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The equine kidneys are remarkable organs, removing waste products such as urea from the body. They are surprisingly robust in the horse, when compared with those of other species, and rarely go wrong.

The left kidney is slightly smaller than the right and positioned a little further back. Shaped like a letter “J”, it is located at the approximate level of the last rib under the lumbar spine — just behind the spleen and between it and the body wall. The heart-shaped right kidney sits under the last two or three ribs on the right-hand side and nestles into the liver, which sits in front of it.

The kidneys receive roughly a quarter of the blood output from the heart. The purpose of the large blood flow through these organs is to enable them to filter the blood, removing waste products and regulating body fluid volumes and concentrations of solutes (dissolvable substances) such as sodium.

The kidneys empty this filtered fluid, now called urine, into the bladder, through a tube called the ureter. The bladder then excretes the waste into the outside world, through a tube called the urethra.

One for the boys

Kidney stones are rock-hard formations, termed nephroliths.

Stones can occur anywhere in the urinary tract. The majority — approximately 60% — are found in the bladder, with around 24% in the urethra, half this amount in the kidneys and just a few in the ureter. Around one in 10 affected horses has stones in multiple sites.

It is reasonable to assume that some of the stones discovered in the bladder have come from the kidney and were simply flushed down through the ureter. The kidney is therefore only one place within the affected horse where stones may be found.

Male horses are more commonly affected than mares, with geldings more likely to develop stones than stallions. While the short, expandable urethra of the mare allows passage of small stones, the longer male urethra narrows as it passes over the ischial arch of the pelvis and can trap stones. Castration potentially results in a gelding having a narrower urethra than that of a stallion.

Stones are uncommon in the horse, however, compared with other domestic species such as cattle, sheep or goats. In one older study, over a 19-year period, urinary tract stones accounted for just 0.11% of all equine diseases found.

With modern diagnostic techniques, such as ultrasound, we are better at diagnosing the disease. This figure may well be slightly higher now, but, to my knowledge, accurate information on this is not available.

Stone formation occurs in two phases. The first stage is called nucleation, where small groups of ions such as calcium either group spontaneously to form a nidus (centre) or collect around a pre-existing nucleus. Dead cells that line the kidney tubules, mucus clots, white blood cells and dead tissue can all act as a nucleus for stone formation.

The second phase of stone development is crystal growth, which occurs when urine remains “supersaturated” with appropriate minerals. Almost all equine stones are composed of calcium carbonate in various forms.

Dehydration, in combination with drugs such as phenylbutazone (bute), is believed to be a factor in the initiation of equine kidney stones, as this can cause cellular death in one part of the kidney. Infection in the kidney is another possible factor.

The role of urinary tract infections in equine stone formation remains poorly understood. While urine samples obtained at the time of diagnosis or stone removal are usually negative for bacteria, the stones themselves can yield bacteria. This suggests that a transient urinary tract infection may be important in some cases.

Spotting signs

The presence of kidney stones may be indicated by one or more signs — weight loss, recurrent colic, decreased performance, blockage of urine from the kidney, renal (kidney) failure and infection of the kidney called pyelonephritis.

Importantly, kidney stones can be incidental findings and may not be the cause of disease at the time of discovery. Proving that they are the problem requires a detailed diagnostic work-up to rule other causes either in our out.

Historically, most horses with kidney stones are in chronic renal failure by the time they are diagnosed. This can be detected on routine bloodwork, which will show an increase in blood urea nitrogen (BUN) and/or creatinine.

More than two-thirds of the kidneys must be damaged for these parameters to be raised, however, meaning that the stones have to be clinically relevant in both kidneys for blood tests to flag up problems.

Urinalysis (urine testing) can be helpful, especially if there is a stone in the bladder, but this may not detect any problems if the stone is in the kidney. Inserting a tiny camera with a flexible tube into the bladder, a procedure known as cystoscopy, may also reveal stones in that location. Unless the ureter is dilated, however, any stones in the kidney may remain undetected.

This leaves abdominal ultrasound as the key diagnostic test to highlight kidney stones, which are seen on the resulting images as areas of white that cast a shadow. Performing an abdominal ultrasound in the horse is challenging, however, requiring a specific machine and expertise, such as a specialist in equine internal medicine or imaging.

Other possible causes of the clinical signs then have to be eliminated. Often, monitoring response to treatment is the only way to determine whether kidney stones are the root of issues such as recurrent colic or weight loss.

If there is evidence of kidney failure, prognosis for treatment of stones is poor. Otherwise, surgical or medical options are available.

One such surgical option involves removing the affected kidney — a big undertaking in a horse that brings with it a number of possible complications including peritonitis, bleeding and abscess formation.

As stones tend to affect both kidneys, removing one organ may not solve the problem. Another consideration is that the remaining kidney may be in the early stages of disease development and therefore vulnerable to renal failure in the future.

Medical treatment options include the use of urinary acidifiers, such as ammonium chloride or ascorbic acid. These acidifiers can maintain urinary tract health, but evidence that they are useful as treatment is lacking.

I have seen a number of patients seemingly improve with their use, however, with repeat ultrasound scans revealing break up of the stones.

Keys to kidney health

While there are few practical measures that will prevent stone formation, dietary modification to reduce the amount of calcium, phosphorus, magnesium and protein in your horse’s feed may reduce risk. Ask your vet or a qualified nutritionist for advice.

It’s important to be vigilant about changes in urinary habits, especially in male horses. Don’t ignore suspicious signs, such as blood in the urine or an interrupted flow that may be accompanied by straining or grunting.

Ensure that your horse is well hydrated at all times, especially during weather as hot as we’ve had this summer. Hydration is crucial after exercise, especially if a horse has sweated profusely. Salt licks or the administration of electrolytes by oral syringe, or both, can also be helpful, especially in a competition environment where a horse may be less inclined to drink.

Medication involving anti-inflammatories and painkillers, such as bute, should only be given to a well-hydrated horse — and only ever under veterinary supervision.

Ref Horse & Hound; 13 September 2018