38
Respiratory Diseases of the Horse
usually produced. It is notoriously difficult to produce immunity against streptococci and
success has not been attained by the use of streptococcal vaccines to prevent strangles
[Edwards, 1935]. The highly infectious nature of the disease and the fact that a strong
immunity is usually established suggest that it is primarily caused by a virus. Bazely and
Battle state that their work was 'conducted in a laboratory which for the past two and a half
years has concerned itself exclusively with the study of hæmolytic streptococci,' and we can
only conclude that no attempt was made to isolate a virus.
Clinical observations and experiments led Arnold [1933] to conclude that strangles is primarily
due to a virus which produces febrile reactions only ; a secondary invasion by streptococci pro-
duce sall the recognized manifestations of the disease. The virus is responsible for the high in-
fectivity and a benign uncomplicated virus infection is sometimes observed. On the other hand,
sporadic case may be due to a bcteria only. The virus disease can be produced by the intra-tracheal
inoculation of filtered strangles pus and this leaves the animal solidly immune to subsequent
infection. A formalized filtrate produced only slight immunity.
Van Dorssen [1939] concluded that 'benign strangles was caused solely by Str. equi.
Richters [1935] isolated true diphtheria bacilli from the unopened lymph-nodes and nasal dis-
charge of some horses affected with malignant strangles, injections of these organisms into
susceptible horses produced inflammation of the nasopharyngeal mucosa and severe gastro-
enteritis. He concludes, however, that neither these organisms nor streptococci are the primary
cause of strangles, but that it is due to a specific virus. It has not been possible to find any
subsequent publication of Richters, substantiating this view, but Beller and Traub [1941] and
Rexroth [1941] reached the same conclusion. Law [1902] noted that, although horses inoculated
with strangles streptococci developed abscesses, there was no evidence that the disease produced
was contagious, but taken as a whole the evidence suggesting that a virus is the primary
ætiological agent in this disease is not perhaps so strong as in those already described.
It has already been mentioned that Krage and Tiedge [1938] isolated S. enteritides, Bact.
viscosum equi and C. diptheria from the organs of horses affected with malignant strangles.
LESIONS IN NERVOUS AND OTHER TISSUES
The conditions described in this review are usually accepted as being respiratory diseases,
but one cannot help remarking that the primary virus infection often exhibits all the features
of a generalized disease. There may be marked impairment of kidney and liver function, and
the predilection of the influenza virus for the intestine is well known. Traub [1941] and
Rexroth [1941] noted pustular and exanthematous lesions during the course of infectious catarrh,
and in an exhaustive clinical study Steffan [1941] demonstrated partial or complete auricular-
ventricular heart block.
The nervous and psychic disturbances which may occur during the course of respiratory
diseases have been noted by most authors. Bemelmans [ 1921 ] attributed many of the primary
disease symptoms to nervous derangement and Steffan [ 1941 ] has made a special study of the
subject. There can be little doubt that there is a relationship between the neural damage
caused and the subsequent onset of roaring, stringhalt and the other nervous affections of the
horse. Ebberbeck's [ 1940 ] pathological investigations have provided much evidence in
support of this view, but it is probable that nutrition [ Mitchell 1936 ] and hereditary disposi-
tion also play their part.
DISCUSSION
The four respiratory diseases have been described as though they were separate entities, and
some of their main features are summarized in Table I, but it must be admitted that, whilst
the typical diseases can be recognized, there are many outbreaks which even experienced workers
are unable to classify [ Theiler, 1918 ].