The cause is Trichostrongylus spp., known as Small Intestinal Worm, Black Scour Worm, Small Hair Worm, and Bankrupt Worm. In Australia the following species have been found: T. axei, T. colubriformis, T. rugatus, T. vitrinus, T. probolurus, T. falculatus, and T. longispicularis, The first four species are commonly met with, the last three are rare. T. axei is found in the abomasum, the other species in the first 10-20 feet of the small intestine. T. colubriformis is the commonest species. This species has been found in the rabbit, pig and man. T. axei occurs in horses. All species may occur in cattle and goats.
The disease is manifested by emaciation and diarrhoea. Anaemia is never severe and in most cases is not present.
The predisposing causes are youth, malnutrition, overcrowding and wet weather during the cooler months of the year. The disease is seldom seen in sheep older than 18-24 months.
Why Trichostrongylus spp. are Important Parasites:
These parasites are very important causes of disease in sheep in all sheep-raising countries of the world because:—
(1) Trichostrongylosis is usually insidious in onset and is frequently overlooked and confused with malnutrition and "seasonal conditions." Actually, the disease usually occurs when the plane of nutrition is low, thereby 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.
(2) 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 worm parasites of sheep.
(3) Treatment with anthelmintics is not highly efficient against Trichostrongylus spp. Even phenothiazine is less effective against these parasites than against the Nodule Worm (Oesophagostomum columbianum). Phenothiazine is, however, the most effective treatment. The widely used copper sulphate-nicotine sulphate mixture is not highly efficient and will rarely remove more than 60-70 per cent. of the worms. Further, treatment will fail completely in about 10 per cent, of sheep—those in which it is not swallowed into the abomasum.
(4) The effects of trichostrongylosis are often semi-permanent. The heavily infested weaner does not thrive, and frequently remains a stunted and uneconomical animal throughout its life.
(5) The fact that trichostrongylosis usually occurs during periods when the plane of nutrition is low renders control difficult.
(6) The early symptoms are not obvious and at post-mortem examination an inexperienced observer may readily overlook a heavy infestation.
Geographical Distribution in Australia: Trichostrongylus spp. are found wherever sheep are found. They are commonest and produce most severe ill-effects in areas receiving a predominantly winter rainfall. Outbreaks have been recorded, however, in most of the sheep raising areas having a mean annual rainfall of 18 inches or over. The Northern Tablelands of N.S.W. and Darling Downs in southern Queensland, although areas having a predominantly summer rainfall receive sufficient winter rain for the annual occurrence of trichostrongylosis.
During a series of wet years, outbreaks were seen on western plains country. e.gs., in the Riverina. These areas, normally receiving about 14 inches of rain per annum, recorded 20 inches and more in consecutive years. It appears that it is necessary to have excess rainfall in consecutive years before outbreaks occur in what is normally a low rainfall region. In this way the level of infestation can be built up in one year, and in the next year continues from a higher level, thereby more readily reaching pathogenic significance.
In low rainfall regions there may be local areas in which Trichostrongylus spp. find suitable conditions for development and survival. Such areas are frontages, flooded country, gilgai country, along leaking bore drains and around shallow tanks. These situations are usually overcrowded by sheep and a heavy contamination with worm eggs occurs.
The development of Trichostrongylus spp. is simple. Eggs are passed in faeces and under suitable conditions of temperature and moisture give rise to a worm-like embryo and hatch in 18-20 hours. Development is slowed by cold, and hatching may be prevented by dryness. After hatching, the first stage larva continues development and gives rise in 4-7 days to the third stage or infective larva which ascends blades of grass when moisture is present. This stage is ingested by grazing sheep and develops further in the small intestine, spending a few days in the mucosa. Growth is rapid and adult worms in the egg producing stage are present about 18 days after infection.
Epidemiology of Trichostrongylosis:
The female worms are not prolific egg producers. This is offset by the marked resistance to dryness and cold of the free-living stages on pastures. Heavy contamination of pastures takes some time to build up and sheep usually pick up larvae in small numbers over a long period. This accounts for the insidious onset of the disease. Accumulations of eggs and larvae may occur, however, and rapidly acquired infestations may be seen, with sudden onset of scouring and very rapid loss of condition. Overstocking and overcrowding facilitate the rapid intake of large numbers of larvae. Overcrowding may be local, e.g., in gilgais, frontages. etc., in low rainfall areas, or general, eg., where large numbers of sheep are kept for extended periods on improved pastures or grazing crops.
Outbreaks depend upon the following: susceptible sheep, climatic factors, management, sources of infective material. Young sheep are highly susceptible; wet, cool weather is very favourable; overstocking, overcrowding and malnutrition predispose; infective material is derived from eggs passed by older sheep in contact with young sheep—usually ewes contaminate pastures for their lambs.
Investigations on epidemiology are preceeding at Armidale, and while at present definite relationships with weather conditions have not been worked out some general relationships and seasonal changes are obvious. The seasonal changes in infestations by Trichostrongylus spp. in the southern parts of the Australian continent may differ from those seen at Armidale, but changes recorded in Tasmania agree fairly well with the Armidale generalisations.
It must be emphasised that post-mortem examinations may give no indication of the seasonal occurrence of trichostrongylosis if at such an examination large numbers of immature worms are found, then it is certain that the infestation was recently acquired within 2-3 weeks. If only adult worms are present it must be remembered that they may hare been there for months.
In epidemiological studies, at Armidale, weekly faecal examinations are made. Fluctuations in worm burden can thereby be traced backand correlated with climatic conditions.
At Armidale, the worm burden with Trichostrongylus spp. begins to increase from April onwards, reaches a peak in July-August and with the coming of spring and improvement of nutrition, decreases rapidly.
Spring lambs weaned into the autumn usually show trichostrongylosis in the winter months. Autumn lambs, on the other hand, run with the ewes during winter and are weaned into the spring and often do not show trichostrongylosis until the next late summer and autumn. Malnutrition in winter—as usually occurs in northern NSW—renders spring lambs specially susceptible. Lack of feed, or the presence of
unsuitable feed, in the summer—as often occurs in southern Australia—renders both spring and autumn lambs specially susceptible.
A wet autumn, followed by a dry winter, and a wet winter followed by a dry or late spring are likely to bring about severe outbreaks.
Lambs running with the ewes seldom show trichostrongylosis although acquiring considerable numbers of worms. The adequate diet maintains a balance between host and parasite, but after weaning the balance is frequently tilted in favour of the parasite.
The ability of embryonated eggs to withstand considerable periods of dry weather, particularly in winter, may lead to accumulations of potentially infective material which may become available for sheep over a short period when rain falls. Mass infestations may thus occur, leading to sudden, acute outbreaks. A short drought of two or three months during cooler months, followed by rain, should always be regarded as a danger period for trichostrongylosis.
Trichostrongylosis is essentially a disease of young sheep and is rarely seen in sheep older than 18-24 months. An exception is seen where sheep from western country, "outside" country, are brought into "Inside" country on slopes or tablelands. In this case, even grown sheep may develop trichostrongylosis; probably because they have never had other than extremely light infestations and have not developed resistance.
The most serious effects of trichostrongylosis are usually exhibited by lambs from weaning at 4-5 months old, until they are 12-18 months old, by which time the majority of sheep have developed resistance. Some adult sheep may harbour relatively large numbers of worms without showing clinical symptoms, and are a source of danger to young sheep which may graze with or immediately after them. Ewes, particularly young ewes which may not have developed a high degree of resistance, or very old ewes which may have lost their resistance, are a source of danger for lambs.
In N.S.W., infestations of the abomasum with T. axei are of much less importance than the intestinal infestations with the other species. In southern states and in New Zealand, T. axei may be of much greater significance. There is no recorded evidence of the pathogenic significance of T. axei and experiments with pure infestations apparently have not been conducted. The symptoms are not known and it is possible that diarrhoea may not be characteristic.
Macroscopic lesions in trichostrongylosis are inconspicuous. There may be some congestion of the duodenal mucosa and the amount of mucus may be increased. Experimentally, by giving a massive dose of larvae (60,000), a fairly marked enteritis was produced.
In contrast to haemonchosis a marked anaemia is rare in trichostrongylosis and the skin and conjunctival membrane never become dead white. On the contrary, in many cases of severe trichostrongylosis the conjunctival membrane is congested. The pink colour of the worms suggest that they do ingest some blood, but they are not vigorous bloodsuckers.
Large numbers of Trichostrongylosis spp. in the duodenum lead to increased irritability of the bowel, resulting in increased bowel movements and persistent diarrhoea. The precise cause of this is not yet known. Toxins and anti-enzymes have been suggested, and there is evidence that digestive processes are greatly disturbed. Once diarrhoea is established, ingesta passes through the alimentary tract very quickly and there is almost certainly insufficient time for normal digestion and absorption and the sheep is virtually starved.
In young, poorly grown weaners, 5,000 Trichostrongylus spp. may produce fatal effects. In well-grown crossbreds, as many as 100,000 worms have been recorded in fatal cases. Disease depends upon numbers factors and their general relationship with predisposing or preventing factors.
Symptoms and Diagnosis:
Scouring and loss of condition in young sheep during the cooler months of the year should always arouse suspicion of trichostrongylosis. Clinical signs of anaemia are usually absent, The onset of trichostrongylosis is usually gradual and there may be a high proportion of young sheep affected and showing symptoms before the owner realises that the disease is present. There may even be a fair number of deaths, spread over many weeks, but because they occur only in ones and twos do not arouse much interest on the part of the owner. As many as 10 per cent. of a weaner flock may die off in this fashion from trichostrongylosis.
Black scours is the characteristic symptoms, but if feed is dry scouring may not be severe, and if feed is not fresh and green, the scour will be yellowish and not black.
Symptoms of loss of condition and scouring may be seen before lambs are weaned, particularly when, following a wet spring and early summer, the feed dries off rapidly and the ewe's milk supply fails. Generally, however, symptoms do not appear until after weaning, when within a few weeks a large proportion of the flock may be affected. Concurrently with the development of diarrhoea the weaners begin to lose condition, becoming weak and poor and difficult to drive.
In the early stages of the disease the conjunctival membrane may be congested, later there may be slight anaemia.
Scouring in young sheep may be due to a wide variety of causes— dietetic and parasitic. Fresh growth of pastures produces diarrhoea for perhaps two or three weeks, but there is no loss of condition. Severe infestations with the Nodule Worm (esophagostomum columbianum) and Large Mouthed Bowel Worm (Chabertia ovina) may produce diarrhoea, but it is seldom of the very fluid type seen in trichostrongylosis. In infestations by Oe. columbianum and C. ovina, the faeces are usually soft and contain large amounts of mucus and sometimes blood. Heavy artificial infestations with immature Nematodirus spp. have shown diarrhoea. In other countries scouring is given as a symptom of infestations with Ostertagia spp. It is not unusual to find concurrent infestations with Trichostrongylus spp. and other worm parasites. In northern N.S.W. it is usual to find Oe. columbianum present, in southern N.S.W. C. ovina and Ostertagia spp. are often present.
Faecal examinations on sheep infested with Trichostrongylus spp. may be deceptive. In the first instance these parasites are poor egg producers and the watery faeces both dilute the eggs and increase the weight of faeces. Low egg counts are usual. In the very early stages of infestation, before scouring begins, counts may be relatively high, but soon decrease as faeces pass through the stages of soft, very soft, diarrhoeic then watery. In heavily infested sheep egg counts seldom exceed 2,000 to 3,000 eggs per gramme. Faecal cultures showing a high percentage of Trichostrongylus spp. larvae exclude other parasites as a cause of the symptoms seen. Even though there may be a high proportion of Haemonchus contortus larva in cultures, Trichostrongylus spp. may still be the principal cause of the condition. Owing to the high egg-producing capacity of H. contortus, a small number of females can lay many more eggs than a much larger number of Trichostrongylus spp. females.
In an experimental case a weaner received a daily dose of 25 H. contortus and 1,000 Trichostrongylus spp. larvae for a long period. As the resulting disease developed the following was recorded—an egg count of 12,000 eggs per gramme and faecal culture showing 70 per cent, H. contortus larvae and 30 per cent. Trichostrongylus spp. larvae. The sheep died a few days later and at post-mortem examination were found 500 H. contortus (insufficient to cause deaths and 22,000 Trichostrongylus colubriformis. A simple calculation from the results of faecal examination shows, of course, that there were 4,000 Trichostrongylus spp. eggs per gramme.
The eggs of Trichostrongylus spp. tend to be elongated and narrow with somewhat pointed ends, but there is considerable overlapping with eggs of other genera and in mixed infestations there can be considerable difficulty in differentiating eggs. Trichostrongylus spp. larvae have very short tails and relatively short bodies—being much shorter than those of Ostertagia spp., the only other species of larvae with very short tails.
At post-mortem examination the carcass is very thin and wasted and there is very little fat present. Rib bones are often weak and break readily. Whether this is related to trichostrongylosis alone or to the concurrent malnutrition, is not quite certain. There are no obvious lesions, but occasionally the duodenum is slightly congested. Unless the examination is made by a trained and careful observer it is possible to overlook fatal infestations with Trichostrongylus spp. owing to their small size and inconspicuous character and the absence of obvious lesions. The first ten feet of the duodenum must be opened and examined carefully. The examination should be made in open sunlight—on dull days, or in a building it is practically impossible to see Trichostrongylus spp. lying on the mucosa. The worms may be seen more readily if the opened intestine is stretched as widely as possible and then held up against the light; the worms are then seen in silhouette. Where any doubt exists, and always in the case of a sheep which has been dead for more than a few hours before the examination is made, the mucosa should be scraped with a knife, the scrappings placed in water and held over a dark background, or scrapings may be smeared on a piece of glass.
Even after considerable experience it is difficult to estimate the number of parasites present. It is possible to form some idea of the degree of infestation by the following procedure. At intervals of about two feet, examine carefully a piece of opened duodenum, about two inches long, by stretching between thumb and fingers and holding up to the light. It is possible to count the worms present in this area if necessary. Usually, however, it is sufficient merely mentally to record whether few, numerous, very numerous, etc. In light infestations the first three or four sections examined, beginning from the pylorus, will show a few worms in each two inch length, but, after a few feet, the numbers decrease rapidly and at 7-8 feet there may be only an occasional worm present. In heavy infestations not only are there many worms in the successive two inch sections examined, but even after having examined 10-12 feet of duodenum the worms are still numerous and may not disappear for perhaps 20-25 feet.
The number of worms present is a useful indication of the seriousness of the infestation, but must be correlated with the age of the sheep and the plane of nutrition. In small experimental lambs 3-4 months old, 5,000 worms may be fatal. In older weaners 20,000-25,000 worms will be fatal in most cases. In the field, counts of 40,000 and 50,000 worms have been recorded. In other countries even larger numbers have been found—usually in British Breeds and crossbreds. When conditions permit, a worm count should be carried out for the post-mortem diagnosis of trichostrongylosis.
When selecting a sheep for a post-mortem examination for the diagnosis of trichostrongylosis, it is preferable not to use a sheep which has died, or one which is about to die and which has been scouring severely, for it is probable that such sheep have passed out many of the worms. An obviously affected sheep, but not a very severely affected sheep, should be selected.
There are a number of factors which render the control of trichostrongylosis difficult. Their importance must be appreciated by all who are concerned with control—graziers and veterinarians alike. The chief of these factors are:-
1. Treatment is far from being highly efficient. The advent of phenothiazine has indeed completely changed this aspect of control. It is very effective against mature Trichostrongylus spp. being much more effective than any other drug or combination of drugs previously used. It is, however, not as effective against Trichostrongylus spp. as it is against Haemonchus contortus and Oesophagostomum columbianum, and it is not very effective against immature Trichostrongylus spp.
Of other drugs, the only combinations known to possess reasonable degrees of efficiency against Trichostrongylus spp. are:—
(a) Tetrachlorethylene administered immediately after a "stimulating" dose of copper sulphate.
(b) Copper sulphate—nicotine sulphate mixture.
Of these (a) is more efficient than (b). Both depend for their success on being swallowed into the abomasum and thus on the efficiency of copper sulphate in stimulating the oesophagal groove reflex. It has been shown that on the average stimulation fails in 10 per cent. of sheep, and, of greater significance, that failure is likely to occur in the same sheep at subsequent treatments.
Leaving phenothiazine out of the question for the time being, it is clear that other treatments will fail in about 10 per cent. of sheep, and that repeated treatments may not be of much value in restoring the "tail" of the mob to the general flock level. Phenothiazine will do this, and when it becomes reasonable in supplies and price, the treatment aspect of control will be very satisfactory.
The copper sulphate-nicotine sulphate mixture has been widely used in the control of trichostrongylosis during the past eight years. When used regularly it maintains the Trichostrongylus spp. population at a non-pathogenic level, but when an outbreak is in progress it often fails. It fails for these reasons:—
(a) It is effective in only 90 per cent. of sheep.
(b) When effective it is not highly efficient, seldom removing more than 60-70 per cent. of worms.
(c) It fails repeatedly in some sheep.
Tetrachlorethylene administered immediately after a dose of copper sulphate is more efficient than the copper sulphate-nicotine sulphate. It fails in 10 per cent. of sheep and is likely to fail again and again in the same individual. Its administration is time consuming because the stimulating dose of copper sulphate must be given separately. Tetrachlorethylene at times produces ill-effects after drenching and care is necessary to avoid some losses.
2. The free-living stages of Trichostrongylus spp. are very resistant to climatic conditions.
Eggs, particularly when fully embryonated, and infective larvae, are very resistant to both cold and dryness. Heat and dryness are the most potent factors in destroying eggs and larvae on pastures. The embryonated eggs are very resistant to dryness, and although they will not hatch, they remain viable. During a dry period there may be an accumulation of eggs on pastures, and this may be followed by mass hatching and development of larvae when rain falls. Thus there may be a sudden heavy infestation of sheep during a short period of time. The marked resistance of eggs and larvae offsets low egg production by the female worms and ensures a high level of contamination of pastures.
3. Intensive grazing without careful management of pastures.
Highly improved pastures and irrigated pastures are likely to suffer overcrowding and, if rotational grazing is not practised, heavy infestations are almost certain to occur. The high plane of nutrition will enable sheep to withstand heavy infestations, but a sudden change to a lower plane or a heavier "swamping" infestation may lead to an outbreak.
4. In the absence of an adequate nutritional level it is difficult to control trichostrongylosis. It is particularly difficult to bring about resolution of an outbreak unless the plane of nutrition can be raised quickly. Outbreaks are not infrequently seen at periods when nutritional conditions are far from good, particularly for young sheep. In northern N.S.W. outbreaks are usually seen in winter, a period of feed scarcity. In southern N.S.W., although the infestation may have been acquired in the cooler months, ill-effects are often delayed until the summer when dry feed is no longer an optimum diet. Provision of supplementary feed is an important part of a plan for the control of trichostrongylosis.
The anthelmintics, in order of value, are: (a) Phenothiazine; (b) tetrachlorethylene after a "stimulating" dose of copper sulphate; (c) Copper sulphate-nicotine sulphate mixture.
Mixtures and Dose Rates.—
(a) Phenothiazine: Against Trichostrongylus spp. it is essential to use a full dose i.e., one greater than that now recommended against Oe. columbianum.
Make a suspension by mixing 1lb. phenothiazine (commercial forms now available are Phenovis, Phenovine, Phenzeen) with 16 fl. ozs. water.
Grown sheep, about 1⅓ fl. ozs., or 38 c.c.; Sheep, 8-12 months, 1fl. oz. (30 c.c. containing 20 grammes phenothiazine); Sheep, 4-8 months, ¾ fl. oz. (25 c.c.).
(b) Tetrachlorethylene following a "stimulating" dose of copper sulphate: A mixture of equal parts of tetrachlorethylene with liquid paraffin can be used, or an emulsion may be prepared. Experiments have not shown any advantages for emulsions either in efficiency or in preventing after-effects.
Dose Rate.—(Of a 50/50 mixture with liquid paraffin).
Grown Sheep, 15 c.c.; Sheep, 12-18 months, 12 c.c.; Sheep, 8-12 months, 8 c.c.; Sheep, 4-8 months, 6 c.c.; Lambs, under 4 months, 4 c.c.
The dose must be preceded by 10 c.c. of 5 per cent. copper sulphate solution, or the sheep's mouth should be swabbed with a 5 per cent solution.
Drenches containing tetrachlorethylene should be administered carefully, for should it be inhaled, sheep are likely to be suffocated. Even when given with due care, immediate after-effects appear in a number of sheep. These effects include staggering gait, weakness of legs, stupor, prostration and, later, bloating. Affected sheep go down and, unless repeatedly roused and placed in a normal lying position, are liable to suffocate, particularly if other affected sheep fall on them. Provided care is taken to rouse the sheep and place them in the normal resting position, there need be no fear of losses.
Following drenching, the sheep should be allowed to go into a large yard and it should be the duty of one person to attend to those showing after-effects. It may be necessary to apply artificial respiration to bad cases. Affected sheep should be lifted onto their feet and encouraged to walk about. They should not be allowed to lie about in cold weather.
(c) Copper Sulphate-Nicotine Sulphate Mixture:
(1) A 2 per cent. solution is made by dissolving 1lb. copper sulphate in five gallons of water and adding 16 fl. ozs. nicotine sulphate.
Grown sheep. 2 fl. ozs, (60 c.c.); Sheep, 12-18 months, 1½ fl. ozs. (45 c.c.); Sheep, 8-12 1 fl. oz. (30 c.c.); Sheep, 4-8 months, ¾ fl. oz.(25 c.c.) Lambs, under 4 months, ½ oz. (15 c.c.).
(2) A 4 per cent. solution is made by dissolving 2lbs. copper sulphate in five gallons of water and adding 32 fl. ozs. nicotine sulphate.
Grown sheep, 1 fl. oz. (30 c,c.); Sheep, 12-18 months, ¾ fl. oz. (25 c.c.); Sheep. 8-12 months, ½ fl. oz. (15 c.c.); For younger sheep use two per cent. solution.
The amount of nicotine sulphate present in the mixture is such that if overdose were given there would be a risk of poisoning; therefore, when using the mixture, great care must be taken in measuring the ingredients and the dose. If sheep are poor and weak the amount of nicotine sulphate should be reduced, using 10-12 fl. ozs. instead of 16 fl. ozs. per lb. of copper sulphate.
Care is necessary during drenching to see that the mucous membrane of the mouth is not injured by the drenching gun or funnel. The end of the nozzle of drenching guns can be protected by enclosing it in a short length of rubber tubing, say two inches long, and allowing this to project about 1/8th inch beyond the end. If the mucous membrane is injured and nicotine sulphate enters the wound, a severe inflammatory reaction affected is caused and a serious abscess often develops. Severely affected sheep usually die.
APPLICATION OF TREATMENT.
Phenothiazine is in short supply and is too expensive for general use. It should be reserved for the "tail" of the weaner flock, and particularly for outbreaks.
Tetrachlorethylene and nicotine sulphate are also in short supply. They should be used sparingly under present conditions, and should be restricted for use against Trichostrongylus spp. There has been a tendency to use copper sulphate-nicotine sulphate mixture rather haphazardly as a general treatment against "worms," often against Haemonchus. Other preparations which are more readily available and much cheaper, eg., copper sulphate copper sulphate-arsenic mixtures, should be used against Haemonchus, particularly the copper sulphate-arsenic mixtures, for treatment of grown sheep. As far as possible, copper sulphate-nicotine sulphate mixture and tetrachlorethylene should not be used for grown sheep but should be reserved for young sheep, thereby conserving supplies and being used where most needed.
It should be noted that there are some preparations on the market containing tertrachlorethylene, usually mixed with carbon tetrachloride and liquid paraffin, the dose rates of which do not contain sufficient tetrachlorethylene for satisfactory efficiency against Trichostrongylus spp. If such mixtures are used for this purpose, the dose should be
increased so that sheep receive amounts of the drug as follows:— Grown sheep, 7.5 c.c.; sheep, 12-18 months, 6 c.c.; sheep 8-12 months, 4 c.c.; sheep 4-8 months, 3 c.c.; and lambs under 4 months, 2 c.c. Further, the dose must be preceded by copper sulphate. If such mixtures are used at increased dose rates, there is a waste of carbon tetrachloride unless sheep are infested with liver fluke.
Treatment can be applied as part of an annual plan of control, and should then be carried out at intervals related to season and management, and should not be used haphazardly. In an annual plan treatment should be carried out whether the sheep are thriving or not, for its aim is to keep down the worm population and thereby decrease the chances of outbreaks.
Treatment must be applied when an outbreak occurs, but treatment alone, unless with phenothiazine, is likely to fail to give rapid resolution and recovery of sheep. Treatment should be combined with movement to fresh paddocks and provision of more or better feed. Further, treatment should be repeated et intervals until the outbreak is controlled—particularly when it is not possible to move infested sheep to uncontaminated paddocks.
GENERAL CONTROL MEASURES.
The following management procedures of value in the control of all gastro-intestinal worm parasites and are of particular value in the control of trichostrongylosis because treatment is not highly efficient and because they aim at improvement of nutrition.
(a) Rotational Grazing and Spelling of Pastures:
Under very favourable conditions eggs and larvae may live on pastures for many months. but under average conditions a very high proportion of them dies off within 3-4 weeks. Spelling a paddock for this period therefore renders it practically free of risk of reinfestation of sheep. Spelling also permits pasture growth and leads to better nutritional conditions. Rotational grazing and spelling can be combined conveniently. If there are two paddocks of about equal size, carrying about the same number of sheep, run all of the sheep in one paddock for three weeks, then transfer them all to the other paddock which has been spelled meanwhile. This can be repeated and can become the usual method of management. An experiment carried out on natural pasture in New England has demonstrated its value. Rotational grazing is of very great importance on improved and irrigated pastures.
(b) Reduction of Stocking:
This practice aids control of parasitic diseases by reducing contamination of pastures and by making more feed available. Many Graziers have found that a reduction of sheep numbers by as much as one third over several years has not resulted in lower wool return, but has actually given better returns and has definitely resulted in better health of sheep. If the whole property cannot have the stocking reduced, the number of breeders and young sheep should be reduced, or else given more room.
(c) Running Weaners with Grown Sheep:
Weaners, the most susceptible sheep as far as parasites are concerned, are often crowded at a relatively high rate of stocking. The owner may argue that because they are small sheep they need less feed than grown sheep. However, their demands for certain food materials are greater than those of grown sheep for the weaner is expected to grow and to produce wool, whereas the grown sheep has only to produce wool.
The weaner population of a paddock should be kept low. This can be achieved by "cross-weaning" or weaning into a wether mob. If a property is "sheep-to-the-acre" country, aim to have half a weaner and half a grown dry sheep per acre rather than one weaner per acre. Grown dry sheep are resistant to Trichostrongylus spp. and as they do not harbour many worms they do not contaminate pastures heavily. Moreover, they probably destroy many of the larvae which they ingest with the grass, whereas the same larvae ingested by a weaner, which is not resistant, will develop into adult worms.
This method of management keeps the population of susceptible sheep low and provides more feed for these sheep.
Adequate nutrition is of very great value in the control of trichostrongylosis. A well-fed sheep develops and maintains resistance. Grazing crops, improved pastures, spelled pastures, rotational grazing and conserved fodder are all means for maintaining adequate nutrition. and they all diminish the need for drenching.
AN 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 Trichostrongylus 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 disease. 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 lambs 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 predominatly summer rainfall, and summer grazing crops, e.g., lucerne, in areas having a predominatly winter rainfall. Improved pasture areas should be set aside for the use of young sheep and legume hay and cereal grains conserved. Rotational grazing should be practised.
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, sheep should be given a run-off onto natural pasture, or hay, chaff or grain supplements should be applied.
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.