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Heartwater

HEARTWATER Heartwater is a specific tick-transmitted disease of cattle, sheep, and goats, caused by an ultravisible virus, and characterized post mortem by an accumulation of liquid in the pericardial sac, a striking lesion from which the popular name has been derived.

Various other names have been applied to the disease by South African farmers, particu larly" Boschziekte" (bush-sickness), Veld sickness, and Inapunga, but the conditions passing under such popular names cannot as a rule be regarded as distinct pathological entities. As has been pointed out by Theiler, a case of Heartwater in which the best-known and most characteristic post-mortem lesion was wanting would probably be diagnosed as Boschziekte or Galziekte, or possibly as" Dronk Galziekte" (drunken gall-sickness), although" Galziekte" could not properly be given as a synonym for Heartwater, as the name is ex tended to all cases of disease (particularly in cattle, but also in sheep) showing a particular train of symptoms, and the majority of bovine cases so described are, of course, due to infection with Anaplasma marginale.

Similarly a case of some other disease, in which post-mortem examination revealed any very noticeable amount of liquid in the pericar dial sac, would often be ascribed to Heartwater, although there might be few or no other features in the case to suggest such a connection. Theiler records that cases of East Coast Fever have often been mistaken for Heartwater (and East Coast Fever is still very frequently diagnosed by farmers as Gall-sickness), and many cases of parasitic invasion, leading to emaciation and the presence of transudates in the serous cavities, have been attributed to the same cause.

History. The disease has probably existed for a long time in such territories as Zululand, Swaziland, and the Transvaal low veld, and Theiler remarks that it has long been recognized by the Transvaal Boers. As regards the Eastern Province of Cape Colony, the disease would appear to be of more recent introduction. Spreull states that" bont" ticks were first seen in the Lower Albany district in 1840, and that small stock began to do badly about 1860, while Lounsbury quotes a report from the magistrate at East London in 1875, in which it was stated that sheep and calves were becoming increasingly difficult to rear in certain parts of his district. Of recent years the distribution of the disease has been considerably affected by the constant dipping of cattle, and the disease has ceased to be of any great importance in many parts of South Africa in which it was formerly very prevalent. On the other hand the very considerable amount of settlement and development which has been occurring during the past few years (and is still going on rapidly) in the low veld areas of the Transvaal, Zululand, and Swaziland, has caused the disease to acquire renewed importance.

Geographical Distribution. In the Union of South Africa the disease is, or has been, more or less widespread in the" low" or" bush" veld of the Transvaal, and in the low-lying warmer parts of the eastern coastal belt, ex tending through the territories of Zululand, Natal, the Transkei, and the Eastern Province of Cape Colony.

Local Distribution. The disease is contracted only in the warm, low-lying" bush-veld, " and only in those parts with actual bush; the local distribution is necessarily dependent on that of the transmitting agent, the bont tick.

The disease is more prevalent, and appears to be more virulent, during the summer months.

Animals Affected. The disease particularly attacks sheep and goats, and in these animals its symptoms and lesions are most character. istically shown. Angora goats are highly susceptible, and merino sheep also contract the disease very severely; merino crosses with less susceptible breeds show more resistance.

The common Kaffir goat and the fat-tailed sheep are found in the Heartwater areas, and probably over a long course of time have become more or less immune (Theiler). It appears, however, that less resistance may be shown by such animals which have been bred for generations in districts quite free from Heart water, but they never show the great suscepti bility exhibited by the breeds previously men tioned. Persian sheep thrive in Heartwater areas, and, although not absolutely immune, are very resistant; in the great majority of cases they merely pass through a febrile reaction, during which the blood is infective for susceptible animals, either by direct inoculation or through the agency of ticks.

As regards cattle, Theiler records that the Transvaal Boers long ago recognized a bovine disease as identical with the Heartwater of small stock, but the connection was not so well recognized in some other areas. Edington first succeeded in inoculating the disease to cattle with virulent goat blood, and Lotmsbury trans mitted the disease to cattle with infected ticks, while Theiler (Annual Report, 1903-4) has given a full and very careful study of a number of cases, both experimental and naturally contracted, and has shown that Heartwater may be a very fatal disease of cattle.

All attempts to convey the disease to horses, both by inoculation and by infestation with infected ticks, have failed.

In all the susceptible species and breeds young animals contract the disease more easily and more severely than adults.

„Etiology. The disease is caused by an organism or virus which is present in the blood during an attack, as can readily be proved by subcutaneous (and still more certainly by intravenous) inoculation into a susceptible animal. The organism has not been demon strated microscopically, nor has it been possible up to the present time to cultivate it in vitro, and although in various experiments it has failed to pass through Berkefeld and Chamber land filters, the organism is presumed to be ultravisible. Inoculation of pericardial liquid may produce the disease, but it frequently fails to do so. The virus cannot be preserved by any known method, and blood drawn from a sick animal retains its virulence only for about forty eight hours. Edington claimed to have shown that Heartwater and African Horse-sickness were the same disease, or different phases of one disease, but this view was strongly com bated by Theiler and Stockman, who were able to bring forward very convincing reasons for not accepting such an hypothesis.

Natural Mode of Transmission.Lounsbury transmitted the disease to goats and sheep, and later to cattle, by means of infected" bont" ticks, and he failed to transmit the disease with any other species of tick. The results of his valuable work on this subject have been thoroughly confirmed, not only by Theiler and other experimental workers, but also by the facts observed in practice. Heartwater has never been identified in any case of disease contracted naturally in any area free from bont ticks, although affected flocks and herds have very frequently been moved from Heartwater districts into areas in which most, or practically all, of the other known South African ticks were abundant.

In view of the great importance of the tick in the spreading of the disease, and in connection with the problem of its suppression, a resume of the recorded facts relating to its habits and life-history may be given here.

Amblyomma hebrceum, the" bout, "" varie gated, " or" tortoise-shell" tick, is one of the largest South African ticks, and the adults are easily recognized, the male and unengorged female by the colours of the scutellum on the back (from which the popular name is derived), and the engorged female by the size attained, which considerably exceeds that of any South African tick other than Hyalomma cegyptium, the" bout - leg" tick. Amblyomma hebrceum is a" dropping off" or" three host" tick.

The engorged female drops from the host, and after a period which varies from about two weeks in the summer to even more than three months in the winter, lays thousands of eggs. According to the temperature prevailing, the eggs may hatch in from ten weeks (in summer) to about ten months, the average time being four to six months, and the hexapod larvae hatched out may shortly afterwards succeed in gaining a host; it has been shown, however, that they may live as long as seven months before feeding. Once on a host the larvae attach and engorge with blood, and they drop off again in most cases in about five to seven days; periods varying from four to twenty days have been recorded. The engorged larvae undergo the first moult on the ground, the time taken for this varying from about one up to four months, according to the season and temperature. The nymph which emerge are eight-legged, like the adults, but have not the scutellum with the mosaic of colours. They can exist without feeding for quite six months, and when a host is gained they remain on it for about four to eight days (periods up to twenty days have been noted), before dropping to the ground and undergoing the second moult, which may occur in from twenty-five to one hundred and sixty days. The adults which emerge can exist for a very long time, certainly over eight months, before feeding.; the sexes mate about five days after being placed on a host.

The female remains on the host from ten to twenty days, but the males may remain for months, and probably mate with several females. Finally the engorged female drops off, and the cycle recommences.

Under the most favourable conditions the complete life cycle requires a period of about nine months, and under unfavourable conditions it may take over a year, and even up to two years.

A. hebrceum is notable amongst South African ticks not only on account of the long period of time required for its complete life cycle, but also for its somewhat stringent requirements as regards natural conditions of climate and shelter. The bont tick requires more favourable conditions with respect to warmth and humidity than the other South African ticks, and it is not found in the high or middle veld, but only in the warm, low lying, generally more humid, " bush-veld." The adults are chiefly found on ruminants, selecting as a rule areas where the skin is softer, such as under the elbows, in the perineal region, and on the udder; the larvae and nymphw are also generally found on ruminants, and attach anywhere.

The bout tick has been found on horses, cattle, sheep, goats, dogs, the" wild dog, " antelopes, and the ostrich, and it may even attack man. Apart from its role in transmitting Heartwater, it may directly cause local troubles, such as suppurating sores on the udder, and suppurating lesions on the teats leading to constriction of the ducts and sloughing, and it may also cause lameness when attached near the feet in sheep and goats (such lameness is very common in many parts of South Africa from infestation with the bout - leg tick, Hyalomma cegyptium).

Lounsbury showed that when a female bout tick engorges on a sick animal, the Heartwater infection does not pass through the eggs, and the larvae hatched out do not transmit the disease. The disease is conveyed by: (a) nymphw which fed on virulent blood during the previous larval stage; (b) adults which fed on virulent blood during the previous nymphal stage; and (c) adults which as nymph fed on non-infected and insusceptible animals, but derived from larvae fed on an infected animal. Apparently the infection acquired by larvae persists to the adult stage only in a proportion of the ticks, but the fact of this possibility has some practical importance in connection with the prophylactic measures adopted. Lounsbury also proved that the infection is ordinarily transmitted within twenty-four hours of the tick's attachment, and that infected nymph and adults transmitted the disease whether they were fed soon after moulting or after a lapse of six months or longer. He failed in a number of attempts to infect ticks by placing them on recovered animals, and this agrees with the negative results of the inoculation of the blood of such animals, obtained by Theiler, Spreull, etc. Apparently the blood may cease to be infective as early as the twentieth day after inoculation; it generally fails to produce a reaction after the lapse of about four to five weeks.

The recovered animal is therefore not a" reservoir" for the virus.

Period of Incubating. In sheep and goats the natural period of incubation, after tick infestation, is generally about fifteen days, but it varies from eleven to eighteen days, and even a period of twenty-three days has been recorded. In cattle the period is about twenty to twenty five days. After the inoculation of virulent blood, the period varies from five to fifteen days, or even longer; the shorter periods are obtained with intravenous inoculation and the use of larger doses of infective material, but the virulence of the particular strain and the degree of suscepti bility of the experimental subject affect the result also.

Symptoms. 1. In Goats and Sheep. The first change is a rise of temperature, and this may continue until 106° F., or even 108°, is attained; the fever generally shows nocturnal remissions, particularly during the first four days, and ultimately it usually drops quite suddenly to normal or subnormal, generally shortly before death, but sometimes on the preceding day (or even longer in the less common subacute type of case). In the cases which recover the fever is usually not quite so high, nor so pro longed, but that is not always the case.

In stabled (experimental) animals the first visible symptoms may be dulness, prostration, and lack of appetite, but under natural condi tions one will probably notice first a tendency to isolation from the rest of the flock when grazing, and to lag behind when the flock is driven. In mild cases there may be no symp toms other than a rise of temperature, and peracute cases occur in which the animals die so quickly that no symptoms develop (other than the rise of temperature); such animals are often found dead on the veld or in the kraal.

In other cases the animal shows dulness, disinclination to move, loss of appetite, cessation of rumination, and, when driven, quickened respiration and a tendency to lie down. Many animals show striking nervous symptoms, which may develop quite early, or only shortly before death; the animal may bleat repeatedly, and a very common symptom is a constant move ment of the jaws as in mastication, while the tongue is repeatedly projected forwards and withdrawn. The animal in some cases is seen to lick the ground; the eyes may squint, and salivation may be observed, foamy saliva appearing about the lips. The animal may turn in circles, and finally drops to the ground, generally lying on the side; the head is gener ally extended forward and upwards, and at times there may be marked opisthotonos. As the attack progresses, there may be localized muscular tremors, or more widespread and violent muscular spasms, and Spreull records a characteristic" galloping" action of the legs.

The nervous symptoms may persist with little change for some considerable time, or they may occur in fits which commence and end quite suddenly; in the latter case an attack may often be excited by any interference or stimulation, and interference during an attack will generally increase the severity of the manifestations. Either constipation or diarrhoea may occur, but in the majority of cases the bowels act normally; during a ner vous attack with muscular spasms, however, normal faeces and urine may be repeatedly passed in small quantities. Death usually follows fairly rapidly, especially if more than one nervous attack has occurred, and it is quite exceptional for an animal to recover after having shown the nervous symptoms at all clearly. Death may be preceded by general convulsions, but in other cases the animal passes into a state of coma.

The disease generally runs its course in from two to six days after the first rise of temperature, but in some cases it is considerably prolonged.

2. In Cattle. The symptoms shown by cattle are very similar indeed to those noted in goats and sheep, and they were well described by Theiler in a study of. both natural and experi mental cases, published in his Annual Report for 1903-4.

As in small stock, there are mild cases in which a temperature reaction may be noted, but in which ordinary clinical symptoms are absent or quite insignificant. Other cases show dulness, prostration, impairment or loss of appetite, cessation of rumination, and other general symptoms. Many cases, however, show striking nervous symptoms, including the peculiar movements of the jaws and tongue already noted in connection with sheep and goats. Other symptoms which may be observed are muscular tremors, nodding of the head, a peculiar blinking of the eyelids, and a glaring eye with injection of the sclerotic vessels. The animal may wander in circles, or if tied to a post may repeatedly encircle it, and it may push with the head into objects; finally it goes down. There may be localized or more or less general muscular tremors, and in the later stages stronger muscular spasms; the legs may move with an action as if trotting. Attacks may be initiated or markedly increased by even slight stimuli, such as a light and touch. In some instances the animal bellows repeatedly, and Theiler records cases of biting of the ground, and even of repeatedly biting the leg on which the head was resting. Finally coma supervenes.

The duration of the disease from the first appearance of fever is usually about six days, but the period of visible illness in fatal cases is generally only a couple of days, or less.

As in small stock, peracute cases occur in which death supervenes so rapidly that no clinical symptoms are noticed; under natural conditions such animals are quite unexpectedly found dead in the veld.

Mortality. This varies considerably, and it is impossible to give any precise figures which would be applicable to all cases. In the first place, the different species attacked show variations in susceptibility according to breed and age, as well as considerable individual variations in resistance, and a greater degree of resistance is likely to be found amongst animals which have been bred in or near Heartwater areas. Moreover, there are, without doubt, strains of varying degrees of virulence naturally

occurring in different localities, and in all cases the mortality (as well as the incidence) is likely to be heavier in summer than in other seasons.

In goats and sheep many experiments have resulted in a mortality of 100 per cent, but under natural conditions the mortality is usually very considerably lower, and about 60 per cent would be a more probable figure; Spreull gives figures of 50 per cent for merino sheep and 80 per cent for Angora goats.

With respect to cattle, the published experi mental data show wide variations, most prob ably due partly to the use of different strains, and partly to the accidental selection in some cases of animals already possessing a considerable degree of resistance. Thus Lounsbury only obtained two deaths after inoculating five calves, and no deaths at all in three adult cattle, whereas Theiler, working in the Transvaal, killed six out of seven inoculated animals ranging from eighteen months to two years in age, and later, after inoculating four three-year-old oxen, he observed fatal reactions in three of them. Theiler's results, and the facts observed in practice, particularly in the Transvaal low veld, show that Heartwater may be a very fatal disease for cattle, and newly imported stock show the greatest susceptibility.

Lesions observed post inortein. On reflecting the skin it is often observed that the vessels are deeply injected; some subcutaneous cedema may occur. Rigor mortis and coagulation of the blood usually take place normally. The characteristic" Heartwater" is generally seen in sheep and goats, especially in natural cases (but it is very frequently absent in cattle). The amount of liquid in the pericardial cavity may show only a slight increase over the normal, or the pericardium may be greatly distended with liquid. In a perfectly fresh carcase the liquid is practically always clear and light yellow in colour, but if the autopsy is made later it may show a reddish tinge, and exceptionally it may have coagulated in the unopened pericardial sac. In any case the liquid generally coagulates well after the peri cardium is opened, or if it is collected in a vessel. Similar exudates may or may not occur in the other serous cavities, particularly the pleural cavity. A case with a good" Heartwater" lesion usually shows some pleural and peritoneal exudate, but this is not necessarily the case.

The lungs may appear normal, but in acute cases generally show some degree of cedema, with the presence of froth in the bronchi, and even in the trachea. The epicarclium may show petechiw, and the endocardium, especially in the left ventricle, often shows numerous petechi, and even extensive ecchymoses. The spleen is generally only slightly enlarged, with pulp of softer consistence than normal. The liver may appear to be normal, but is usually congested, and the kidneys also sometimes show congestion. The bladder and urine show no obvious change.

The abomasum may show little, but there is often some degree of gastritis; the mucous membrane may be thickened and oedematous, and it may show diffuse hyperaemia, numerous scattered petechiw, or exceptionally hmorrhagic patches and streaks. The small intestine may also show little or nothing abnormal to the naked eye, but it usually shows some inflam matory changes which may affect the whole length of the bowel or only certain parts. The mucosa is generally thickened, and may ex hibit changes varying from a diffuse reddening or mere injection of vascular" arborizations, " to the presence of collections of small petechi, or less commonly, the occurrence of well marked patches or streaks of congestion, or even hmorrhagic patches. Such changes, when present, are generally better marked in the ileum, in its posterior portion, and in this situation Theiler has even seen necrosis of the mucous membrane. Peyer's patches show no abnormality. In the large intestine one may fmd changes similar to those seen in the small bowel, but they are often absent.

Some differences are to be seen between the naked-eye lesions observed in cattle and those which occur in sheep and goats. The typical" Heartwater" is generally observed in natural cases in sheep and goats, although it is not quite constant. It is less constantly found in sheep or goats inoculated with virulent bovine blood, and it is quite frequently absent in cattle, although in a minority of cases it constitutes a striking feature. The gastro-enteritis tends to he more severe in cattle, and Theiler has remarked that in certain cases the enteritis seen may even recall the lesions occurring in Rinderpest, apart from the fact that Peyer's patches are not involved in the former case.

No characteristic microscopic lesions have been described; the blood in an uncomplicated case shows no abnormality.

Diagnosis. (a) In all cases one is naturally guided to some extent by considerations of locality and season, and by the history of the outbreak. Thus one could safely eliminate Heartwater in connection with any outbreak occurring on the high veld, unless the history showed that the affected animals had been brought from low bush-veld within the usual incubation period of that disease.

(b) The clinical symptoms may be almost diagnostic, when the nervous symptoms are well developed, and the association of such symptoms with a high temperature is of some significance, although individual cases might be seen only in the later stages, when the tempera ture had fallen to normal or sub-normal. On the other hand, in many cases no positive diagnosis could be made on clinical grounds, particularly in those cases in which death occurs before any symptoms have been noted.

Apart from the nervous symptoms, which may not be present or at all well marked, the symptoms shown are common to a number of febrile diseases which are quite likely to occur in areas liable to Heartwater, and no safe diagnosis can be based on their presence.

Certain other specific febrile diseases occurring in South Africa are apt to give rise to nervous symptoms in cattle; for instance some excite ment, bellowing, and an inclination to charge any human being or animal approaching, may be seen in an acute case of Redwater, but these symptoms are not quite the same as those seen in a typical Heartwater case. Similar symptoms of excitement are seen in some cases (loosely termed" Gall Sickness") in which the temperature is little, if at all raised, and in which all the well-known specific diseases can be excluded; such cases are presumed to be due to the ingestion of some poisonous plant, and, as in acute Redwater, the localized nervous symptoms seen in Heartwater (such as the peculiar movements of jaws, tongue, and eyelids) are not shown. There are certain outbreaks, however, which are also believed to be due to some form of vegetable poisoning, and in which nervous symptoms more closely resembling those of Heartwater are prominent, and most of the nervous manifestations mentioned might be exhibited in some cases of Snake-bite (these would, however, be only sporadic cases).

In most of these cases there are features which would at least help to differentiate them from Heartwater cases, and this applies par ticularly to the temperature.

(c) The post-mortem lesions may also be very typical, and if combined with a characteristic clinical history, will then leave very little room for doubt. In sheep and goats a really typical post-mortem picture is not likely to be found in any known condition other than Heartwater, but in atypical cases the lesions present are very similar to those which may be encountered in a variety of other conditions, and particularly in conditions of as yet undetermined origin, believed in many cases to be associated with the ingestion of poisonous plants.

A specific tick-transmitted Gastro-enteritis of sheep described by Montgomery in East Africa should also be remembered in this connection.

There should, of course, be no difficulty in differentiating cases of debility due to parasitic invasion, with poor condition, anemia, the ab sence of acute lesions, and the presence in the serous cavities of thin, usually incoagulable transudates.

In cattle the post-mortem lesions found are very frequently atypical, and most or all of the lesions are similar to those encountered in other more or less common conditions. Thus similar lesions may be found in acute cases of Redwater and in East Coast Fever, although these conditions are likely to show other lesions not found in Heartwater (such as the usually greatly enlarged spleen in Redwater), and can, of course, be differentiated microscopically. Such lesions are also found in some peracute cases of Lamziekte, and in cases which have at present to be ascribed to vegetable poisoning.

(d) No microscopic diagnosis can be made. As previously stated, Heartwater produces no recognizable blood changes, but it should be remarked that most of the cattle which contract the disease are already immune to several other South African diseases, and harbour various protozoa which are liable to multiply during Heartwater reactions, and therefore to be detected in blood smears. This is particularly the case with P- mutans, which can very frequently be found in practice, and P bigeminum, Anaplasma marginale, Spirochota theileri, and Trypanosoma theileri have all been noted in similar cases (see Theiler, Annual Report, G.V.B. Transvaal, 1908-9, page 59).

(e) In all cases of doubt, when it is considered important to ensure an accurate diagnosis, recourse may be had to the inoculation, with the blood of sick animals, of goats or sheep, kept under close observation, and under conditions excluding the possibility of the accidental contraction of Heartwater, or of any probable disease liable to be confused with it. As previously stated, goats or sheep inoculated with virulent bovine blood may fail to show the most characteristic lesions, but if the virus is passed from sheep to sheep (or goat) for a few generations, the typical picture is soon obtained.

Those particularly interested in this question should refer to the account given by Theiler (Annual Report, 1903-4) of the inoculations and sub-inoculations made by him in order to prove that a certain Ox 448 suffered from naturally contracted Heartwater. The example m question shows clearly how a number of inoculations may have to be made in order to overcome the difficulties arising from the pro duction of further atypical reactions and lesions m susceptible animals, and from the accidental selection as experimental subjects of animals possessing a considerable degree of resistance.

Preventive Measures. The most important of the prophylactic measures needed are naturally those directed towards the eradication of the bont tick, and of all such measures constant short-interval dipping of cattle is the most successful. Reference to the facts established in connection with the life-history of the bont tick will show that to be sure of catching every tick that may infest the animals at any time, the cattle should be dipped at least every four days. In practice five-day dipping will catch the great majority of the ticks which are picked up, and this interval is now the one most commonly adopted in Natal and the Transvaal (where the dipping is primarily directed against the ticks which transmit East Coast Fever).

Fortnightly dipping will naturally kill many ticks, and will ultimately reduce their numbers to insignificant proportions, but five-day dipping is much to be preferred, and is far more certain and very much more rapid in its effect.

The dipping fluid should have sodium arsenite as its basis, and the" Laboratory dip" of Watkins - Pitchford, or a solution of arsenite of soda of corresponding strength, is to be recommended, the strength varying according to the dipping interval chosen (see" East Coast Fever").

Such dipping has proved very successful in many areas in reducing the number of ticks of all species, and in reducing the incidence of Heartwater to quite insignificant proportions.

In a badly infested bush-veld area, however, it cannot be expected that complete eradication of the bont tick will be attained within a few months or a year. The comparatively very long periods during which the tick in its various stages may live before gaining a host, and the long periods necessary for the laying and hatch ing of the eggs, render it possible for the eradica tion to be long delayed. Another important fact is that an infected larva may, as a nymph, feed on some insusceptible (and undipped) host, such as a dog or wild antelope, and transmit the infection in its adult stage. In view of the long periods which may be passed in the various stages before the adult is developed, and of the long survival period of the adult, it is obvious that some cases of Heartwater would still be liable to crop up even eighteen months or more after the institution of regular five-day dipping of all cattle, and the presence on the same veld of any susceptible goats and sheep (which, of course, are not subjected to short-interval dipping) would naturally extend the period during which such cases were liable to occur.

The above-mentioned facts do not merely indicate theoretical possibilities, but explain what has actually been found to occur in a number of instances in recent years, during which many low-veld areas, previously un settled, have been laid out in farms or taken up in larger blocks for ranching.

Another prophylactic measure is the wide spread and old-established practice of grass burning; from some points of view this practice is quite harmful, but under certain conditions it appears to be necessary at the present time, and without doubt many ticks may be destroyed in this way, the number depending largely on the season. Only those ticks which are actually on the grass are likely to be destroyed, and during the winter and early spring comparatively few ticks are to be found there. The later in the season the burning is done, the more tick destruction is likely to be effected, but in most cases the farmer is guided, in his selection of the season, more by considerations relating to the provision of fresh grazing.

It would also be possible to starve out the ticks, provided that a limited and enclosed piece of veld could be kept entirely free from any of the hosts of the bont tick, but it would not be easy in practice to exclude all possible hosts for a sufficient length of time (a minimum of fourteen to fifteen months).

Such methods of tick eradication as ploughing and cultivation, and dressing the land with various substances, have obviously an extremely limited application in the areas likely to be affected with Heartwater.

The oldest and simplest method of checking an outbreak is to move the animals, and when it is possible to move the flock or herd at once to higher ground, where the bont tick cannot multiply or thrive, no further cases are con tracted. Where it is impossible to move to high veld, and it is desired to move on to ground believed to be more or less free from Heartwater, although infested with the bont tick or liable to become so, recourse may be had to the " quarantine camp" method largely used in connection with East Coast Fever. In this case two successive moves would be required, the animals being kept for three to four weeks in each of the two places, and all affected animals picked out and returned to the original ground (where the rapid termination did not render this unnecessary).

No certain method of

artificial immunization has yet been worked out and applied on a large scale, and the success which has been obtained with dipping has rendered such a process less urgently required than was formerly the case, although it would be by no means unwelcome or unnecessary. Spreull obtained some success in a number of experiments in which the blood of recovering cases was used as a vaceine, although he found the immunity produced in many cases to be insufficient to protect against natural infection; later he injected small doses of virulent blood subcutaneously.

Theiler showed that it was possible, by hyper immunization of recovered cattle, sheep, and goats with virulent blood, to produce a pro tective serum, but he did not continue the experiments to the point of determining all the details necessary for an exact process of immunization suitable for application in the field. His work indicates that such a method is quite feasible, but our inability at present to preserve the virus in vitro for longer than forty-eight hours is a practical difficulty, and one would have to take into consideration the natural occurrence of strains with varying degrees of virulence.

Curative Treatment. No drug has been found to give any promising or useful results with this disease. In cases which are not too acute, good nursing will increase the chance of recovery, and such measures as isolation, placing the animal in a cool shady spot, and the provision of green food are indicated. W. H. A.

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