34
Respiratory Diseases of the Horse
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, 1935 b].
It will be seen from the foregoing summary that outbreaks of diseases 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 classification 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 satisfact-
ory method of classification is on an aetiological basis. Considerable 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 aetiological basis, and it can
be definitely ascertained that a given outbreak of disease is due to a certain 'virus'.
EQUINE
INFLUENZA
(Pink eye, influenza catarrhalis, typhoid fever, pferdestaupe)
'Malignant epidemics' which occurred about 1815 and the stringent methods which were em-
ployed to control them have been described by Youatt [1831]. Three out 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 edizootic
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 pneumonia were confused by the early workers. The con-
fusion 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
Frohner, 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 [1935 a]. It is felt, however, that no change should be made in the
nomenclature of the equine diseases.
The clinical manifestation of influenza vary considerably, and detailed descriptions have been
given by Law [1902], Friedberger and Frohner [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
mucous membrances are reddened and there may be some nasal discharge. The conjunctiv
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 long railway journeys
the mortality may be 100 per cent. It would appear that the influenza virus can attack any
of the body tissues ; there is always intestinal inflamation, and Friedberger and Frohner [1908]
state that the principal changes are in the organs of digestion. Apart from the virulence of the
virus the type of disease depends on environmental conditions, management and on the species
of the bacterial invaders.