46
REPORT OF THE IMPERIAL INSTITUTE OF VETERINARY RESEARCH,
It was stated in the preceding Report that attempts were being made at the labora-
tory to exploit further possibilities in the development of more efficacious preventive
agents on the assumption that strangles was in reality streptococcic in origin. Manifold
laboratory investigations indicate that the pathogenesis of strangles is associated with
streptococci, for in samples of pus taken freshly from a lesion streptococci can almost
invariably be isolated in large numbers to the exclusion of other types of visible micro-
organisms ; sometimes no organisms can be isolated, but in such cases it is reasonable
to believe that the streptococci associated with causation have been destroyed. The
streptococci, however, when subjected to comparative microscopic and cultural examina¬
tion (especially in regard to their hæmolytic effects and capacity for fermenting sugars),
do not appear to be homogeneous in type. A few colonies examined would appear to be
of fæcal origin (Streptococcus fæcalis), and these may be ruled out as having any casual
association with the disease. The streptococci isolated are mainly of the so-called
hæmolytic type, but even among them there is some variation when they are classified
according to their other cultural characteristics. Thus, among a batch of 23 " strains "
among a large number examined and classified according to GORDON'S system, there were
(i) Streptococcus pyogenes, 11, (ii) Str. equinus, 7, (iii) hæmolytic streptococci not classi-
fiable but apparently of the same type and did not ferment any sugars, 3, and (iv) Str.
fæcalis, 2. These organisms would therefore correspond largely with the ordinary strep-
tococcal flora of wounds. However, the recent researches of ROSENOW in particular
would appear to indicate that streptococci are capable of developing into pathogenic
agents with marked specific affinities for certain tissues, in which they set up character-
istic morbid processes, although these organisms are not readily distinguished from strep-
tococci incriminated with other processes by laboratory tests. Further, the researches
of BROCO-ROUSSEU, FORGEOT, and URBAIN upon the strangles streptococci after propaga-
tion in culture media indicate that the organisms change rapidly in antigenic properties
outside the affected host, as witnessed by the capacity of the organisms to " deviate
complement" when tested against the sera of horses that have recovered from strangles.
It would therefore seem that streptococci restored to association with equine tissues might
themselves or in the form of the products elaborated by them in contact with the tissues
constitute an antigen capable of becoming an efficacious " vaccine," if in reality protec-
tion against streptococcic invasion signifies protection against strangles. It is apparently
on an assumption of this kind that various authors have in the past adopted prophy-
lactic measures based upon the injection of strangles pus, or pus produced artificially by
inoculation of equines with streptococci, rendered innocuous by the action of ether or
other antiseptics, for prophylaxis. The observations upon which the efficacy of such
measures is proclaimed would appear, however, to be insufficiently controlled. Never-
theless, in view of the success of measures undertaken against blackquarter in cattle by
the use of products of this nature in the shape of so-called " aggressin," and, according
to the recent statements of American workers, against hæmorrhagic septicæmia by the
use of a comparable product, it seemed worth to try an analogous method for strangles
prevention.
Hence, three strains of strangles streptococci were taken, representing each of the
three cultural hæmolytic groups noted above and exalted by passage intramuscularly in
donkeys and horses. From the muscle lesion of the animals that succumbed to infection
the juice was pressed out, preserved by the addition of antiseptic, and filtered bacteria-
free. Some tests executed at the laboratory, though it is admitted that they were not
performed perhaps sufficiently extensively, seemed to indicate that both the filtered and
unfiltered products were capable of evoking enough response in inoculated animals to
withstand fatal doses of virulent material, in the form of the fresh muscle juice or a mixture
of the strains of streptococci used, when injected subsequently. Young donkeys seemed
to be very suitable for such tests.
A brief reference to the results obtained in some of these tests is worth notice, which
were controlled by inoculating animals not treated with the streptococcus " aggressin "
simultaneously with the animals so treated with the same test material; an endeavour
was made to obtain a still larger measure of control by using for the tests in addi-
tion to untreated animals, ones that had been " vaccinated " with " normal aggressin "
composed of a mixture of the expressed muscle juice from a healthy donkey and a
healthy pony foal, obtained in the same manner as the strangles aggressin.
Test No. 1 (16th December 1924). Vaccinated on 24th November 1924 (with 20
c.c. filtered, or 10 c.c. unfiltered aggressin). Test material was virulent streptococcic
muscle juice injected intramuscularly—
(a) Strangles unfiltered aggressin. One pony foal and one donkey foal, both lived.
(b) Strangles filtered aggressin. One pony foal and one donkey foal, both live.