Parasitic Disease of Farm Animals

New Zealand Journal of Agriculture, Volume 59, Issue 6, 15 December 1939, Page 485

Parasitic Disease of Farm Animals

Practical Problems Which Confront The Farmer in Avoiding Losses

E. L. TAYLOR,

Ministry of Agriculture and Fisheries, Waybridge, England.

—mt—mt——tin—tin—mi——nu—tut——mt——mt——nti——iiti^—t«j» i^>u— nn—mi—nn— uh— mi——im—mi—mi—mi——nu——iiii^—i f ? Written by an English veterin- | ? arian, this article will be of 1 I great . interest to farmers, and I I especially sheep farmers, in that I I the subject is widely covered j | and is explained attractively in j j simple, everyday language, j j Most of his contentions are I = readily applicable to New Zea- § I land conditions. ! 1 I i|in— ——iin——mt——-nil——mi——nit-—-—>llll——iin——uejfr

Contributed to a symposium on Practical Problems of Animal Production., British Association for the Advancement of Science, Section M (Agriculture), Cambridge, August 23, 1938, and reprinted from the “Empire Journal of Experimental Agriculture” and “The Veterinary Journal.”

nnHE close association between paraTIE close association between parasitic disease and animal nutrition will be more easily understood if we carry ourselves back in ■ imagination to the beginnings of man’s interference with the balance of Nature. Let us admit at the outset that throughout the stretches of evolutionary time everything ran comparatively , smoothly until man made his appearance, and that the advent of his intelligence was of the nature of a calamity to all other zoological species. z We may suppose that early man was in some respects a gentleman, in that he spent his days in hunting and his nights in feasting. No doubt he sometimes returned from his hunting expeditions with . young animals for his children to play with, and later, recognising the possibilities of providing through this tneans for his own comfort and security, he contrived to encourage the breeding of certain useful animals in captivity, and so set himself the task of protecting his tame animals from other beasts and providing for their food requirements. Through some such process as this primitive man came to exchange what must have been a very dashing if uncertain sportsman’s life for the humdrum cares of the husbandman.

Where the Blame Lies If we would lay the blame for the present prevalence of parasitic disease upon any particular individual it must be upon that early genius who was the first to think of husbanding the patches of grazing near his own home, fencing them round to keep his domesticated animals from wandering, and reserving the choice grass for them alone. That man may be regarded as the first agricultural sinner,

who deranged the natural order of things in the Garden of Eden, where the grazing animals had hitherto lived in peaceful equilibrium with their parasites. \ /. One'might describe the change by saying that the primordial state was one of harmonious community of happy worms in happy sheep, but since man’s interference upset the balance one only too frequently sees a threatened annihilation of both. A satisfactory readjustment, bringing the, „ level of parasitic infestation within the tolerance of . the host, still awaits discovery. ...

Normal Inhabitants It is generally admitted among graziers that the diseases caused by parasitic worms are the most economically important of all with which they have to contend. Most of us have been brought up to think that parasites are nasty features and that their only product is disease, so that it may come as a surprise to many to know that the very parasites which may give rise to so much trouble by excessive multiplication are normal inhabitants of healthy animals, guests of their alimentary tract. Disease is as harmful to the parasites as it is to their hosts, as . it may lead to the death of the whole, community. Parasitic disease is, therefore, to be regarded as a flaw in the environment and a departure from the

primitive conditions to which the parasites had adapted themselves, rather than to a predaceous attack by small creatures upon a larger one. So long as the number of worms in an animal remains reasonably and “naturally” small, the balance of the ecological unit is maintained (that is, the animal remains reasonably healthy) and a harmonious relationship continues, but disease follows where the numbers of the parasite community become excessively great. The increase of parasites, as we shall see, is essentially linked with progress in the improvement of pastures. Worm diseases are man-made, and the artificial readjustment of the balance between the grazing animal and its parasites looms very large in the practical problems of animal production. ' This readjustment will come only through a detailed knowledge of the reactions of the parasites both during their free life in the pastures and during their parasitic life in the grazing animal.

Development The outline of the life history of the parasites is - comparatively straightforward. The adult female worms, situated in the lumen of the intestine, lay eggs which pass out of the grazing animal and on to the pastures. In the course of about 24 hours these hatch and give rise to minute larvae which, during the course of a week, cast their skin twice and reach the infective stage, at which they are ready to proceed with parasitic life,as soon as they happen to be picked up by a suitable grazing animal. For the proper understanding of the development of worm diseases there are three important points which require special consideration. The first of these is the inability of parasitic worms to multiply in the body of the animal in which they are living. This constitutes an important peculiarity ip

the propagation -of worm diseases, distinguishing them from the kind of disease caused by bacterial or protozoan parasites, which are capable of multiplication within the body of the animal in which they are living. Where parasites can multiply in this way there is some reason for supposing that the initial infection is a most important step in the disease-produc-ing process. If, however, every individual parasite requires to be picked up from outside, as is the case with parasitic worms, the relative importance of the initial introduction of infection appears to be very little indeed, and is of no practical consequence at all where, as with the parasites under discussion, all communities of animals, both.. healthy as well as diseased, are invariably infected. In such an instance as this it must be concluded that the environment of the grazing animal,, which so largely governs the increase of the parasites, is of paramount importance.

Egg-pro duction Powers The second important point to consider is the remarkable egg-producing

powers of the worms. The egg-produc-tion of some parasitic worms is numbered in tens of thousands daily per individual female, and although most of the worms with which we are concerned probably produce 1 less, the daily output of eggs by infected grazing animals is very great indeed. A healthy sheep carrying an infection of trichostrongyles (intestinal worms) may pass some 200,000 to 400,000 eggs daily, and a diseased sheep between 9,000,000 and 30,000,000; a healthy horse carrying only a light infestation of strongyloid worms passes 1,000,000 to 2,000,000 eggs daily, and a diseased horse more than 50,000,000. There is thus seen to be ample opportunity for the transmission of large numbers of worms under crowded grazing conditions, and this . enormous output of eggs, which earlier in evolutionary time was developed to ensure the continuity of the species, is quite unsuited to present agricultural conditions. . , The third important point to . consider is the longevity of the infective larvae of the worms in the herbage. In the infective stage the larvae are

ensheathed in a second outer skin, the result of incomplete moulting, and in consequence are endowed with great powers of resistance against cold, dryness, or other adverse forces acting upon them. They are also able to survive for an inconveniently long time in the herbage. Patience may be said to to be one of their vices, for some of them can wait 12 months, or even two years in exceptional cases, for a. passing host. '

Evil Effects of Overcrowding In view of the enormous output of eggs, , even by healthy animals, and the longevity of the larvae on the pasture, it is clear that under agricultural grazing conditions there»is liable to be a more or less rapid accumulation of infective material during the whole of the time that a pasture is occupied by grazing animals. The evil effects of overcrowding may now be more fully, appreciated. It is obvious that the more animals that are put on to a field the more eggs will be disseminated, and the more infective larvae picked up. But the rate of increase is actually even greater than it

appears,, as it has been shown that there is a tendency for the parasites to increase as the square of the number of animals per unit area. With all these forces in favour of numerical increase of parasites, one may well ask by what miracle the animals in our fields ever escape parasitic disease. The logical explanation of miracles has always been a stiff proposition, and much of the research work which is being carried on at the present time into the helminthiases (worm. infestation) of grazing animals may be regarded as an endeavour to explain this one. Proceeding along the mathematical line, as in the analysis of the effects of overcrowding, we might consider some such - unit as “sheep-per-acre week” with a view to easy comparison of the amount of grazing sustained by various pastures. The influence of factors other than the grazing animal and its particular kind of parasites upon an increase of parasite population is so great, however, that a unit such as this would be quite useless for estimating the amount of infection in a field. Indeed, these third factors are of such dominating importance that it is hoped that a thorough understanding of their influence may enable us, some day, to bring the worm disease of grazing animals under our control.

“Third Factors” Now let us consider the “third factors” governing the increase of parasitic worms. (a) Immunity.— First of all there. is the question of immunity is, the resistance of the grazing animal to the development of the infective larvae which it acquires while grazing on infected herbage. 1 The study of the immunity reactions to worm-infection has attracted much attention among workers in parasitology during the past decade, and many interesting points., have been revealed. In general, it may be stated that adult animals are much more resistant both to the results, of worm-infestation and to. the actual development of the worms than are immature animals. Although adults are never entirely immune and usually carry a few parasitic worms throughout their lives, their resistance to new infection is very considerable, so that they may be able to graze with impunity under conditions that would lead to fatal disease among susceptible young animals. ,

This resistance is not absolute, however, and poor feeding or a deficient diet, combined .with a high rate of intake or infective larvae, will break down the resistance, and severe disease frequently occurs among adult stock under those circumstances.

Regular Dosing (b) Regular Anthelmintic Medication for the Destruction of Adult Worms.— Since we are dealing first with the inhabiting influences upon the parasitic stages of the worms, it will be convenient to make a short mention here of the use of the medicaments known as anthelmintics (worm medicines). The control of parasitic worms by the use of cheap, safe, easily administered and effective drugs is the means of control most favoured by the farmer, who finds the restrictions imposed on him for the control of the parasites by other means most irksome and difficult to . fit in with the management of his stock. At the end of the last century,

upon the advice of a South African veterinary surgeon, the farmers there began to use copper sulphate for the control of the “twisted wireworm,” and so saved the South African sheep farming industry, which was threatened with ruin as a result of the enormous numerical increase of that parasite. Were it not for regular dosings with copper sulphate, which is now used all over the world for the limitation of the increase of the twisted wireworm, it would be impossible in many parts to rear sheep. This medicament is the only one yet known for use in grazing ' ,o^l'J.l mi WI’llWI I tuj'.' gJa'Ol. illrflMHl''A .if —■g ’"■■J animals that answers the requirements of being cheap, safe, easily administered, and effective, but, unfortunately, it serves only against this one species of parasitic , worm. Our knowledge of anthelmintic medication as a means of controlling, the worm population and preventing disease is therefore very limited at the present time and, with the exception of the twisted wireworm and one or

two other species, we must depend upon other measures for preventing the excessive multiplication of the worms. Larvae in Pastures (c) Factors Acting upon the Larvae in the Pastures. — The factors influencing the increase of the parasites next to be considered are those concerning the development of their free-living stages on the ground. These may conveniently be considered under four •headings:— ' ' , (1) Factors affecting the development of larvae in the pastures. / (2) Factors affecting the longevity , of the larvae. (3) Factors affecting the relative concentration of larvae in the pastures and their accessibility to the grazing animal. (4) Factors affecting the disappearance of larvae from the pastures.

Moisture and Drought (1) Soil moisture is popularly supposed to have a marked influence, and “wet” pastures are usually thought to be' “wormy” pastures. There appears to be no valid reason for this supposition, however, and it may reasonably be argued that “moist” pastures are just as dangerous as “wet” ones for the development of diseases caused by nematode worms, the only real difference. being between “moist” pastures and “very dry” pastures such as occur in arid countries. Drought produces rather an interesting result in that it hinders the development of some species of larvae without destroying them, the embryonated eggs remaining viable but not hatching until moister conditions recur. This sometimes leads to the accumulation of a large amount of potential infective material on the pastures during times of drought and a mass release of infective larvae when wet weather appears. This sequence of weather conditions occasionally leads to. the widespread appearance of worm disease in apparently epidemic form. Rain assists larval development by supplying abundant moisture, and although heavy rain washes many larvae off the herbage, they soon climb on to it again. ■' . Frost has no action whatever in destroying the infective larvae, but through hindering their development its effects are similar to those of drought in that the accumulation , of potential -infection is followed by a mass development of infective larvae as soon as warmer weather conditions recur —rather like the sudden invasion

of our roads by week-end motorists as soon as the weather decides to be warm. ' - Trampling of faeces into the soil, such as occurs where sheep are penned together, is distinctly favourable to larval development.

Most Lethal Force (2) As the most lethal of the natural forces acting upon the ensheathed resistant larvae is dryness, the. relative moisture of the soil and of the atmosphere doubtless plays a part. Observations made at Weybridge have shown

that the larvae which climb high into the herbage may die within 24 hours in dry weather, but that those which remain close to the ground retain their vitality for months. The nature of the herbage is also of some importance, the longevity of the larvae probably being greater in thick, matted herbage which keeps them near to the ground than in a first or second year’s grass ley, or in clover where they are able to climb away from the soil and come into contact with the drying influence of the air.

(3) The amount of herbage in a field has a marked influence on the number of infective worm larvae picked up by . a grazing animal, an important point which had not been recognised until some observations on the larval-content of pastures were carried out at Weybridge necessitating the employment of the unit “larvae per pound of herbage.” . It was then realised that the herbage 4 may be regarded as a diluent to the larvae. Where herbage is long, and plentiful, there tends to be a comparatively small larval-content in a rumen full of grass, but where herbage is scarce, as during times of drought’ and' on many farms during ordinary weather towards the end of the summer,,a high concentration of larvae is likely to be contained: '

Kind of Herbage The kind of herbage has an interesting influence upon the accessibility of -larvae to the grazing animal, larvae being able to climb more easily on to some kinds of herbage than on to others. The structure of a shoot of grass, for instance, is such that the greater part of the infective larvae tend to remain near to the soil, being turned off the- stem and along the under side of the lower leaves. In : clover, however, there is nothing to prevent a venturesome larva from climbing up the petiole and on to the trifoliate lamella of the leaf, and it is there that the great majority are actually found. This factor is of particular significance where sheep are concerned. The highly selective grazing of these animals is now generally recognised, and it is probable that while picking the better part of the young growth of the grass plants they tend to leave the bulk of the worm larvae behind on the coarse under-leaves, but while in clover, and selecting the green upper part of the leaf, they are taking the part which , carries most of the worm larvae. It is suggested that this may be the mechanism which has led to the increased incidence of parasitic gastritis in Northumberland, following the extensive use of basic slag and the con-, sequent increase of wild white clover in the. pastures. Patchiness of pasture' and time of day also have an influence upon the rate of intake of infection. -

Rest from Grazing 1 (4) The duration of a rest from grazing by infected animals has an

obvious influence upon the amount of infective material remaining in a pasture. Observations made at Weybridge have shown, however, that the natural death of the larvae begins almost as soon as they have reached the infective stage, some of those which climb high into the grass being killed within a few hours by the drying action of sun and wind.A more 'important consideration, however, is the effect of a rest upon the “number of larvae per pound of herbage.” This effect, may be very great, particularly at a time of the year when .the grass is making ,a rapid growth, 1 and it is probably on account of the growth of grass as much as the death of the larvae that efit. is derived through, a rest from grazing. Manuring with nitrates by forcing a rapid. growth of grass to dilute the larvae may . produce a similar . reduction in : the number of larvae per pound of herbage.

Eating-off Process Of all the methods employed for the destruction of infective larvae in a pasture, however, doubtless the best is the eating-off of the infective herbage by animals which are unsuitable for the development , of the particular parasites , concerned. Larvae which are picked up by the wrong host animal in this way are destroyed, and, as an illustration of the effectiveness of the procedure, reference might

be made to a certain paddock attached to . a public thoroughbred stud which was found to carry some . 6000 “redworm” ... larvae per pound of herbage. Each of the several sheep placed there to clear up the infection would therefore collect and destroy some 90,000 larvae daily which might otherwise remain to infect the horses. ' ’ '' Haymaking is doubtless very effective for the removal of larvae from an infected field. Ploughing is probably not nearly so effective as is generally supposed, as the plough does not completely invert the soil surface, and leaves a space between the furrows through which it is both easy and natural for the larvae to migrate. Harrowing can have little effect, and burning of the grass can only be carried out in particularly rough herbage where trouble from helminths (worms) is unlikely to arise. . Destruction .of larvae through the application of lime, salt, kainit, or other substances to the pasture has little or no effect. .

ConclusionsTo say that parasitic worms constitute the principal controlling factor in achievement of maximum, animal population through , grassland farming is stating a truth’ that there can be no denying. On looking into sheep farming practices— it is the sheep farmers who bear the .brunt of the

loss from parasitic worms— is surprising to find how many of the age-old procedures of penning sheep, feeding them, and moving them have their origin in the control of parasitic worms. This reason is, of course, generally unknown to the farmer, who has developed the methods through a long and costly period of trial and error. The oft-repeated adage that “sheep should never hear the church bells ring twice in the'same field” doubtless refers to the period of six or seven days required for the development of the infective larvae on the grass. The closed system of folding sheep over arable land with the movement of the pens every three days, and the farmers’ belief that sheep which are being folded should never be allowed to “run back” on to land grazed. some days previously, refer to the same factor. The removal of young sheep from rich, crowded pasture on to clean land at the end of the summer refers to the accumulation of worm-in-fection on- those pastures, as also does the restriction of the number of sheep to the acre, which is so forward a question in every far m e r’s mind, and sets a very definite limit to the. profit that can be made out of an acre of sheep land. -

Number of Sheep Carried It is no exaggeration to say. that every good farmer knows almost to a sheep how many, his land will carry, and that if he exceeds that number in a normal year they will cease to thrive and may even become diseased and die. It seems quite clear that the evil effects of overstocking and the parasitic disease that is brought on through keeping too many sheep is representative of some flaw in. the adjustment of the. parasites to the grazing animals, which may' be described in popular terms as an upset of the “balance of Nature.”

Fortunately for the farmer, the powerful resistance to the development of . parasites which the grazing animals ultimately acquire (provided that they do not die in the process) gives this balance a considerable amount of elasticity, without which it would be impossible to crowd sheep together, even with the limited success we now attain. The farmer, however, stretches' the elasticity of this ecological (health limit). balance to the very limit in order to make a profit out of his land, and trouble frequently results. 1 . Lowering of Resistance The number of sheep per acre is only one of the, variable factors- in the

ecological unit of parasite, pasture, and grazing animal. With one or two rare exceptions, a variation in the virulence of the parasitic worms (apart from differences which are peculiar to the various species) is not known to occur, but a lowering of the resistance of the grazing animal as may be brought about through deficient food, or resulting from' the increased susceptibility of a group of animals - containing too many immature individuals, are common causes I of increased wormburden. This same result can just as easily be brought about by the derangement of a wise rotation of grazing, or the shortage of pasture and. the poverty of

its nutritive qualities that often occur towards the end of the summer. What may justly be regarded as reasonable stocking during an ordinary year may constitute overcrowding during a drought. Indeed, the variable factors are very numerous, and’ there are .llltiiiiwi flwu i~ Um jam TwiiiimHiiiW miww mm, w many small alterations, unknown to the farmer’s rule of thumb, which favour an increase of the parasites beyond the broad limits of the natural balance, and disease results.

Loss from Parasites . \ The loss to the country from these parasites is very, great indeed, both from outbreaks of obvious disease and from bad thriving. Figures are difficult to obtain, but I am aware of one farmer who lost between £4OOO and £5OOO as a result of parasitic gastritis during the course of five years’ arable sheep farming, and of 43 sheep farmers in Kent who, on inquiry, were clearly shown to have lost more than £ 10,000 between them during an epidemic of the disease in the Irought of 1933-34. ■ . ? "X;' So long as the farmer stocks his “—— 1 land to the maximum and remains ignorant of the factors which influence the increase of parasitic worms, these losses will continue. Eradication of the parasites is quite.out of the question at the present time, so that we must do what we can to control their increase.Recent observations on the factors which lead to this end are distinctly helpful, but the struggle to keep the numbers of parasites down is likely .to be a long , one, and the danger, of worm disease will dog the progress of greater stock • production' through grassland improvement until practical . methods of eradicating the parasites have ultimately been found. '’ ’:V M .