Articles "Approaches to Managing Gastrointestinal Nematode Parasites..." Article Index

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By: A. David Scarfe
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Gastrointestinal parasites in sheep and goats can, and in some circumstances do, cause severe losses. These range from decreased utilization of feeds in unthrifty animals to death. Good management practices, including prevention, control and treatment of gastrointestinal parasites, can prevent or reduce these losses. This paper is intended to aid both the producer and veterinary practitioner in these aspects.

Goats and sheep have numerous gastrointestinal parasites, many of which are shared by both species. The most important include coccidia (a protozoan), bacteria and viruses, nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Among the gastrointestinal parasites, nematodes present the greatest potential problems, of which the barberpole worm (Haemonchus contortus), is the most important. Haemonchus is also referred to as the stomach worm or bankrupt worm for obvious reasons. Other species of nematodes occasionally cause economic loss to producers, but these pale in significance compared to the damage caused by Haemonchus. In general, most management and therapeutic methods apply equally to all or most gastrointestinal nematodes and, except in specific circumstances, the principals discussed here will apply to other gastrointestinal nematodes as well as Haemonchus.

Management Approaches
Prevention, rather than cure, is the philosophy used in developing control programs against gastrointestinal nematodes. It should be assumed that worms cannot be eradicated from the environment and livestock will continually be reinfected. However, infections can be limited to the extent that they will not cause economic loss to the producer. A combination of treatment and management is usually necessary to achieve control. One approach is the strategic use of anthelmintics. Anthelmintics are used at a time when most of the total worm population is within the goat and not on the pasture, such as when goats are moved from a contaminated pasture to a parasite-free or nearly free pasture. Pastures become parasite-free when they have been tilled or given prolonged rest at a suitable time of year or were grazed by animals which are not satisfactory hosts for the target parasite species.

A few points to emphasize: The disease caused byHaemonchus is usually due to the animal's inability to naturally overcome adverse effects of the parasites. It is almost impossible to totally eliminate parasites in a herd, but the simple presence of a parasite in an animal does not indicate disease. Only when parasite loads become excessive or when an animal's natural immunity to disease becomes suppressed (such as with stress, starvation, etc.), will it show the symptoms of disease. It is seldom cost effective to have a parasite management system that only treats a disease rather than to try to prevent it before it has occurred. Further, the hidden costs of sub-clinical diseases (before you see the symptoms) are likely to be enormous. For example, the cost of decreased reproductive efficiency, decreased milk, meat or fiber production, make high parasite loads a real problem. Moreover, in warm, wet climates parasite transmission is high and, secondly, animals are rapidly reinfected with large numbers of parasites when overgrazing, crowded pens or traps, fecal contaminated feed bunks, etc. occurs. Any control measures need to consider these facts.

Approaches to Controlling Parasites
Control measures require a knowledge of important parts of the life cycle of the parasites. A few points should be emphasized here. Each adult Haemonchus has a tremendous egg laying potential (5000 eggs or more per day). Eggs passed in the feces contaminate the environment. These hatch and pass through three larval stages, the third being infective to sheep and goats. Desiccation or extremes of heat or cold are detrimental to the development of eggs or larvae in the environment. Transmission of most parasites is seasonal. This seasonality is related to environmental temperatures and rainfall.

Haemonchus also has the ability to undergo I>hypobiosis. Hypobiosis is an important mechanism to survive adverse weather conditions when worms within the host become metabolically inactive within the gastric cells of the abomasum; this can be equated to dormancy. As a result, we see an increase in transmission between spring to fall, usually with a peak in early summer. Much of the use of de-worming drugs and management practices are aimed at reducing parasite loads and keeping the loads relatively low. Occasionally a producer may request a fecal sample be examined to "see if there are any parasites". Seldom is this done with a good understanding of the biology of the parasites and, all too frequently, producers loss production income. Only occasionally does a producer work closely with a veterinarian in a parasite monitoring program that is designed to measure actual parasite burdens and set up a well planned management scheme for the appropriate use of drugs, pasture rotation and other management tools.

Use of Anthelmintic Drugs
There are generally four ways anthelminthic drugs can be used: strategically; tactically; suppressively; and for salvaging animals.

Strategic treatments, primarily aimed at hypobiotic (dormant) worms in late winter for spring kidding females, have proven effective in controlling worm burdens during the warm weather transmission season. In some situations, a single de-worming may be sufficient to keep the level of parasitism below the economic threshold for the entire season. This approach has been fairly successful in very arid climates and where livestock are able to forage extensively over extremely large areas.

Tactical treatments, when weather conditions have been favorable for the transmission of parasites, eliminate worms from the gut before they have the opportunity to reproduce and further contaminate the environment. The timing of tactical de-worming may be based on recent rainfall when transmission periodically increases or, secondly, prior to moving animals from a contaminated area to one less contaminated. In both cases, the use of appropriate drugs three to five days before such an event is useful. Occasionally, tactical treatment may be based on increasing fecal egg counts. There is a fairly linear relationship between the number of adult Haemonchus and fecal worm egg counts in small ruminants. If the counts are used appropriately in conjunction with knowledge of seasonal transmission, they can be one of the most useful and economically viable management tools. For example, if the mean egg count of 10 to 15 randomly selected animals is above 500 eggs per gram of feces during the spring, the herd should be de-wormed even though there are no signs of hemonchosis. Treatment at this time, especially when accompanied by movement to parasite-free pastures, may prevent an outbreak of disease. During the fall, the average count would have to be above 200 eggs/gm to recommend treatment, as the transmission of Haemonchus will soon cease for the year. Unfortunately, in some parts of the country where the winter is very short and relatively warm and moist, it is difficult to predict when transmission rates decrease or when they will rise again. Tactical de-worming is, therefore, best done in conjunction with appropriate parasite monitoring and adjustments in management practices.

Suppressive anthelmintic treatments may also be given at regular intervals. Most frequently this is done with little or no change in management. This may be quite appropriate in situations or conditions in which transmission rates are relatively low. To be completely effective, this must be done before the worms, which are acquired since the last de-worming, become reproducing adults themselves. This interval is approximately 3 weeks. However, this method of parasite control is expensive and fails to utilize the host's defenses where they are applicable. For example, in very intensively operated herds where de-worming is routinely done at three to four week intervals, the cost of the drugs, without considering labor, may amount to $70 or more per 100 head, per month. Suppressive de-worming is probably the most effective means of keeping parasite numbers lowered for a period of time. However, this method will also eventually lead to resistance to the anthelminthic(s) used much more rapidly than if other strategies of control are utilized.

One point to consider here is alternating the use of different drugs. It is considered by this author, and several expert parasitologists, that rapid rotation of different drugs is ill-advised as this will lead to resistance of multiple drugs - something that the small ruminant industries certainly do not need. If alternating drugs is unavoidable, it may be advisable to change to a different class of drugs. Where large numbers of animals are confined to limited grazing and pastures can be neither rested nor alternately grazed by other species, or tilled, suppressive de-worming may have to be used.

Salvage (treatment to save lives, not control parasites) is a frequently used anthelmintic strategy in small ruminants. This is treatment in the face of disease. The animals are frequently anemic, may have diarrhea, bottle jaw or swelling (edema) along their ventral abdomen due to blood loss from the parasites. While many animals have the genetic ability to resist high infection rates, when disease is evident this resistance has been compromised. Although anthelmintics may remove thousands of worms from each of the treated animals, the pastures from which they came have billions of larvae awaiting ingestion. Under these circumstances, treatments at 2 week intervals may have to be practiced until weather conditions are no longer favorable for transmission. Unfortunately, once clinical disease is seen in a goat, it may take two or three months for it to return to a physiologically normal state, during which time it can be easily overcome by further parasitism or other diseases. The overall loss of productivity (lack of growth, decreased hair or wool, decreased milk production, increased nutritional demand) is enormous and cannot be long tolerated by any producer.

Management Practices
Overgrazing is never an appropriate management tool; producers are encouraged to avoid it at all costs. Over-grazed areas are characterized by two features: insufficient food and high transmission rate of parasites. Both significantly reduce profits from any livestock operation. Animals in poor condition cannot ward off disease easily, and they certainly do not produce maximally. From the parasite's point of view, fewer blades of grass available guarantee each infective larva a greater chance of ending up in the goat. Thus, overgrazing creates a situation where more larvae are concentrated on each blade; more worms in the goat produce more eggs, etc.

Over the years, there have been advocates of pasture rotation schemes to aid in the control of parasitic disease. For the most part, pasture rotation schemes, featuring increased stocking density, tend to increase populations of parasites. However, the improved nutritional status of the animals, due to rotational grazing providing more succulent and nutritious grazing, helps compensate for deleterious effects of the increased numbers of parasites. Pasture rotation, as a single practice for reducing parasites, is largely ineffective and may actually increase parasite loads in goats. This is particularly true for short term, rapid rotation schemes.

On the other hand, pasture rotation many decrease parasite numbers in deferred grazing systems if the pasture is rested for at least 6 months during the cool season and 3 months in the warm part of the year. Anything less than this is unlikely to effectively reduce larval populations. Contrarily, if pastures are tilled and replanted, most of the infective larvae would have succumbed to the effects of ultraviolet radiation and desiccation by the time forage regrowth had occurred. Studies comparing various deferred grazing systems in west Texas range lands have thus far not shown significant difference in the levels of parasites acquired among various management systems.

Alternate grazing of two or more ruminant species has been shown to be of value in controlling some species of parasites. When the range is shared by several grazing species, the competition for nutrients is usually intraspecific (between individuals of the same species). In other words, sheep, goats, and cattle seldom compete for the same type of grazing because the species prefer different types and lengths of forage. This affects parasite loads of each grazing species as transmission is dependent on ingesting the parasite larvae on certain parts of the forage. When sheep and goats are grazing in brushy country, sheep will tend to graze closer to ground level and goats will browse brushy herbage. In these circumstances, sheep may suffer from severe parasitic disease while the goats are relatively unscathed. On the other hand, when goats are forced to graze the same land as sheep without the opportunity to browse, the same species of parasites may devastate the goat population while the sheep are less affected.

Alternative grazing by different species that do not share the same parasites simply reduces the available parasites when the susceptible species is returned to do the pasture. On horse farms in Kentucky, cattle or sheep are often used to graze horse pastures before returning the horses to pasture. This reduces the horse parasites from the pasture without any harm to the sheep or cattle. Unfortunately, this practice in which sheep and goats are alternated on the same pasture is not likely to be as effective because they share many of the same species.

Alternate use is a variation of alternate grazing in which the land is used for other purposes between grazing of livestock; during such times, parasite contamination is actively reduced. A very successful approach has been adopted by some producers in which pastures are subdivided and the animals are intensively grazed at relatively high stocking rates for a relatively short time when the forage is in the young, active growing stage. As soon as this area is depleted or after a relatively short period of time, (e.g., two to three weeks), all the animals are moved to the next area. The first area is left to produce harvestable hay which, when baled and removed, eliminates most of the infective larvae. If this type of rotation is carefully planned, the animals can be returned to the original pasture when the new growth, after haying, is most nutritious. The producer gains from improved goat nutrition, decreased parasite loads, and reduced labor and drug cost.

Drought, good nutrition, bare soil, alternate species grazing, dung destroying insects, etc. may all contribute to the demise of parasites. Parasitic disease is an important limiting factor, and the judicious use of anthelmintics is essential. At this time there are no effective anthelmintics approved for use in small ruminants.

Resistance of Haemonchus to thiabendazole is widespread on the Edwards Plateau (Texas) and this resistance is also present to other drugs in this family (benzimidazoles, see Tables 1 & 2). One of the older anthelmintics, phenothiazine, is still apparently quite effective on some ranches in west Texas, but it cannot be recommended elsewhere. The use of low level phenothiazine as a preventive of hemonchosis has also not been thoroughly investigated in recent years in other areas.

Several anthelmintics are effective against benzimidazole resistant strains of Haemonchus. Levamisole appears to be effective in most areas, but reports of resistance to levamisole have validity. Recently, resistance to ivermectin has been reported in Texas and South Africa. When resistance is encountered, it is necessary to change to an anthelmintic with a different mode of activity. Anthelmintics such as levamisole, morantel or ivermectin have different modes of activity than the benzimidazoles and should be used when resistance is encountered. Because of the resistance of parasites to anthelmintics, it is imperative that goats be examined following use of anthelmintics to determine if the drugs used are truly effective. It is necessary to make egg counts before and after the use of an anthelmintic to determine if it is effective. If possible, some goats should remain as controls to determine if environmental factors may be contributing to parasite loss. Collection of fecal samples from 10 to 15 individuals of each class of animals on a ranch i.e., does, kids, etc., at the time of treatment and again in 7 to 10 days, will give a good estimate of the value of the anthelmintic used. Ideally, this should be done yearly on each ranch. Effectiveness of an anthelmintic would be acceptable if 90% or greater reduction in the mean egg counts before and after treatment occurs. If the reduction were less than this, it can be assumed that the anthelmintic is either not as effective as desired or it was incorrectly administered, i.e., wrong dose, out-dated drug, delivered to the wrong site etc. If an individual animal in a treated herd have no reduction in egg counts while others do, it can be assumed that the individual was missed or the anthelminthic was incorrectly administered.

If resistant is encountered to an anthelmintic, it will do no good to retain that drug or related drugs for use on the premises where resistance is seen. An example is the thiabendazole resistant strain of Haemonchus from the Edwards Plateau research station. Thiabendazole had not been used on the station for 20 years and, recently, when thiabendazole was used against that population of worms, there was only a 50% reduction in egg counts after treatment; the progeny of the surviving worms were nearly 100% resistant to thiabendazole. They also showed resistance to other benzimidazole anthelmintics. Resistance to levamisole may not last as long as with the benzimidazoles, but the genetic link to resistance to this class of anthelmintics is unknown.

Rotation of anthelmintics during a grazing season will not aid in preventing resistance, but may well lead to multiple resistance. Research indicates, but does not prove, that annual rotation of anthelmintics may aid in slowing the appearance of resistance. For this reason I usually recommend using only one anthelminthic drug, which has been shown to be effective in a particular herd, for the whole season (usually a year) and then, if necessary, change to another product (with a different mode of action) the next season.

Treatment of goats that are showing signs of disease has been de-emphasized here, primarily for economic reasons. If disease is seen, management practices need to be closely examined and, if necessary, drug resistance should be determined. It is frequently assumed, often correctly, that an emaciated or severely underweight animal is heavily parasitized. The opposite is not necessarily true because well fleshed animals, with a high body condition score and not showing typical signs, can carry extremely heavy loads of parasites. Occasionally, these animals may rapidly develop clinical signs and die before a producer can make a link between parasitism and the disease. Without doubt, regularly monitoring parasite loads in a herd will reduce these losses.

In general, treatment of diseased animals should include the use of effective drugs, good nutritional support and a reduction of stress. In a severely anemic animal, the simple stress of handling may raise the demand for oxygen sufficiently that it expires from anoxia. In severe circumstances, other medical treatments, such as blood transfusion and dehydration therapy, may have to be implemented. This is seldom economically justifiable, except in the case of valuable animals.

Table 1.
Anthelmintics used for small ruminant gastrointestinal parasites that are currently available in the United States.
Class of Compound Compound Trade Name*
phenothiazine phenothiazine various
benzimidazole thiabendazole TBZ
mebendazole Telmin
fenbendazole Panacur
oxfendazole Benzelmin
oxibendazole Anthelcide
albendazole Valbazen
pro-benzimidazole febantel Rintel
imidothiazole levamisole Levasol
tetrahydropyrimidine morantel Rumital
organophosphate haloxon Loxon
avermectin ivermectin Ivomec
Many are not at this time approved for use in small ruminants.
*Any reference to commercial products is made with the understanding that no discrimination is intended and no endorsements are made by the author or Tuskegee University, or are implied.

Table 2.
Relative spectrum of anthelmintic activity. The activity indicated may not be accurate for a given flock and annual re-evaluation of activity against Haemonchus is recommended.
Anthelmintic Haemonchus Gastrointestinal
phenothiazine + - -
thiabendazole + ++++ -
mebendazole ++ ++++ ++
fenbendazole ++ ++++ ++++
oxfendazole ++ ++++ ++++
oxibendazole + ++++ -
albendazole ++++ ++++ ++++
febantel ++ ++++ -
haloxon* +++ ++++ -
levamisole +++ ++++ -
morantel +++ ++++ -
ivermectin ++++ ++++ -
*Toxic to Suffolk sheep and Angora goats.
Most of the products listed are not approved for use in small ruminants in the United States.

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