This research study is essentially concerned with the etiology of magnesium
ammonium phosphate urolithiasis in dogs, particularly the role of bacterial infection
of the urinary tract on the formation of calculi* The problem was investigated
from four aspects by carrying out} I* a clinical and bacteriological study of dogs,
cats and sheep with urolithiasis) II* a study of the structure of canine calculi)
III* tests on bacteria isolated from dogs with urolithiasis) IV* attempts to
reproduce calculi experimentally in vitro in canine urine and in vivo in dogs.
I. URINARY TRACT INFECTION IN RELATION TO UROLITHIASIS IN DOGS AND OTHER ANIMALS
A review of the literature showed that other workers had noted that magnesium
ammonium phosphate calculi were frequently associated with bacterial infection of the
urinary tract in dogs and man* Differing opinions were expressed on infection in cats
and sheep with urolithiasis. Staphylococci were recognised as the main type of
organism recovered from dogs but workers studying urolithiasis in man have found a
wider variety of organisms involved.
In the present study urinary tract infection was present in 82% of dogs with
magnesium ammonium phosphate calculi and in 22% of dogs with calculi composed of
calcium salts. Infection was absent from all dogs with cystine calculi.
In 44 cases of magnesium ammonium phosphate calculi staphylococci were isolated
on 31 occasions, streptococci on 7, Proteus species on 3 and other organ!sue were
found on 5 occasions. In 8 cases the urine was sterile.
Crystals were removed from 43 magnesium ammonium phosphate calculi and sections
were prepared of the remaining organic matrices. In 41 of these sections bodies
resembling bacterial cocci were present. In 33 instances these calculi were associated
with staphylococcal or streptococcal infection of urine, while in the remaining cases
the urine was either sterile or infected with organisms of a different typo.
Dogs with urolithiasis were treated by surgical removal of calculi and antibiotic
therapy. Their post-operative progress was followed by clinical and radiological
In animals with magnesium ammonium phosphate stones at the initial episode,
recurrence of calculi occurred in 30% of cases while in dogs with calculi composed of
calcium salts or cystine the recurrence rates were 77% and 75% respectively. At
follow-up examination 11 dogs were infected with staphylococci and 9 of these had
recurrent calculi composed of magnesium ammonium phosphate.
These studies confirmed the association between staphylococcal infection and
magnesium ammonium phosphate urolithiasis in dogs. Bacteria did not appear to be
involved in the deposition of magnesium aamoniuis phosphate crystals in the urinary
tracts of 10 cuts or 4 sheep.
II. PHYSICAL CHARACERISTICS OF CANINE URINARY CALCULI
Calculi retrieved from dogs with urolithiasis were weighed and their shape recorded.
Thin sections of the stones were prepared and the internal structure examined maeroscopieaily and microradiographically. The mean weight of magnesium ammonium phosphate
calculi from female dogs was 19.87g and from male animals 0.98g.
The number of magnesium ammonium phosphate calculi and their shape, size and
surface texture appeared to be inter-related. Single calculi were usually of a
spherical shape while large multiple calculi tended to be tetrahedra.
Examination of the internal structure of magnesium ammonium phosphate calculi
Use other side if necessary.
shoved that nuclei were occasionally present but did not seem to be ait mportant
cause of urolithiasis* Many of the calculi were composed of bands of crystals
of alternating composition. Central fissures were also noted in several calculi.
III. PROPERTIES OF BACTERIA ASSOCIATED WITH MAGNESIUM AMMONIUM PHOSPHATE
Review of the literature showed that earlier workers considered the production
of urease by bacteria and its action in raising urinary pH was important in the
etiology of urolithiases by reducing the solubility of magnesium ammonium phosphate
in urine. Some bacteria can produce phosphatase and the possible effect on
calculi was studied. Urease was produced by 33 out of 34 strains of staphylococci
and by all 3 strains of Proteus tested. Four strains of streptococci and 2 strains
of E. coli were urease negative. Phosphatase was produced by 33 out of 34 strains
of staphylococci but not by the other organisms isolated from dogs with calculi.
Urease and phosphatase could influence the solubility of magnesium ammonium
phosphate in urine but neither enzyme was produced by all staphylococci isolated
from dogs with calculi.
Tests for the production of coagulase, haemolysis and pigment production
were carried out on 34 strains of staphylococci. Thirty-one strains produced
coagulase, 29 strains haemolysed sheep blood, 8 strains haemolysed horse blood
and 2 strains produced golden pigment.
IV. EXPERIMENTAL STUDIES ON UROLITHIASIS
The literature on experimental production of calculi in vitro and in vivo
was reviewed. It was found that dogs have been seldom used in experimental work.
When calculi were placed in sterile canine urine in vitro they became smaller
and when placed in urine infected with staphylococci or Proteus organisms little
change occurred in the calculi but there was a heavy deposit of magnesium ammonium
phosphate crystals from the urine.
Experimental bladder infection with staphylococci was established in 3 dogs
and 1 of these developed a magnesium ammonium phosphate calculus.
As a result of these investigations it was concluded that urinary tract
infection with staphylococci is an important factor in magnesium ammonium phosphate
urolithiasis. The production of urease by bacteria leads to elevation of the
urinary pH and causes deposition of magnesium ammonium phosphate crystals but the
mechanism by which these crystals aggregate to form calculi was not established.