Surra

By Ch. Wardell Stiles, Ph. D.

Professor of Zoology, United States Public Health Service.

Surra is not known to occur in the United States, but it is more or less common in the Philippine Islands and India. It is caused by a microscopic, flagellate animal parasite, known as Trypanosoma evansi, 20 to 34 µ long by 1 to 2 µ broad, which lives in the blood and destroys the red blood corpuscles. In general the disease is very similar to and belongs in the same general class with tsetse-fly disease, or nagana, of Africa and mal de caderas, of South America.

Surra is a wet-weather disease, occurring chiefly during or immediately after heavy rainfalls, floods, or inundations.

Surra attacks especially horses, asses, and mules, but it may occur in carabao, camels, elephants, cats, and dogs, and has been transmitted to cattle, buffaloes, sheep, goats, rabbits, guinea pigs, rats, and monkeys. No birds, reptiles, amphibia (frogs, etc.), or fish are known to suffer from it. It attacks both male and female animals, young and old. Australian breeds of horses and white and gray mules are said to be more susceptible than animals of other breeds and color.

Surra in equines and camels is said to be an invariably fatal disease, but cattle occasionally recover from it. There is no history of a definite onset of the disease, and the condition is progressive, usually with a number of relapses. The period of incubation may vary somewhat; in experimental cases it is from 2 to 75 (usually 6 to 8) days, according to conditions. The duration varies with the species of animal attacked, their age, and general condition. The average duration in the horse is reported at less than two months, though some cases may terminate fatally in less than one to two weeks.

Method of infection.—All evidence now available seems to indicate that surra is strictly a wound disease, namely, that the parasite may enter the body only through a wound of some kind. Apparently by far the most common method is through wounds produced by biting flies whose mouth parts are moist with the infected blood of some animal bitten by the same flies immediately before biting the healthy animal. Crows may also transmit the infection by pecking at sores on a diseased animal, soiling their beaks with blood, and transferring this infected blood to a healthy animal. Likewise, if a scratch is made on a horse and then infected blood is rubbed on the scratch, the horse will become diseased. If, in experiment, infected blood is fed to a healthy animal, the latter may contract surra in case it has an abraded or wounded spot in the mouth; but if no part of the lining of the alimentary canal is wounded, infection does not take place. Thus dogs and cats may contract the disease by wounding the lining of the mouth (as with splinters of bone) while feeding on the carcasses of surra subjects. All available evidence indicates that under normal conditions of pregnancy the disease is not transmitted from mother to fetus.

There is a popular view that surra may be contracted by drinking stagnant water and by eating grass and other vegetation grown upon land subject to inundation, but there is no good experimental evidence to support this view: Probably the correct interpretation of the facts cited in support of this theory is that biting flies are numerous around stagnant water and in inundated pastures; hence, that a great number of possible transmitters of the disease are present in these places.

Symptoms.—The invasion of this disease when contracted naturally is usually marked by symptoms of a trivial character; the skin feels hot, and there may be more or less fever; there is also slight loss of appetite, and the animal appears dull and stumbles during action; early a symptom sometimes appears which may be the first intimation of the animal's indisposition, and which, as a guide to diagnosis, is of great importance; it is the presence of a general or localized urticarial eruption. If the blood is examined microscopically, it may be found to present a normal appearance; but in the majority of cases a few small, rapidly moving organisms will be observed, giving to the blood, as it passes among the corpuscles, a peculiar, vibrating movement, which if once observed will not easily be forgotten. If the parasite has not been discovered in the blood for several days, the symptoms mentioned above may be the only ones noticed, and, as a rule, when treated with febrifuges, the horse quickly improves in health and the appetite returns. This condition does not last for more than a few days, when the animal is again observed to present a dull and dejected appearance, and on examination well-marked symptoms are found; the skin is hot, the temperature more or less elevated—101.7° to 104° F.; the pulse full and frequent—56 to 64 beats a minute; the visible mucous membranes may appear clean, but the conjunctival membranes, especially those covering the membrana nictitans, are usually the seat of dark-red patches of ecchymosis, varying in size in different animals. There is more or less thirst and slight loss of appetite; the animal eats its grain and green grass, but leaves all or a portion of the hay with which it has been supplied. At the same time there are slight catarrhal symptoms present, including lacrimation and a little mucous discharge from the nostrils. Occasionally at this period of the disease the submaxillary glands may be found enlarged and perhaps somewhat tender on manipulation. One symptom is markedly absent, namely, the presence of rigors or the objective sign of chilliness. In addition, it will be noted that there is some swelling and edema of the legs, generally between the fetlock and the hock, which pits but is not painful on pressure, and in case of horses there may be also some swelling of the sheath at this stage of the disease. When the fever and concomitant symptoms have declared themselves for a short period, one thing becomes especially noticeable in every animal attacked, namely, the rapidity with which it loses flesh. If the blood has been examined microscopically during the second period of fever, at first a few parasites will have been observed in it, which day by day increase in number and reach a maximum, where they remain for a varying period, or at once suddenly or gradually disappear during the period of apyrexia. After the fever and the accompanying symptoms have for the second time been present for a few days—the period varying from one to six—the animal is found to have lost the dull, dejected appearance and to look bright. The temperature has fallen and, in some cases, has attained normal or even subnormal limits. The visible mucous membranes are clean, and the conjunctival petechiæ begin to fade; the pulse, however, will be found to be weak and thready in character, but the appetite excellent, and, in fact, if it were not for the loss of flesh and slight edema of the legs, there would be little to show that the animal was sick. Unfortunately, however, this condition does not continue for any great length of time, for again the temperature is elevated; in the course of a few hours the thermometer registers a still higher degree, the animal is dull and dejected, and by the following day the visible mucous membranes present a yellow tinge; large ecchymoses, dark in color, appear on the conjunctival membranes, the action of the heart is irritable, the pulse full and quick, or at times intermittent, and regurgitation may be observed in the jugulars, the breathing is quickened, and the individual respirations are shallow. On watching an animal in this condition it may be noticed that it takes seven or eight very short inspirations, followed by a much more prolonged and sonorous one; at the same time the breathing is more abdominal than thoracic in character. On examination of the legs it will be found that the swelling and edema have increased considerably, and that on the under surface of the abdomen, where previously it was confined to the sheath, it has now commenced to spread forward along the subcutaneous tissue between the skin and the muscles. During the whole of this time the appetite will have varied little, and the evacuations will be only slightly, if at all, altered in character. In the blood a repetition of the previous events takes place, the parasites make their appearance and increase to a maximum and again suddenly or gradually disappear, according to the length of the fever period. These periods, alternating with and without fever, may go on for a considerable time. The progress of the disease is variable and greatly depends upon the condition of the animal attacked, the weak one succumbing very rapidly, but each return of the fever brings with it, as a rule, an increase in the severity of the symptoms. There is increased yellowness of the membranes, fresh crops of petechiæ on the conjunctiva, a collection of gelatinous material at the inner angle, which at times becomes red in color from an admixture of blood, and which on microscopic examination is found to contain a varying number of the surra parasites; increased swelling and edema of the extremities and abdomen, which now extends between the fore limbs and up the chest. During this time the wasting has been steadily progressive, especially of the muscles of the back and those surrounding the hip joint and the glutei.

Toward the termination of the disease it will be noticed that an animal is disinclined to move, and when made to do so there is manifest loss of power over the hind quarters, somewhat simulating a slight partial paralysis, and the hind quarters of the animal reel from side to side. In connection with this it may be noted that frequently there is paralysis of the sphincter ani and a dilated condition of the anus. These symptoms taken together point to some interference with the normal functions of the spinal cord in the lower dorsal and lumbar regions, and are probably owing to pressure caused by an exudation within the spinal membranes. In many cases shortly before death the heart's action becomes exceedingly violent, shaking the whole frame at each beat, so that the sound can be heard at some distance from the animal. In some of these cases the animal may suddenly drop dead; in others the emaciation and weakness become so pronounced that it falls to the ground, and, after a short struggle, succumbs to the disease. In other cases, again, the animal falls to the ground and appears to be suffering from acute pain, struggles violently, sweat covers the body, and respiration is very hurried. The struggles soon exhaust the patient's strength, and for a time it lies quiet; soon, however, the struggles commence again, continuing until death occurs. In some cases the appetite is voracious.

The symptoms of the disease as observed in experimentally inoculated animals are as follows: Twenty-four hours after the subcutaneous injection of a small quantity of surra blood, in the great majority of cases, a small circumscribed and somewhat raised swelling is noticed at the seat of the inoculation. After forty-eight hours the tumor has increased in size and is accompanied with some edema; it presents a certain amount of tension of the parts involved, and is generally tender on manipulation. These conditions continue to increase, until by the fourth day the tumor may measure 3 or 4 inches in; one direction by 2 or 3 in the other, and raised to the extent of an inch or an inch and a half above the surrounding tissues, or in some cases the tumor presents an almost circular form throughout. It will be also found that, if the tumor is firmly grasped, it is not fixed, but can be lifted up from the subcutaneous tissue. According to the nature and quantity of the inoculated blood, these symptoms rapidly present themselves, and either attain a maximum or are retarded until, varying from the fourth to the thirteenth day, the tumor at the seat of inoculation will be found to have lost a certain amount of its tension and tenderness. From this date the swelling and edema gradually begin to grow less, until finally, after a period of 10 to 14 days, the only sign left of the former swelling is a slight thickening of the skin over the point of the injection; but at the moment when the tension and tenderness of the parts at the seat of inoculation become suddenly decreased a symptom of the utmost clinical importance takes place, namely, at that moment the parasite of surra enters the blood of the general circulation.

The temperature on the day of inoculation, and, in fact, for several days afterwards, may remain normal in character, there being only a few degrees difference between the morning and evening observations. In other cases there may be a slight rise from the first evening, and a gradual progressive rise until the swelling at the seat of inoculation shows signs of reduction in size, when the temperature generally takes a decided rise again, and may attain 104° or 105.8° F. This elevation will last a varying period of from two to six days, and on the day following its onset the ordinary symptoms of fever will be noticed, and in addition there will be petechiæ on the conjunctival membranes, lacrimation, a slight mucous discharge from the nose, and in severe cases some edema of the lower portion of the legs, and perhaps of the sheath in horses. At the termination of the period of fever the temperature will be found to have fallen to normal or nearly so; the animal will present a brighter aspect, and there is every appearance of its return to health; in a few days, however, the animal again appears dull and half asleep; the temperature is elevated, a relapse takes place, and a repetition of all the symptoms in the primary paroxysm, including the reappearance of the parasite, is observed.

Diagnosis.—A diagnosis may also be established by the complement-fixation or agglutination tests with the sera from suspected animals. This, however, can be carried out only in laboratories and requires special facilities for its execution.

Treatment.—No satisfactory treatment is known. Intravenous injections of Fowler's solution of arsenic give temporary relief, but relapses occur. In view of the great economic importance of this disease, it would not be advisable to attempt to treat any sporadic cases should they occur in this country. On the contrary, the animals should be slaughtered immediately and their carcasses promptly burned.