Pathology of the urinary system

UROLITHIASIS

 

 

   ∙ 

the formation of solid or semisolid concretions (“stones”) called urinary calculi or simply uroliths; 

these can be found anywhere along the urinary tract; 

   ∙ 

uroliths are composed of a wide variety of minerals, often mixed with proteinaceous material. 

   ∙ 

uroliths also vary considerably in diameter (from sand-like particles to 10 cm), shape and 

consistency, depending on their chemical composition; 

   ∙ 

formation of uroliths can be facilitated by the presence of an organic matrix (e.g. exudate, bacteria, or 

necrotic cells) acting as a nidus for the formation and aggregation of crystals of minerals, but this is 

not necessary; 

 

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23

 

   ∙ 

some examples: 

   - 

very high incidence of calculi in sheep on diets high in phosphate; 

 

   - 

high levels of silica in native (unimproved) pastures grazed by cattle in western Canada; 

major cause of urinary tract obstruction; formation of calculi is favoured by low urine 

production in hot weather; addition of salt to the ration ensures high water consumption; 

 

   - 

calculi with a high carbonate content associated with very alkaline urine, seen in ruminants 

consuming high-oxalate plants or clover-dominated pastures; 

 

   - 

calculi of struvite (complex crystals composed mainly of magnesium, ammonium and 

phosphate): 

- in dogs, ureases produced by bacteria (e.g. 

Staphylococcus

, 

Proteus

) in bladder 

infections induce a supersaturation of urine with struvite by increasing the 

concentration of ammonium ions and increasing the urine pH (which increases the 

availability of phosphate ions and decreases the solubility of struvite); 

 

 

 

 

 

 

 

 

 

 

 

 

 

-  the same process can also occur in cats, but, in this species, calculi contaminated 

with bacteria are much less common than are sterile struvite uroliths (see: Feline 

Urologic Syndrome); 

 

   - 

calculi of calcium oxalate in dogs and cats: tend to form in acidic urine, therefore are not 

usually associated with a urinary tract infection; predisposed by metabolic disorders that cause 

hypercalcemia and, thus, excessive urinary excretion of calcium, e.g. hyperparathyroidism, 

hypervitaminosis D, hypercalcemia of malignancy; 

 

          Struvite and calcium oxalate are the two most common types of urolith in dogs and cats. 

 

   - 

struvite calculi in feedlot cattle on high-grain (rich in phosphorus) rations: gritty sludge with a 

high proportion of organic matrix; inhibition of urethral growth by early castration 

predisposes to obstruction by the calculi; 

NH

2

|

O=C  +  

urease

CO

2

+ N H

3

NH

4

+

+ OH

-

|

+ H

2

O

NH

2

(urea)

H

3

PO

4

+ 3OH

-

PO

4

-3

+ 3H

2

O

NH

2

|

O=C  +  

urease

CO

2

+ N H

3

NH

4

+

+ OH

-

|

+ H

2

O

NH

2

(urea)

H

3

PO

4

+ 3OH

-

PO

4

-3

+ 3H

2

O

 

---

 

 

24

 

 

   - 

renal excretion of abnormally high levels of uric acid in Dalmatian dogs, due to an inherited 

deficiency in uptake of uric acid by hepatocytes and, thus, incomplete conversion of uric acid 

to allantoin, which is more soluble; for similar reasons, any form of severe hepatic 

dysfunction may predispose to urate uroliths in other breeds of dogs as well as in cats; 

 

   - 

defective renal tubular reabsorption of cystine in some dogs, especially Dachshunds; cystine 

precipitates in acid urine; calculi occur in males only, but cystinuria is recorded in females; 

 

   ∙ 

urethral obstruction is common in male animals and in particular sites such as the ischial arch or 

sigmoid flexure of bulls, the vermiform appendage of rams, the os penis of dogs, and anywhere along 

the urethra of tom cats; 

   ∙ 

clinical signs of calculi: dysuria (difficult urination), stranguria (painful urination), pollakiuria 

(frequent urination of small volumes), hematuria, rupture of the urinary bladder. 

 

   ∙ 

possible consequences: 

 

 

 

 

 

 

 

 

 

 

   - 

none, the urolith is passed in the urine; 

 

   - 

hydronephrosis if the urolith is lodged in one of the ureters; 

 

   - 

chronic cystitis, from irritation by uroliths or from urine stasis (which favors bacterial growth) 

secondary to partial urinary obstruction; 

 

   - 

bladder distension secondary to complete obstruction of the urethra  

➝

 bladder rupture (

➝

 

uroperitoneum / chemical [urine-induced] peritonitis), or functional obstruction of both ureters  

→ death from uremia in 3-6 days; 

 

   - 

acute hemorrhagic urethritis 

➝

 possible urethral perforation and urine leakage at the site of 

obstruction  

➝

 severe cellulitis.