The cause of this disease is the entrance into the blood of a species of protozoa - the Trypanosoma equipedum. This is a unicellular organism having a flagellum by which it is capable of considerable activity. So far as at present known it is the only trypanosoma which is not transmitted by a biting insect. The common mode of access of the virus is through the medium of the external genital organs during the act of copulation. The spread of the malady is effected more especially by stallions when going from mare to mare during the breeding season.
It has been induced experimentally, by Nocard, with matter from a diseased centre in the spinal cord, and previously, by Herting, with discharges from the vaginal mucous membrane.
The period of incubation is said to extend from eight to twenty-eight days. At the expiration of the term a discharge issues from the penis of the male or the vulva of the female, after which small red spots appear on the glans or vulva, and on these arise vesicles or blisters, which break, leaving a small spreading sore or ulcer. As the disease progresses the enlargement spreads from the penis to the sheath, and maybe to the scrotum and testicles. In those instances where the disease first attacks the urethral canal a muco-purulent discharge issues from the end of the penis, and urine is discharged with some difficulty.
A similar eruption appears on the vaginal mucous membrane of the mare, associated with more or less considerable swelling of the labia, and an ichorous discharge. If the eruption is severe the irritation may extend to the mammary gland and induce the formation of abscesses upon or in it.
Both stallions and mares exhibit marked sexual excitement in the course of the early stages of the disease, the former by frequent erections of the penis, and the latter by exhibiting signs of oestrum. The local manifestations may subside, or altogether disappear, after a time varying from several weeks to months, but this is not to be taken as an indication of the termination of the disease, for it is at this time that the general health begins to give way to the ravages of the virus. There is now dul-ness and progressive weakness, shown by shifting the weight first from one leg and then the other. An eruption of the character of urticaria, but usually more enduring, appears about the neck, or the chest and shoulders, or the croup, or on all of these parts in succession, disappearing on the one as it arises on the others. The chief features of the later stages of the disease are progressive paralysis and wasting of the body.
The animal knuckles over at the joints, crosses his legs in walking, trips, or trails one or both hind-limbs, and ultimately loses all power behind.
Facial paralysis is sometimes present, when the eyelids droop and the lips hang pendulous and immovable. Finally, emaciation becomes extreme, and death results from blood-poisoning and exhaustion at the expiration of various periods from one to two years.
Death results in 70 to 80 per cent of cases, and having regard to the chronic lingering nature of the malady, and the decrepit useless state to which an animal is reduced by the disease, there can be no real advantage in attempting a cure. To destroy the animal at the outset is the most rational procedure.