Thought that some of this may be of interest.  It is from UROLITHIASIS
IN DOMESTIC ANIMALS
S.P. DiBartola, DVM which is a very extensive document and has lots I
need to study intensively.  Trimmed two parts down to bare essentials
to avoid copyright infringement.  There are all sorts of useful things in
there and it is one the web so you can find the download at
www.vet.ohio-state.edu/docs/VM602/notes/lith.doc
 
>The role of diet in the pathogenesis of most types of urolithiasis in
>dogs an cats is unknown.  Dietary manipulation may be of benefit in the
>management of struvite, urate, cystine, and silicate urolithiasis in
>dogs...
 
Much later in the document:
 
>A.  Cystine urolithiasis
>1.  Cystine stones are uncommon in dogs, rare in cats, and have not
>    been reported in horses or cattle...
 
>2.  Cystine stones have been reported in many breeds of dog...
 
>3.  In most studies, cystine stones are found almost exclusively in
>    male dogs. However both male and female Newfoundlands...].
 
Note both genders here in ferrets for whom they seem to be even more rare
(though who knows if cases may have been missed).
 
>4. Affected dogs usually are middle aged (4 to 6 years at presentation).
 
Note both here are young, not middle aged.
 
>5. Canine cystinuria is an inherited disorder of renal tubular
>transport involving cystine or cystine and other amino acids (...the
>so-called "COLA" group of amino acids).  Not all dogs with cystinuria
>develop urolithiasis.  Therefore, cystinuria is considered to be a
>predisposing rather than a primary causative factor.
 
Notice that the genetic condition is NOT thought to be the primary
causative factor in dogs though it plays a part in predisposing dogs
to getting the stones.  Notice, too, that it is described as "a"
predisposing factor rather than "the".
 
> 6.  ...the suspicion that cystinuria is genetically heterogenous in
>dogs as it is in humans in whom both type I and "non-type I" cystinuria
>occur.
>
>7. Cystinuria decreases in severity with age...
 
>8. Cystine stones are composed entirely of cystine. ...
 
>9. They are small, spherical, and light yellow, brown, or green in
>   color.
 
>10. They occur most commonly in the bladder and urethra and usually
>    are multiple...
 
Singular in the female whose stone did go to pathologist.
 
>6. The recurrence rate for cystine urolithiasis may be as high as
>   47-75%.
 
That fits for a the ferret of a person who wrote.  Luckily they worked
out an effective medical routine.  (See the FHL posts.)
 
>7. When it occurs, urinary tract infection usually is a complication
>of cystine urolithiasis rather than a predisposing cause.
 
Important to know because infection can cause some other types of stones.
 
>8. Cystine crystals have a characteristic hexagonal shape, and when
>   observed in urine should be considered abnormal...
 
The symptoms noted for the other animals do NOT include constipation,
difficulty defecating, and thin stools, but those occurred with both of
these sweethearts.  So, keep in mind that (as with so many other things
in which ferrets differ) this is one way ferrets differ.
 
Oh, one other way they differ is which of the kidney values should scare
the most with hydroneprosis.  Specific gravity is one of the very most
important to watch in ferrets.  If it stays okay when some things which
cause worry in other animals gets bad it may still be able to save the
kidneys as our vet did for Hilbert.
[Posted in FML issue 4570]