Horse worms and wormers - the Facts

INDEX:
1. Understand the worms
2. The Anthelmintic Drugs we Currently Use
3. A Short History Lesson – on why we developed worm resistance
4. Parasite Resistance to Anthelmintics
5. How To Tell if You Have Resistant Worms
6. Modern Anthelmintic Programs
7. Management Activities That Make a Difference
8. Choice of Worming Product
9. What products Are Currently Available in Australia?
10. Understand the Ivermectin Terminology
Dr. Grahame Best


All horses have worms.

Internal parasites are a significant threat to the health of horses, particularly in young or undernourished animals.

The horse is susceptible to more than 70 internal parasites, and commonly hosts several species of worms at any time, in addition to many external parasites that can cause irritation and disease.

All of these parasites contribute to ill thrift and reduced performance. Many are serious pathogens which produce clinical disease and death. Many of the life cycles are complex and prolonged, and control measures must generally be lifelong on a very regular basis.

At this time, control depends heavily on anthelmintic drug use, repeated regularly to provide an effective preventive worm control strategy. There are also many husbandry and management measures which complement the effective use of anthelmintic drug treatments. Sadly, these management measures are frequently not considered, or implemented, with the resulting failure, or partial failure of a worming program.

If you are having trouble visualising the scope of the horse worm problem, consider this -
The common reliable method of determining the actual level of worm infestation in horses is the faecal egg count. If this count were to show a parasite egg count of about 2,000 eggs per gram of faeces – a common finding- that horse could easily release 25 million worm eggs per day into the environment.

Horse owners have been made well aware of the impact of helminth disease on horse health, and of the need to eliminate internal parasites in horses for many decades. The volume of information available, and differing recommendations made for the control of worms, has regularly created confusion and uncertainty, often to the detriment of the horse population.

Regular horse industry surveys have confirmed that about 98.6% of equine operations dewormed horses as a general preventative measure sometime in the past 12 month period. About 49% of these worm at least 4 times during that year.

No studies show that horse owners anywhere fully understand the details of what they are doing, what worms they are trying to target, the life cycles of the worms, the best times to strategically target individual worms, or the most appropriate drug class to use to control the worms at that time of year.

Many owners are simply influenced by the available marketing information at the time, and often don’t have enough information available to make informed decisions about worming measures. Still others are solely influenced by product price, and they may not even choose the appropriate drug for the job at hand.

The prevailing attitude for most horse owners is now effectively a “zero worm tolerance” approach because of the availability of safe, effective anthelmintics which are easy to administer on a regular basis. Unfortunately, this approach has led to a situation where anthelmintic resistance is now extremely important, both from a horse health perspective, as well as from a marketing angle. Owners are regularly bombarded with marketing information about the ability of individual products to overcome resistance, but they have little understanding of the actual science behind this information.

Let’s try to fix that right now.

The marketing issues related to worming horses are now focussed primarily on resistance of worms to the available drugs. There is little emphasis on actually understanding parasite life cycles and individual drug actions.

This article is written in the hope that you will have enough technical information to enable you to make an informed decision on how, and when, to worm your horse effectively.

Firstly, we need to understand the actual internal parasites we are trying to control.

1. UNDERSTAND THE WORMS

There are 5 major types of equine internal parasites:
Large Strongyles,
Small Strongyles,
Ascarids,
Bots,
Pinworms,
and then several more minor parasites, including Tapeworms. There are also some important arthropods which can now be controlled simultaneously (ticks, lice, etc)

To understand how varied horse internal parasites are, let’s first have a quick look at the organs they live in, or can be found in. This will help you to understand why control can be difficult, and ongoing:

Stomach
Stomach Worm Draschia megastoma, Habronema spp
Stomach Hair Worm – Trichostrongylus axei
Bot Fly larvae – Gasterophilus spp

Small Intestine
Large Roundworm – Parascaris equorum
Threadworm – Strongyloides westeri
Tapeworm – Anoplocephala spp

Large Intestine
Strongyles: Large Strongyles – Strongylus vulgaris (bloodworm), S.equinus, S.edentatus
Small Strongyles – Cyathostomes – over 40 species
Itch Worm Oxyuris equi
Tapeworm Anoplocephala spp

Lungs
Lungworm Dictyocaulus arnfeldi
Large Roundworm Larvae Parascaris equorum

Liver
Large Strongyle Larvae Strongylus spp.

Eye
Stomach Worm larvae Habronema, Draschia spp

Ligaments
Nuchal Ligament Worm Onchocerca cervicalis, O.gutterosa

Skin
Microfilaria Onchocerca spp
Intestinal Threadworm Larvae Strongyloides westeri

Mouth
Bot Fly Larvae Gasterophilus spp
Brain
Stomach Worm Larvae Habronema, Draschia spp

Arteries
Bloodworm Strongylus vulgaris

Other Tissues
Migrating stages of large strongyles, especially S.edentatus and S.equinus may be found in many tissues, including pancreas, kidney, testes, ovary, abdominal cavity

In addition to these parasites, the list of external parasites of horses includes Flies, Lice, Ticks, Mites and Midges.

The most damaging horse internal parasites are the large strongyles and ascarids (roundworms), sometimes causing permanent damage that can profoundly affect equine performance.

Now let’s look at the actual worms, and some of the important aspects of their lifecycles:

Large Strongyles. (also known as bloodworms, redworms)

Strongylus vulgaris is the most significant worm in this group. Also Strongylus edentatus and Strongylus equinus

The most common and destructive internal parasite of horses, the large strongyles have traditionally been considered as the most dangerous of horse worms because adult worms consume large amounts of blood, causing anaemia, weakness, loss of appetite, diarrhoea, and damage to the internal lining of the gut, and because the immature larvae, before they mature and settle in the large intestine, migrate to the branches of the intestinal (anterior mesenteric) arteries where they cause damage, irritation and aneurism. An aneurism is a bulging, weakened blood vessel wall which may alter the flow of blood to the bowel or may rupture, causing death by internal bleeding. If these intestinal blood vessels are blocked by blood clots, serious colics follow, typically after feeding or exercise.

Seen in horses of all ages, except in very young foals, large strongyles range in size from 1 to 5cm. The mature female lays eggs which are passed in faeces, and the infective larvae hatch to contaminate grass and hay.

It takes about 6 months for the immature strongyles to complete their tissue migration before they settle as adult worms in the large intestine. Foals under 6 months of age may thus have immature, migrating strongyles in tissue, but not yet in the gut.

Not all wormers are effective against the immature or migrating tissue stages of this worm. Adults are chiefly in the caecum and large colon, attached to the mucosal lining. Immature worms are mostly in blood clots and the walls of arteries, or in nodules in the bowel wall., It takes 5-7 months from ingestion to development of egg laying adults.

S.edentatus larvae tend to migrate more to the liver and peritoneal tissues. It takes about 11 months for this parasite life cycle. They can cause tissue damage to liver, lungs and bowel, and do cause diarrhoea, colic and anaemias.

Large Roundworms (Ascarids) (Parascaris equorum)

These live in the small intestine and grow to about the thickness of a pencil and 20-25cm long. They cause illness primarily in foals and young horses, with mares being the main source of infection for foals. These worms often cause unthriftiness and weakness in foals, and very occasionally bowel obstructions and ruptures can occur. Female worms produce about 100,000 eggs per day, and larvae are very resistant, lasting years on pasture

Found mostly in the Duodenum (small intestine), large strongyles are most frequently a problem in young horses. They do not suck blood, but much of their damage is due to the migration of the immature worms throughout body tissues. Small larvae are ingested by the horse, hatch in the intestine, burrow into the intestinal wall and migrate to the liver through the blood. They can reach the heart and enter the small airways of the lungs and the trachea, and are then coughed up and swallowed, to mature in the intestine. Lung tissue can be extensively damaged, and large numbers of ascarids can block the intestines, especially in foals. Damage to lung tissue is usually permanent.

It takes about 10-12 weeks for Ascarids to complete their life cycle. Most foals become infected by larvae shortly after birth, with worms maturing when foals are about 2.5 to 3 months old. Foals must be treated from 8 weeks of age to control this parasite, with repeated treatments at 6-8 week intervals for benzimidazole drenches or 8-10 weeks for ivermectin products. Older horses develop immunity to these roundworms, and are much less affected, generally.

Small Strongyles (Cyathostomes)

There are at least 40 species of small strongyles (previously known as the Trichonema group), and they are now the most important equine internal parasite because this group has developed significant resistance to many anthelmintics. Small Strongyles limit their migration to the intestinal lining, where many encysted small strongyles may remain unaffected by anthelmintic treatment for long periods (often for many years). When young strongyles emerge from their protected cysts, colics can often occur.

The life cycle of these parasites is continuous, and can complete in 6 weeks, so treatment needs to be regular, every 8-10 weeks, year round, to effectively control them. Virtually 100% of horses are infected with some species of small strongyles.

Clinical signs of infection with small strongyles generally include ill thrift, anaemia, loss of appetite and diarrhoea, but are non-specific.

Numbers of these small strongyles are lower in adult horses because immunity does develop in time. The small strongyles are much less harmful than the large strongyles (Strongylus spp) because the larvae only migrate into the lining of the intestine, not into tissues such as lung and liver, and they then encyst in the gut lining. Large numbers of small strongyles can emerge from the cysts, especially in late winter or early spring, or after deworming. (Worm treatment typically kills all adults living free in the intestine, but often does not kill encysted larvae in the gut wall, (depending on the drug class used), which can survive worming treatments easily. When all adults are killed, large numbers of larvae are then released from the cysts, to replace the free living adults in the gut)
.
Resistance to all anthelmintic drug classes except the ivermectins and moxidectin (the macrocytic lactone drug group) has been recorded world wide with Cyathostomes (small strongyles).

There is a lifelong susceptibility to cyathostomes, and they can cause clinical disease in any age of horse during any season. For this reason, strategic, or “once a year” type treatments are not suggested, as the life cycle is continuous year-round.

Keep in mind that the small strongyles have a much shorter development time than the large strongyles (6 weeks compared to 6-12 months).

Threadworms (Strongyloides westeri)

These affect foals under 6 months mostly, causing diarrhoea. Eggs are seldom seen in faecal examination of adult horses. Foals obtain this infection through larvae which are present in the mare’s milk from 4-40 days after foaling.(This is one cause of “foal heat” diarrhoea.) Foals can become severely infected by 2-3 weeks of age, showing diarrhoea, indigestion and unthriftiness, so preventive treatment in intensively managed horse areas may need to commence at a very young age. This is best started by treating the mare with an ivermectin product within 24 hours of foaling. Foals can be treated from 3 weeks with ivermectin or oxibendazole products.
Eggs passed in faeces actually contain an actively moving larva which can hatch in a few hours only in warm conditions. In the mare this parasite remains in mammary gland tissue, being passed in milk from about day 4 of lactation up to the 50th day. While foals are commonly infected through milk, and this lifecycle completes within 12 days, contamination also occurs through the mouth or by skin penetration of larvae, which then migrate through lungs and trachea.

Because this is such an important issue in very young foals, it is strongly advised to worm all mares 3-4 weeks before foaling, and again within 24 hours after foaling, to reduce transmission of this parasite to the foal through milk. Use a very efficient wormer such as a macrocyclic lactone for these important preventive treatments.

Small Stomach Worm (Trichostrongylus axei)

This is the only equine internal parasite that can also survive in cattle and sheep. This is important if you use cattle to graze horse pastures periodically to reduce pasture contamination in horses. In this case the cattle should be wormed as part of the pasture rotation procedure to prevent Trichostrongylus re-infestation of horses.
The eggs of this parasite hatch in a few hours from faeces. Infective larvae develop in about one week, and infection occurs when horses ingest larvae off pasture. Worms mature in only a few weeks. Heavy infections cause stomach irritation. Easily controlled by most worm products

Pinworms (Oxyuris equi)

Adult Pinworms live in the large intestine (colon and caecum), and rectum of horses. Females migrate to the anus at night to deposit eggs in the anal region. This can cause intense irritation to the area, with the horse rubbing its tail. This irritation comes from the sticky material holding eggs onto hair around the anus. Yearlings and 2 year olds often suffer from pinworms. This parasite takes about 5 months to complete a life cycle, and is usually controlled effectively in programs to treat P.equorum or small strongyles. Pinworms are probably the least damaging of all equine parasites, apart from the skin irritation.

Bots (Gasterophilus sp)

These are not worms: they are the larvae of flies. Active in summer and autumn, female flies lay their eggs (up to 900 in 3 hours) and glue them to the hairs of the horse’s body – typically on the front legs. These flies cause significant irritation to horses, which are often very restless when bot flies are present. The eggs are hatched soon after they are laid, when they are rubbed by the warm, moist lips of the horse. These eggs can remain in place for weeks, but as soon as stimulated the larvae emerge and attach to the mucous membrane lining of the mouth, penetrating the membrane anywhere in the lips, mouth and tongue. They burrow there for a short time and then migrate to the stomach where they attach to the lining of the stomach with strong mouth hooks.
All species of bots require about 10-11 months to develop, eventually passing out in faeces to complete another life cycle. Large numbers of bots in the stomach may damage the stomach lining, reducing nutrient absorption and increasing irritation.
Bot drenches should generally be administered twice yearly, once in mid to late summer, and again in mid to late winter. In Australia, bot treatment occurs in about May and September. Any bot eggs on the hair coat should also be removed at the same time as treatment occurs.

Tapeworms (Anoplocaphala perfiolata)

These are usually acquired by the ingestion of small forage mites (Oribatid mites) present in pasture. Tapeworms live in the last of the small intestine and the large intestine. They can increase the risk of colic and impactions, and are now effectively treated with products containing the drug Praziquantel, or double-dosed Pyrantel pamoate. The tapeworm lifecycle is 6 months, so annual treatment may often be adequate, but is more usually twice yearly. Regarded as generally harmless, but Tapeworms can gather at the ileocaecal valve region of the large intestine, causing ulceration, blockage, or colics.

Stomach Worms (Habronema muscae, Draschia megastoma)
These worms lie in mucus in the stomach. Eggs pass in faeces and are ingested by maggots of flies including the house fly, stable fly, bush fly, buffalo fly. Horses ingest flies which fall into feed or water troughs, or from liberation of larvae when flies are feeding around lips, or where larvae are ingested from around “fly sores” on the body, where adult flies have been active. Fly control is essential to fully manage this parasite. The major issue with these worms is that larvae can contaminate skin wounds when f\infected flies are feeding around wounds. This larval infestation of skin wounds causes significant irritation and prevents wound healing.

Lungworm (Dictyocaulus arnfeldi)
These small worms are found in the airways (bronchi) of horses, and especially donkeys.
Eggs are coughed up and swallowed, where larvae then travel to lungs where they mature. After ingestion, they can begin egg laying in 35-40 days.
The donkey is the normal host for the Lungworm, and normally harbours heavy infestations without many signs or coughing. Horses are mostly at risk only when grazed with donkeys.

Onchocerca
Larvae are found in the neck, shoulders and ligament of the neck. These worms lay live larvae, and the microfilariae are often found in skin and subcutaneous tissues. The microfilariae are ingested by midges when they feed, and deposited on skin, wounds, and even the eyes when midges feed about 3 weeks later. There are high infection rates in northern Australia. Again, these larvae cause a skin irritation in wounds that are infected when midges feed. The condition is known as Cutaneous Onchocerciasis, and can be quite important in northern Australian regions, especially in hot or humid weather.

2. THE ANTHELMINTIC DRUGS WE CURRENTLY USE

Not all dewormers are equally effective in controlling the internal parasites of horses, and many of them will not take adequate control of the immature migrating larval stages in tissues, or of encysted Cyathostome (small strongyle) stages in the gut lining. It is important to have an understanding of how each drug group works – and what it controls.

The anthelmintic drugs we commonly use to treat almost all horses fall into three distinct drug classes.

Drugs within each drug class exert their effects on the parasite in a similar manner, so resistance to one drug in the class will imply resistance to all other drugs in the class.

a) Benzimidazoles (the older “white” drenches)

This class kills parasites quickly and offers broad spectrum nematode protection with very low toxicity to the horse, including large and small strongyles, ascarids and pinworms, but there is now significant resistance to this drug class in the Small Strongyle (Cyathostome) group.

Drugs in the Benzimidazole class include Thiabendazole, Oxibendazole (Anthelcide EQ), Fenbendazole (Panacur), Oxfenbendazole (Benzelmin), Mebendazole (Telmin).

The most effective of this group are those drugs with the longest half-life in the body, such as oxfendazole, fenbendazole and albendazole, because they are not rapidly broken down to inactive products. In horses the BZ drugs are characterised by effective removal of 90-100% of mature strongyles, but migrating larvae are more difficult to control, and may require higher doses over longer periods.

Because of the development of resistance to this drug class by small strongyles, the whole benzimidazole group are now used mostly as rotational wormers, or “winter wormers”, in rotation with an ivermectin class product, for example.

In the absence of any small strongyle resistance on specific properties, this benzimidazole group is still an effective, safe, broad spectrum wormer. Use of these products is still worthwhile, particularly if a faecal egg count is used periodically to determine the presence or absence of any parasite resistance to benzimidazole drugs.

b) Tetrahydropyrimidines (Pyrantels and Morantel)

This class kills parasites slowly by causing paralysis in a wide range of nematodes including large and small strongyles, ascarids and pinworms.

Drugs in the Pyrantel class include Pyrantel pamoate (Strongid), Pyrantel tartrate and Morantel. The Pyrantel drugs are effective against most adult roundworms, so have a role in rotational worming programs. When used to treat tapeworm, Pyrantel products are effective at twice the normal recommended dose.

There is well recorded small strongyle resistance to this drug class, although not quite as prevalent as to the benzimidazole group. These drugs are usually used in combination with a second drug class in similar fashion to benzimidazoles. In the absence of proven small strongyle resistance they can still be very effective wormers, particularly in rotation programs.

c) Macrocyclic Lactones (Ivermectins)

This drug class causes paralysis in parasites, including a broad range of nematodes and arthropods, affecting large and small strongyles, benzimidazole resistant small strongyles, ascarids, pinworms, bots and summer-sore causing parasites.

This drug class includes avermectins and milbemycins. There is no difference between avermectins and milbemycins in mode of action. The avermectins in commercial use are ivermectin and abamectin, and the available milbemycin is moxidectin.
The avermectins and moxidectin have a potent, broad antiparasitic spectrum at low dose levels, and they are active against many immature and mature nematodes, including encysted larvae and arthropods.
None of this class of drug is effective against cestodes (tapeworms) or flukes.
Ivermectin given to horses orally as a paste or liquid at 0.2mg/kg is highly effective against Trichostrongylus axei, Parascaris equorum, Oxyuris equi, Strongylus vulgaris, S.edentatus, Dictyocaulus arnfeldi, and adults of Strongylus equinus, Triodontophorus spp, Habronema muscae, Strongyloides westeri, bots (Gasterophilus spp). It is also effective against larvae of Habronema muscae, Draschia megastoma, and Onchocerca spp. Many studies with ivermectin have demonstrated 100% efficacy against arterial larval stages of Strongylus sp., and the Small Strongyles as well.
This is the most recent drug class for equine parasite control, and ivermectin has now been used for 20 years in horses.

To date, there is no recorded small strongyle resistance to ivermectin.

Moxidectin alone has the ability to kill encysted cyathostome larvae in the gut lining, according to manufacturer literature.

The interval between ivermectin treatments can be up to 10 weeks, and in a single dose ivermectin is effective against larval and adult stages of all important parasites of horses, including small redworms (Cyathostomes, or small strongyles).

Ivermectin wormers control parasites that are resistant to benzimidazole (BZ) wormers. No worms that infect horses have been found to be resistant to ivermectin anywhere in the world at this time (2005).

Ivermectin wormers also control bots in a single dose. Ivermectins have no activity against tapeworms and flukes.

d) Combined Macrocyclic Lactones

This group is a combination of two different drug classes which paralyses nematodes and some arthropods (insects), and breaks down the membranes of tapeworms.

This drug combination protects against large and small strongyles, benzimidazole resistant small strongyles, ascarids, pinworms, bots, summer-sore causing parasites and tapeworms.
Drugs in this class include combinations of ivermectin & praziquantel, and moxidectin & praziquantel, currently

Praziquantel provides very effective treatment for all tapeworms in a single dose, usually given twice a year.
Products containing Ivermectin & Praziquantel can be used all year as they are also an effective treatment against bots, roundworms, and summer-sore producing Habronema spp.

It is important to remember that all of the molecules within the same chemical class work in similar ways.
Therefore, it is absolutely essential that wormer rotation occurs not within brand names, but between chemical classes. This practice helps reduce the chances of developed resistance in parasites.

Examples of Commonly Used Anthelmintics for Horses
Mean Efficiency (%)
Class Anthelmintic Bots Ascarids Large Strongyles Small Strongyles Pinworms
Avermectin Ivermectin 99 100 100 100 100
Milbemycin Moxidectin 90 100 100 100 100
Benzimidazole Fenbendazole 0 85 95-97 97 97
Oxibendazole 0 85 97 97 97
Pyrimidines Pyrantel pamoate 0 95 70-77 95 50

Unless resistance has developed in a parasite worm population, all of these compounds above have a good level of efficacy against adults and larval stages in the horse’s intestine. Only ivermectin and moxidectin have good efficacy against larval stages of parasites migrating in thee wall of the intestine or in the blood supply to the intestinal tract.

A Short History Lesson (on how we developed worm resistance)

In the 1960’s, the benzimidazole group (BZ) of anthelmintics became available, with good broad spectrum activity against almost all nematode parasites in horses. At this time, the approach to worming led to recommendations for treatment of horses every 6-8 weeks, in an attempt to control Strongylus vulgaris in particular. This program was extremely successful, with a dramatic reduction in colic cases. By the early 1980’s it was evident that S.vulgaris infection was becoming very uncommon, and that the Small Strongyle (Cyathostomes) class of internal parasite was frequently accounting for virtually 100% of worm egg output in grazing horses.

Following the introduction of ivermectin in 1981, which is highly effective against migrating larval stages of Strongylus spp, a further dramatic reduction in prevalence and intensity of S. vulgaris occurred. As a result, S.vulgaris is no longer considered an important cause of colic or tissue damage in horses, and is uncommonly diagnosed.

The problem is that regular treatment at set intervals in this manner profoundly affected strongyle worm populations. When BZ drenches were first introduced, Cyathostomes (small strongyles) were regarded as nothing more than “nuisance” parasites compared to the highly damaging S.vulgaris.

It wasn’t long before reports of resistance to Thiabendazole (the first BZ drug) in Cyathostomes began to appear. Resistance to other BZ drugs quickly followed, and now resistance to the Pyrantel class has also developed. Pyrantel has been used as an equine anthelmintic since the 1970’s, yet it is only in recent years that Pyrantel resistance has been reported.

Horse owners world wide are now in the position where Cyathostomes have high resistance to all commonly use anthelmintics except the avermectin class (macrocyclic lactones)

Cyathostomes (small strongyles) are now considered the primary parasitic pathogen of horses. Their resistance to most commonly used anthelmintics is compounded by the fact that immunity to these parasites is slow to develop and is not complete, so most horses require regular anthelmintic treatment throughout their lives, yet there is only one currently effective drug class which shows no resistance. While not causing overwhelming tissue damage like the Strongylus spp, cyathostomes cause far more problems when larvae are released from their almost impenetrable cysts within the intestinal walls after deworming kills all adults in the gut.

Of the 3 major drug classes mentioned earlier, resistance by Cyathostomes to the Benzimidazoles is the most common and widespread

Ivermectin was first introduced as an equine anthelmintic in 1981, and remained the only macrocyclic lactone drug used in horses until moxidectin was introduced in the late 1990’s. Despite being used for 20 years as an equine anthelmintic, and being the most commonly administered anthelmintic drug, there are still no reports of ivermectin resistance by equine parasites. Ivermectin efficacy against equine nematode parasites remains at 99-100% to this day.

3. PARASITE RESISTANCE to ANTHELMINTICS

The important thing to remember when considering resistance is that resistance is only recorded for the Cyathostome (Small Strongyle) parasite class in horses to date. These are the parasites which survive in cysts in the lining of the intestinal wall, and are generally impervious to drugs while encysted. Because they are encysted, and are released in large numbers after any worm treatment, preventive treatment for horses has always focussed on a regular interval worming program to control these parasites.

This resistance is primarily to all Benzimidazole class drugs, otherwise known as the older “white” drenches, and to a lesser degree to the Pyrantel class of drugs. The use of any drug in these classes on resistant parasites will result in ineffective treatment. That is largely why current marketing programs from many drug manufacturers recommend the use of ivermectin/moxidectin class drugs to effectively control cyathostomes.

The only class of drugs where there is no resistance currently is with the ivermectins. This drug class includes ivermectin, avermectin, abamectin and moxidectin. These drugs are all very similar, with only very minor differences in efficacy and safety. There is real concern that resistance to this drug class will develop in horse parasites, as it has in sheep and cattle, but 20 years after the introduction of ivermectin, and over 10 years after the introduction of moxidectin, there is still no resistance evident.

From an equine parasite management perspective, it makes sense to use an ivermectin or moxidectin product to ensure effective control of cyathostomes. The drug mixed with the ivermectin or moxidectin is generally Praziquantel, to allow effective treatment of tapeworms as well.

All of this talk of small strongyle resistance is somewhat academic if the simple measure of faecal egg counts is conducted periodically to actually determine if you do have a resistant worm problem. If you don’t, then the simple benzimidazole drugs and pyrantel/morantel class are perfectly adequate for roundworm and strongyle control. These drugs are usually cheaper, and are all very safe for horses.

The real advantage of the macrocyclic lactone group (ivermectins) is that they do also control bots, and are usually in combination with praziquantel to control tapeworms as well. They do have a significant role to control resistant small strongyles when that resistance has actually been confirmed in your horses.

4. HOW TO TELL IF YOU HAVE RESISTANT WORMS

The only effective method to determine the level of resistance to particular anthelmintic drug classes in your horse(s) is to have your veterinarian arrange a Faecal Egg Count from fresh faeces before and/or after your horse has been dewormed. This will indicate the number of worm eggs still being shed in the faeces. The number of eggs shed immediately after deworming will indicate the degree of worm ‘kill” produced by that particular drug class used. In general terms, a kill of only 70-80% will strongly indicate a degree of anthelmintic resistance.
Faecal Egg Counts are cheap, yet very effective management tools to monitor internal parasite burdens and worming effectiveness.
As there are so many small strongyle species, it is easy to go one step further if you need to identify the actual worm species by having your veterinarian arrange for a larval culture from a laboratory. This will actually put a name on the specific species of worm which may be causing difficulty for you.

5. MODERN ANTHELMINTIC PROGRAMS

Planning a parasite control program for your horses can be confusing, especially when recommendations advise alternating the wormer used. What wormers do you use? Why?

Remember that rotating the dewormer – not by brand, but by class of drug – can prevent parasites from developing resistance, and can mean the difference between an effective treatment and a totally ineffective one.

Rotating a dewormer may be done in an interval program, or in a seasonal, or strategic, program.

Age is Important
Young horses are more prone to parasite infections than older adults. Adults frequently have very high worm egg counts in faeces and can house thousands of adult worms, but will appear to be in excellent physical shape when compared to a similarly infected yearling. With age, the immune system of the older horse helps prevent parasite migration and the damage inflicted by parasites in the gastrointestinal tract. Different species of parasites are important for horses less than one year of age compared with those with medical significance after the first year of life.

Young Horses Most Susceptible to Worms
A worming program will vary with the age of horses: a parasite program for adult horses will aim to control bots, tapeworms and large and small strongyles and will focus more on strategic treatments based on seasonal rises in parasite populations, but programs for horses under 2 years of age need to control Parascaris equorum (roundworms), Oxyuris equi (Pinworms) and Strongyloides westeri as well. If large strongyles and roundworms are not controlled in young horses, the parasites could enter the tissue migratory phase causing long lasting tissue damage. Strongyloides westeri is a problem on farms with a lot of horses, and is best controlled by dosing the mare with ivermectin less than 24 hours after foaling, for example. Foals can then be treated at 3 weeks of age with ivermectin or oxibendazole.(if there is no resistance to benzimidazole drugs).

Because younger horses are exposed to roundworm and cyathostome eggs year-round, they require an interval based deworming program, beginning at 8 weeks, with earlier treatment for Strongyloides if required. It is especially important to commence worm treatment of foals by worming the mare 3-4 weeks before foaling, and again within 24 hours after foaling, if Strongyloides westeri is a problem, as this worm is passed from mare to foal in milk.

The older preventive practice of worming adult horses at fixed intervals is now being replaced with the suggestion of properly timed anthelmintic treatments (varying with the drug used, the parasite, and the geographic region), coupled with sound hygiene, good horse husbandry and pasture management to reduce the number of drug treatments required, as well as to reduce environmental contamination with cyathostome eggs and larvae.

The once common practice of rotating drugs with each treatment should cease at once, as it does not appear to slow the development of resistance, and may actually increase the rate at which resistance develops by selecting for resistance to more than one drug simultaneously.

6. MANAGEMENT ACTIVITIES THAT CAN MAKE A DIFFERENCE

Annual Rotation of Drugs is Important
Every effort should be made to ensure slow (or annual) rotation of drug classes to ensure delaying resistance. In simple terms, use one anthelmintic class for one year. A different class is then used for the following year.
Depending on the drug class, this may not control tapeworms or bots. Inclusion of drugs for these parasites at the appropriate time is therefore acceptable and recommended.
If resistance to a particular drug class has been demonstrated by faecal egg counts on a particular property, this drug class should not be used in a slow rotation program for about 5 years.

Current research suggests that the best approach for delaying the selection for resistance is to treat simultaneously with two chemically different anthelmintic drugs.

Correct Dose is Vital
It is essential to always use the correct drug dose, and this requires a close estimate of bodyweight. It is better to overestimate bodyweight than to underestimate it.
Dose All Horses At The Same Time
All horses on the property should be on the same deworming program, and should be treated at the same time, with the same drug

Quarantine and Treat New Arrivals with an Effective Drug
Any new arrival on a property should be treated immediately on arrival, and quarantined until worm treatment is effective. Anthelmintic resistance most commonly establishes on a property with the arrival of a new horse that is infected with drug resistant parasites. This quarantine treatment must use an effective drug, because the common practice of treatment on arrival with a cheap anthelmintic may actually accelerate the spread of resistance if the treated horse is infected with worms resistant to that drug, or be totally non-effective if the wrong drug class is used because of existing resistance. In these cases, horses will shed pure resistant worm eggs for weeks after treatment as larvae hatch from the encysted sites in the intestinal wall.
It is imperative to realise that most available drug treatments do not kill the mucosal encysted larval stages, which are usually much more numerous than the free living adult worms in the gut lumen. For that reason alone, it is suggested that quarantine treatment of new arrivals should use an efficient drug class with no known resistance, such as the macrocyclic lactones (ivermectin, abamectin, moxidectin)

Because shedding of larvae will occur for weeks after an introductory treatment on arrival on the property, it makes more sense to treat several times after arrival with a recognised effective anthelmintic. Do not allow treated horses access to common pasture for at least 3 days after treatment.

Practice Mixed Grazing with Sheep or Cattle, if Possible
Practice mixed grazing with sheep or cattle to reduce pasture worm egg contamination, as horse worm species do not survive in sheep or cattle (except Trichostrongylus axei, as mentioned elsewhere).

Keep Stables and Yards Clean
Pick up manure regularly

How Do You Determine The Interval for proper Re-Treatment?
Each drug class has a very stable time interval during which treatment will keep faeces relatively free of worm eggs. This interval varies with the active drugs, as some wormers remove only adult worms, while others remove adults and larvae. This time interval to re-appearance of eggs in faeces becomes the Dosing Interval for that drug class, and knowledge of that interval allows owners to predict accurately when worming is due again if pasture contamination is to be prevented.;
In the absence of resistance, reasonable times for the re-appearance of eggs in faeces after treatment are as follows;
Benzimidazoles 4-5 weeks
Pyrantel 4-5 weeks
Ivermectin 8-10 weeks
Moxidectin 12 weeks

Simply by moving from drenching with benzimidazoles to ivermectin based products, we increase the interval for drenching from 4-5 weeks to 8-10 weeks, thus reducing frequency of drenching.

It should be noted that it takes 2 to 3 years of regular preventive drenching at the correct interval depending on the drug used, to deplete the pool of encysted cyathostomes (small strongyles) in the lining of the intestinal wall. This pool of encysted cyathostomes always survives a deworming treatment, and large numbers hatch shortly after the deworming, replacing the adults killed by treatment.

In practice. Most deworming programs will focus on 6 dewormings at 2 month intervals. Boticides and tapeworm treatments will occur twice yearly during these regular dewormings. Foal treatments generally begin at 8 weeks of age, continuing at 2 monthly intervals.

Pasture Management is Crucial to Prevention
Sound pasture management is critical to good animal health.

Until you know otherwise, consider all adult horses a threat to pastures by passing eggs in their faeces.

Following a deworming treatment the adult parasites in the intestine are killed, and the passage of eggs in the faeces stops for a while. In the meantime, new adult worms develop from larval stages in the horse’s intestinal wall and start to produce eggs again after a short period. These eggs are passed onto grass, and must mature on the ground for a period before becoming infective to other horses. The time taken for this to occur depends primarily on climate, with the period to become infective being much shorter in hot, humid weather.

Picking up manure from pasture twice weekly (or even daily in high rainfall regions) controls strongyle infections just as effectively as anthelmintic drug preparations in most cases, but is labour intensive.

Harrowing manure regularly can be of assistance to dry paddock manure out and kill eggs. This effort can be rewarding, especially when resistant cyathostomes (small strongyles) are present, as these are only transmitted via contaminated pasture. Parasites in pasture are killed rapidly during periods of hot, dry weather.

Daily cleaning of stables and yards to remove faeces and worm eggs is very important – both to reduce egg contamination, and to reduce fly and insect worry.

Avoid high stocking rates, as higher densities require more frequent anthelmintic dosing.

Rotate pastures if at all possible

Young horses are much more susceptible to gastrointestinal parasites than older horses (which have developed more immunity). Give young horses the cleanest pastures, and don’t graze them with other age groups. Assume the pastures young horses graze are heavily contaminated after their removal.

To control Roundworms in young horses
Mares should be treated with an ivermectin product 3-4 weeks before, and again within 24 hours of foaling.
All foals should be treated at 6-8 weeks of age at the latest.
Then treat all foals regularly at 6-8 week intervals for the first year of life, irrespective of the anthelmintic drug used. Deworming foals throughout the year, rather than in specific seasons, is warranted as foals ingest infective eggs in the environment, particularly in stables, throughout the year. Roundworm eggs can remain viable in the environment for months to years. Attention to cleaning stalls and stables will help to reduce the worm egg burden

To Control Cyathostomes (Small Strongyles)
Cyathostomes are primarily acquired by horses at pasture, so horses only need deworming for this parasite when on pasture, in contrast to roundworms (ascarids).
Because cyathostomes continually produce eggs, control centres around regular deworming at about 8 week intervals. (moxidectin claims a 13 week interval), especially in young horses.
If resistant small strongyles are a known problem, use an effective drug class for treatment of young horses.
Use all other management procedures mentioned in this article.

In Summary, effective worm management practices include;
* Do not feed off the ground
* Regularly clean stables and yards. Remove faeces from paddocks if at all possible.
* Avoid overcrowding of pastures
* Avoid wet pastures, especially with young horses.
* Periodically graze another species on horse paddocks to break the equine parasite life cycle.
* Use proper effective drugs at the correct dose rate, and at the correct time.
* Accurately estimate horse bodyweight to ensure correct dosing.
* Deworm all animals in a group at once
* Deworm pregnant mares within 30 days of foaling, and again 24 hours after foaling to minimise transmission of worms to foals.
* Keep lactating mares and foals on a rigid treatment schedule every 6-8 weeks
* Rotate classes of anthelmintic drugs annually
* Don’t run young horses with adults if possible.
* Strategically treat bots and tapeworms twice yearly

7. CHOICE of WORMING PRODUCT

Effective equine anthelmintics are available as oral liquids, pastes and gels, and as pellets or powders. Oral pastes are far more popular and convenient for owners now.

How an anthelmintic is administered has almost no bearing on its effectiveness in this modern era of efficient drugs, oral anthelmintic paste and gel formulations, and other dosing alternatives including pellets and oral liquids.

In general, as long as the following criteria are met, regardless of the route of administration (stomach tube, oral, mixed in feed), effective deworming should occur;

1. The correct amount, determined by an accurate estimation of the horse’s weight.
2. Complete retention / consumption of the dose administered
3. The anthelmintic selected must be highly effective against the parasite(s) affecting the horse

There is no longer any real requirement for stomach tubing of horses to achieve thorough worming of horses.

8. WHAT PRODUCTS ARE CURRENTLY AVAILABLE IN AUSTRALIA?

The 2005 IVS lists the following 34 anthelmintic products available for horses in Australia. Remember that they all fall into only one of 3 groups of active drugs, in general terms.

To simplify the list, products will be listed by the active drug class so you can see how a long, confusing list can be reduced to a reasonable level so that your choice of drug is based on solid technical information.

In the long run, selection of a product from any of the three major groups will come down to price, or ease of application (a smaller, more ‘ergonomic” syringe that fits a small hand), low volume dose, or the particular flavouring offered, as it will become evident very quickly that there is little differentiation technically between the many choices within each drug class.

1. BENZIMIDAZOLE GROUP DRUGS

Anthelcide EQ Horse Wormer Suspension (Ranvet)
Contains Oxibendazole 100g/L
Indicated for susceptible nonmigrating large strongyles, small strongyles, pinworms, roundworms, threadworms . Safe for use in foals, pregnant mares and stallions.
Comments:
This is a benzimidazole class drug, and resistant cyathostomes may not respond to treatment. Drugs for bot and tapeworm control may be added to the suspension. Used effectively as a “winter wormer” or rotational wormer.

Benzelmin Paste (Jurox)
Contains Oxfendazole 185g/kg
Indications as for other benzimidazole drugs in this list. Tapeworm and bot treatment may be added
Comments:
A benzimidazole class anthelmintic as an oral paste

Bomatak C (Pharm Tech)
Contains Oxfendazole 90.6g/L
Indications claim activity against stomach hair worm adult & immatures, roundworms, pinworms, large strongyles and susceptible small strongyles.
Comments:
An oral liquid. This is a benzimidazole class drug

Equinox Orange Tube (value Plus)
Contains Oxfendazole 185g/kg as an oral paste 32.6g syringe
Indications claim activity against large roundworm adult & immatures, pinworm, small & large stomach worm, susceptible small strongyles. Kills immature and adult large strongyles in gut, tissue stages of S.edentatus
Comments:
A benzimidazole class drug.

Oximinth Horse Worming Paste (Virbac)
Contains Oxibendazole 1g/5mL as a 25mL oral paste syringe
Indications claim it is active against susceptible small strongyle adults and larvae, is ovicidal, and at twice recommended rate will kill Strongyloides in foals where first treatment required at 4 weeks of age. Also effective against bloodworms, pinworms and roundworms
Comments:
A benzimidazole class drug

Oximinth Plus Horse Worming Paste (Virbac)
Contains Oxibendazole 1g/5mL, dichlorvos 1g/5mL in a 25mL oral paste syringe
Indications as for Oximinth above, plus bots
Comments:
A benzimidazole class drug plus Dichlorvos for bot control
Marketed as effective bot control over winter months by killing bots in stomach, thus breaking life cycle. Should be strategically used at start and end of the bot season

Panacur 100 (Intervet)
Contains Fenbendazole 100g/L as an oral liquid
Indications include mature & immature stages of hair worms lungworm, strongylus spp, cyathostomes, ascarids, pinworms, Strongyloides westeri. Safe for use in pregnant mares, stallions and foals
Comments:
This is a benzimidazole class drug

Strategy-T Paste (Vetsearch)
Contains Oxfendazole 6g/30mL plus Pyrantel embonate 7.8g/30mL in a 30mL oral paste syringe.
Indications claim that these two drugs act synergistically for high efficacy against small strongyles (including benzimidazole resistant strains), large strongyles, pinworms, large roundworms and tapeworms
Comments:
This is a benzimidazole class drug with a tetrahydropyrimidine class drug (Pyrantel). It is frequently marketed as an alternative for rotational worm treatment.

Worma Drench (Farnam) (International Animal Health)
Contains Oxfendazole 100g/L as an oral liquid
Comments:
This is a benzimidazole class drug

Worma Paste (Farnam) (International Animal Health)
Contains Oxfendazole 107.2mg/g plus piperazine dihydrochloride 214.4mg/g
Comments:
This is a benzimidazole class drug with added piperazine, an older anthelmintic class.

Worma and Bot Paste (Farnam) (International Animal Health)
Contains Oxfendazole 28.4mg/g, trichlorfon 454.4mg/g in a 40g oral paste syringe
Indications are for control of mature large strongyles, susceptible small strongyles, mature & immature roundworms, stomach hair worm, pinworm, plus bots
Comments:
This is a benzimidazole class drug with added Trichlorfon for bot control

2. TETRAHYDROPYRIMIDINE GROUP DRUGS (Pyrantel and Morantels)

Equiban Granules (Pfizer)
Contains Morantel tartrate 115mg/g as oral granules
Indications are gut dwelling large roundworms and strongyles, small strongyles, pinworm, and good activity against tapeworm. Safe for pregnant mares and foals over one week
Comments:
This is a tetrahydropyrimidine class drug (Pyrantel/Morantel). Granules to be added to a small amount of feed.

Equiban Paste (Pfizer)
Contains Morantel Tartrate 150mg/g in a 35g oral paste syringe
Indications as for Equiban granules
Comments:
A tetrahydropyrimidine class drug as an oral paste

3. MACROCYCLIC LACTONE GROUP DRUGS

a) Ivermectins / Abamectins

Equimec (Merial)
Contains Ivermectin 16.7g/kg as a low dose volume oral paste syringe 6.42g
Indications claim activity against large strongyles, adult & arterial stages of S.vulgaris, adult & tissue stages of S.edentatus, adult S.equinus, adult small strongyles, including benzimidazole resistant strains of adult & immature cyathostomes, adult & immature pinworms, roundworms, adult hairworms, Onchocerca spp, bots, lungworm and Strongyloides westeri adults. Control of skin lesions caused by Habronema spp and Draschia spp
Comments:
Ivermectin alone has no activity against tapeworms, but good broad spectrum activity against major worms. A macrocyclic lactone class drug

Equimec Liquid (Merial)
Contains Ivermectin 10g/L as a liquid for oral dosing
Indications and comments as for Equimec.
A macrocyclic lactone class drug

Equiminth (Virbac)
Contains Abamectin 3.7mg/g in an oatmeal base 32.4g oral paste syringe
Indications claim activity against large & small strongyles, pinworm, hairworm, threadworm, bots, ascarids, Onchocerca spp, lungworm. Up to 100% kill of bots and worms in horses, except tapeworm.
Comments:
A macrocyclic lactone class product. No added praziquantel for tapeworm control.

Equiminth Ivermectin Liquid (Virbac)
Contains Ivermectin 10mg/mL as a liquid for oral dosing.
Indications as for Equiminth
Comments:
A macrocyclic lactone class drug with no added praziquantel for tapeworm control.

Eraquell Pellets (Virbac)
Contains Ivermectin 120mg/30g sachet as pellets for inclusion in feed
Indications as for ivermectin
Comments:
Pellets for inclusion in feed. A macrocyclic lactone class product.

Imax LV (Pharm Tech)
Contains Ivermectin 10mg/mL in a liquid 100mL bottle for oral use
Indications: as for other ivermectin products
Comments:
A macrocyclic lactone with no praziquantel for tapeworms.

MecWormer & Bot (International Animal Health)
Contains Abamectin 3.64mg/g as an oral 33g paste syringe
Indications as for other ivermectins
Comments:
A macrocyclic lactone class drug. No praziquantel

Noromectin Paste (Norbrook)
Contains Ivermectin in a low dose 6.42g oral paste syringe
Indications as for ivermectins
Comments:
A macrocyclic lactone

Promectin Worm & Bot Paste for Horses (Jurox)
Contains Abamectin 3.7mg/g
Indications are that it treats roundworms (including arterial larval stages of Strongylus vulgaris and benzimidazole resistant small strongyles), bots and skin lesions caused by Habronema, Onchocerca spp and Draschia spp
Comments:
Abamectin and ivermectin are in the same macrocyclic lactone class, acting in a similar manner. There is no claim for tapeworm treatment by ivermectin products.

Valumec Green Tube (Value Plus)
Contains Abamectin 3.7mg/g
Indications are similar to all other ivermectin class products, with no tapeworm claims.
Comments:
A macrocyclic lactone class drug

Worm-IT (Kelato Animal Health)
Contains Abamectin 3.54mg/g in a 33g oral paste syringe
Indications similar to all other ivermectins
Comments:
A macrocyclic lactone class drug

b) Combination Abamectin/Ivermectin Plus Praziquantel

Equimax (Virbac)
Contains Abamectin 3.7mg/g, Praziquantel 46.2mg/g in an oatmeal base as a 32.4g oral dose syringe.
Indications claim efficacy against tapeworm (up to 100% kill in 24 hours), large & small strongyles, pinworm, stomach hairworm, threadworm, bots, ascarids, Onchocerca spp, lungworm
Comments:
This is a macrocyclic lactone class drug plus praziquantel. Has wide activity against all major species.
Manufacturer claims that abamectin is the natural ivermectin used, where all other ivermectins, and abamectins are semi-synthetic.

Equimax Liquid Allwormer (Virbac)
Contains Abamectin 0.8g/L, Praziquantel 10g/L
Indications as for Equimax. This is a liquid for oral dosing
Comments:
A macrocyclic lactone class drug plus praziquantel. Wide activity against all major species.

Equimax LV (Virbac)
Contains Ivermectin 16.7mg/g, Praziquantel 140mg/g in a low dose volume 6.42g oral paste syringe
Indications as for Equimax.
Comments:
A macrocyclic lactone class drug plus praziquantel in low volume dose oral paste presentation.

Genesis Equine (Ancare)
Contains Ivermectin 8g/kg plus Praziquantel 100g/kg in a 15g oral paste syringe
Indications are treatment and control of roundworms, tapeworms and bots in horses and skin lesions caused by Habronema and Onchocerca
Comments:
A broad spectrum macrocyclic lactone plus praziquantel oral paste

Imax Gold (Pharm Tech)
Contains Ivermectin 10mg/mL, Praziquantel 75mg/mL as oral liquid
Indications as for other ivermectin/praziquantel products for tapeworms, roundworms, bots, summer sores
Comments:
A macrocyclic lactone plus praziqantel oral liquid

Promectin Plus Allwormer Paste for Horses (Jurox)
Contains Abamectin 3.7mg/g, Praziquantel 46.2mg/g in a 32.4g oral paste syringe
Indications are as an efficient broad spectrum wormer to treat tapeworms (adult & immature) adult & larval arterial stages of large strongyles, small strongyles including benzimidazole resistant worms (adult & immature), pinworms, ascarids (adult & immature) stomach worms, Onchocerca spp, bots, lungworms and intestinal threadworm. Also controls skin lesions caused by Habronema and Draschia spp, and cutaneous larvae & microfilariae of Onchocerca spp
Comments:
This is a macrocyclic lactone drug plus praziquantel, with broad spectrum activity against all major worms.

Promectin Plus Red Tube (Value Plus)
Contains Abamectin 3.7mg/g and Praziquantel 46.2mg/g
All other information is the same as for Promectin Plus Allwormer
Comments:
This is a macrocyclic lactone class drug plus praziquantel for tapeworm control

c) Combination Abamectin/Ivermectin plus another drug class

AMMO Allwormer Paste for Horses (Nature Vet)
Contains Abamectin 4mg/g plus Morantel tartrate 167mg/g in a 32.5g oral paste syringe.
Indicated for tapeworm, roundworm (including arterial larval stages of Strongylus vulgaris and benzimidazole resistant small strongyles), bots and skin lesions caused by Habronema and Draschia sp (Summer Sores) and Onchocerca sp microfilariae (Cutaneous onchocerciasis)
Comment:
This product contains no benzimidazole group drug, and does provide active drugs from both the Macrocyclic Lactone (ivermectin) group, plus the Tetrahydropyrimidine (Pyrantel/Morantel) drug group.
Marketed as the ideal rotational wormer for use from 6 weeks of age.

d) Combination Moxidectin plus Praziquantel

Equest Plus Tape Gel (Fort Dodge)
Contains Moxidectin 20mg/g, Praziquantel 125mg/g as an oral gel 12.2g syringe
Indications include tapeworm, roundworms (including arterial larval stages of Strongylus vulgaris), adult and late encysted stages of small strongyles, bots and Habronema spp.
Comments:
A Macrocyclic lactone class drug with wide efficacy, plus Praziquantel for tapeworm activity. Moxidectin has slightly longer effect in tissues than ivermectins.

Marketed as the only long acting horse wormer, with treatment recommended after 14-16 weeks. Also the only horse wormer to have a claim to control encysted small strongyles in a single dose. Safe for mares, stallions and foals over 4 weeks.

OTHER SEPARATE DRUG CLASSES

Neguvon Soluble Powder Anthelmintic Boticide (Bayer)
Contains Trichlorfon 800g/kg to control bots in horses
Comments:
Rarely used now in the era of ivermectins and oral paste treatments. Was used in stomach tubes regularly before ivermectins. Is still of use for combination with benzimidazole class drugs to provide strategic bot control at certain times of the year.

Observations:

One third of common equine anthelmintics available in Australia belong to the macrocyclic lactone drug class, and most of these are formulated with praziquantel, to provide tapeworm activity

There is no real difference in activity of ivermectin and abamectin. Both have the same claims for activity.

Moxidectin is still in the macrocyclic lactone group, but has the best efficacy claims from the manufacturer for encysted small strongyles over ivermectin and abamectin. This may be of no real advantage as long as a regular deworming program is based on efficient drugs such as ivermectin/abamectin because larval worms will hatch from cysts after deworming for up to 3 years. As larval worms reach the intestinal tract and mature, they will be killed by the ivermectins or abamectins which are used regularly throughout the year. Scientists suggest considering drugs such as Moxidectin as “last resort” drugs which should be held in reserve, to some degree, in case resistance to the ivermectins surfaces in the future.

The benzimidazole drug class are still available, in spite of widespread small strongyle resistance. These drugs are still effective for most other large strongyles, ascarids, pinworms, etc, so resistance is only of major concern with small strongyles and benzimidazoles, and, to a lesser degree, between tetrahydropyrimidines (pyrantel and morantel) and small strongyles.

The benzimidazole drugs still have a valuable role as rotational drugs, especially after long term use of ivermectins or moxidectin, which tends to reduce resistant worm populations, allowing the BZ drugs to retain some activity.

There is no resistance to ivermectins or abamectins reported in horse parasites.

There is no value in rotating from ivermectin to abamectin or moxidectin, as all of these drugs are in the same drug class, and have the same mode of action.

The major difference in the drug groups is that the macrocyclic lactones have a far better broad spectrum effect against all major parasites, including immature and larval stages, and have added activity against skin lesions caused by Habronema and Draschia larvae as well as Onchocerca spp. This drug class also assists by reducing lice, tick and other external parasite loads.

Understand the Ivermectin Terminology

The three names, Avermectin, Abamectin and Ivermectin are used to describe the same class of compounds. For simplicity, they are usually called the Avermectins.

Avermectins are insecticidal and miticidal compounds derived from the soil bacterium Streptomyces avermitilis, originally discovered in Japan. They are commonly used as livestock and human anthelmintics as well.
Avermectin is the natural fermentation product of this bacterium. This is classified as practically non-toxic to mammals by the US EPA.

These compounds all act by interfering with the nervous system of the parasite, causing paralysis.

Abamectin is a mixture of avermectins, including 80% avermectin B1a, and <20% avermectin B1b. Both of these components have very similar biological and toxicological properties.

Abamectin is the natural fermentation product of the bacterium mentioned above. It is non-toxic to birds and mammals, but highly toxic to fish and aquatic invertebrates, hence the warnings on labels to avoid water contamination. This is commonly used in equine anthelmintics.

Ivermectin contains a mixture of >90% avermectin B1a and <10% avermectin B1b. It is a semi-synthetic derivative of abamectin.

For all intents and purposes, these compounds all have the same activity, with only very minor variations in solubility and efficacy.