ABORTION, CONTAGIOUS, an infective disease of cat tle, horses and sheep which frequently causes premature birth in otherwise healthy pregnant females. In Great Britain the condi tion is most frequent and of greatest economic importance in cattle.
The condition is widespread and is more prevalent where ani mal husbandry and dairy-farming are common. Although the disease is not usually fatal to the cow, temporary illness and loss of flesh occur; lactation is markedly interfered with ; and many cows do not conceive again for prolonged periods, or may remain permanently sterile. At one time it was thought that the bull was the chief factor in the spread of the disease but this view is not now widely held, though it has been shown that the bull may con tract the disease, and that lesions may exist in the seminal vesicles and testes, so that during coitus infection may be conveyed to the cow. The present-day belief is that the ingestion is the commonest method of contracting the disease, the principal source of the causal bacillus being discharges of an infected animal per vaginam. These discharges may retain their infectivity for considerable periods. In addition, the udder acts as a reservoir for the organ ism, which can remain located in this site without giving rise to alterations in the mammary gland; during lactation, the bacillus finds its way into the milk and may be present in the faecal matter of calves born from and reared on infected cows. Once contami nated material becomes deposited on pastures where healthy cattle are grazed, or in yards where they feed off the ground, infection is certain to follow. In non-infected herds the introduction of an infected animal is a fertile source whence the disease may spread rapidly. Naturally occurring cases of the disease are mostly con fined to cattle but outbreaks have been recorded in sheep and swine. Experimentally, the disease may be transmitted to many species of animals both large and small.
Whether natural infection occurs by ingestion and entrance into the blood stream, or by way of the vagina, the bacillus reaches the uterus, which is a predilection site in the pregnant female. Here a variable amount of exudate collects between the lining membrane of the uterus and the chorionic layer of the placenta ranging from yellowish to dark brown in colour, and from thin and.pus-like to thick, tenacious and glutinous in consistence. The connective tissue underlying the chorion, and also the umbilical cord, undergo alterations and become infiltrated. As a result of these changes the foetus dies in utero or is expelled. If expulsion does not occur, mummification may supervene and retention may ensue over a prolonged period.
In aborting cows, the placenta is infiltrated with a yellowish gelatinous material and whitish mucoid flakes are sometimes pres ent, the foetal cotyledons are partly or entirely yellowish and macerated. The tissues of the foetus may be abnormally moist and the great cavities may hold quantities of blood-stained serous fluid. Infected pregnant cows may abort at any period of gesta tion but around the fifth to the sixth months is more common ; occasionally, infected cows may undergo a normal pregnancy. Generally, in herds where the outbreak is of recent origin, abor tion occurs earlier than when the affection is of longer duration.
Secondary infection of the uterus may follow, especially if the foetal membranes are retained and a condition of sapraemia or septicaemia may be set up (see SEPsis). Although putridity is not a character of contagious abortion, the secondary infections quickly lead to a very offensive condition of the retained placenta. As a consequence of these changes, animals become affected with a chronic endometritis with sterility as a sequel.
Some animals may become pregnant to abort a second time, but it is more usual for the animal to carry her calf to a normal parturition and to abort at the subsequent pregnancy. Sta tistics show that the majority of cows do not abort more than once. In the early months of pregnancy the calves are born dead; later, they may be living, and from the eighth month many are reared. Although the cow may only abort once, she may remain a carrier of the disease for a considerable time.
Diagnosis of the disease may be arrived at by the microscopi cal examination of the uncontaminated discharges and diseased placental membranes. Two blood tests are also available, the agglutination and the complement-fixation tests; the technique of the former has been clearly defined by M'Fadyean and Stock man and the method is now extensively used in diagnosis. Blood, taken from the jugular vein of an animal may be sent to a veter inary pathological laboratory, and the result will show whether the animal is affected with contagious abortion. It is necessary to re peat the test after a few weeks as single tests are open to error when dealing with recently infected animals. The method is of value in eliminating affected animals from herds, and in prevent ing the introduction of newly purchased, infected animals, into clean herds. As a means of preventing spread of the disease the agglutination test is of great value; the healthy animals are sep arated from the reactors and kept in strict isolation; their numbers are augmented out of their own progeny, or by purchased animals, which have not reacted positively to the agglutination test.
Where this method is not adopted animals showing signs of abortion should be strictly isolated. Very rigid disinfection should be carried out especially after abortion has occurred. The foetus and the placenta should be burnt and all discharges thoroughly disinfected. The uterus may be treated with medicated douches and the external genitalia kept as aseptic as possible. In such herds the preputial hairs of the bull should be clipped and the prepuce should be irrigated with a suitable disinfectant after each service.
Every case of abortion should be regarded as of the contagious variety unless proved to be otherwise by sero-diagnostic tests. In badly infected herds every effort should be made in order that aborted cows may conceive again, so that an immune herd may be acquired eventually.
Methods of vaccination have been introduced. Both dead and living abortion bacilli have been used in the endeavour to estab lish an immunity. In England, the use of the living vaccine has been advised by the Ministry of Agriculture and Fisheries. The method consists in the subcutaneous inoculation of a massive dose of living abortion bacilli into the virgin heifer, or the non-pregnant cow. Intercourse is not allowed for eight weeks. The object is to establish that degree of immunity which a natural attack confers, and to prevent the undesirable sequelae which occur in the pregnant animal; the process should be repeated annually until the disease is controlled. Vaccination should not be resorted to in herds which are only very slightly affected, or in herds which are free of the condition.
In some countries attempts have been made to control the disease by legislative methods.
Mares.—The aetiology of this condition has been studied in various parts of the world and although several organisms have been described, the bacillus abortivo-equinus is now held to be the cause of the majority of outbreaks of mare abortion.
In Great Britain the subject has been studied by M'Fadyean and Edwards, and their observations indicate that the commonest cause of abortion in mares and joint-ill in foals is the B. abortivo equinus..
The bacillus is small and pleomorphic ; it may resemble a coccus, or occur in short, plump bacillary forms. It is motile, non-sporing and Gram-negative. It grows readily on artificial media and on agar its surface culture is characteristic. Experimentally, the dis ease can be transmitted to other farm animals and to small animals.
The agglutination and complement-fixation tests are applicable for diagnostic purposes.
The symptoms and lesions are in some respects similar to those observed in cattle but the placenta usually shows less alterations.
Mares are more liable to secondary complications such as laminitis, synovitis and pneumonia.
Sheep.—A vibrio has been described as the chief cause of out breaks of abortion occurring in pregnant ewes (M'Fadyean and Stockman) ; these slightly curved organisms are motile and rarely exceed 314 in length. They are Gram-negative and stain readily with the ordinary aniline dyes. They may be cultivated under somewhat similar conditions to the bacillus of contagious abor tion of cattle and their distribution in naturally occurring cases follows somewhat the same lines. A similar vibrio has also been isolated from cows, which have aborted, but outbreaks of abor tion in cattle are not usually regarded as being vibrionic in nature. Infection may take place per os or per vaginam and the act of abortion usually follows earlier in ewes than in cows.
(A. R. S.)