Haematuria is a relative common problem of adult horses. There are a number of potential causes (see below). Investigation should include collection of a urine sample for cytology and potentially culture. Ideally the sample is collected sterilely using a catheter, but if this is not available then a mid-stream free catch should be done. Ultrasound is an important adjunct in these cases, as the kidneys may be the source of the red blood cells. Rectal palpation can also be rewarding, particularly searching for ureteral or bladder (cystic) calculi. Finally, endoscopy is critical for determining the source of bleeding in many cases.
The timing of haematuria is helpful in indicating the source of red blood cells. If the blood appears throughout the urine stream it is likely that the source is the kidney(s) or bladder. Haematuria at the beginning of urination is usually due to damage in the distal urethra. Blood at the end of urination is due to damage in the proximal urethra or neck of the bladder.
How to differentiate blood from haemoglobin and myoglobin.
There are several circumstances where the urine may be discoloured. Visual inspection and the use of urine dipsticks is often unable to differentiate blood, haemoglobin and myoglobin. Haemoglobinuria is a consequence to intravascular breakdown of red blood cell (haemolysis). Myoglobinuria is due to myoglobin release from damaged muscle cells. With haematuria when the urine is spun in a centrifuge the red blood cells will form a pellet in the bottom of a tube, in contrast to haemoglobin or myoglobin where the sample will remain discoloured. Haemoglobin will only be present in urine if the plasma concentration of free haemoglobin is high (> 1.0 g/L), therefore the plasma will also be pink. Ammonium sulphate when added to the urine (2.8 grams to 5 mL of urine) and thoroughly mixed and then centrifuged for 10 minutes at 3000 rpm will cause any haemoglobin to precipitate. If the urine sample remains red/brown after centrifugation then it is myoglobin.
Some important causes of blood in the urine are:
- Idiopathic renal haematuria
- Urethral tears
- Blister beetle toxicosis (Cantharidin)
- Neoplasia of the urinary tract
- Exercise-associated haematuria
- Urolithiasis (often worse after exercise)
- Non-steroidal anti-inflammatory drugs (medullary crest or renal papillary necrosis)
- Urinary tract infections (kidney, bladder)
Idiopathic renal haematuria
This is a syndrome of sudden onset passage of blood and blood clots from one or both kidneys. It is potentially life threatening. There is a strong breed predisposition, with more than half of the reported cases occurring in Arabian horses. The underlying cause is not known.
The diagnosis is based on endoscopy of the bladder to confirm bleeding from one or both ureters. Ultrasound of both kidneys is also important. An important differential diagnosis is adenocarcinoma of the kidney where ultrasound reveals a large abnormal kidney.
Blood transfusions may be necessary although are clearly not curative. Some people have seen a beneficial effect of corticosteroids. Nephrectomy can be curative, although there is a real possibility that the remaining kidney could become affected in the future.
Urethral tears
Typically these occur in stallions and geldings in the proximal urethra at the level of the ischial arch. They cause blood in the urine at the end of urination. The majority of the time the blood is bright red. Less commonly some geldings will have dark blood at the beginning of urination. The horses show no signs of pain or distress associated with urination. The condition is more commonly reported in Quarter Horses, although other breeds can be affected. The condition is widely recognized as a cause of haemospermia in stallions. It is thought that tears occur due to excessive pressure within the corpus spongiosum penis (CSP) during urination or ejaculation. It is plausible that the decrease in intraluminal urethral pressure occurs at the end of urination when pressure inside the CSP remains high, facilitating movement of blood through the tear or ‘fistula’ into the urethra.
Urethral tears are diagnosed through endoscopy, where the tear can be seen on the dorsocaudal aspect of the urethra at the level of the ischial arch. Most cases will resolve spontaneously, but if it persists for more than 4 weeks a temporary surgical procedure may lead to healing. this involves performing an incision into the CSP to the level, but not into, the urethra.
Blister beetle toxicosis (Cantharidin)
Blister beetles are found in the Midwest USA, most commonly associated with alfalfa (lucerne) hay. Blister beetles (Epicuata species) produce an odourless colourless toxin known as cantharidin. Contact with humans can lead to the formation of skin blisters, hence the name blister beetle. Ingestion of blister beetles results in a range of clinical signs, including oral ulcers, colic and diarrhoea. The toxin can also inflammation of the heart muscle (myocarditis). There are potent effects in the urinary tract, including haemorrhagic cystitis. Classically affected horses become hypocalcaemic, and commonly develop synchronous diaphragmatic flutter (thumps). Blister beetles are not found in Australia.
Neoplasia of the urinary tract
There are several different cancers that can affect the urinary tract leading to haematuria, including adenocarcinoma of the kidney (see above). Squamous cell carcinoma (SCC) is the most common, followed by transitional cell carcinoma and leiomyosarcoma of the bladder, and lymphosarcoma. Blood from the distal urethra could be due to SCC, melanoma, sarcoids, habronemiasis (summer sores).
Exercise-associated haematuria
Passage of red blood cells through glomerular barrier in the kidney occurs normally in equine athletes during exercise. Usually the cells are in low number and do not grossly affect the colour of the urine. Occasionally, there may be enough to cause gross haematuria. Small erosions may also form in the bladder in response to concussive trauma of the abdominal viscera during exercise. This can also lead to haematuria.
Urolithiasis (Stones, calculi)
The bladder is the most common site for the formation of calculi. Cystic calculi lead to a number of clinical signs, including haematuria, pyuria, stranguria, pollakiuria, and incontinence. The haematuria is usually more apparent after exercise, due to trauma of the calculi moving against the bladder mucosa.
Non-steroidal anti-inflammatory drugs
Non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used in equine practice. This class of drugs inhibits the formation of prostaglandins. Two important prostaglandins are PgE2 and PgI2 (prostacyclin). These are important vasodilators, critical for maintaining blood flow to the kidneys, particularly when animals are volume deprived (e.g., dehydration). Blood flow in the normal kidney is predominately directed to the cortex where filtration occurs. The renal cortex receives 80-90% of total renal blood flow. Consequently the effects of NSAIDs are more likely to occur in the medulla, which only receives 10-20% of renal blood flow. (It is important to note that renal cortical disease can also occur – interstitial nephritis). The resultant lesion is renal medullary crest necrosis (aka renal papillary necrosis). Phenylbutazone is the drug most commonly associated with renal side-effects.
Gross or microscopic haematuria is characteristic of medullary crest necrosis. A hyperechoic band parallel to the corticomedullary junction on ultrasound, known as the renal medullary rim sign, is also consistent with NSAID toxicity.
Tags: Renal