The liver may be normal (Griffith Evans), or especially during a paroxysm (Lingard) congested, inflamed and enlarged. The pancreas is usually normal.
The kidneys are petechiated, congested, œdematous, or the seat of blood extravasations.
Instances have been noted of cerebral and meningeal œdema (Steel, Lingard), encrease of fluid in the arachnoid or lateral ventricles, or in the spinal cord, with gelatinoid exudation in the lumbar portion (Lingard). Like other tissues, in advanced stages the nerve centres are usually anæmic (Geo. H. Evans).
Incubation. As made out by inoculation cases this extends from two to eight days, according to the dose—five to seven days being the most common. Infection by inoculation subcutem in mules showed a hyperthermia on the fifth day; or by ingestion (feeding the virus) on the sixth day (Steel). Longer (alleged) incubation depends mainly on the first slight paroxysms having been overlooked, or set down for the frequent bilious and icteric condition which is common in mules and horses in India (Steel), or to a later infection by insects or otherwise.
Symptoms. In experimental cases a small raised swelling in the seat of the inoculation, appears in 24 hours, encreasing to 2 to 4 inches in diameter, and 1 to 1½ inch high by the 4th day, and loosely connected with the parts beneath. From the 4th to the 14th day it decreases in size and softens and general symptoms set in.
In casual cases these general symptoms are the first to be observed. There is a transient fever 102° to 104°, highest toward night and without preliminary chill, hot mouth and skin, dulness, sluggishness, inappetence, yellowness of the mucosæ, petechiæ on conjunctiva or vulva, and sometimes nodules like those of urticaria on the skin. After a day or two these symptoms subside, the temperature is 101° F., or below, the mucosæ clear and pale, and spirit and appetite nearly normal. These slight first paroxysms are rarely seen by the veterinarian, having been looked upon as one of the oft-recurring bilious attacks of a hot climate. The remission lasts for 3 to 10 days when the second paroxysm sets in, like the first but often more marked: temperature 102° to 104°, eyes especially the membrana nictitans petechiated, epiphora, slight catarrh from nose or vulva, it may be stocking of the legs, or pitting swelling under the breast bone, or abdomen, or in the sheath. Like the first, the second paroxysm subsides, and after another interval a third sets in, to be followed in like manner by a fourth, a fifth and so on, if the patient survives. With each the symptoms become more pronounced, the mucosæ are left more pale and bloodless, debility and weakness are greater, emaciation is more marked, œdema of the limbs or body more extensive, hyperthermia may reach 105° or more, the pulse is weaker and the heart more liable to palpitation, and the respirations may reach 50 or 60 per minute. Yet in sparely built animals dropsy may be entirely absent (Steel). Steel often found superficial, circumscribed ulcers on the tongue, inner sides of the lips, nose, eye, or vulva, beginning as epithelial degeneration, followed by superficial erosion and early healing. Sometimes, similar erosions appeared on the skin. Generative excitement may be present, the mare appears to be in heat, while the horse has erections, which are supposed to depend directly on the implication of the generative centre in the lumbar myelon. The submaxillary glands sometimes swell and even suppurate and discharge a gluey pus (Griffith Evans). The bowels are usually costive at first, the fæces may be glazed, but in advanced stages they become soft, pultaceous and fœtid. The urine at first normal in amount, becomes later abundant or even profuse (Griffith Evans, Ranking, Nariman and Vaz, Lingard). It is at first yellow and turbid, later of a dingy green or greenish yellow. Sometimes it diminishes as the disease advances. It may contain bile, albumen, or even casts (G. H. Evans), though the latter appear to be exceptional. The reaction varies, sugar is absent, and the parasite has not been found in it. Appetite though interfered with during the paroxysms, remains fair or even voracious in the intervals and the animals may eat to the last (Burke). Thirst usually encreases with the advance of the disease, in keeping with the free urinary secretion. Rumbling and gurgling of the bowels are common and even tympany at times (Lingard).
In the advanced stages the picture is one of great anæmia, marasmus and general debility. When moved the animal will stumble over the slightest obstacle, even the litter, recovering himself with effort and difficulty. If he should fall he is liable to remain down indefinitely, the side next the ground becoming drenched with sweat though there is no general perspiration. The hair becomes encreasingly dry, withered and erect, the skin dry, powdery, rigid and more and more firmly adherent to the bones and muscles, losing all its natural pliancy and mellowness, and becoming like that of a dead animal. It is bloodless, and sloughs readily over the prominent bones, where compressed or bruised in lying, owing to the lack of nutritive and reparatory action. The visible mucosæ are absolutely bloodless. The muscles as a whole are wasted to an extreme degree, but this atrophy is most marked in the back and loins, along the longissimus dorsi and in the quarter in the gluteal muscles. The patient may remain recumbent, from sheer weakness, for a length of time at the last, or he may get up after a long recumbency and stand to the end.
Death may occur early with general anasarca and extreme hyperthermia (110° F.). In the great majority it appears to result largely from perforation of the stomach, clots in the heart, or general debility and heart failure.
Diagnosis. With symptoms such as are above described the discovery of the trypanosoma in the blood completes the diagnosis. Trypanosoma is found in dourine, nagana and other affections so that the discovery of it alone would not be conclusive as to the existence of surra. Nor can the discovery of the parasite always be made at the first or second attempt. The swarm of mature trypanosomata is found with the advent of a paroxysm, and as the veterinarian is often called during the decline of the attack the parasites have already retired and elude his investigation. It becomes needful to take the temperature and examine the blood daily sometimes for eight or ten days, and when with a sudden rise of temperature he finds also a swarming of the mature trypanosomata, the diagnosis is perfect. A drop of blood placed on a cover glass, pressed down upon the slide, and placed under the microscope, will show the parasite with eel-like movements among the blood globules. There may be very few during the first or second paroxysm, but they become numerous and very obvious as the disease gains its height. Evans recommends to use defibrinated blood.
They may be dried rapidly on a cover glass, fixed in absolute alcohol one or two minutes, then stained ten minutes in a mixture of the two following liquids united just before using: