Information provided by Sarah Reuss, VMD, DACVIM
What are bladder stones?
Bladder stones are a concretion of minerals. In horses, they are primarily made of calcium. In other species like goats (and cats), they are primarily what we call struvite, which is magnesium ammonium phosphate. In horses, the stones are most often found in the bladder and called cystoliths. But stones can also occur in the kidney (nephroliths), ureter (ureteroliths), or urethra (urethroliths).
What are clinical signs of bladder stones?
The most common sign in horses is blood in the urine, especially after exercise. Horses may also show low grade signs of colic or abdominal pain. Fortunately, horses rarely have stones that fully obstruct their urine flow. On the other hand, small ruminants can have stones that fully block their urethra and make them unable to urinate. They will show more obvious straining, pain, and vocalization when trying to urinate. The disease is much more common in males of every species because of their longer, narrower urethra. Females can generally pass sludge and small stones before they become large enough to cause a problem.
What causes bladder stones?
Bladder stones often start with a “nidus” or a cluster of cells. These cells may be there because of a urinary tract infection or because of kidney damage from drugs like phenylbutazone and flunixin (Banamine). Once that cluster of cells is there, then layers of mineral keep getting deposited concentrically until a stone is formed. The different types of stones that different species can make relate to their diet and urine pH. Horses take in a lot of calcium with forage and therefore excrete a lot of calcium in their urine. They also have a lot of mucus in their urinary tract to try to keep stones from forming. That calcium and mucus is why even normal horse urine is often cloudy. But if the amount of calcium that needs to be excreted overwhelms that mucus production and finds a cluster of cells to grab on to, then a stone will begin to form. All herbivore urine also has a high pH (is alkaline) which precipitates formation of calcium carbonate stones.
In our small ruminants, their stones are primarily made of phosphate. This is also affected by their diet and urine pH. Goats that eat a lot of concentrate often consume too much phosphorus relative to calcium. They then have to excrete that extra phosphorus through their urinary tract instead of through their saliva which is their primary means of excretion if they are chewing on a lot of hay and browse. Their urine is also very alkaline, so once a stone starts it will keep growing.
How are bladder stones diagnosed and treated?
In horses, bladder stones can often be identified by transrectal palpation of the bladder. Ultrasound and endoscopy can help confirm the size, number, and location of stones. Cystoscopy can be used to look for stones in the urethra, and also to verify that urine is coming from both kidneys into the bladder (making a stone in that kidney or ureter less likely). Ultrasound is especially helpful to look at the kidneys for any signs of underlying kidney disease or stones within the kidney itself. In males, some degree of surgery is usually necessary to remove the stone from the bladder. Sometimes this can be done by making a small incision in the urethra just below the anus (called a perineal urethrostomy) with the horse just sedated and with local lidocaine. With larger stones, general anesthesia and a midline incision into the abdomen (like with colic surgery) may be necessary to access the bladder to remove the stone. Lithotripsy can also be used to break stones into smaller pieces so that they can be removed without surgery.
In goats and sheep, the stones are usually stuck in the urethra. They can be diagnosed based primarily on consistent signs in a male animal, but ultrasound can confirm that the bladder is full and unable to empty. Medical treatment may be attempted but is usually unsuccessful. Surgery consists of general anesthesia and making either a temporary (tube cystotomy) or permanent (marsupialization) connection directly between the bladder and the outside.
How can bladder stones be prevented?
Keeping urine dilute may help prevent stone formation. So any tricks you can use to keep your horses drinking lots of water will help (flavoring water, feeding mashes, salt supplementation). Minimizing use of drugs like NSAIDs that are damaging to the kidney will also help. While not necessary for every normal horse, in a horse that has had a stone before, we recommend removing legume hays like alfalfa from their diet as these are high in calcium. We also try to make their urine acidic to dissolve calcium crystals as they form. Things like vitamin C and ammonium chloride have been used, but recently we have had success in a few cases using a feed supplement designed for dairy cattle.
In goats, making sure that the bulk of their diet is forage is incredibly important. Any concentrate fed to them should have a calcium to phosphorus ratio of greater than 2:1. Castration should also be delayed in goats until at least 6 months as testosterone may influence the diameter of their urethra. And just like in horses, make sure they are drinking plenty of water. Acidification of their urine will also help. Historically we have used ammonium chloride, but most goats don’t eat it well. We are currently conducting trials with SoyChlor (the feed supplement designed for dairy cattle) to see if it will effectively acidify urine, as most animals will eat this product readily.