JOHN
FRANCIS
35
Research work on the aetiology 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 pneumonia form streptococci and perhaps
also staphylococci 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. enteri¬
tidis, Bact. viscosum equi and C. diphtheria from the organs of horses affected with ' malignant
strangles,' and it is possible that these organisms 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, husky cough may be
heard Friedberger and Frohner [1908] and Bemelmans [1921] both state that there is in-
testinal inflammation, 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 responsible 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 Schutz nor Lignieres 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 Luhrs [1910-12, cit. Edwards, 1935a] showed that this disease was
caused by virus which could only be found in the lungs and at an early stage of the disease.
After the primary rise of temperature of 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 described what was probably an outbreak of this
disease and states that streptococcal vaccines were valuable in reducing mortality. The
experiments of Luhrs [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.