bladder to expel the sludge helps speed the process. In some rabbits, urinary catheterization under anesthesia is necessary to allow complete flushing of the bladder.
Once the offending calcium sludge or stones are removed we focus on prevention. As mentioned earlier, a rabbit’s blood and urine calcium levels reflect dietary intake of calcium. Decreasing dietary calcium will directly lower serum calcium levels and subsequently the amount of calcium excreted in the urine. To reduce calcium intake, grass hay (timothy, oat, orchard grass) and green vegetables should be the primary diet and the amount of pellets reduced to 1-2 ounces per day. Oxbow’s timothy hay and high fiber Bunny Basics T pellets are both low in calcium and recommended. In the vegetable arena, collards, chicory greens, lambs quarter, and mustard spinach are higher in calcium than other choices and are best avoided. Please note, alfalfa hay is also high in calcium which is why we recommend the grass hays.
The fiber rabbits need for nutrition, proper digestion and to aid in normal dental wear comes from feeding grass hays. Variety is the spice of life and that includes the hay you feed your bunny. Oxbow Animal Health Products offers a variety of hays and Pet Care Veterinary Hospital recommends the timothy, orchard grass, oat or botanical hays for the adult rabbit.
References
1. Redrobe S. Calcium Metabolism in Rabbits Seminars in Avian and Exotic Practice, 2002.
2. Cheeke PR. Nutrition of the Domestic Rabbit
3. Whary, Mart T, Peper, Randell L. Calcium Carbonate Urolithiasis in a Rabbit. Laboratory Animal Science 1994
4. Paul-Murphy, J. Urinary Tract Diseases and Disorders Proceedings HRS Veterinary Conference 1998
