The very great pathogenic and economic importance of the nematode parasites of sheep is obvious to all who have had contact with the sheep-raising industry. Losses due to parasitic diseases must be relatively enormous, but for a number of reasons an exact estimate is practically impossible. Parasitic diseases caused by nematodes are insidious, and rarely spectacular, but constitute a constant drain on the vitality and productiveness of the host animal. It is impossible in the time available to deal with all the nematode parasites of sheep. It is intended, therefore, to discuss control measures in general. Special measures will be discussed in relation only to three of the parasites. Haemonchus contortus, Trichostrongylus spp. and Oesophagostomum columbianum, since it is considered that these are the most important of the nematodes which infest sheep in Australia.
(a) Haemonchus contortus is an important cause of disease in sheep for the following reasons:
(i) The female produces relatively enormous numbers of eggs, about 5,000 every 24 hours, so that a very heavy degree of contamination of pastures can be built up in a relatively short period of time. Even a lightly infested sheep will heavily contaminate pastures. e.g., a sheep harbouring 300 females of the species may disseminate 1,500,000 eggs every 24 hours.
(ii) Haemonchosis is often of sudden onset, and deaths may also occur suddenly. Experimentally, sheep have died within three weeks of receiving a massive dose of infective larvae, having been actually bled to death by the developing worms. In the field, sudden outbreaks have been observed, with deaths within four or five weeks of contracting the infection.
(iii) There are no obvious symptoms unless sheep are handled or driven about in paddocks. Loss of condition is not seen in acute haemonchosis, and sheep in prime condition may die. Close examination of sheep will reveal anaemia, and driving them with dogs will show a lack of stamina that casual observation would not suggest.
(iv) A high plane of nutrition is not very effective in preventing haemonchosis c.f. trichostrongylosis. Outbreaks of haemonchosis commonly occur at times of the year when the plane of nutrition is high, and even when it is rising, e.g. spring outbreaks on Northern Tablelands of N.S.W.
(v) Haemonchosis frequently affects sheep of all ages in the flock, c.f. Trichostrongylosis which is seldom serious in sheep older than 18-24 months.
The great pathogenic significance of H. contortus should be more than offset by the high degree of efficiency of certain anthelmintics against this parasite. Control should therefore be a relatively easy procedure of which anthelmintic medication is the most important aspect. Unfortunately, however, treatment is frequently delayed until sheep are already heavily infested and pastures grossly contaminated.
(b) Trichostrongylus spp. are important nematode of sheep for a number of reasons:
(i) Trichostrongylosis is usually insidious in onset and is frequently overlooked and confused with inadequate nutrition and "seasonal conditions." Actually, this disease usually occurs when the plane of nutrition is low, thus adding to the confusion. Failure of young sheep to thrive is probably very often associated with relatively minor degrees of trichostrongylosis, but is seldom ascribed to this cause.
(ii) The eggs and larvae of Trichostrongylus spp. on pastures are very resistant to climatic conditions, remaining viable under conditions of dryness and cold which are sufficient to destroy the free-living stages of most nematode parasites of sheep.
(iii) Treatment with anthelmintics is not highly efficient against Trichostrongylus spp. Even the now widely used mixture of copper sulphate and nicotine sulphate is not highly efficient. One must not expect individual treatments to remove more than 70-75% of these parasites and one must expect treatment to fail completely in certain individuals in which the drugs fail to be swallowed direct into the abomasum. The number of cases in which the oesophageal reflex fails is very variable, but probably avenges from 10-20%.
(iv) The effects of trichostrongylosis are often semi-permanent, that is, the heavily infested weaner does not thrive, and this check to growth and development is frequently reflected in later life by stunted growth and poor development.
(v) The fact that trichostrongylosis often occurs during periods when the plane of nutrition is low renders control very difficult. In northern N.S.W. outbreaks frequently occur during the winter months when natural pastures are at their lowest productive capacity, and unless supplementary feeding can be quickly adopted, losses are severe.
(c) Oesophagostomum columbianum is an extremely important nematode parasite of sheep for the following reasons:
(i) It may, and frequently does, produce pathogenic effects throughout the life of the sheep. The adult worms severely damage the mucosa of the colon, while the nodules produced by the larval stages severely interfere with movements of both large and small intestines.
(ii) With massive infestations oesophagostomiasis is rapidly fatal, deaths frequently resulting from ulcerative colitis. Chronic cases continue for many months, and are accompanied by very serious interference with growth, development and productivity.
(iii) Nodules may persist throughout the life of the sheep, and in certain districts inability to fatten old sheep appears to be closely associated with the presence of numerous nodules in the large intestine.
(iv) In oesopitagostomiasis the existence of resistance on the part of the sheep does not appear to be an advantage. In the young or non-resistant sheep the larvae remain in the bowel wall for short periods only and usually do not give rise to nodule formation. In older, resistant sheep, however, the larvae remain in the mucosa for long periods, up to three months, frequently wander in the sub-mucosa and give rise to extensive nodule formation. In such sheep it appears that most of the larvae fail to return to the lumen of the colon, and one finds gross nodule formation associated with few or even no adult worms. Each year the sheep acquires further infections, and the resistance mechanism results in nodule formation becoming yearly more extensive.
(v) Antheimintic treatment by mouth is very unsatisfactory. Treatment with enemas is very effective, but is slow and tedious, and unless carefully and patiently applied may give unsatisfactory results and even cause a small percentage of losses.
(vi) Damage to the intestines produced by nodules prevents their use for the manufacture of sausage casings or "gut." Losses of this type are more considerable than is generally realised, and have called forth complaints from a number of manufacturers.
(d) Other Nematode Parasites of Sheep.
Of the remaining nematode parasites of sheep in Australia, probably Chabertia ovina and Dictyocaulus filaria are the most important. Ostertagia spp. are undoubtedly of serious pathogenic importance when present in large numbers. The exact importance of Nematodirus spp. and Cooperia spp. has not been determined, but there are recent observations from USA, which suggest that even in light infestations, Cooperia spp. may result in failure of the sheep to utilise food materials efficiently.
Large numbers of Trichuris ovis produce obvious lesions in the caecum, but this parasite has not been incriminated as a cause of death. Oesophagostomum venulosum is seldom met with in large numbers, and does not appear to be of much importance. Strongyloides papillosus does not appear to be of importance, though massive experimental infestations in young lambs have shown definite pathogenic effects. Mullerius capillaris produces obvious lesions in the lungs, but has not been incriminated as a serious pathogen.
It seems very unlikely that nematode parasites will ever be eradicated. Moreover, it is quite apparent that the diseases produced by such parasites are rapidly increasing in importance, and are liable to increase even more rapidly as animal production becomes more and more intensive. Improved pastures and high rates of stocking are necessarily associated, and while the improvement in the plane of nutrition tends to offset the increased worm burdens acquired under conditions of intensive stocking, there must be a balance somewhere between nutrition and the worm burden which can be tolerated. This balance can only be preserved by correct management procedures. e.g. spelling, rotation, mowing, etc., and at present our knowledge of such procedure is sadly deficient. Further, it has already been pointed out that outbreaks of haemonchosis may occur under conditions where the plane of nutrition is high. Again, a high plane of nutrition will not protect sheep from the ravages of the larvae of Oesophagostomum columbianum in producing nodules in the bowel wall.
In a report prepared for the International Committee on the Control of Parasitic Diseases, the late Dr. Maurice Hall, of U.S.A. stated "that it is assumed that parasites must be controlled rather than that we must accept and adjust to parasitism, and that parasites in general are harmful and not either harmless or beneficial." With this statement we must agree, and we must therefore devise control measures for all parasites—not overlooking those whose exact pathogenic importance has not yet been determined, Finally, an ageing soil and deterioration of natural pastures leading to imperfect nutrition and development of the sheep, favour an increase in nematode infestations.
Satisfactory control depends upon:
(i) Prompt, early, correct diagnosis.
(ii) Immediate treatment.
(iii) Immediate removal from sources of re-infestation.
(iv) Extensive knowledge of the epidemiology of diseases produced by different parasites, and by the same parasite in different districts.
(v) Improved extension service so that each stockowner receives detailed and definite recommendations for control measures on his own property, such recommendations depending upon a thorough inspection of the property and stock and thorough enquiry into methods of management.
Control includes two chief activities, treatment and prophylaxis.
Treatment is necessary—
(a) To save the host animal.
(b) To reduce the output of pre-infective material and so reduce contamination of pastures.
In relation to treatment, two questions arise:
(a) When to treat.
(b) What anthelmintic to use.
When to treat depends upon a knowledge of epidemiology, which in turn is based on the nature of the climate, pastures, management, and class and age of sheep concerned.
The selection of an anthelmintic depends on many factors, but a discussion of details of these factors, important though they may be, is not possible in the time available. They may be summarised as follows:
(1) Efficiency against the species concerned—efficiency against both immature and mature parasites.
(2) Safety for the sheep.
(4) Ease of administration, handling, storage, transport, etc.
Prophylactic measures may be designed to achieve one or both of two objectives:
(i) To prevent infection.
(ii) Prevent development of disease although infection has taken place.
Preventive measures are therefore directed along five chief lines:
(1) Reduction of the output of eggs or larvae.
(2) Reduction of the intake of infective material.
(3) Anticipation of outbreaks of parasitic disease and immediate adoption of control measures.
(4) Rendering and maintaining the host animal as insusceptible as possible; that is to say, insusceptible both to infection and to the effects of infection (infestation)
(5) Protection of certain animals.
Let us now consider in a general way the methods which may be adopted to further these aims.
(1) Reduction of the Output of Eggs or Larvae.
(a) By regular treatment with efficient anthelmintics.
(b) By elimination of "carriers" by treatment, segregation, disposal or slaughter.
(c) By special attention to ewes so that the environment of the lamb will not be heavily contaminated.
(d) By treatment during periods when little or no fresh infection will be acquired.
(2) Reduction of the Intake of Infective Material.
(a) By reduction of the output of pre-infective material through measures outlined above.
(b) By exercising methods of management designed to avoid heavy contamination of grazing areas:
(i) Rotation and spelling.
(ii) Avoid overstocking and overgrazing.
(iii) When feeding off crops, do not permit sheep to regraze contaminated areas; keep them moving forward over fresh areas.
(iv) Subdivision of pastures to ensure uniform grazing over the whole area and to avoid overgrazing (and hence overcrowding) of special areas which will develop into "danger zones" for infection.
(v) Avoid patchy distribution of pasture plants, controlling areas not grazed by sheep by grazing with other animals, mowing, burning, etc. Patchy distribution leads to overgrazing of certain areas and development of "danger zones" for infection.
(c) By provision of adequate pasture, nutrition and water so that sheep are not forced to graze or drink in a dangerous manner. Sheep should not have to:
(i) Graze for too great a length of time — the longer the grazing time the greater the intake of infective material.
(ii) Graze too close to the ground — the greater amount of parasitic material lies close to the ground. The sheep is naturally a close grazing animal, and in sheep husbandry it is desirable to maintain pasture growth at a relatively short level. Overstocking and overgrazing usually result in sheep eating plants down to ground level, and it is in such instances that the danger of infection is increased.
(iii) Graze in damp, marshy areas.
(iv) Drink from shallow, stagnant pools.
(d) By destruction of parasitic material in faeces or on pastures:
(i) Application of parasitical materials. e.g, calcium cyanamide. Further work is necessary before the value of such methods can be assessed.
(ii) Burning. It is doubtful whether burning pastures actually destroys much parasitic material, for the reason that situations where such material is likely to accumulate are usually damp and would not burn. Burning, however, may destroy protective cover and permit destruction of parasitic material by climatic agencies, and by destroying dead growth of grass and encouraging new growth attracts animals to graze on burnt areas, thus relieving overcrowding on areas which would not burn. In addition, burnt areas soon provide fresh pasture growth, which improves the general plane of nutrition.
(iii) Allowing natural climatic agencies to destroy parasitic material through the effects of sunshine, heat, cold, frost on temporarily unstocked areas. Areas need not be completely unstocked. e.g. horses can be used on sheep pastures, and vice versa. In controlling certain parasites of sheep, cattle may be used while a paddock is being spelled from sheep. It should be noted that cattle do not become infected with Oesophagostomum columbianum, but do become infected with Haemonchus contortus and Trichostrongylus app.
(e) By permitting stock to graze only at times when a minimum of infective material is available to them. Under natural conditions in Australia, where large areas and flocks are predominant, this method of control is generally impracticable; but on smaller areas and particularly when improved pastures, grazing crops or irrigated pastures are concerned, this aspect is worth consideration. Dangerous periods for grazing are considered to be early morning and night while dew is on the grass, and after light misty showers and during dull humid weather when dew or rain persists on pasture or crop.
(3) Anticipation of Outbreaks of Parasitic Disease and Immediate Adoption of Control Measures.
The effective application of this measure of control necessitates a much greater knowledge of epidemiology than that which now exists. However, there are certain general relationships which are known. e.g. haemonchosis occurs in spring, summer and autumn; trichostrongylosis in autumn, winter and spring; oesophagostomiasis in late summer, autumn and winter. An important consideration is that the outbreak of disease may not coincide with the period during which the infection was acquired. In all cases there is a lag of at least three or four weeks during which the parasites are developing to maturity. In haemonchosis the outbreak commonly follows closely the period of acquisition of infection. In trichostrongylosis and particularly in oesophagostomiasis, the outbreak is frequently separated by a long period, even two or three months, from the period of acquisition of infection.
This aspect of control may be summarised under the following:
(a) Correlation of climatic conditions with survival, accumulation, distribution, aggregation and accessibility of infective material, i.e. ecological aspects of epidemiology.
(b) Correlation of management procedures with conditions liable to result in parasitic disease, i.e. management aspects of epidemiology.
(c) Correlation of certain special occurrences and practices with conditions liable to result in outbreaks.
(i) Travelling stock.
(ii) Agistment stock going to or returning from contaminated pastures.
(iii) Use of "hospital" paddocks.
(iv) Occurrence of abnormal seasons.
(v)Transmission of parasites to sheep from other animals, e.g., cattle and rabbits.
(4) Rendering and Maintaining the Host Animal as Insusceptible as Possible. [That is to say, insusceptible both to infection and to the effects of infection (i.e. infestation)].
Insusceptibility to infection implies the existence of immunity; insusceptibility to infestation implies the existence of resistance. With our present understanding of the immunity-resistance phenomena of nematode parasitism we cannot initiate immunity, but we can establish and maintain resistance. This we aim to achieve by:
(a) Maintenance of as high a plane of general nutrition as possible, particularly for sheep of susceptible ages or in susceptible states (young sheep, breeding ewes).
(b) Provision of special food supplements to counteract special effects in infestation, e.g. iron, copper and cobalt in anaemia in districts where these minerals are deficient in the diet.
(c) Control of other diseases e.g. pink-eye, and condition which lower the general resistance, e.g., pink-eye, grass seed infestation, scabby mouth.
(5) Protection of Certain Animals.
Sheep which are specially susceptible to parasitic diseases are young animals up to the age of about 18-24 months, breeding ewes and very old sheep. Rams, if overworked and underfed, are also liable to become highly susceptible.
(a) Young Sheep. Lambs running with their dams seldom manifest parasitic disease, probably because their diet is ideal and they do not spend long periods in grazing. Nevertheless, observations in the field suggest that such lambs frequently acquire heavy worm burdens before weaning, and exhibit disease only after weaning when there has been a sudden change in nutrition. The source of infection is the ewe, the pregnant and lactating ewe is also particularly susceptible to infection. Regular treatment of ewes during pregnancy and lactation is therefore the first essential in protecting the lamb. Such treatment also protects the ewe. After weaning, sources of infection are the weaners themselves or infested sheep which have grazed paddocks into which weaners may be moved. Rotational grazing and spelling as well as regular treatment are necessary to protect the weaner. Young sheep should never be moved into paddocks known to have been recently carrying heavily infested sheep or cattle. Periods during which temporary teeth are being lost and permanent teeth are erupting are times when young sheep may become highly susceptible to infection. Treatment and provision of adequate nutritional conditions are necessary at such periods (12-18 months when the permanent central incisors are erupting, and 18-24 months when the first, second and third molars are replaced by permanent teeth and the sixth permanent molars are cut).
(b) Breeding Ewes. Young ewes and very old ewes are apt to become heavily infested during pregnancy and lactation, and even though they may not suffer severely themselves, they serve to contaminate pasture for their lambs. Regular treatment and provision of adequate nutrition are necessary for these ewes.
(c) Very Old Sheep. These sheep appear to lose the age resistance usually seen in adult animals. This loss of resistance may be associated with imperfect nutrition following loss of and wearing of teeth. Regular treatment and suitable, as well as adequate, feed are necessary to protect these sheep.
(d) Sheep from Districts where Parasitic Diseases are not Endemic.
These animals may not have become infected and so developed a degree of resistance in their youth. When introduced into areas where severe nematode infestation is endemic, such sheep, irrespective of their age, are liable to become heavily infested Regular treatments will protect them.
The control measures already discussed have been of a general nature. An understanding of them is essential to anyone having to deal with the control of diseases produced by nematode parasites. General recommendations of this nature are not of much value to the man who is vitally concerned with control, that is to the grazier. General information is of little value to him; he requires detailed specific recommendations, such recommendations having due regard to specific parasites, specific districts, and even specific properties. It is often difficult to make definite recommendations; but as far as possible, and as far as our present limited knowledge of epidemiology will allow, one should always attempt to do so.
It is often necessary to differentiate two chief kinds of worm infestation:
(a) Acquired rapidly by intake of large amounts of infective material during a short period of time, usually from accumulations of infective material resulting from:
(i) Heavily stocked areas not spelled long enough before introducing fresh stock.
(ii) Material accumulated in dry weather and not becoming infective until rain falls
(iii) Material aggregated by being washed by rain water, flooding, etc., or collected by other agencies.
This kind of infestation leads to acute disease with sudden onset and appearance of clinical symptoms and death after a short time.
(b) Acquired slowly by intake of small amounts of infective material during a more or less extended period, usually by auto-infection through grazing over the same area for a long period. This kind of infestation leads to chronic disease with insidious onset, which is often confused with and associated with, dietetic factors.
Haemonchosis is a disease of late spring, summer and early autumn. It is most severe in areas receiving a predominantly summer rainfall. In Australia it is most severe in Central Queensland, Darling Downs (Qld), northern tablelands and north-western slopes of N.S.W. Severe outbreaks do occur, however, on central and southern tablelands and central and south-western slopes of N.S.W. and in Victoria. Occasionally severe outbreaks have been recorded on the north-western plains of N.S.W. following a sequence of "wet"" summers.
(1) Annual Plan of Control (Referring chiefly to areas having a regularly high summer rainfall).
(a) Ewes to Lamb in Spring.
These sheep should be treated at least twice during the last two months of pregnancy. These treatments will protect the ewes themselves as well as reducing the amount of infective material available for the lambs. Alter lambing, the ewes should be treated at regular three to four weekly intervals until the lambs are weaned. The length of the interval depends upon climatic conditions; chiefly the nature and amount of rainfall. If the summer is very dry, the interval may be further extended. For these treatments the copper sulphate-nicotine sulphate mixture should be used.
(b) Lambs born in Spring. Treatment should begin not later than weaning time. In a "wet" summer, with weather conditions specially favourable for development of haemonchosis, it may be necessary to treat lambs once or even twice before weaning. For these treatments, the copper sulphate-nicotine sulphate mixture should be used. Lambs should be weaned into paddocks which have not been stocked with sheep for at least one month.
(c) All Young Sheep (Sheep under two years old).
These sheep should be treated at regular intervals of 3-4 weeks during spring, summer and autumn. The intervals depend on weather conditions, but in no case should the interval exceed five weeks. The copper sulphate-nicotine sulphate mixture should be used.
(d) Adult Non-Breeding Sheep.
These sheep, ideally, should receive treatments as stated above for young sheep, but the periods between treatment can be extended. They should not exceed 6-7 weeks, however. The copper sulphate-nicotine sulphate mixture of carbon tetrachloride should be used.
(e) All Sheep.
All sheep should be treated several times during the colder winter months (June, July, August). This applies particularly to the higher tableland areas where the winter is severe and frosts are common. The object of winter treatments is to remove Haemonchus contortus from the sheep at a time when little or no fresh infection is likely to be acquired. Winter treatments should result in a very considerable decrease in the contamination of spring pastures. Treatment with carbon tetrachloride as carried out in winter against Fasciola hepatica will assist in the control of haemonchosis.
(2) Dealing with an Outbreak.
All sheep should be treated immediately and, if possible, moved to pastures which are not likely to be heavily contaminated. Treatment should be repeated within 14 days in order to kill worms which have matured since the first treatment. It has recently been found that the anthelmintics commonly used in the treatment of haemonchosis are relatively ineffective against immature H. contortus. Copper sulphate appears to be the least effective. The mixture of copper sulphate and nicotine sulphate, however, appears to be reasonably effective against immature worms. If sheep cannot be moved to fresh pastures, treatment must be repeated at short intervals, not exceeding 14 days, until weather conditions favourable for development of haemonchosis give way to hot, dry conditions.
It must be remembered that if sheep are treated with any anthelmintic other than carbon tetrachloride, treatment will be ineffective in a certain proportion of animals owing to the drug having been swallowed into the rumen. Thus there is likely to be a number of animals which will not recover and may even die, and which will in any case serve to continue the heavy contamination of pastures with eggs. Carbon tetrachloride, in young sheep at any rate, is highly efficient and rarely fails to destroy over 90% of the adult H. contortus in every sheep. Its efficiency in adult sheep is not as constantly high as in young sheep. Copper sulphate in the usually recommended doses is very erratic in its efficiency against H. contortus in adult sheep. It thus appears that in an outbreak of haemonchosis, one should treat young sheep with carbon tetrachloride and adult sheep with the copper-sulphate-nicotine sulphate mixture. At the same time, it should be remembered that the latter mixture appears to be contra-indicated for sheep which are markedly anaemic.
Trichostrongylosis may occur in districts having a wide variety of climatic conditions. Outbreaks have been recorded in most of the sheep raising areas of Australia. Usually, however, trichostrongylosis is a disease of areas having a predominantly winter rainfall or a relatively high winter rainfall. e.g. the New England tableland of NSW. Trichostrongylosis usually occurs during the cooler months of the year, in autumn, winter and spring. However, infection may be acquired in spring and manifest its effects in summer, when the plane of nutrition has been depressed.
At the present time there is only one combination of drugs which can be recommended against Trichostrongylus spp., that is the copper sulphate-nicotine sulphate mixture. It must be remembered that this anthelmintic mixture is far from being 100% efficient. One must expect to average only about 70-75% efficiency, and one must expect treatment to fail completely in those animals in which the mixture is swallowed into the rumen. The relative lack of a high degree of efficiency is an additional reason for repeated treatments, particularly of those individuals which fail to respond as indicated by continued diarrhoea and failure to improve in condition.
(1) Annual Plan of Control (referring chiefly to areas in which trichostrongylosis is liable to occur every year).
(a) Before Lambing.
Ewes, especially young ewes which may not yet have developed resistance to Trichostmmulus spp., and very old ewes which may have lost their resistance, should be treated at least twice during the last two months of pregnancy.
(b) Lambing to Weaning.
It may be necessary to treat lambs once or twice during this period in districts where trichostrongylosis is severe and where weather conditions are favourable for development of the discase. In most cases it is desirable to treat lambs once, a few weeks before weaning. If ewes have been treated before lambing, it should not be necessary to treat them more than once during the lambing to weaning period. Lambs should be weaned into paddocks which have been spelled from sheep and cattle for at least 3-4 weeks. Lambs should be treated at or about weaning time.
(c) After Weaning.
Weaners and other young sheep should be treated at regular intervals of 3-4 weeks during autumn, winter and spring, unless exceptionally dry conditions prevail. The free living stages of Trichostrongylus spp. are very resistant to cold (e.g. infective larvae have been recovered from pasture after seventeen consecutive frosts). Dryness may inhibit development but may not kill, so that during a short dry period of a few weeks parasitic material may accumulate and, following rains, there may be mass development into infective larvae and sudden massive infection of sheep. Regular winter treatments will protect young sheep against both gradual and sudden infections. Nutritional supplements should be provided for young sheep. These may take the form of winter grazing crops, e.g. oats, in areas having a predominantly summer rainfall, and summer grazing crops. e.g lucerne, in areas having a predominantly winter rainfall. Improved pasture areas should be set aside for the use of young sheep, and legume hay and cereal grain conserved. Rotational grazing should be practiced.
(2) Dealing with an Outbreak.
All sheep should be treated and, if possible, moved from contaminated pastures. The plane of nutrition must be improved immediately. Where an outbreak occurs on young rapidly growing, improved pastures, natural pasture and hay and grain supplements should be utilised, being alternated with the rapidly growing pasture. Where these adjuncts are not available, and in very severe outbreaks even where they are available, consideration should be given to segregation of the most severely affected sheep. Isolating them on bare cultivation land or in yards and hand-feeding them entirely. In all cases, treatment must be repeated at intervals, short at first (10-14 days), and then extending to 3-4 weeks, until the outbreak subsides.
(4) Oesophagostomiasis (Oesophagostomum columbianum).
Oesophagostomum columbianum is practically confined to areas having a predominantly summer rainfall. It is commonest, and outbreaks of the disease produced are most severe, on the northern tablelands and near north-western slopes of N.S.W., and the Darling Downs and country between the Downs and the N.S.W. border in southern Queensland. Outbreaks have been recorded in several other areas in N.S.W. (Barraba, Scone-Murrurundi, Moss Vale-Mittagong, Coolah, Braidwood) and in these areas the parasite appears to be well established. Isolated outbreaks have been recorded in southern N.S.W., but the parasite does not appear to have become permanently established there. It is possible that this nematode may find suitable conditions in irrigation areas.
Oesophagostomiasis is associated with "wet" summers and autumns. The infection is acquired during the summer months, but frequently the disease is not manifested until the following winter when nutritional conditions have deteriorated. Acute outbreaks associated with mass infection may occur in late summer or autumn. The after-effects of infection, the nodules in the bowel wall, may manifest their presence in any period of deficient nutrition.
(i) Annual Plan of Control (referring chiefly to areas having a predominantly high summer rainfall).
At present there is only one method of treatment which is highly and constantly effective, namely, the use of a watery solution of sodium arsenite as an enema. Application of this method is slow and laborious, but this is compensated for by the high degree of efficiency obtained.
Treatment may be applied with either or both of two objectives; cure of severely infested sheep, or as part of a general plan of control. As a curative measure, treatment is applied whenever symptoms are observed.
In a plan of control, treatment should be carried out at certain periods which are related to the epidemiology of the disease. There are two chief periods at which treatment should be applied, one in late autumn (early May), and one in the second month of winter (July). The late autumn treatment will remove worms acquired in summer and early autumn, and will prevent sheep suffering ill-effects during the winter months when natural pastures have deteriorated. The late winter treatment is designed to remove worms at a time when re-infection is at a minimum. It is probable that in an average winter there would be no re-infection since the free-living stages of Oe. columbianum are not resistant to cold.
The late winter treatment should considerably reduce the worm burden of sheep and hence the egg output later in the year when conditions suitable for re-infection again occur. A winter treatment is very desirable for ewes which are to lamb in the spring, but it appears that enema treatment should not be applied later than the second month of pregnancy. Ewes due to lamb in September-October should therefore be treated about the end of June. A grazing crop should be provided for feeding off during winter months. Fresh green feed in itself results in sheep passing a number of these parasites and, in addition, it is desirable to provide sheep with fresh green feed before enema treatment in order to soften the contents of the large intestine. Following treatment, sheep should be placed in paddocks which have been spelled from sheep for at least one month.
(ii) Dealing with an Outbreak.
Treatment should be carried out immediately and the plane of nutrition should be raised. Provision of fresh, green feed is particularly desirable. It may be necessary to repeat treatment within a month to destroy worms which have emerged from nodules meanwhile.
The control of diseases caused by nematode parasites in sheep is discussed in general terms, attention being directed to the value of epidemiological studies in correlating outbreaks with climatic and other factors.
Special control measures, as an annual plan of control and as a special procedure for dealing with an outbreak, are discussed in relation to the diseases produced by Haemonchus contortus, Trichostrongylus spp. and Oesophagostomum columbianum. These are considered to be the nematode parasites of greatest pathogenic and economic importance in sheep in Australia.