Page 895 - Small Animal Clinical Nutrition 5th Edition

Feline Lower Urinary Tract Diseases 927

Usually one mineral type predominates; however, the composi-
VetBooks.ir tion may be mixed in some uroliths and plugs. Different min-
eral types may be dispersed throughout the urolith (i.e., mixed
urolith) or organized into separate, discrete bands or layers (i.e.,
compound urolith). The most common mineral types identi-
fied in feline uroliths are struvite (magnesium ammonium
phosphate) (Figures 46-1 and 46-2) and calcium oxalate
(Figure 46-3) (Houston et al, 2003; Cannon et al, 2007). a
Rarely, uroliths are composed of non-mineral substances (e.g.,
dried solidified blood) (Westropp et al, 2006). Although there
have been changes in trends over the past 25 years, struvite and
calcium oxalate have remained the most common uroliths in
cats (Table 46-3). The most recently collected data reveal that
struvite is the most common feline urolith followed by calcium
oxalate and purine (e.g., urate) (Table 46-4). Since 1981, stru-
vite has consistently been the most common mineral type iden- Figure 46-1. Urinary bladder from a cat with struvite urolithiasis.
tified in urethral plugs, representing 81 to 87% of plugs ana- Note the thickened urinary bladder wall and several struvite uroliths
lyzed in the U.S. and Canada (Table 46-5) (Houston et al, within the urinary bladder lumen.
2003). a
Over the past 25 years, several changing trends have been
noted in occurrence of feline uroliths (Cannon et al, 2007;
a
Picavet et al, 2007). In 1981, 78% of feline uroliths evaluated
at the Minnesota Urolith Center (University of Minnesota)
a
were struvite and only 2% were calcium oxalate. From 1994 to
2001, however, occurrence of calcium oxalate uroliths increased
to 55% and struvite decreased to 33%. In 1994, 77% of uroliths
from cats in Benelux (Belgium, The Netherlands and Lux-
emburg) were struvite and 12% were calcium oxalate; however, Figure 46-2. Penis and urethra from a cat with urethral obstruction.
by 2003, struvite uroliths had decreased to 32% and calcium Note the small struvite urolith (white arrow) in the penile urethra.
oxalate had increased to 61% (Picavet et al, 2007). At the Uroliths consist of small amounts of matrix and macroscopic crys-
talline mineral concretions. They are less common causes of urethral
Canadian Urolith Centre (University of Guelph, Ontario),
obstruction in male cats than urethral plugs (Figure 46-12).
approximately 50% of feline urinary bladder uroliths analyzed
from 1998 to 2003 were calcium oxalate and 44% were struvite
(Houston et al, 2003). Since 2001, the number of struvite
uroliths analyzed at the Minnesota Urolith Center has consis-
tently increased whereas the number of calcium oxalate uroliths
has decreased (Figure 46-4). At the Gerald V. Ling Urinary
Stone Analysis Laboratory (University of California, Davis),
struvite-containing uroliths were the predominant mineral type
analyzed from 1985 to 1993. Thereafter, calcium oxalate be-
came more common (Cannon et al, 2007). From 2002 to 2004;
however, 44% of feline uroliths submitted were struvite and
40% were calcium oxalate. The cause(s) for these changing
trends is unknown and needs further study.
Urethral obstruction is a complication of both uroliths and
urethral plugs, particularly in male cats (Bovee et al, 1979;
Kruger et al, 1991; Gerber et al, 2007). During the past 20
Figure 46-3. Calcium oxalate dihydrate urolith removed from the
years, the number of perineal urethrostomies performed at vet- urinary bladder of a cat with hematuria and dysuria.
erinary teaching hospitals in the U.S. and Canada has declined,
which has paralleled a similar decline in the frequency of ure- Although data from urolith centers are helpful, this informa-
thral obstructions, urethral plugs or urethroliths (Figure 46-5) tion cannot be used to determine incidence (i.e., rate of occur-
(Lekcharoensuk et al, 2002). These trends coincide with wide- rence of new cases in the population) or prevalence (i.e., total
spread use of specially formulated foods to minimize struvite number of urolith cases during a given time) of urolith types.
crystalluria in cats. This is important considering that struvite Not all cats with uroliths are diagnosed or treated (e.g.,they may
has consistently been the predominant mineral type in feline not receive veterinary care). In addition, not all uroliths are sub-
urethral plugs during the same time period. mitted for quantitative analysis and not all laboratories routine-