236
Respiratory Diseases of the Horse
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 examinations 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 cávity 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 of
the symptoms and the mortality probably depend,
therefore, on the virulence of the secondary organisms
and above all on the care which is taken of the horses
as soon as the first rise of temperature occurs. This
was fully realized by Fitzwygram [ 1894 ], who
showed that the ' incipient attack ' could be most
readily detected by the use of a clinical thermometer.
The Royal Army Veterinary Corps made use of this
fact during the last war, and as a result the losses
from respiratory diseases were greatly reduced
[ Edwards, 1935b ].
It will be seen from the foregoing summary that
outbreaks of disease caused by any one virus may
differ greatly in their severity and also in the clinical
symptoms presented, and this is probably why
there is confusion regarding the clinical classifica-
tion of equine respiratory diseases ; a confusion which
was not apparently diminished by the experience
of the last war. At that time, however, knowledge
of human respiratory diseases was in no better state,
and in 1918-19 influenza killed ten to twenty million
people [ Geddes-Smith, 1941 ]. On the other hand,
it is well known that human ' influenza ' may be a
very mild disease, and the only satisfactory method
of classification is on an ætiological basis. Con-
siderable progress has already been made in this
direction [ Francis, T., 1941]. Our knowledge of
equine respiratory diseases will probably only become
clarified when they can be classified on an ætiological
basis, and it can be definitely ascertained that a
given outbreak of disease is due to a certain ' virus',
EQUNIE INFLUENZA
(Pink eye, influenza catarrhalis, typhoid fever,
pferdestaupe)
' Malignant epidemics ' which occurred about, 1815
and the stringent methods which were employed to
control them have been described by Youatt [ 1831].
Three out of five horses which were attacked died.
and the clinical symptoms indicate that the disease
was a severe form of equine influenza. He also
describes equine influenza as it existed in this country
during the latter part of the nineteenth century.
Law [ 1902 ] has described in great detail the epizootic
which paralyzed horse transport in the U.S.A. in
1872-73. Williams [1924] discussed other early
epidemics and also described a severe outbreak in
India in which the mortality in most groups of horses
varied from 1 to 10 per cent but was occasionally as
high as 20 per cent. Equine influenza caused serious
inconvenience and loss during the last war [ Todd
and Soutar 1938; Udall, 1938; Anon., 1941 ], and
it is still fairly prevalent in India.
Equine influenza and contagious penumonia were
confused by the early workers. The confusion that
existed in the Army veterinary services was described
by M' Fadyean [ 1889 ]. The two diseases had,
however, been' clearly differentiated by Falke, 1862
[ see Friedberger and Fröhner, 1908]. The position
was again confused by the work of Ligniers [see
Nocard and Leclainche, 1903 ], who claimed that
both diseases were primarily due to pasteurella
organisms. Bemelmans [ 1921 ] also distinguished
clearly between the two diseases, but emphasized
that equine contagious pneumonia closely resembled
human influenza. This similarity has also been
described by Edwards [ 1935a]. It is felt, however,
that no change should be made in the nomenclature
of the equine diseases.
The clinical manifestations of influenza vary
considerably, and detailed descriptions have been
given by Law [ 1902 ], Friedberger and Fröhner
[1908]. A characteristic feature is the marked
prostration, and muscular and cardiac weakness
during the early febrile stage. The temperature
curve is much more erratic than in equine pneumonia.
There is usually a harsh cough, the muccus mem-
brances are reddened and there may be some
nasal discharge. The conjunctiva assumes a deep
red to mahogany colour, the eyelids are usually
swollen and œdematous. The disease may not
progress beyond this stage and the mortality is
then very low, but under adverse conditions swellings
are often seen in the subcutaneous tissues of the
limbs and secondary intestinal or pneumonic
symptoms develop; Law [1902] states that in