Your horse’s urinary system: what you need to know

The urinary system is responsible for excreting the liquid waste products that are filtered out of the bloodstream, including urea (from which urine takes its name), salts and excess sugars.

This constant filtering process is carried out by two bean-shaped kidneys, each about 15cm long, which lie near the horse’s backbone and within his abdominal cavity.

The entire volume of a horse’s blood passes through the kidneys many times a day, but only a tiny fraction is converted into urine.

The bladder lies within the pelvic cavity and has elastic walls that can expand tremendously without bursting. As it collects urine and distends, nerve endings sensitive to pressure begin to signal to the brain that the bladder is full.

The horse then relaxes a circular muscle at the bladder outlet and contracts the bladder muscle to expel the urine.

What’s normal?

Colour, concentration and smell are three readily identifiable features of urine that vary according to a number of factors. Some factors are physiological and entirely normal, producing urine that is:

•  Pale-coloured

Horses with an unlimited supply of water will have a more dilute, paler and less pungent urine than those whose access is restricted.

• Dark yellow

Water intake can be reduced in winter, especially if the water trough freezes over, leading to a more concentrated and darker-coloured urine. The same can happen with excessive sweating in hot weather.

• Orange-red

Horses have varying amounts of compounds called urocatechins in their urine. These can be oxidised by light after being passed and turn orange to red in colour. Discolouration seen on bedding or snow is sometimes mistaken for bloody urine.

• Cloudy

Grazing on calcium carbonate-rich broad-leafed plants or being fed alfalfa will produce a high level of calcium salts in the urine, giving it a cloudy or even milky appearance.

While humans and dogs tend to regulate calcium absorption from the diet at the level of the intestine, horses tend to absorb excessive calcium from the intestine and 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 settle at the bottom in the manner of snowflakes in a souvenir snow globe.

• Foamy

Horse urine is very bubbly when first passed, due to the large amount of mucus it contains. The role of mucus in urine is to act as a lubricant that can help prevent calcium carbonate crystals from forming into stones. A concentrated urine sample can be quite viscid (syrupy). Mucus is secreted from the innermost portion of the kidney, which is the start of the drainage system through the ureter.

• Smelly

The waste products in urine include the urea and ammonium ions, two breakdown products of protein. The more protein a horse has in his diet, the more urea and ammonium he will produce and excrete in urine — hence the ammonia smell in the stable.

The urinary tract of some horses will contain bacteria that possess the enzyme urease, which splits urea into its two constituent ammonia molecules. These horses will produce more ammonia and a stronger smell.

Spotting problems

Any changes in the urine’s usual colour, clarity or frequency could point to problems, so it’s sensible to monitor your horse’s habits.

Urine can be affected by pathological conditions — those caused by disease — and may be used as a guide to diagnose or suggest their presence. Seek your vet’s advice if urine is:

• Little and often, or blood-stained

Horses suffering from cystitis pass small, often frequent quantities of urine that may contain blood.

This common urinary tract condition more typically affects mares because the shorter female urethra allows infection to gain access to the bladder more easily.

Analysis of the urine by a vet using a “urostick” quick-dip laboratory test will reveal high levels of protein and cells, while the presence of blood demonstrates the likelihood of an ongoing bladder infection. The urine may be cultured, so that the infection can be identified and the antibiotic with the highest probability of success selected.

• Infrequent or uncomfortable

Urolithiasis is the formation of urinary stones called calculi in the kidneys or, more commonly, the bladder. These may cause difficult urination, discomfort and even colic.

A total blockage can be fatal, due to the metabolic problems that occur if the bladder bursts and urine accumulates in the abdomen.

• Very dark-coloured

Dark red or brown urine could point to muscle strain or more serious disease. Rhabdomyolysis, often called “tying up”, causes inflammation in the skeletal muscle and a change in the permeability of the muscle cell membranes. Large quantities of protein peculiar to muscle tissue are then voided into the blood and consequently excreted through the urine.

Urine in an affected horse is very dark. A urostick test will reveal high protein levels which, in addition to muscle rigidity and reluctance to move, indicate the presence of rhabdomyolysis.

The often fatal condition seasonal pasture myopathy (previously known as atypical myopathy) is the result of muscle breakdown caused by a toxin present in sycamore seeds and saplings. Horses that consume the toxin and develop the condition will display muscle weakness, tremors and a desire to lie down — signs frequently mistaken for those of colic.

They will also void dark-coloured urine, caused by the presence of muscle pigment (myoglobin), which is released from damaged muscle cells into the blood and removed by the kidneys.

Where seasonal pasture myopathy has progressed to obvious changes in the urine, the prognosis is poor.

Ref: Horse & Hound; 5 November 2015