SELECTED ARTICLES
A REVIEW OF THE RESPIRATORY DISEASES OF THE HORSE
BY JOHN FRANCIS, B. Sc., M.R.C.V.S.
(Reprinted from The Journal of the Royal Army Veterinary Corps, Vol. 14, No. 1, November 1942)
R
EADING the older veterinary authors, e.g. Smith, [1897 ] ; [1902] ; Friedberger. [1908],
one is struck by the scope and accuracy of their clinical and pathological observations. A
vast amount of research, particularly in Germany and France, has been carried out on the
respiratory diseases of the horse, but most of it was done before the days of the motor transport;
that is, before the methods of research into the aetiology of bacterial and virus diseases had
been perfected, and as this group of diseases is particularly difficult to study it is not surprising
that the subject is still some what confused. However, after having paid some attention to
the question of predisposing causes and secondary bacterial invasion an attempt will be made
to assess the evidence that there are four respiratory diseases of the horse each primarily due
to a specific virus infection.
It is far from easy to show experimentally that adverse environmental conditions will cause
the onset of pneumonia, but all clinical observers appear to agree that they are of great import-
ance in precipitating attacks of equine respiratory diseases. In a very fine clinical article on
'Pneumonia and Pleurisy in the Horse,' the late Sir Frederick Smith [1897] quotes Hunting as
saying that ' want of condition, plus fast work, causes exhaustion, and then we only want cold,
changeable weather to produce diseases of the lungs.' Smith believed exhaustion to be a 'direct
exciting' cause of pneumonia, especially in young, soft horses, whereas older horses seem to
possess a comparative immunity against some of the 'causes or conditions' which so freely con-
tribute to the production of the disease in the young, Smith realized that the febrile attacks
from which nearly all young horses suffer when first brought into stables may be associated
with the process of acquiring immunity to the 'pneumonia microbe.' Pneumonia chiefly affects
stabled horses and the great importance of good hygienic conditions is shown by the fact that
five years after the introduction of sanitary science' into the French Army the annual incid-
ence of pneumonia had dropped from 110 to 46 per thousand horses, and four years later to little
more than 3½ per thousand. Long rail and sea journeys also predispose to pneumonia. Coleman
(Principal of the Royal Veterinray College, 1794-1839) laid great emphasis on the importance
of well-ventilated stable, but unfortunately he did not display equally good judgement on many
other subjects.
In parentheses, it may be said that the clinical methods of examining the chest, and the
'respiratory sounds,' described by Smith will be of great interest to all clinicians working with
large domestic animals. He found the phonendoscope to be by far the best instrument as an
aid to diagnosis, and the extreme care with which he conducted his exminations can be judged
from the fact that they were carried out at night with sawdust as bedding to 'insure quietude '
The best types of modern stethoscopes have been described in the Lancet [1941].
Koch [cit. M' Fadyean 1938] found that normal horses harboured diplo-streptococci (see later)
in the nasal passages. Boyer [1919], in an examination of the nose and mouth of normal horses,
found that 8 per cent were carriers of 'Bact. Viscosus' and 12 per cent of 'Str. pyogenes' and
'Pseud. pyocyanea' respectively. Hignett [1940] isolated group C streptococci from the nasal
cavity of normal horses.
There can be little doubt that one or other of a number of bacterial species may be present
in the nasopharynx and produce disease when host resistance is lowered. Sporadic cases of
pneumonia, nasal catarrh or strangles probably arise in this way, but severe outbreaks usually
occur only when large numbers of young horses, or horses which have not previously been exposed
to infection, are brought to remount depots or sales yards The infectious nature of these
outbreaks is probably due to a virus and, although the uncomplicated disease usually produces
immunity, all observers agree that in itself it is very mild and may pass unnoticed. The severity
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