Urolithiasis is a rare condition in horses that involves the formation of bladder stones within the horse’s urinary tract. These urinary stones or uroliths are hard collections of crystals or mineral deposits, which can lead to various health complications. [1][2][3]

The most common type of urinary stone in horses is calcium carbonate crystals found within the bladder. Bladder infections often trigger the formation of these stones. [1][2][3]

Symptoms of urolithiasis vary based on where the stones form in the urinary tract. Horses may exhibit subtle signs such as weight loss and reduced appetite, or more overt symptoms such as difficulty urinating and the presence of blood in the urine. [1][2][3]

Veterinarians will typically diagnose urinary stones with a physical examination, including rectal palpation. In certain cases, other diagnostic tools may be needed. [2][3]

Urolithiasis is a condition that requires prompt attention and appropriate management to ensure the well-being of the affected horse. The goal of treatment is to eliminate the stones from the urinary tract and prevent recurrence of this condition. [2][3]

Urolithiasis in Horses

Urolithiasis in horses typically results from the accumulation of mineral salts, primarily calcium, in the urinary tract. [1][3]

Bladder stones typically affect horses that are at least 10 years old, but they can also affect younger horses. Male horses, especially geldings, are the most susceptible to developing urinary stones. [5][6]

Some horses can also have uroliths in their kidneys, which are harder to detect. If stones enter the bladder and continue into the narrow tube known as the urethra, they can get stuck and obstruct the normal flow of urine. [2][3]

Urinary blockages are more common in other species of animals. In horses, males, specifically geldings, are more commonly affected by this issue than females. [2]

Usually, there is only one stone present in the urinary tract at a time. However, if a stone has one smooth side and one rough side, it indicates the presence of more than one stone rubbing together. [3]

Risk Factors

Several factors can contribute to the development of urinary tract stones in equines:

  • Diet: An imbalanced diet with excessive levels of certain minerals, particularly calcium and phosphorus, can predispose horses to urolithiasis. Diets that are high in alfalfa hay or legumes may contribute to bladder stone formation.
  • Dehydration: Insufficient water intake can lead to higher mineral concentrations in urine, which increases the risk of stone formation. Horses that do not have access to clean, fresh water are more susceptible to urolithiasis.
  • Age and Gender: Urolithiasis is more commonly seen in male horses, particularly geldings. This is because geldings have narrower urethras, making it easier for the stones to become lodged in the urinary tract.
  • Breed Predisposition: Certain horse breeds, such as Miniature Horses, are presumed to be more prone to urolithiasis due to differences in conformation. However, genetics are not a known risk factor for uroliths in horses. [3]
  • NSAID Use: Prolonged anti-inflammatory (NSAID) administration can lead to kidney damage, which may contribute to the formation of kidney and bladder stones. [2][6]

Types of Bladder Stones

Uroliths in horses can be made of different minerals that form crystals in the urinary tract. These mineral compositions can vary depending on the specific conditions and dietary factors influencing stone formation.

The most common bladder stones in horses are made of calcium carbonate, a combination of calcium and carbonate ions. [1][2] Stones formed from this type of crystal typically have a rough surface and can crumble when touched. [3]

In some cases the bladder fails to completely empty of urine, typically due to neurological disorders. This type is known as “sabulous” urolithiasis, and is characterized by sandy sediment in a distended bladder. [1][3]

In rare cases, stones may be formed of calcium phosphate, which consists of calcium and phosphate ions. These stones are typically grey-white, smooth, and are not easy to break apart. [1]

Stones made of struvite crystals are even less common in horses. These bladder stones are more common in cats and dogs. [2][7]

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Clinical Signs

Recognizing the signs of urolithiasis in horses is crucial for early diagnosis and treatment. Common symptoms include: [1][3][6][8][9]

  • Weight loss
  • Fever
  • Red, bloody urine
  • Straining to urinate
  • Frequent urination
  • Dribbling urine
  • Only passing small amounts of urine
  • Grunting during urination
  • Colic
  • Sweating
  • Restlessness
  • Decreased appetite
  • Anorexia
  • Difficulty passing manure
  • Hindlimb weakness

The severity of clinical signs varies depending on the location of the stones within the urinary tract. Some symptoms may also be more noticeable after exercise or towards the end of the urination. [3][4][9]

Horses that dribble urine can develop skin sores, also called urine scald. [2] Signs associated with urine scald include redness and wetness of the skin around the perineum of females or hindlimbs in males.

Causes of Urinary Stones

Urinary stones form in horses due to a combination of factors related to urinary tract function and mineral balance.

  1. Minerals in Urine: Horses normally excrete large amounts of minerals, such as calcium carbonate, in their urine. When the concentration of minerals in the urine becomes too high, calcium and other minerals can precipitate out of the urine and form tiny crystals. These crystals serve as the initial building blocks for bladder stones. [1][2][3]
  2. Crystal Aggregation: Over time, these tiny crystals can aggregate and stick together to form larger, solid structures within the bladder. This process is facilitated by the presence of organic materials and proteins in the urine, which can act as a matrix for crystal aggregation. [1][2][3]
  3. Inflammatory Cells: Damaged tissue and inflammatory cells in the urinary tract also provide suitable surfaces for crystals to attach to and start forming stones. Urinary tract infections increase the presence of damaged tissue and cells within the urinary tract, raising the risk of urolithiasis. [3][6]
  4. Stone Growth: Once crystals start to grow in the urinary tract, the alkaline (high pH) urine of horses favors the continued formation of these stones. If urine remains in the bladder for too long, there is a higher likelihood of crystals coming into contact with each other, promoting further aggregation. [3]

If stones are not removed from the bladder, they can result in a persistent cycle of urolithiasis. Rough-surfaced stones can damage the bladder tissue, creating new surfaces for additional stone formation. The presence of bacteria in stones may indicate a concurrent bladder infection. [1]

Secondary Conditions

Urolithiasis can pose a significant health risk to horses, potentially causing discomfort, pain, and severe complications if left untreated.

Secondary conditions associated with bladder stones include urinary obstruction, chronic kidney failure and bladder rupture.

Urinary Obstruction

The most common secondary condition associated with urolithiasis is urinary obstruction. This can occur when a stone becomes lodged in the narrow urethra on its way out of the bladder, resulting in a blockage. Stones that become stuck can partially or completely obstruct the flow of urine. [3]

Male horses are at a higher risk for urinary blockage because their urethra is narrower than females. In males, the most common location for stones to become lodged is the part of the urethra that curves around the pelvis, known as the ischial arch. [3]

Chronic Renal Failure

Horses that have bladder stones should be assessed for stones elsewhere in the urinary system. Although less common, some horses may also have stones in their kidneys or ureters (tubes connecting the kidneys to the bladder). Kidney stones can lead to chronic kidney failure. Weight loss is the most common symptom in horses with chronic renal failure due to uroliths. [1]

Any condition that damages the kidneys, producing dead cells that minerals can bind to, can lead to kidney stones. Chronic use of NSAIDs and dehydration are the most common risk factors for kidney stones. [10][13]

Bladder Rupture

If a bladder stone obstructs the outflow of urine, the horse’s bladder will continue to fill and potentially rupture.

A ruptured bladder can result in a serious condition known as uroperitoneum, in which urine leaks into the abdomen. Uroperitoneum causes severe electrolyte imbalances and requires urgent care. [1]

Diagnosis

The diagnosis of urinary stone in horses typically involves a combination of clinical evaluation, imaging studies, and laboratory tests. Veterinarians often begin by conducting a thorough physical examination to assess the horse’s overall condition and look for signs such as discomfort, straining to urinate, or blood in the urine.

Bladder stones are usually easily palpated on physical examination. If a firm, oval mass can be felt within the bladder on rectal palpation, a diagnosis of urolithiasis is made.

A thorough pelvic floor exam must be performed on rectal palpation to avoid missing smaller bladder stones. [2][3] Ultrasound can also be used during the examination to help identify bladder stones that cannot be diagnosed on rectal palpation alone. [2][3]

Diagnosing Urinary Obstruction

If a stone completely obstructs the outflow of urine, the horse will present with an extremely full bladder. However, if the horse can still pass urine, a small empty bladder is more commonly observed.

A small tube (urinary catheter) can be passed into the urethra to identify the approximate location of the stones causing the obstruction. Alternatively, a small camera can be passed up the urethra to visualize the presence of a stone. [2]

Before using a camera to examine the bladder for stones, the bladder must first be emptied. Urine present in the bladder can cover and hide the stones. [3]

Small cameras passed into the bladder can also assess if urine is flowing properly from the kidneys. Improper flow of urine into the bladder indicates that there may be a kidney issue causing the obstruction. [3]

Treatment

Treatment of urolithiasis in horses focuses on removing the stones from the urinary tract. The choice of procedure is typically made by the veterinarian, taking into account factors such as the horse’s gender, overall health, and size of the stones. [3][8][11]

Culturing the urine after the stone is removed is recommended to identify any underlying bladder infection. It is also recommended to send stones for laboratory analysis to guide future preventative measures. [2]

Treatment in Mares

The urethra in mares is much larger than in male horses. Smaller bladder stones may pass in the urine spontaneously, without producing symptoms.

If a bladder stone becomes stuck in a mare’s urethra, manual removal may be performed to eliminate the stone, often avoiding the need for surgical intervention. [1][3]

The procedure for manual removal involves sedation and an epidural to block sensation in the hind end. After that, the urethra can be gently widened by hand to remove the stone. [3]

Manual removal is usually conducted when the stone is smaller than 10 cm across. For larger stones, a laser is first used to break them into smaller pieces, which can either be passed in the urine, rinsed out, or removed by hand. [2][3][11]

Treatment in Males

For male horses, surgical removal of bladder stones without breaking them into smaller pieces is generally recommended. This avoids further tissue damage from sharp stones passing through the narrow urethra. [2][3]

Surgery to remove bladder stones usually requires general anesthesia, because an incision in the body wall is needed to visualize the bladder. Surgery aided by cameras (laparoscopic surgery) is also an option for veterinarians if the equipment is available. [3][11]

For owners under financial constraints, standing sedation can be used. In this procedure, stones are broken into tiny pieces using a laser, and a small incision is made in the urethra to collect the stones. If the stones are close enough to the end of the penis, specialized tools can be used to remove them by hand. [3][8][11]

Supportive Care

In addition to removal of the urinary stones, other treatments and supportive care are routinely administered, including: [6][11]

  • Antibiotics
  • Providing intravenous (IV) fluids
  • Regular cleaning of the genital region and hindlimbs to avoid skin damage due to urine
  • Feeding a hay-based diet low in calcium to prevent the formation of stones
  • Providing oral electrolytes or salt to encourage drinking more water

Prognosis

The prognosis for horses with urinary stones depends on the location of the stone and whether the horse is affected by other medical conditions. [1]

Stones that completely obstruct the passage of urine require immediate attention for a favorable outcome. Otherwise, secondary complications can occur, resulting in a poorer prognosis. [4]

Horses with underlying chronic renal failure or other kidney issues have a poor prognosis. [1][6][9]

Recurrence of bladder stones is common, with up to 40% of horses developing urolithiasis again after treatment. [1][6]

Procedures that involve breaking urinary stones into smaller pieces and allowing them to pass naturally result in the highest risk of recurrence. This is because there is a high likelihood that not all stones will be passed in the urine. [1][6][11]

Surgical removal has the highest success rate of available treatments, but poses risks if general anesthesia is required. Complications associated with surgical removal include: [6][9][11]

  • Leakage of urine into the abdominal cavity
  • Infection of the abdominal cavity
  • Surgical site infection
  • Further narrowing of the urethra due to inflammation and healing

Prevention

Your horse’s diet and management play an important role in preventing the development of uroliths. If your horse has a history of bladder stones, work with an equine nutritionist to formulate an appropriate prevention diet.

Feeding Program

Balancing the mineral composition in your horse’s diet is essential for preventing the formation of uroliths. Avoid calcium-rich forages and feeds, such as alfalfa, other legumes, and beet pulp. [2][3][12]

Low-calcium diets can effectively prevent stone formation in horses with recurrent urolithiasis. Feeding oat hay may also reduce the risk of urinary stones compared to grass hay. [12]

To determine the calcium content of your horse’s diet, it is critical to submit a forage sample for analysis. A forage report will provide levels of calcium and phosphorus which an equine nutritionist can use to ensure the ratio of these minerals is correct and the levels are suitable for your horse.

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Hydration

Increasing your horse’s water intake helps to lower the concentration of calcium carbonate crystals in the urine. Increasing water consumption also reduces the amount of time urine spends sitting in the bladder. [3]

You can encourage hydration by adding 1 – 2 ounces of plain loose salt to your horse’s daily feed, and providing free-choice loose salt in your horse’s environment. Avoid salt blocks as they may not provide sufficient salt intake.

Ensure your horse has fresh, clean water available at all times and provide adequate warm water during colder months.

Medications

Several medications promote the excretion of calcium carbonate in urine, thereby reducing the risk of urolith formation. These medications work by making the horse’s urine more acidic (lower pH).

The following have been fed to horses to acidify the urine: [2][3][12]

  • Ammonium chloride (50-200 mg per kg daily)
  • Ammonium sulfate (200-300 mg per kg daily)
  • Methionine (1 g per kg daily)
  • Ascorbic acid (1-2 g per kg daily)

However, the efficacy of these compounds has not been proven and may be unpalatable at these doses. [3] In addition, taking some of these medications long-term can lead to serious conditions such as osteoporosis, especially in athletic horses. Veterinary guidance is necessary for the proper use of any medication. [2]

In horses with sabulous urolithiasis, medications such as bethanechol chloride may be used to aid in emptying the bladder. Bethanechol chloride is administered orally at a dose of 0.25 – 0.75 mg per kg between 2 – 4 times daily. [3]

Summary

  • Urolithiasis in horses refers to the presence of stones within the bladder and urinary tract. These stones are formed from aggregations of calcium carbonate crystals, which occur naturally in the bladder of healthy horses. [1][2][3]
  • Conditions such as urinary tract infections can predispose horses to developing bladder stones. Middle-aged, geldings are most frequently affected by urolithiasis. [1][2][3][6]
  • Symptoms of bladder stones include straining to urinate, dribbling urine and blood in the urine. Your veterinarian will diagnose urolithiasis based on rectal palpation or other diagnostic tools. [1][2][3][6]
  • Treatment focuses on removal of the stones from the urinary tract, often using surgical techniques. However, there is a high recurrence risk. [1][2][3]
  • Diet and management play a role in preventing the formation of bladder stones. Horses prone to urolithiasis may be given medications to increase excretion of calcium carbonate. [1][2][3]

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References

  1. Duesterdieck-Zellmer K.F., Equine Urolithiasis. Veterinary Clinics of North America: Equine Practice. 2007. View Summary
  2. Jones M., Urolithiasis in Horses – Urinary System. Merck Veterinary Manual. Accessed Oct. 26, 2023.
  3. Edwards B. and Archer D., Diagnosis and Treatment of Urolithiasis in Horses. In Practice. 2011.
  4. DeBowes R.M., Surgical Management of Urolithiasis. Veterinary Clinics of North America: Equine Practice. 1988. View Summary
  5. Carleton C.L., Blackwell’s Five-Minute Veterinary Consult Clinical Companion: Equine Theriogenology. John Wiley & Sons, Incorporated. 2011.
  6. Lavoie J.-P. and Hinchcliff K.W., Blackwell’s five-minute veterinary consult: equine. 3nd ed. Wiley-Blackwell. 2019.
  7. Reuss, S. Bladder Stones. University of Florida. Accessed Nov. 02, 2023.
  8. Laverty S. et al., Urolithiasis in 68 Horses. Veterinary Surgery. 1992. View Summary
  9. Holt P.E. and Pearson H., Urolithiasis in the Horse — A Review of 13 Cases. Equine Veterinary Journal. 1984. View Summary
  10. Macbeth B.J., Obstructive Urolithiasis, Unilateral Hydronephrosis, and Probable Nephrolithiasis in a 12-Year Old Clydesdale Gelding. Can Vet J. 2008. View Summary
  11. Hawkins J.F., Surgical Treatment of Urolithiasis in Male Horses. Equine Veterinary Education. 2013.
  12. Remillard R.L. et al., Dietary Management of Cystic Calculi in a Horse. Journal of Equine Veterinary Science. 1992.
  13. Murillo, D.F.B. et al. Severe unilateral nephrolithiasis and recurrent colic in a horse. Vet Record. 2023.