40
Respirntory Diseases of the Horse
also discussed the difficulty of differentiating strangles from the various catarrhal affections of
horses.
Beller and Traub [ 1941 ] noted that strangles and infectious catarrh might occur together in
the same horse and after a very careful study put forward the interesting suggestion that they
are both primarily due to the same virus and that the different symptoms are due to different
secondary invaders or predisposition of the horse. Rexroth [ 1941 ], as a result of clinical and
patholygical investigations, supported this view.
Luhrs [ 1933 ]. whilst admitting that no one can mistake true epidemic equine influenza,
states that there are many outbreaks of disease which one cannot easily classify and Law
[ 1902 ], who was obviously familiar with the epidemic disease, says that in the slighter cases
the lesions are often confined to the anterior part of the respiratory organs which are congested
and red, the lesions may extend to the bronchi. In human medicine there are similar differences
between epidemic and sporadic influenza, and, although in some outbreaks influenza ' A ' or
' B ' virus can be identified, in other clinically identical outbreaks no virus can be isolated.
It would appear that, whilst the main clinical entities—strangles, infectious catarrh and
influenza—can be recognized, we still do not know whether they are caused by closely allied or
distinct viruses.
SUMMARY AND CONCLUSIONS
An attempt has been made to assess the evidence concerning the ætiology of the equine
respiratory diseases, and it is tentatively concluded that contagious pneumonia is a distinct
entity primarily due to a virus. There is no doubt that viruses are also responsible for out-
breaks of infectious catarrh and influenza, and probably also of strangles.
It appears possible that strangles and infectious catarrh are both primarily due to the same
or a similar virus, and there is no evidence which proves beyond doubt that the virus of
influenza is distinct.
In any of the above diseases the uncomplicated virus infection may pass almost unnoticed,
although producing immunity to subsequent infection. Climatic conditions and methods of
management, however, have a great influence on the course of the disease ; under adverse
circumstances, a variety of bacteria may produce serious complications, and there is little doubt
that individual animals may develop more or less characteristic symptoms when suffering from a
simple bacterial infection.
The results reported by Stableforth [ 1939 ], Hignett [ 1940 ], Hignett and King [1940 ], as
well as other workers abroad, have demonstrated the value of the sulphonamide group of drugs ie
the treatment of pneumonia. The apparent specific action of neosalvarsan and the sulphonamidu
group cannot, however, be accepted as evidence that bacteria are the primary ætiological agents.
Methods of immunization, if they could be evolved, might be effective and economical.
During the last great war it would appear that research was carried out by a few enthusiasts,
but Edwards [ 1935b ] has said that the need for a trained research cadre attached to the
R.A.V.C. was apparently not realized until the end, and it never came into being ; a great op-
portunity to investigate the respiratory disease of the horse was lost.
Knowledge concerning the viruses, hæmolytic streptococci, methods of drying and preserving
biological materials, and the technique of investigating respiratory diseases has greatly improved
since the last war. It is also felt that one of the three methods which have been used to prepare
swine-fever vaccine [ see Francis, J. 1941 ], two of which have also been successfully used in the
control of rinderpest, might very well be adapted particularly for the preparation of an equine
influenza vaccine. In this disease, and possibly also in infectious catarrh, virus is known to
circulate in the blood. It appears, therefore, that if the need arose, and the necessary facilities
were granted, an efficient research cadre working in conjunction with clinicians could do much
to clarify our knowledge of equine respiratory diseases and possibly evolve methods of specific
immunization. One gains the impression that the clinical and pathological investigations which