Incubation appears to be longer than in equine influenza, varying in different cases from 3 to 10 days.
Symptoms. These vary greatly in different cases, mild and severe. Some, in the same stable with the severe cases, simply refuse food, are a little sluggish in work, cough, have hyperthermia (104° to 106° F.), respirations 20–25, pulse 60, with conjunctiva only moderately yellow, a slightly yellowish discharge from the nose, and no observable lung consolidation. The temperature descends to normal in three to five days, the symptoms generally abate, and the animal may be convalescent in eight or ten days.
In the more severe cases there may be seen a shivering fit, or it may pass unobserved. Then the first morbid phenomenon is usually a rapid and extreme elevation of temperature which may reach 104° or 106° F. in a few hours. With this there is great impairment or complete loss of appetite, and a loss of life and energy. In some cases the depression, stupor and muscular weakness suggest influenza but this is not the rule. Still more rare is infiltration of the eyelids and free watering of the eyes, yet in the absence of this, drooping of the upper eyelids is not uncommon. The respiration may be accelerated and short, from 20 to 30 per minute, and the pulse, which is usually small and weak in spite of the fever, may rise to 50 or 70 per minute. The breathing may be trembling or distinctly interrupted in the course of inhalation or exhalation, short and with no interval between inspiration and expiration. Cough may or may not be a marked feature, heard at long intervals only in some cases and frequent and painful in others. It is liable to be dry and husky rather than hard, loose or gurgling. The eye and to a less extent the nasal and buccal mucosa tend to show a yellowish shade, and this may even at an early stage show a distinct brownish orange, or even a dark mahogany hue. Yet dropsy of the lids or even epiphora are uncommon. A yellowish discharge from the nose is an almost constant feature and this may dry up into a yellow crust on the floor of the anterior nares and adjacent skin. The percussion and auscultatory indications of lung consolidation are rarely obtainable before the end of the second or third day and when at all extensive can usually be detected on both sides. Trasbot considers the double pneumonia as almost pathognomonic of contagious pneumonia. When confined mainly to the lower parts of the lungs and occurring in isolated areas, with lung tissue still pervious to air in the intervals, it comes more nearly to being so. Crepitation round the border of consolidated areas, is a more marked feature than in equal consolidations in influenza. It often becomes inaudible again as the disease advances. Blowing murmurs, coarse mucous râles, heart and intestinal sounds can often be heard with unusual clearness, in unusual situations, when an area of consolidated lung is immediately beneath. A transient dry friction sound of commencing pleurisy is sometimes detected over a tender intercostal area, but soon giving place to the uniform quiet of effusion rising to a given horizontal level. Later still there may be the creaking sound of organizing false membranes in process of being stretched, and which is so often confounded with crepitation. The indications of pneumothorax (tympanitic resonance, and metallic tinkling), are rare. In advanced stages there may be tympanitic sound from the cavities of abscesses or the sacs containing sequestra.
The urine is always scanty and high colored and may at times prove red and hæmorrhagic. Albuminuria is usually present when the disease is at its height. The same is true of uric acid, which replaces the hippuric acid, in cases of high fever and complete abstinence so that the products are drawn from the disintegrating tissues alone. The returning appetite, and the restoration of a neutral or alkaline condition of the urine, therefore tend to occur simultaneously, and to mark improvement.
Great tenderness of the throat, protrusion of the nose, and difficulty of swallowing mark the localization of the lesions on the pharynx and larynx. It is liable to be accompanied by the introduction of exudation and food materials into the larynx and trachea with the occurrence of inhalation bronchitis and pulmonary gangrene.
Symptoms of pericarditis, endocarditis and myocarditis, are especially common in the more severe types of the disease. With soft, weak or imperceptible pulse and tumultuous heart beats they may be suspected, and further indications are a transient friction sound, synchronous with the beat of the heart, intermissions, murmurs with first or second heart sound, and an increasingly low, distant, or muffled heart beat, as pericardial fluid accumulates. As in the case of troubles with the kidneys or liver, stocking of the legs, or dropsical swellings elsewhere may appear.
Exceptionally, acute nervous symptoms may appear, due to functional derangements caused by circulation of the toxins and metabolic products, or even to congestion or inflammation of the brain or its membranes. This may occur at the outset of the disease indicating the election of the nerve centres for the colonization of the microbe, and advancing to a rapidly fatal issue (Friedberger and Fröhner). It may set in with hepatization (Rey), or it may coincide with pulmonary gangrene (Cadeac). There may be merely dulness, prostration, or stupor; or trembling, unsteady gait, or falling; there may be rolling of the eyes, or amaurosis, or vertigo occurring intermittently; or there may be epileptic attacks or paraplegia.
Course. In moderate cases the disease may last from two to three weeks, and in well conditioned horses, with strong constitutions, tends to recovery. On the third to the eighth day all the symptoms appear better, appetite, expression, alertness, breathing, pulsation and temperature. The temperature which has been a degree, or more, higher in the afternoon than in the morning, remains about the same from morning to night, or is even slightly lowered; it is lower still next morning and in two or three days may have reached 101°, still rising a little in the afternoon. The pulmonary exudate is usually quickly absorbed though less so than in favorable cases of influenza. Convalescence may be completed by the end of the third or fourth week.
In violent and fatal cases the general symptoms tend to encrease in violence, though the temperature may descend to 103°, and in the final collapse to 100° or lower. In a mare presented at the college clinic after three weeks illness and treatment elsewhere, prostration was extreme, the head rested in the manger, the nose discharged a fetid, glairy, frothy liquid, with grumous, bloody debris; breath offensive; pulse 92, almost imperceptible; respirations 30, very labored; nostrils widely dilated, flapping; temperature in vagina 103.6°, anus was open with constant ingress and egress of air, and a watery glairy, frothy discharge; extensive dropsy under the sternum; percussion and auscultation indicated consolidation of lungs from the lower border up, crepitation, creaking, and loud clucking bronchial sound. The mare survived for forty-eight hours, the temperature descending to 100.5° in the morning and rising to 102° and upward in the afternoon. At the necropsy the right lung was consolidated throughout, the left had pervious areas anteriorly and posteriorly; there were large areas of infarction, necrosis, with encystment, caseation, and lobular and perilobular exudation and hepatization.
Liver, enlarged, tense in its capsule, but soft and friable on its cut surface, with areas of softening and necrosis. Both kidneys congested, with pale zones of necrosis; right enlarged. Beside the substernal exudate, there was extensive hæmorrhagic exudate between the serratus magnus and ribs. The blood, very dark, brightened on exposure to air.