The urinary tract is a vital system that should never be taken for granted; if a problem develops, it can rapidly become a crisis

(Editor’s Note: Contributing to this article was Hal Schott, DVM, PhD, Dipl. ACVIM, associate professor in the Department of Large Animal Clinical Sciences at Michigan State University.)

The body produces many wastes and byproducts of energy production that must be regularly eliminated. The lungs dispose of gaseous wastes and carbon dioxide, and the digestive tract takes care of the solid wastes from feed. The urinary system excretes the liquid wastes containing nitrogen, salts, excess sugars, and other substances that are filtered out of the bloodstream.

Urinary tract problems can range from fatal (kidney failure, bladder rupture, etc.) to minor (irritations that clear up on their own, or an infection that can be readily treated). The kidneys filter the blood and kidney damage can result in impaired filtering function, which might be life-threatening, depending on the extent of damage. A bladder stone might pass, and the horse will be fine, or it might plug the urethra, prevent urine from passing, and cause eventual rupture of the bladder. The main signs of urinary disease include abnormality in urine output, pain or discomfort, and difficulty urinating.

Overview of the Tract

The body functions in a sea of fluids. Blood acts as the transportation system, taking oxygen and nutrients to all cells and bringing back waste products to be eliminated. The kidneys constantly filter the blood, removing some substances, absorbing some that shouldn’t leave the body, and collecting wastes. The portion of fluid–and its concentrated waste products­–filtered out by the kidneys becomes urine.

The urinary system consists of two kidneys, each with small tube called a ureter that leads from the kidney to the bladder (a storage organ for urine). A tube called the urethra leads from the bladder to the urethral opening on the outside of the body, through which the urine is excreted.

The horse’s kidneys are chocolate-colored organs about six to seven inches long and four to six inches wide. They lie near the top of the abdominal cavity on each side of the backbone. The entire volume of a horse’s blood passes through the kidneys many times per day, but only a tiny fraction of the fluid filtered from blood is converted into urine.

Inside the kidneys are millions of tiny ball-shaped filters called glomeruli that are packed tightly into the thick, curved outer portion of the kidneys. Each glomerulus is a tufted network of intricately laced capillaries or tiny blood vessels that lies inside a tiny capsule (Bowman’s capsule) that collects the fluid filtered from the blood. The filtered fluid then flows from Bowman’s capsule into the kidney tubules, which extend from the outer portion of the kidney to deep within the kidneys. The horse’s entire blood volume travels through these filters 200-300 times a day, resulting in production of around 300-400 gallons of filtrate daily. The fluids passing through these filters contain many dissolved materials–salts, sugars, chemicals, and nitrogenous (nitrogen-containing) wastes. Some of these are indispensable to the body and are retained. Some are harmful and must be removed.

The kidney tubules perform this important sorting function, gradually concentrating the residues into urine. They also regulate the vital acid-base balance, fluid volume, and electrolyte balances that must be kept within extremely narrow limits. For instance, the body’s salt balance is closely regulated by the kidneys. Any extra salt is excreted, and necessary salt and fluid are returned to the bloodstream along with valuable substances such as crucial mineral electrolytes and sugars.

All but one or two gallons of this filtered fluid are returned to the blood each day. This small amount not needed by the body, containing nitrogenous wastes (byproducts of body metabolism) and any excess salts or sugars, travels down the tubules into cuplike chambers that merge and open into the funnel-shaped sac (renal pelvis) that drains urine into the ureter for passage into the bladder.

The bladder is like a large, muscular balloon lying within the pelvic cavity. The bladder walls are extremely elastic, and they can expand tremendously without bursting. When full, the bladder can even extend forward into the abdominal cavity almost as far as the horse’s navel, but he usually empties it before that point.

The urethra, the passageway from the bladder to the outside, is relatively short in mares, and it is longer and narrower in geldings and stallions. As urine collects in the bladder and distends it, nerve endings sensitive to pressure begin to signal the brain that the bladder is full, and the horse decides to urinate. To do so, he relaxes a circular muscle at the bladder outlet and contracts the bladder muscle to expel the urine.

Sometimes a horse will urinate frequently even though the bladder is not full. A mare in heat will urinate small dribbles while passing secretions from the vagina. This is one of her ways to signal a stallion that she is in heat–he is able to smell the vaginal secretions within the urine. Inflammation or infection in the bladder can also stimulate the nerve endings to signal a desire to urinate even when the bladder is not full. The inflammation or infection also can make the act of urination difficult or painful.

Urinary Stones

Horses tend to have fewer urinary problems than other domestic animals, but one of the problems they might occasionally suffer from is urolithiasis–the formation of urinary stones called calculi. These stones can form in the kidney or in the bladder (bladder stones are more common), causing difficult urination, discomfort, and colic. If there is total blockage, they might result in the death of the horse from metabolic problems that occur if the bladder bursts and urine accumulates in the abdomen.

Urine is a complex and supersaturated solution of many substances, including mineral salts that might precipitate out of solution into crystals under certain conditions; kidney/bladder stones are formed by combination of many small crystals into a larger conglomerate. Variation in the mineral content of feeds and water can influence the formation of stones. In some instances, a change in the acid-base relationship (pH) in the thick “soup” of urine can also trigger formation of stones. If you have ever seen your horse pass urine, you likely have recognized a couple of differences from the urine voided by dogs (or by yourself). First, horse urine is very cloudy. Cloudiness is a consequence of the large amount of calcium carbonate crystals that horses normally excrete in urine. While humans and dogs tend to regulate calcium absorption from the diet at the level of the intestine, horses tend to absorb excessive amounts of calcium from the intestine and must eliminate it via urine. If urine is collected in a transparent container and allowed to sit for a few minutes, these calcium carbonate crystals will actually settle to the bottom of the container­–imagine one of those travel souvenirs that you shake to disperse “snowflakes” throughout the water, only to have them settle at the base of the Empire State Building. Second, horse urine is very bubbly (almost foamy) when it is first passed. This is due to the large amount of mucus in horse urine. In fact, a concentrated urine sample can be quite stringy and viscid (thick). Mucus is secreted from the innermost portion of the kidney (the renal pelvis), which is the start of the drainage system through the ureter. Mucus acts as a lubricant that can help prevent small calcium carbonate crystals from forming into stones.

Horses fed hays high in calcium (for example, alfalfa or clover) have a larger amount of calcium carbonate crystals in their urine. Not surprisingly, the vast majority of equine uroliths (calculi) are conglomerations of calcium carbonate crystals that can harden into one or more rough-surfaced, yellow-to-brown-colored stones. Although the stones are as “hard as rock,” they are rather fragile.

A diet high in phosphorus (usually due to large amounts of grain) also can make conditions just right for formation of stones rich in phosphate. These phosphate stones often have a smooth surface and are much more dense that the more common calcium carbonate stones. Other factors that make conditions more favorable for producing stones are urinary tract infections or anything that causes a horse to not drink enough water (causing urine to become more concentrated). The more concentrated the urine, the more likely crystals will precipitate, although horses that become dehydrated do not routinely develop stones.

Normal urine, in all species, contains mucoproteins. Mucoproteins are important compounds in urine that help defend against entry of bacteria and regulate inflammation. However, when conditions are favorable for stone formation, these mucoproteins also act as cement to help calcium carbonate crystals clump together to form a stone. High mineral content from feed or water sources and an alkaline urine pH can also predispose horses to stone formation. Horse urine is normally alkaline, and this makes a more favorable environment for minerals to crystallize out of solution if the urine has a heavy concentration of mineral salts.

Once some of the concentrated salts crystallize and a stone gets started, it can grow larger and larger as more salts collect around it. A bladder stone might cause discomfort in and irritation of the bladder. Presence of a stone in the bladder can also make the horse more prone to bladder infection. If this kind of problem is suspected, a veterinarian can check the bladder by palpation (putting a gloved hand into the horse’s rectum and carefully feeling the bladder and its contents) and determining whether there is a bladder stone.

Small stones are usually passed with the urine and don’t cause problems, but larger ones can block the tract and create painful and difficult urination. Sometimes a small stone is passed out of the bladder and lodges in the urethra, obstructing the flow of urine. This rarely happens in mares because their urethra is so short and large. But in the stallion or gelding, blockage is more common; the passage is longer and there is a narrowing of the urethra where the penis passes out of the pelvic canal and curves downward. Obstructions can occur at any point in the urinary tract, but are found most commonly at this narrowest place just under the tail. Some horse owners think geldings castrated young are more apt to have problems than later-cut geldings, but there is no scientific evidence of this (there are no adverse effects from early castration). Stones can also form in the kidney, but in horses this is very rare; they are much more likely to be found in the bladder (and bladder stones are not very common).

Symptoms of Urinary Stones

Clinical signs of urinary stones will vary according to the location of the stone. If a stone blocks the bladder outlet or gets caught in the urethra and plugs it, the bladder (and part of the urethra if the stone is lodged in that tube) becomes distended with urine. The affected horse tries to urinate and he cannot, or he merely dribbles small amounts of urine. He shows signs of difficult urination–groaning, grunting, and straining–and is often mildly to severely colicky. A notable difference from true colic is that the penis will often be persistently dropped in the horse with obstruction of the bladder and urethra. A horse with bladder stones might have blood in the urine after exercise because the movement jostles the bladder wall between the pelvic bones and the stones, causing further irritation and bleeding.

The affected animal will often take a stretching stance for urination, but he will keep that stance for a long time, passing only small amounts of urine or none at all. The mare with an obstructing stone might dribble urine instead of putting it out in a stream, and the constant dribbling can irritate and scald her perineal area, eventually leaving the skin around her vulva and down the hind legs sore and hairless. A stallion or gelding with this problem might have skin irritation and scalding around the sheath opening and on the front of the lower hind limbs.

A horse with a urinary stone might pace nervously or refuse feed and water; he might be too uncomfortable to eat. Because of the pressure and pain created by the distended bladder and/or urethra, he could be colicky and get up and down often. Sometimes a horse will back up a few steps, tread with the hind feet, paw or stamp, kick at the belly, constantly switch the tail (sometimes very forcefully, in angry frustration), bite at the flanks, or strain as though constipated. All signs might immediately disappear if the stone is passed, only to recur again at a later time if another stone causes blockage. If the stone is not passed and is completely obstructing urine flow, the bladder will rupture unless the problem is corrected. Diagnosis can be confirmed by rectal examination; the veterinarian will be able to feel the distended bladder. Treatment usually consists of trying to remove the obstruction, which is usually done surgically because it is nearly impossible to move a stone with a catheter.

If the stone is in the bladder, surgery might be necessary to remove it, or the veterinarian might try to shatter a large stone with a laser; the resulting tiny fragments can be passed in the urine. The laser is introduced via an endoscope that is passed into the bladder.

Preventing Urinary Stones

Some types of feed can make a horse more prone to develop urinary calculi, but not all horses on these feeds will have problems. A horse that tends to create stones might not be able to handle a diet high in calcium (such as alfalfa) or too high in phosphorus (grain). Staying away from these feeds can alleviate future problems with that horse. Occasionally water with a very high mineral content causes the stones; if you suspect the water, have it tested.

Make sure the horse always has plenty of fresh, clean water. This can sometimes help prevent formation of stones. If the horse drinks an adequate amount of water, he is less likely to have concentrated urine, and the urinary salts will be less likely to precipitate out and form stones.

If your horse has problems with urinary stones, your veterinarian might have some suggestions to help you prevent continuous trouble. There is some value in trying to acidify the urine. In humans afflicted with kidney and bladder stones, some swear by drinking lots of grapefruit juice (as well as drinking plenty of water and avoiding the types of foods that are high in calcium or phosphates). Stones (in horses and humans) tend to form more readily in an alkaline environment and do not form in acid. A slightly acidic diet also helps horses troubled with enterolith formation in the intestines (clumping of small particles around a foreign body such as a piece of wood, hay twine, pebble, or even a horse hair in the digestive tract, creating an ever-growing “stone”). Daily administration of a small amount of apple cider vinegar added to the grain helps prevent intestinal stones; enteroliths will not form in an acid environment. But there is no evidence that this will help keep a horse from getting urinary stones.

One of the problems–and one of the reasons we don’t have good medical treatments for equine urinary stones–is that it is very difficult to acidify the urine of a normal horse. The important thing is to just have the diet analyzed and ensure your horse is getting the lowest possible calcium intake.

If a horse shows signs that make you suspect urinary stones, have your veterinarian examine the horse immediately before damage or rupture creates a life-threatening situation. If stones are passed or removed, your veterinarian can have the chemical composition of the stone (and the horse’s urine) analyzed to give you clues as to the exact cause (which minerals are to blame). There might be a way to limit or prevent further stone formation by altering the horse’s feed or water. Unfortunately, horses that develop stones, especially bladder stones, can become repeat offenders. About 25% of horses that have stones removed have additional stones form over the subsequent months to years. Although unproven, this suggests that some horses might be genetically predisposed to stone formation, perhaps by having different amounts of mucus in their urine.

Urinary Tract Infection

Infection of the urinary system is sometimes introduced from external sources, traveling up the urethra into the bladder and on into the kidneys. The tract can also become infected by extension of a problem in surrounding tissues or by bacteria traveling through the bloodstream Acute infection in the tract can cause severe pain; infection in the kidneys (although even much rarer than formation of stones) can create such pain that the affected horse is reluctant to move and travels stiffly and awkwardly. The horse is depressed and dull with a poor appetite, and she often has a fever.

Bladder infection (cystitis) can make the horse urinate frequently and painfully. There might also be pus or blood in the urine. If the bladder infection is not treated, it can lead to a kidney infection, resulting in kidney failure. If the kidneys become badly damaged, they lose their ability to filter toxins and wastes from the blood. Persistent accumulation of these waste products in blood and body tissues results in chronic renal (kidney) failure. If this happens, the horse becomes dull and lethargic and slowly loses weight over a period of months to years, until euthanasia becomes warranted.

Urinary tract infections can be treated if the condition is properly diagnosed. If infection is suspected, a sample of urine can be collected and cultured to see what type of infectious organism is involved. A sensitivity test can be conducted to see which antibiotic would be most effective against it. Treated with the proper antibiotics, most urinary tract infections can be cleared up.

It is fairly easy to get a urine sample from a horse. Mares are easy to catheterize (because of their short urethra), and stallions or geldings are not difficult. An easy way to get a sample from a stallion or gelding is to hang a clean gallon milk jug (with the bottom cut out and the lid kept on) over the prepuce. It can be tied there (over the back) and you will get a sample from the horse within a few hours. Most veterinarians prefer to not use drugs (such as diuretics) to induce urination because the sample collected is often abnormally dilute.

Take-Home Message

The urinary tract is a vital system that should never be taken for granted. If a problem develops, it can rapidly become a crisis. Damaged kidneys, a ruptured bladder, or acute or chronic kidney failure are all potentially fatal. If a horse shows discomfort when urinating, or if he urinates too frequently, have your veterinarian check the animal. Most urinary problems can be corrected, if they are caught before major damage occurs.