A low form of catarrhal disease affecting the horse has for a long time been distinguished from common catarrh by the term influenza or distemper. It has existed in this country as far back as veterinary history extends, prevailing generally in the spring of the year, reappearing sometimes in the autumn.
Influenza attracted a special attention in 1872, in consequence of an extensive outbreak in the United States, causing enormous loss among horses belonging to the tramway companies. In consequence of the swelling and inflammation of the eyes, which is in many cases one of the symptoms, it acquired the name of "pink eye", but at the time the American veterinary authorities admitted that there was nothing in its character to distinguish it from the old form of influenza, and they ascribed the excessive mortality to the injudicious treatment to which sick animals were subjected. The tramway companies insisted on keeping them at work after the indications of the disease had appeared, with the result which has been referred to.
The new term " pink eye" was accepted in this country as evidence of the existence of a new disease among horses in the States, and considerable alarm was created among horse-owners, who realized the risk of importing the disease from America, an event which really occurred, but not until several cases of what was considered to be "pink eye" had been detected in different parts of England. It may be observed here that the swelling and redness of the eye had always been recognized as a symptom in certain cases of influenza, and the older practitioners insisted on this fact when the attempt was made to substitute "pink eye" for influenza. The popular feeling, however, was too strong for them, and to this day cases of influenza, associated with swelling of the eyelids and redness of the lining membrane, continue to be described as cases of "pink eye".
As influenza affects a considerable number of animals which are associated together in large establishments, the idea naturally arose that the disease was spread by contagion. In opposition to this theory there is the evidence that one case may be followed by another at the other end of a large stable. In other instances a large number of animals are attacked simultaneously. The disease also appears in certain localities, in which it rages for a time and then rapidly ceases without any apparent reason. In favour of the contagion theory it is urged that the introduction of a sick horse into a healthy stable is followed by the propagation of the disease, and that the introduction of horses into infected stables has, at least in many instances, been followed by an outbreak of the malady. It is undoubtedly expedient in practice to treat the disease as one of the contagia.
Influenza is distinguished from common catarrh by the very marked depression and weakness which attend the early stages. The appetite is impaired, the temperature rises, the pulse gradually increases in frequency, and the depression becomes more pronounced as the disease advances. Swelling of the eyelids and redness of the mucous membrane are also observed, and sometimes the swelling extends to the limbs, or maybe the head. Discharge from the nostrils may, or, as in some cases, may not, be present. Muscular pain and weakness is shown by the feeble gait and the constant change of posture and resting of first one limb and then another.
In other cases the disease appears to be concentrated in the digestive system, affecting the liver chiefly, when the membranes of the eyes and nose become yellow, the feces hard and coated with a glairy mucus. In other instances the respiratory organs seem to suffer most, and some of the worst cases terminate in a gangrenous pneumonia.