When your Secretary invited me to address you and was good enough to leave the subject to my own choice, I decided to take the opportunity thus offered to discuss with you a disease of sheep that is not very widely known and which has never yet been diagnosed with certainty on the mainland of Australia. It may seem to you that, this being the case, it is a disease in which you have no reason to be particularly interested, but I hope to change your opinion if that is so. There is almost a probability that the disease does occur in Australia, but for various reasons has not been recognised, and possibly when its symptoms are described some of you may recall having seen such cases.
Circling disease was seen from time to time in New Zealand for many years. It did not cause striking losses, but in some areas men would lose odd sheep from it during the summer and autumn months. Because the deaths were comparatively few and were scattered over a period of some months, they were not regarded seriously and it was seldom we heard of it except after the sheep had died. The losses were ascribed to gid (Coenurus cerebralis cysts), grub in the head (Oestrus ovis), bad feed, Impaction of the omasum, and so on. In 1930, however, an opportunity occurred to investigate cases or this sort properly and several affected sheep were seen before death so that the symptoms could be studied.
In the first stages the animal is merely dull and disinclined to feed normally with its mates. When driven, it is seen to hold the head in an abnormal position, generally slightly to one side, and to travel in a wide circle either to the right or the left. If caught and examined more closely, it is usually found that the eyes are tending to weep and the conjunctiva is congested. An a rule, one eye—that on the side to which the animal circles, is more obviously affected than the other. As the disease develops, the sheep becomes more somnolent and the circle in which it moves decreases in size until eventually it turns round almost on its own axis. Later it is unable to stand at all, and dies shortly after this stage is reached. The time from the onset of symptoms till death occurs varies from about two days up to a week, but about three or four days seems to be the usual duration of the illness. As far as my experience has gone with sheep, the disease has been invariably fatal. Sheep of any age may be affected.
Post-mortem examination reveals no very obvious cause of death. If one considers the symptoms described they indicate a disease involving the brain, yet examination of the brain will show nothing that is obvious to the naked eye—the meninges may appear somewhat engorged, but there is nothing definite about it. The lesions of the disease are readily seen, however, if sections are prepared from the mid-brain and examined histologically: but that, of course, involves sending specimens to a laboratory and waiting some days for a report.
There is one valuable aid to diagnosis of this disease which can he carried out quite simply in the field, and it will be as well to mention it at this stage. If the cerebro-spinal fluid of a sheep affected with circling disease is examined it will be found that, Instead of being absolutely water clear, as it is in a normal animal, it shows some degree of cloudiness. There may he merely a trace, which only a close examination will detect: or on the other hand, the fluid may be so opaque that it looks almost milky. For examination to detect cloudiness, it is best to collect the fluid in a pasteur pipette and such a specimen can be procured for examination quite simply, either from the living sheep or after its death. In the live animal a two-inch hypodermic needle of fairly wide bore is inserted into the spinal canal by pushing the needle downwards in the midline of the back between the last lumbar vertebra and the anterior spine of the sacrum.
The cerebro-spinal fluid will rise into the shoulder of the needle when its point has entered the right distance, and it can easily be aspirated thence into a pasteur pipette. It is essential to have the animal in the right position and well restrained, otherwise a sudden struggle tends to break the needle. If a sample of the fluid is to be collected from a dead sheep or one killed for post-mortem examination, the simplest way to procure it is to stab the tip of a pasteur pipette into the spinal canal through the membrane that cover the under surface of the point between the skull and the first neck bone—the atlas. There are two points to bear in mind in this connection however—one is to insert the tip of the pipette through the membrane carefully as there is a small vein just inside and if this is pierced blood will pass into the cerebro-spinal fluid, making an examination for opacity impossible. The other point is that if a sheep is being killed for examination in a case suspected of being circling disease, its neck must not he broken; if it is, the cerebro-spinal fluid usually escapes or becomes so blood-stained that no proper sample is procurable. It is therefore best merely to bleed the animal out.
If symptoms suggest circling disease it will be necessary to forward suitable specimens to the laboratory for examination and a few words on this score may be of service. The laboratory staff will require the pipette of cerebro-spinal fluid for further examination and it will also need to make a bacteriological and histological examination of certain parts of the brain. For these purposes the brain is best removed whole from the cranium and it should then be cut in half, lengthwise. One half should be forwarded wrapped in cloth that has first been saturated in 5%, or 10% formal saline and then thoroughly wrung out to remove all excess of fluid. This will serve to destroy bacterial contaminations on the surface of the specimen, but will leave unharmed any bacteria in the brain substance itself. The other half should be cut lengthwise into slices about half an inch thick and sent off immersed in 5% or 10% formal saline.
One can pass over the details of the laboratory examination. It will suffice to say that if the case was one of bacterial encephalitis or circling disease, sections of the mid-brain will show accumulations of abnormal cells round the blood vessels and small scattered foci of pus cells, in some of which a few bacteria may be seen.
Bacteriological examination will give a growth of the organisms which are the cause of the disease.
The causal organism is somewhat peculiar in the wide range of animals that it is capable of attacking and in the varied nature of the lesions which it may cause. That is an added and important reason for keeping a watch for the occurrence of the infection here in Australia. Another somewhat extraordinary feature of this type of infection is that although it is now known to occur in England, America, South Africa, New Zealand and possibly in India, it is only since 1926 that it has gradually come to be recognised.
In that year it was described as the cause of mortality in rabbits and guinea pigs in England. Infection apparently took place from the feed or water, and brain lesions did not occur. At about the same time it was found to be the cause of death among certain ground rodents (gerbilles) on the African veldt. Here again infection appeared to take place by the mouth, and the lesions were mainly found in the liver. In 1931 we found it to be the cause of encephalitis of sheep in New Zealand, and a year or two later similar cases in sheep and also in cattle were discovered in the United States. A few crises of encephalitis in human beings have been found due to it within the last few years, most of them in America but at least one in Scotland. In 1936 some cases were described in domestic fowls in America, the lesions in this species being in the form of extensive necrosis of the heart wall, and recently an English worker has encountered further cases in poultry showing oedema of the tissues and liver lesions. (This seems to be a more acute form of the disease in poultry and caused somewhat heavy losses).
Lastly, a correspondent in America informs me that he has encountered an outbreak of circling disease on a stud goat farm. We may yet hear of it attacking pigs, horses, dogs and cats. The name of the bacterium that causes these various lesions in the different species is Listerella monocytogenes.
As has been mentioned, the evidence suggests that in rabbits, guinea pigs and the ground rodents of Africa (gerbilles), infection takes place through the alimentary tract, but the path of infection in the various other species that are affected is as yet unknown. Evidence from a small series of experiments with sheep in New Zealand suggested that infection probably took place from the nasal passages, and this, together with the seasonal occurrence of the disease and the fact that in almost all cases examined there were a number of the minute first stage larvae of Oestrus ovis present in the nasal mucous membrane, naturally focussed attention on the larvae as possibly playing a part. Cases do occur, however, in which none of these larvae can be found, and some of the American cases have happened at a time when there should be no first stage oestrus larvae in the nostrils at all (late winter).
When circling disease occurs, both stock owners and stock inspectors naturally ask what can be done about it. Unfortunately, there is nothing that can he done in our present state of knowledge, but it is some consolation to know that losses are usually quite slight, ranging from under 1% up to 5% or 8% in exceptional cases. It is probable that a method of immunising sheep against it could he devised, but except in rare instances, which could not be foreseen, the cost of immunisation would not be warranted. On the other hand, if the means by which the infection gains entrance to animal body could be elucidated, control measures other than by immunisation might be possible.
In conclusion, one must remind you that there are certain conditions that occur among sheep from time to time that could be mistaken for circling disease. The principal ones are as follows:—
Suppuration of the middle ear, with or without infection of the adjacent meninges and brain. This can he detected quite readily by a careful post-mortem examination. In most casess the animal holds the head to one side and tends to move in a circle. Usually they show these symptoms for a much longer period than in circling disease, but if infection spreads to the brain death may occur rapidly and the case may be quite indistinguishahle from it till an autopsy is held.
Gid (Coenurus cerebralis). The animal generally moves in a circle, but the condition persists for a long period and the cerebro-spinal fluid is quite clear. Often the position of the cyst can he detected by bulging and softening of the overlying cranial bones. There is no reliable evidence that this condition occurs in Australia.
False gid. This is a disease in which brain symptoms, often including circlings, are said to be caused by large numbers of mature oestrus ovis larvae in the cranial sinuses. I know of no thorough inquiry into such cases and it seems at least possible that they are actually cases of circling disease, the Oestrus larvae being subsidiary. It requires detailed investigation.
Acute digestive disturbances. A condition commonly called "staggers," and which is accompanied by cerebral symptoms that might suggest a brain infection, sometimes occurs among sheep placed suddenly on very rich feed. The cerebro-spinal fluid in such cases is quite clear, however: if taken in time, sedatives and a purge will save many of the affected animals, and a change of feed usually prevents further cases developing.
Pregnancy toxaemia in ewes sometimes gives a clinical picture that is very similar to circling disease and, although it is unusual, there are instances where specimens of cerebro-spinal fluid and brain tissue may have to be examined before a final decision can be made.
In this brief account of bacterial encephalitis of sheep, or circling disease, serves to stimulate your interest in the possible occurence of such an infection in Australia, either in sheep or other livestock such as goats, cattle or poultry, it will have served its purpose. Should any of you have encountered cases that may be of that nature it would be of great interest to me to hear of your experiences.