JOHN
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237
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 Fröhner
[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.
Research work on the ætiology of influenza has
been reviewed by Edwards [1935a] and by Todd
and Souter [1938]. It is a highly infectious virus
disease and may be set up by the subcutaneous
inoculation of infected blood [this fact has been
confirmed by Dale and Dollahite, 1939 ] or by direct
or indirect contact. Saliva may be infective for at
least eight months after recovery and it has been
reported that a stallion transmitted infection by
coitus for a period of six years. One attack of the
disease confers a long-lasting immunity, but animals
remain susceptible to equine contagious pneumonia.
Little attention has been paid to the problem of
secondary bacterial invasion in equine influenza.
It is probable, however, that in the pneumonic form
streptococci and perhaps also staphylococcr invade
the lungs, whilst the good results obtained by Marshal
and Lee [ cit. Udall, 1938] following the use of
pasteurella biological products suggest that this
organism may also be of pathological importance.
Krag and Tiedge [1938] isolated S. enteritidis,
Bact. viscosum equi and C. diphtheria from the organs
of horses affected with ' malignant strangles. ' and
it is possible that these organi ms are of importance
in the intestinal form of equine influenza.
EQUINE CONTAGIOUS PNEUMONIA
(Influenza pectoralis. contagious pleuropneumonia,
brustseuche.)
The long incubation period (up to thirty days),
irregular spread, lack of subcutaneous swellings or
marked depressions during the early febrile stage
and the regular temperature curve are said to
differentiate this disease from equine influenza.
There is often a yellowish discharge from the nose
which is pathognomonic a dry, nusky cough may
be heard. Friedberger and Fröhner [1908] and
Bemelmans [1921 ] both state that there is intestinal
inflamation,. but manifest symptoms of intestinal
disease are rare. Following the fall of the primary
fever and particularly if the animal is worked,
varying degrees of pneumonia and pleurisy may
develop. The mortality may be as high as 20 per
cent., and for many years the disease has caused
serious losses amongst Army horses; the period of the
last war was no exception [Udall, 1938 ].
The research work which established the ætiology
of this disease has been reviewed by Edwards [ 1935a ]
and again by M'Fadyean [1938]. The disease
has two causes which act separately and are respon-
sible for different stages in the illness and different
lesions... The virus is the veritable cause in the sense
that it is responsible for the spread of the disease...,
Law [ 1902 ] noted that neither Schütz nor Lignières
had shown the experimental diseases produced by
them, following the inoculation of bacteria, to be
contagious and therefore did not accept their claim
to have discovered the true, cause of pneumonia or
influenza. Gaffky and Lührs [ 1910-12, cit. Edwards,
1935a] showed that this disease was caused by a.
virus which could only be found in the lungs and at
an early stage of the disease. After the primary
rise of temperature the virus disappears from the
lungs and the horse may recover without any very
obvious signs of the disease. On the other hand,
particularly if the horse is worked, bacteria may
invade the lungs and a secondary rise in temperature
with typical pneumonia will occur. It will be seen
that this work anticipated the researches on human
influenza by many years, and this was probably the
first respiratory disease in which a ' dual causation '
was discovered. Infection can only be transmitted
by direct contact or by rubbing lung tissue from a
horse in the early stages of the disease on to the nasal
or buccal mucosa, and even then the incubation
period may be as long as 39 days. This may
explain why the disease has been considered
to be non-infectious by some clinical observers,
including Smith [ 1897 ], although the course of the
disease as described by him suggests that he was
dealing with brustseuche. Fraser [ 1938 ] has des-
cribed what was probably an outbreak of this disease
and states that streptococcal vaccines were valuable in
reducing mortality. The experiments of Lührs [1928,
cit. Edwards, 1935a] showed that animals which
had received virulent lung tissue subcutaneously
were resistant to infection by intranasal or intrabuccal
rubbing of infective bronchial mucus which almost
invariably produced disease in control animals.
The question of secondary bacterial invasion is
somewhat confused. M'Fadyean [ 1938] assumed
that the organisms isolated by Schütz [ 1887 ] and
used in his attempts to produce brustseuche were
typical streptococci. This appears to be incorrect
for an examination of the original article shows that
the organism was a gram-negative bacterium which
often occurred in pair and was usually encapsulated.
Friedberger and Fröhner [ 1908] also state that the
' diplo-bacteria' of Schütz are gram-negative. It
can be concluded with reasonable certainty that
these were not pasteurella organisms. Nocard and
Leclainche [ 1903 ] say that although the organism
was originally described as gram-negative, it is now
known to be gram-positive. This cannot be
accepted, however, because his article on strangles
[ Schütz, 1888] makes it quite clear that he was
I