Renal Disease in Horses

To help me revise my university work, I am trying to make it more fun. Here is a blog about renal disease in the horse.

Disease in the horse may present with dysuria (abnormal urination), which can refer to a variety of urine signs- pollakiuria (frequent voiding of small amounts), stranguria (straining to urinate). These signs may also be confused with a mare in oestrus 'winking' behaviour, as urine dribbles in some specific urine diseases which is known as 'scald.' A mare in heat will also urinate frequently. 

Some urinal disease of the horses are:

Diseases which affect geldings primarily are urethritis, uroliths and calculi.

 In the older horse, urethritis presents with a malodorous and swelled sheath. This is generically due to the build up of concretions 'beans' of smegma in the urethral fossa. Treatment of this comprises topical antibiotics and cleaning the sheath. 

Uroliths are uncommon which are stones in the BLADDER of the horse. Stones in the kidney and ureter are even rarer (and are generally due to infection, and necrosis from NSAID toxicity in the medulla). Stones come in TWO forms- and are principally made up of calcium carbonate in the horse. They are either yellow green or white (more phosphate). Sabulous stones also exist, which are secondary to bladder paralysis due to calcium sludge.

Cystic and urethral calculi are more common. They can also be found in the mare. Clinical signs include 'posturing' to urinate, possible colic signs, urine scald and weight loss. Stones are probably painful I imagine! Diagnosis is by palpation of the stone or endoscopically. They are generally removed surgically are medical dissolution such as in the dog is a lot harder in the horse to get enough ration in the diet. With males the surgical removal technique is perineal urethrotomy, or laparotomy. With females, it is a lot simpler. An epidural and removal by hands is generally a more amenable approach. The problem with such stones, and with it being hard to change the diet, recurrence is common. Give antibiotics post  removal, and avoid alfalfa hay which is higher in calcium content. Salt will increase water consumption and aid diuresis. 

Polyuria and Polydipsia is a very frustrating clinical sign that can be associated with a whole range of clinical issues. Normal water intake is 20Litres/day rising to 90L if hot weather/vigorous exercise. The main differentials for PU/PD are:

1. Renal failure/disease.
2. Cushing's disease (PPID)- induces a glycosuria and osmotic diuresis, with ADH antagonism by cortisol, and possible decreased ADH from pituitary adenoma impingement. 
3. Psychogenic (a lot of the polydipsia in dogs is psychogenic). 

The different differentials must be differentiated between.

1. Is the horse azotaemic? Azotaemia is generally a clinical sign of renal disease, but pre-renal causes may include hypovolaemia, endotoxaemia,volume redistribution etc. Isosthenuric urine indicates renal disease (1.008-1.012 SG). A very dilute urine rules out renal disease.

2. Not azotaemic= water deprivation test to test the integrity of ADH. Withold water and monitor 12 hourly, stop if the SG >1.025, >12-15% body weight loss or increase TP (dehydration) or azotaemia occurs. If the horse can concentrate normally- suspect psychogenic causes or cushings disease. 

3. Rule out cushings- low dose dexamethosone suppression, resting ACTH, and TRH responsive ACTH. 

Tier 1- low dose dexamethosone (overnight) or resting ACTH. Low dose dex suppression has a associated risk of laminitis as exacerbates insulin resistance (insulin resistance via excess glucocorticoids antagonistic effect). Test cortisol with this test 20 hours later. Abnormal if >40 post dexamethosone left, or <70% suppression. Resting ACTH levels will be increased in resting ACTH but glucocorticoid levels will generally be normal as ACTH is mainly biologically inactive in the horse. 

Tier 2- ACTH response to TRH stimulation- unstressed horse, needs freezing within 3 hours, lab correction for seasonal levels (PPID horse has increased levels of ACTH in autumn but normal horses also have this increased level so needs lab correction). ACTH >100= positive for PPID. This is due to decreased receptor sensitivity to normal hypothalamic releasing hormones and increased sensitivity to pituitary stimulated by TRH.

4. If doesn't concentrate can use a modified water deprivation test.

5. ADH response. Response= central diabetes insipidus. No response= nephrogenic diabetes insipidus. 

Red Urine 

Red urine is a common finding in the horse associated with a variety of different conditions.

1. Haematuria. Associated with cystitis, pyelonephritis or uroliths, neoplasia and nSAID toxicity. Ischial arch/terminal urethral defect in males. Idiopathic renal causes are common with haemorrhage, and blood clots seen, can be severe and unilateral in cause. In such instances a unilateral nephrectomy may be performed. 

2. Pigmenturia- myoglobin- myopathy exercise associated. Increased muscle enzymes CK and AST will be seen. Pasture associated atypical myoglobinuria- ill, high CK with VPCs on ECG. Poor prognosis. 

3. Haemaglobinuria- IMHA, secondary to penicillin/strep equi. Can also be neonatal isoerythrolysis, EIA, oxidative injury, end stage kidney or iatrogenic.