Gastrointestinal roundworms, usually abbreviated as GIN (= GastroIntestinal Nematodes) are found worldwide and affect all kinds of livestock (also dogs, cats, poultry and horses), but particularly cattle sheep and goats.

There are about a dozen nematode genera (each genus with several species) that infect ruminants, with a different geographic distribution, depending on the climatic conditions (cold, moderate, arid, humid, subtropical, tropical, etc.), the ecologic environment (abundance of wildlife, vegetation, etc.) and the type of property (cattle, sheep, goats, mixed; extensive-intensive; outdoors-indoors, etc).

Strategic pasture management can reduce the incidence of gastrointestinal roundworms in livestock

The times when periodical undifferentiated administration of wormers was enough to keep GIN under control in a cattle or sheep property are over in most of the world. This is due to the steady spreading and strengthening of worm resistance to anthelmintics worldwide, particularly in sheep and goats, but increasingly in cattle and horses too.

It has taken more than 25 years between the discovery of the two last chemical classes of anthelmintics with different modes of action: ivermectin, the first macrocyclic lactone was introduced in 1981; monepantel, the first aminoacetonitrile derivative, was introduced in 2009. Derquantel, another nematicide with a specific mode of action, was introduced shortly after monepantel. And everything suggests that innovation in the field of livestock anthelmintics its not accelarating but slowing down.

In numerous sheep farms worldwide monepantel and derquantel (only in combination with abamectin) are currently the only anthelmintics that have no cross-resistance with other wormers, i.e. the only one that really works against GIN (so far neither product has been introduced for cattle). Unfortunately, resistance to monepantel has been reported already in several countries (e.g. New Zealand, Australia, Brazil) and reduced efficacy of the derquantel/abamectin product have been reported in Argentina.

The question is not whether worms will develop resistance to monepantel and/or derquantel elsewhere, but when. And it is likely to be very soon, since the first case of monepantel resistance in New Zealand was reported after only two years of using monepantel.

The past experience shows that first cases of resistance were reported in various countries already 4 to 5 years after the launch of a new chemical class of anthelmintics. 10 to 15 years later resistance had strengthened and spread to a significant number of properties. 25 to 30 years later, resistance made many of these compounds completely useless in many properties worldwide, even mixtures of active ingredients of different chemical classes (multi-resistance). But once a worm population has already developed resistance to one chemical class, resistance to another chemical class (i.e. multi-resistance) is likely to develop faster.

For this and other reasons it is widely acknowledged that successful control of GIN in ruminants in endemic regions (i.e. where the environmental conditions favor their development) absolutely requires a full set of measures that include:

In any case, whatever more specific preventative measures that are recommended by local or regional veterinary authorities should be known and followed.

Visit the section on Resistance in this site to lear more about it.

Knowing the worm species that infect the property

Roundworm (1), fluke (2) tapeworm (3)It is highly recommended that each producer knows the predominant GIN species in his property. There are many reasons for it.

A trivial feature of most parasitic worms that infect livestock and other domestic animals is that, in contrast with the external parasites (ticks, flies, etc.) you don't see them. If the animals become sick, or if their performance falls without an obvious reason, investigations to find out whether they are infected with GIN may be undertaken. But if there are no clinical signs, you can think that the animals are worm-free, when in fact they are not.

The reason is that low worm burdens are often well tolerated by adult livestock that has developed natural immunity. But in spite of the absence of clinical signs the property is actually infected. The risk is that you find it out too late. Because unexpected events can lead to a sudden increase in the worm population, which on its turn can cause sudden outbreaks that can be very harmful, especially for young animals. The risk of such events to happen and the potential damage they can cause depend strongly on the worm species in the property. Thus the need for knowing know which GIN species are there and to estimate what could happen if...

In addition, many potential preventative measures depend on the predominant worm species in a given property, because each species has its own life cycle (duration, preferred habitats, survival in the environment, intermediate hosts, etc.) and most preventative measures aim at breaking or at least disturbing the life cycles of the worms. This is why it is very useful to know which worm species are predominant in the property.

Finally, numerous anthelmintics treatment regimes (dosing, when to treat, which animals to treat,) depend on the worm species to be controlled. If you don't known which species affect your livestock, you may be overdosing and spending too much in anthelmintics, or underdosing and leaving some animals insufficiently protected.

The best way to know which GIN infect a property is the analysis of fecal samples by an experienced laboratory. This has become a routine and affordable service in numerous countries (most of Europe, North America, Australia, New Zealand, etc.) and each property should do such an analysis periodically. This reason for periodic analysis is that the worm-mix may change in a property: due to changing climatic conditions (global warming...), to modifications of the grazing and pasture management practices or of the animals or breeds raised, to development of worm resistance to anthelmintics that can make certain species to become predominant, etc.

Unfortunately, a service for fecal sample analysis is not available or not affordable for many producers in numerous less developed countries in Africa, Asia and Latin America. Nevertheless, experienced veterinarians can often help finding out the "problem worms" in a property based on the clinical symptoms. Experience from neighboring properties can be sometimes helpful as well.

Adapting herd and pasture management practices to prevent infections with gastrointestinal roundworms

It is widely acknowledged that eradication of gastrointestinal roundworms from a region or a property is mostly not possible, or at least not sustainable. One reason is that numerous GINs are also natural parasites of wild animals that share the pastures with livestock and act as reservoirs for parasites, because they are not and cannot be systematically treated with anthelmintics. Another reason is that incoming stock in a property can easily re-introduce parasites that had been eliminated, and livestock movements within a property or a region (e.g. from summer to winter pastures) contribute to spread the worms as well. Finally, free-living stages (eggs larvae, etc.) of several parasites are highly resistant to adverse environmental conditions and can survive and remain infective in a property for years.

Since eradication is not a realistic option, efforts must focus on:

  • Reducing the parasite load of a property below the economic threshold level, and/or to
  • Protecting those animals more likely to suffer from GIN infections (e.g. young stock).

Adequate pasture and herd management can help to reduce the level of infestation. This is a key for preventing sudden acute outbreaks in a property, which are the most damaging, especially for young stock. They also help to reduce the amount of wormers needed in a property and the workload required for their administration.

Each property is a different case. What makes sense and is feasible in a given property, may not work or be unfeasible in another one. Or what makes sense in spring may not make sense in autumn. Therefore each property has to develop its own strategy. This article summarizes the most important elements of such a strategy and shows some practical examples. They have to be adapted and complemented based on the local conditions, experience and possibilities, always following official recommendations of the local veterinary authorities.

Ensure a healthy heard

Healthy animals are more resistant to gastrointestinal roundworms.

Healthy livestock suffers less from GIN infections and is more likely to develop it's immunity to many GIN and other parasites. It may support a certain number of parasites without becoming sick and with little to no impact in its productive performance (weight gains, milk output, reproduction, etc.). Ensuring food quantity and quality (vitamins, minerals, etc.) is essential for livestock health and has high priority, especially during the dry season in tropical and subtropical regions, or during winter in the cold regions. Protein-rich food is particularly important for ensuring the proper functioning of the immune system.

However, factors other than poor food can also weaken the acquired immunity of adult livestock: certain medications (e.g. steroids that can be immunosuppressive); stress due to excessive movements, crowding, cold or heat, excessive ectoparasite burdens (ticks, flies, etc.), etc.; certain physiological processes such as pregnancy, parturition and lactation. Animals suffering from viral or bacterial infections are more likely to be weaker and therefore less resistant to GIN infections.

In any case, even healthy livestock develops some degree of immunity only to certain GIN species, not to all of them. And these immunity needs years of exposure to the parasites to be acquired. If animals are not exposed to such parasites, they won't develop any natural defenses. Or if they have too many parasites, they will become sick anyway. Obviously, young stock lacks an adequate natural immunity to GIN (and other parasites) because they had not lived long enough to be exposed to the parasites and, consequently are more susceptible of becoming sick, sometimes severely sick with fatal consequences.

Keep the pastures free of infective worms

Once pastures have become infected with GIN, they will remain infective for months and even years, depending on the worm species and the climatic conditions. This is particularly true if wild ruminants (deer, antelopes, buffaloes, camelids, etc.) share the pastures with livestock: they are adequate hosts for most GIN that affect livestock. But even if eradication is not possible or not sustainable, some practices help to reduce the degree of pasture contamination with GIN and to keep it low. Deciding which option is best for a given property depends on its structure but also on the knowledge of the predominant worm species and their life cycles.

Resting pastures during several months (i.e. keeping them free of livestock) will cause infective larvae of some GIN to die before they find a host and reproduce. This is due to the fact that larvae in the pastures are often killed by sunlight, dryness, cold, etc. This is what happened in the past during transhumance of sheep herds and still occurs to some extent where nomadic animal husbandry is practiced. This has to be done during those months of the year with the most adverse conditions for worm survival (the hottest, the dryest, etc.). This depends not only on the weather, but also on the season, the landscape, the type of soils (water retention, permeability, drainage, etc,), the grass species, whether wild ruminants have access or not to the resting pastures, etc. However, some GIN can survive for months and even years under adverse conditions. And resting pastures may not be feasible for simple economic reasons. A particular case is the alternate use of land for crop and livestock production, e.g. with a yearly or half-yearly alternation.

Rotational grazing is a another particular case of resting pasture that is often practiced to ensure good pasture quality. It consists in dividing the pastures in a number of paddocks that are grazed one after the other. This shortens the time a paddock is grazed and allows it to rest for a relatively long period of time, which will kill many worm larvae. Research in Uruguay showed that 90 days resting were the minimum to achieve a significant reduction of pasture contamination with infective larvae of sheep GIN under the local climatic conditions. In regions with a tropical climate good results have been achieved on sheep with 30 to 40 days resting preceded by 4 to 5 days grazing. Cattle pastures seem to need longer resting times, perhaps because the cow pad allows a longer survival of worm larvae than the dryer sheep droppings.

Alternate grazing is another option consisting in letting different age groups of livestock (e.g. calves followed by cows) or different livestock species (e.g. cattle, sheep, horses) to graze successively different paddocks. This is based on two facts: young animals are more susceptible to GIN infections than older animals that have developed some degree of immunity; and some GIN species are host specific, i.e. the species that infect cattle are not parasitic for sheep, and vice-versa. It is also known that cattle are more likely to develop resistance to parasitic worms than sheep. For this reason cattle can be let first into some pastures to act as "vacuum cleaners" of free-living worm stages, followed by sheep. However, alternate grazing is not advisable in properties where GIN predominate that are parasitic for both cattle and sheep (and horses).

Grazing stubble fields after the harvest. This practice typical of nomadic husbandry makes a lot of sense in the context of GIN prevention. Such fields are free of worms because they have not been visited by animals that allow maintaining the life cycle of the worms, and because plowing can substantially reduce the survival of infective larvae in the fields. This practice is also beneficial for most croplands.

Plowing and re-seeding the pastures with grass will diminish the number of infective larvae, but won't eliminate them. A better option is to grow any other crop and to re-seed it with grass only after the harvest. This would kill most of the GIN infective larvae.

Keeping the pastures as dry as possible can significantly reduce the number of infective worms. Permanent or recurrent water surfaces (ponds, lakes, pools, ditches, pits, moats, irrigation channels, surroundings of water points, periodically flooded zones, etc.) allow development of huge numbers of infective larvae of certain GIN species and also of those worms that have snails as intermediate hosts (e.g. liver flukes). Consequently, whatever eliminates such humid spots or shortens the time they remain humid will diminish the contamination with GIN.

Reduce the risk of animals being infected

Numerous management practices reduce the exposure of livestock to free-living infective worms stages and thus the risk of becoming infected.

Avoid an excessive stock density on pasture. Pasture on places where animals concentrate is more likely to be highly contaminated with infective worms. The reason is that the excrements more or less loaded with eggs or larvae will continuously re-infect the environment, which will be quite humid due to the abundant urine. For these reason overstocking must be avoided. This is also important because harm caused by the worms is often proportional to the number of worms that infect an animal. Therefore: the more stock, the more worms are on pasture, and the more harmful worm infections will be. And the opposite is also true, the fewer the worms, the better they are supported by livestock, and the more likely that animals develop their own natural immunity.

Periodocally changing the resting places for the night reduces the time that animals spend in highly contaminated areas.

Adequate pasture rotation also reduces the time that animals spend in contaminated areas. Each time they get into a new paddock, it should be clean of worms, or at least cleaner than the previous one.

Placing food troughs with feed or hay in places where the animals concentrate reduces the chance that they graze in these highly contaminated places.

Such humid places may support high numbers of parasitic worms. Picture from

Keep livestock away from humid zones using appropriate fencing. Humid grass is more likely to be contaminated because infective larvae of many species can swim in the water that covers the grass blades. For the same reason, restricting grazing at dawn or dusk when dew wets the pastures can reduce the number of infective larvae ingested by livestock. 

Separate young stock from old stock as soon as possible, because young stock is likely to suffer more from GIN infections, and old stock is more likely to be infected and thus transmit the worms to the younger stock. Old stock may seem to be worm-free because having developed some natural tolerance or immunity they don't get sick and consequently they don't show clinical signs. But this does not mean that they are worm-free. Actually, in most cases such animals are "resilient" (i.e. infected but still shedding worm eggs and thus contaminating the pastures) and not "resistant" (i.e. perhaps infected but not shedding eggs). For the same reason young stock should not be allowed to share the pastures with wild ruminants, which very probably are infected as well.

Keep cowpats intact until they dry up, or make it easier that this happens in the property. Most GIN can complete their development to infective larvae inside the cowpats if it remains humid for a minimum period of time. But to be eaten by cattle, larvae have to leave the cowpat and disseminate in the surrounding pasture, because cattle do not eat grass contaminated with excrements if they have an alternative. This means that the risk of infection for livestock is lower in dry zones where the cowpats, either intact or trampled, will dry up quickly before larvae can leave it. This also means that high humidity (abundant rain, irrigation, flooding, etc.) and mechanical breaking of the cowpats (trampled through livestock or machines) makes it easier for the larvae to leave the cowpats into the surrounding pasture, which increases the chance that livestock ingests them and becomes infected.

Favor grazing in rich and tall pastures. This is due to the fact that, once they get older, infective larvae of several GIN species concentrate at the base of the grass blade and in the soil. Consequently, livestock grazing on short grass is more likely to ingest infected larvae than livestock grazing on tall grass. In addition, short grass is often less nutritious than tall grass and livestock grazing on short grass tends to be underfed. The same applies to recently sown pastures.

Ensure appropriate hygiene indoors. Some GIN species (e.g. Nematodirus) can complete their life cycle from egg to infective larvae indoors in hay or bedding mixed with excrements. And infective larvae of many species (e.g. Dictyocaulus, Haemonchus, Oesophagostomum, Ostertagia, etc.) can survive for days in humid hay, freshly mowed grass, humid bedding, etc. To reduce the risk that livestock ingests these larvae indoors, facilities should be regularly cleaned, the droppings removed, the beddings replaced and all should be kept as dry as possible. Hay from contaminated pastures should be brought in the stables only after it has thoroughly dried out.

Ensuring best use of anthelmintics

Most wormers for livestock contain one or more anthelmintic compounds effective against the major GIN. Used correctly they can keep these worms under control, i.e. below the economic threshold level, provided they are used correctly. Correct use absolutely requires to strictly following the use instructions in the product label. Don't trust products with unclear, equivocal or vague use instructions. But be aware that even correctly used, no wormer will kill all the worms forever, even if the worms are not yet resistant to the anthelmintic active ingredients in the wormer.

Read the product label carefully

A good product label must contain specific information on:

  • Which worm species it controls (i.e. its spectrum of efficacy)
  • Which developmental stages of each worm it controls: adults, immature stages, inhibited (or dormant) larvae, eggs, etc.

Don't trust a product that simply says it controls "gastrointestinal worms".

It must also indicate the recommended dose, which can be different for various worm species, type of livestock (age, weight, etc.).

Depending on the product, its label will also indicate the residual effect or protection period, i.e. how long it will protect treated animals against re-infestation, which is often different for various parasite species. Some product labels may recommend certain treatment regimes (how often to treat the animals) but most do not because they must be determined based on the specific characteristics of each property (type of livestock, predominant worm species, ecologic environment, climatic conditions, etc).

It is useful to know the manufacturers cannot include in the label all the information on the product that is available. Additional information is often found in leaflets, product brochures, or websites of the manufacturers or veterinary authorities. It is highly recommended to read such additional information. They often contain useful practical advice for producers.

Find out the spectrum of activity of the product, i.e. which worm species it controls

This is information is usually shown in the so called "indications", which tell for which parasites in which target animals (e.g. cattle, sheep, goats, etc.) the product has been approved by the regulatory authorities. 

No wormer controls all the worms that can infect livestock, and not even all the development stages of each worm. All have a more or less limited spectrum of activity. This is why it is crucial to know which worm species are predominant in a given property, and which worm species are controlled by a particular wormer. Otherwise the wrong product may be selected, or it may be used incorrectly, which can have dramatic consequences in case of sudden outbreaks.

If a product is not approved for a given indication, i.e. if a particular worm species is not mentioned in the label, it can be due to various reasons. It is useful to know, that approval of an anthelmintic indication by the regulatory authorities requires that the manufacturer demonstrates and documents (with corresponding study reports) at least 80% efficacy (sometimes at least 90%) against each parasite species in each target animal, and that the dose required to achieve this level of efficacy is safe for livestock.

Knowing this, it is easy to understand why a product is indicated for some uses and not for other ones. On simple reason is that the active ingredients in a wormer are not at all effective against such worms (e.g. macrocyclic lactones are not effective at all against flukes or tapeworms). Another reason can be that efficacy against some worms does not reach the 80% minimum at the recommended dose. In this case some product labels will say, "aids in the control of...", others will say nothing. Another reason can be that the manufacturer did not deliver adequate studies to support an indication. This is often due to a cost-benefit analysis. Almost no company will spend millions in the development of product indications for minor markets (e.g. dairy sheep, goats, minor species such as rabbits, etc.) or unfrequent parasites.

Obviously, no manufacturer will tell in the product label why it is not indicated against this or that. In most cases they won't tell either, which % efficacy (80%, 85%, etc.) can be expected against this or that parasite, unless it is very high (e.g. >95%, or >99%) or at least superior to most competitors. However such information may be available in product leaflets and brochures. But, be aware that in such commercial publications most manufacturers will tell the truth, but not the whole truth. They will show those pieces of information that put their product in a better shape than the main competitors, and retain those pieces of information that don't.

It is possible that a particular product is effective against parasites in targets animals for which it is not approved. For some vastly used active ingredients (e.g. ivermectin, levamisole, albendazole, etc.) there is abundant information (dosing, efficacy, etc.) in the scientific literature. In most countries veterinary doctors are allowed to prescribe off-label uses based on this information and always under their own responsibility, which must consider not only efficacy, but also safety for livestock, for consumers (residues!) and for the environment.

Find out if the product controls immature stages as well, including migrating and inhibited larvae

Whether a wormer controls only adult worms, or also immature stages is a key feature of any anthelmintic product. Depending on the worm species, immature stages (larvae) may be:

  • Recently ingested larvae underway to their predilection site but still in the lumen of the stomach or the gut
  • Larvae migrating through various other organs (liver, lungs, blood vessels, etc.) before they reach the gastrointestinal system and complete development to adults.
  • Inhibited larvae (arrested, hypobiotic, etc.) installed somewhere in the host's organism and waiting to resume development upon better development conditions.

It is important to know that larvae, particularly migrating larvae can be very harmful for livestock, because they may damage essential organs (liver, lungs, etc.). And numerous active ingredients are ineffective against any stages that are not in the stomach or the gut. The reason is that these active ingredients are not absorbed into blood but remain only in the stomach or the gut and consequently do not reach the organs where the migrating larvae are at the time of administration. Such active ingredients often have no residual effect or only a very short residual effect, i.e. when these migrating larvae finally reach the gut or the stomach, the effect of the wormer will be over.

>Something similar happens with the inhibited larvae of some species (e.g. of the genera Bunostomum, Cooperia, Haemonchus, Oesophagostomum, Ostertagia, etc.).

Most product labels indicate the type of worms they control. However, most won't say something like: "WARNING: this product does not control inhibited larvae". They simply say, "This product controls adult worms..." As a general rule, if a product label does not explicitly say that the product controls migrating and/or inhibited (hypobiotic, arrested) larvae, assume it does not. 

Find out the residual effect of a wormer

The residual effect is the time after administration that a product will protect the treated animals against re-infection. It is usually expressed in days or weeks. It may anything between zero (a few hours) and more than 3 months (e.g. some slow-release boluses, some highly concentrated injectables with macrocyclic lactones). Some wormers (e.g. those with benzimidazoles and many other for oral delivery containing levamisolepiperazine, morantel, etc.) have no residual effect or only a very short one. They kill the worms (adults and sometimes larvae as well) in an animal more or less quickly after administration, but won't protect the treated animal against re-infestation.

A long residual effect allows longer treatment intervals, which is often a key benefit of a wormer. But it is usually associated with long withholding periods (several weeks and even months).

Administer the product correctly

Strictly follow the use instructions in the product label

This should be self-explaining, because the risks of not doing it are significant: from inefficacy (waste resources) to poisoning of livestock.

Several successive operations have to be done correctly to ensure that each animal is treated at the right dose. It's enough that one goes wrong for the whole treatment to fail.

If required, process the product correctly before delivery

If the product needs some previous processing (diluting, mixing, shaking, etc.) follow the label instructions. Shaking is crucial for many drenches (e.g. those with benzimidazoles, called "white drenches" in some countries). The reason is that the active ingredients are not soluble in water. The product is not a solution but a suspension: the active ingredient "swims" in the vehicle (mostly water), but tends to sediment in the bottom of the container. If it is not shaken before use, some animals will be overdosed and other animals underdosed.

Make sure that weigjt estimation is correct! Picture taken from

Weigh the animals correctly

Delivery of most wormers must be done on a weight basis, often expressed (for liquids) in ml of a product per kg body weight (e.g. 5 ml finished product for every 50 kg body weight). Correct use obviously requires knowing the weight of the animals to be treated. Best is to group the animals according to their weights and to treat them together. Many producers have an excellent "eye" to estimate the weight of the animals. Nevertheless it is highly recommended to physically weigh a few animals of such groups to confirm that the estimation is correct.

Calibrate the application equipment (drench guns, syringes, etc.)

Drenchesinjectables and pour-ons are delivered with more or less sophisticated devices. To ensure that they really deliver the desired volume they must be calibrated each time they are used, especially if they have not been used for months. Many pieces of such devices (joints, tubing, pistons, etc.) are exposed to normal wear and may become inaccurate with time.

Checking whether they deliver the adjusted volume is rather easy. Before delivery of the product to the animals, adjust it e.g. to 20 ml, load and empty the device ten times in a graduating cylinder using the product to be administered, and chek that the graduated cylinder really contains 200 ml. Doing it with water can be inaccurate because wormers may me more or less viscous than water, which can falsify the result.

If such delivery devices are powered, it must be checked that the pressure remains constant. When large containers are used (e.g. 5 L drench or pour-on containers) it is important to compensate the progressive vacuum that builds up in the container by simply opening and closing it periodically. Otherwise the vacuum can disturb the accuracy of the delivered volume.

If a delivery device does not work properly, replace it. The cost of such replacement will be much smaller than the potential damage that can follow an incorrect administration.

Ensure that each animals gets the full dose

Even if all previous steps were done correctly, things still can go wrong, e.g. if an animal vomits, or regurgitates part of the delivered dose. Or if a pour-on is delivered by windy weather (part of the product may be blown away) or by rainy weather (part of the product may run off).

For more specific indications regarding correct administration read the specific articles on the major delivery forms.

Use the anthelmintics strategically

Strategic use of anthelmintics means treating livestock with wormers in a selective way instead of indiscriminately treating all the animals at fix intervals, regardless of their age, location, season, etc. There are three major approaches that can be combined:

  • Treating when it has the strongest impact on the worm population (e.g. interrupting or delaying population build-up at the beginning of the season; reduce worm transmission among animals, etc.)
  • Protecting the most valuable stock or those animals that are more likely to suffer from worm infections, e.g. young stock, pregnant animals, etc.
  • Reducing the amount of anthelmintics used in a property, which has not only obvious economic reasons, but is also highly desirable in order to delay development of worm resistance.

Specific strategies have to be determined based on regional or local environmental and climatic conditions, prevalent worm-mix, type of farming, etc.  Basically each property has to design its own strategy, if possible following official recommendations from local veterinary authorities.

In the following some examples of strategic use are described.

Quarantine all stock brought into the property and treat it (once or more times, depending on the product) with adequate anthelmintics. This is crucial to avoid introducing new worm species in the property, or worm strains that are resistant to anthelmintics. Don't let this new stock come together with the old stock until you are sure or at least can assume that it is worm free.

Protect young stock selectively during their first grazing season. The reason is that they still don't have developed their own immunity because they had not been in contact with the worms yet. Exposed to highly infected pastures without an adequate protection acute sudden outbreaks can be extremely harmful, even fatal for such stock. Adequate anthelmintic treatment will not keep this stock absolutely worm free. But infections will remain low, at a level that still allows a healthy growth, and at the same time allows this young stock developing their own natural defenses progressively.

Use a group of animals treated with anthelmintics as "worm cleaners" of a delimited area. The first animals that graze a paddock will ingest most of the infective larvae in the pastures. Adult stock adequately treated can to some extent act as "vacuum cleaners" of the worms in the pastures. If this is done long enough, a paddock can become almost worm-free and may be grazed later by stock that does not need to be treated.

Selectively protect susceptible stock (i.e. stock that has never been exposed to worms) with adequate anthelmintic treatments. This can be advisable when stock is displaced from worm-free to worm-infected parts of a property. Consider that stock bred elsewhere and introduce in a property is likely to have been exposed to other worms than those in the property, especially if it comes from very different climatic regions.

In regions with moderate climate and cold winters, it makes a lot of sense to concentrate the use of anthelmintics in spring (to prevent or at least delay or reduce the build-up of high worm populations in the pastures) and autumn (to reduce the number or overwintering larvae).

Alternatives to anthelmintics

Vaccines and Biological Control

Hundreds of millions of USD have been invested in research on non-chemical control methods, mainly on vaccines and biological control alternatives. The practical results are so far very disappointing. Not even a handful of vaccines (of variable efficacy) have reached the market (e.g. against Echinococcus granulosus, Dictyocaulus spp, and Haemonchus spp) and so far there are no reliable biological products (fungi, bacteria, viruses, etc.) in most of the markets.

Research on these non-chemical control methods continues and there will be breakthroughs. But nobody knows when. And unfortunately, the problem with GIN in ruminants is that infections involve always several worm species, so-called mixed infections. But most if not all biological control methods are very specific. This means that even if successful vaccines against e.g. two worm species are developed, three or more species will still require using classic anthelmintics. And the same biological mechanisms that drive GIN resistance development to chemicals can cause the appearance of resistance to non-chemicals, if they are used as intensively as chemicals.

For additional information on vaccines against external and internal parasites of livestock click here.

Medicinal plants with anthelmintic efficacy

Many plants (extracts, powders, etc.) have been traditionally used in various regions of the world to "control" worms on livestock, humans and other domestic animals. If you have a good experience, keep using them. Alternatives to anthelmintics are always welcome, because they help delaying the appearance of worm resistance to anthelmintics.

But don't expect miracles. Medicinal plants with some kind of anthelmintic efficacy are usually not adequate for handling acute worm infections of livestock, but can help reducing their incidence and perhaps contribute to keep them below the economic threshold level. 

medicinal plants with anthelmintic efficacy click here.

Introduce breeds of livestock resistant to GIN

It is well known that some cattle and sheep breeds are naturally resistant to certain GIN (and other parasites). It is also known that within the same breed, individual animals are more capable of developing natural worm resistance than others. However it must be distinguished between resistant animals and resilient animals.

Resistant animals support infections without becoming sick, and the worms are not capable of completing development in such animals. This means that these animals do not shed eggs and consequently do not re-infest the pastures.

Resilient animals support infections without becoming sick, but the worms are capable of completing development in such animals. This means that these animals will continue re-infesting the pastures with worm eggs.

This is a significant difference! E.g. resistant adult animals can graze together with young stock and will not contaminate them. Resilient animals grazing together with young stock will contaminate young stock very quickly.

The only way to find out whether an animal is resistant or resilient is to take fecal samples and analyze them for worm eggs.

Theoretically, each producer can start a program to select his animals for worm resistance. However, this is not feasible without a specific know-how and requires significant resources and time to be successful. In most places this is rather done research institutions or large breeders. For most producers the best option would be buying "resistant" stock sold by professional breeders to progressively replace their old worm-susceptible stock. Unfortunately such an option is not yet available in most countries. And so far, little is known yet on other features of such worm-resistant breeds, i.e. whether they are as performing as non-resistant breeds regarding other numerous crucial economic parameters (reproductive potential, growth, wool quantity and quality, milk production, etc).

Integrated worm control

Integrated Worm Control is a particular case of Integrated Pest Management (IPM). IPM consists in approaching parasite control from a global perspective, beyond the simple use of pesticides to kill the parasites. It means combining whatever structural, management and ecologic measures that help reducing the parasite populations in a given ecosystem.

Gastrointestinal worms, livestock, intermediate hosts if involved, wildlife, pastures with associated vegetation, and the particular landscape, all build a strongly interdependent ecosystem in each property. Indoor operations represent also particular ecosystems.

Integrated Worm Control means "attacking" the worms with different "weapons" at various "fronts", not only with anthelmintics administered to livestock. Integrated worm control, as IPM, requires designing specific programs for particular regions with similar ecologic, climatic and husbandry conditions, involving all the weapons previously mentioned in this article that make sense for such particular regions. One important objective of Integrated Worm Control is to become less dependent on anthelmintics: for economic and ecological reasons, but also to reduce its use in order to prevent or at least delay the appearance or worm resistance to anthelmintics.

Each producer should develop an Integrated Worm Control program for its property that takes into account more specific regional recommendations and the particular features of the property. Experienced veterinarians and/or government officials can support producers in designing such Integrated Worm Control programms.

Click here to learn more about Integrated Pest Management.