Nixon Equine Vets Worming Advice

Horses can be infected by three groups of internal parasites, roundworms, tapeworms and bots. Infections with any of these parasites can be responsible for weight loss, diarrhoea, poor performance and general ill thrift in horses. More seriously, worms are a major cause of colic, which is the single largest cause of mortality in horses.

Effective horse worming

1. Common parasitism: tapeworms and redworms

2. Wormer groups: choosing wormers and resistance

3. Effective horse worming

4. Worming programmes

5. Pasture worm control

1. Common parasitism

The 2 most common features of parasitism currently relate to tapeworms and redworms:

Increased tapeworm parasitism with worsening risk of colic can be lethal. These worms are not susceptible to routine wormers but require active ingredients such as Pyrantel or Praziquantel.  The larvae are carried in grass mites and hibernated from them when digested. It is usually necessary to does at the end of summer grazing and in some situations e.g. when hay is made from grass land where horses have grazed at some time, at the end of winter feeding.

The lifecycle peculiarity of the small redworm larvae which spend some time in, as distinct from “on” the gut wall AND those ingested in September onwards, lie dormant therein until February / March, when they leave en-masse with damaging effect. There is only one drug which will “get at” these with just one dose; moxidectin.

2. Horse wormer groups

The number of different horse wormer products licensed for use in the UK, each with a different “active ingredient”, can make choosing the most appropriate horse wormer to use at certain times of the year rather confusing for the horse owner.  Basically the active ingredients in all horse wormer products for equine parasite control belong to one of just 4 Anthelmintic horse wormer groups:

1. Macrocyclic lactones (i.e. ivermectin and moxidectin)

2. Benzimidazoles (i.e. fenbendazole)

3. Pyrantel

4. Praziquantel

Which horse wormer product / active ingredient group should you choose?

Worming becomes complicated because no group of wormer on its own treats all horse parasite groups. This means that to control all parasites that infect horses a combination of anthelmintic groups must be used throughout the year. The table below provides details of the activity of these different wormers. Not all wormers have equal levels of control, plus some have levels of resistance that compromise their efficacy. This means that despite your best intentions you may not be protecting your horse from the ill effects of a heavy parasite infection.

Worm resistance to horse wormer ingredients

Resistance to a wormer is the ability of some worms in a population to survive a dose of wormer that should normally be lethal to them. In any given population of worms, a very small number may exhibit a natural ability to tolerate a dose of wormer. This feature will be passed on to their off-spring generation after generation, gradually increasing the number of resistant worms in a population.

If left unchecked resistance to a particular wormer chemical can soon become widespread on equine premises. Once resistance to a particular wormer ingredient is present, all horses kept on those premises may become infected with resistant worms. It then becomes vital to treat these horses with a different wormer ingredient, which has a different mode of action, to stop the spread of resistance. Almost every report of resistance has been in small redworms. Confirmed resistance in small redworms has been widely reported in 21 countries across the world i.e:

1. Benzimidazoles (i.e fenbendazole) Small redworm resistance to benzimidazole wormers is widespread. Some of the latest surveys have demonstrated resistance in 80- 100% of equine premises (Fisher et al. 1992, French et al. 2002, Kaplan et al 2004).As a result the use of benzimidazoles as a routine wormer is not recommended unless faecal egg counts are used to confirm that the worms your horse is infected with are susceptible to it.

2. Pyrantel There have been a few documented reports of resistance to pyrantel in the UK. Resistance to pyrantel is more widespread in America, where it has been common practice administer daily low doses of pyrantel in horse’s feed.

3. Macrocyclic lactones (ivermectin and moxidectin) No confirmed reports of resistance.

4. Praziquantel No reports of resistance in tapeworms.

The more frequently worms are exposed to the same wormer group (“active ingredient”) the greater the population of resistant worms will become. This is the process of natural selection. Resistant worms will survive with fewer susceptible worms in the population. For this reason wormers should not be used any more frequently than the manufacturer recommends. The development of resistance can therefore be minimised by changing to a horse wormer with a different active ingredient group; say every 12 months.

3. Effective horse worming

Worm all new horses on arrival at a yard and keep them off pasture for 24 hours. If it is winter or the worming history of the horse is poor or unknown then a treatment for encysted small redworms should also be given. This will ensure that the new horse does not contaminate the pasture with these parasites.

Worm all horses that graze together with the same wormer at the same time. This will help to prevent multiple wormer resistance from establishing on premises. It also reduces pasture contamination as all the horses stop shedding worm eggs for some time after dosing.

Try to keep young horses grazing separately from older horses. Young horses carry the highest parasite burdens and are responsible for the majority of pasture contamination. These horses may need to be wormed more frequently than older horses.

If these tips are adhered to it is possible to reduce the number of wormer treatments your horse requires each year. Faecal egg counts can be performed on a sample of droppings to ensure that levels of infection have been kept to a minimum.

Use the correct dose of wormer for your horse’s bodyweight at the correct interval

Horse wormers are dosed according to the body weight of the horse. If horses are under-dosed, the worms will be exposed to a sub-lethal dose of the drug. This may leave a population of worms that is able to tolerate the wormer. In addition the effectiveness of the wormer is compromised and heavily infected horses may be left at risk of worm related diseases.

Often horses are unintentionally under-dosed due to poor estimations of their body weight. Horse owners tend to under- rather than over-estimate their horse’s bodyweight. Ideally, your horse should be weighed accurately on a weighbridge, but if this is not available then a weigh tape or weight estimation formula can be used.

The majority of syringes treat up to 600kg however some treat less than this! A few products however, e.g. the new Equimax syringe, will treat a horse up to 700 KG. It is important to check the bodyweight that the wormer you are using treats and dose at the recommended interval for the drug being used.

4. Worming Programmes

A 1 year worming programme

We advise the use of EQUEST (moxidectin) in winter and summer and EQUEST PRAMOX in spring and autumn, or an equivalent active ingredient product dosed 4 times per year. The “active ingredient” in EQUEST is moxidectin, which is slowly excreted from intestinal tissues into the inner wall where they are absorbed by the worms. Such delayed excretion continues for up to 13 weeks.

Rotational strategic worm control programmes

One of the key techniques for minimising resistance is to rotate the group or type of wormer you use over the summer grazing season on an annual or biannual basis. By rotating your routine summer wormer, the worms are exposed to a different chemical with a different mode of action, thereby reducing selection pressure for resistance to one particular wormer group.

Although there are many different brand names of wormers available, several of them contain the same active ingredient. When you rotate your wormer it is essential to ensure you are choosing a different active ingredient, not simply a different brand. As there is widespread resistance to benzimidazole (fenbendazole) wormers, these cannot be used as rotation wormers unless a significant level of efficacy has been demonstrated with faecal egg counts. With this in mind the most suitable rotation is between the macrocyclic lactones (ivermectin or moxidectin) and pyrantel.

A 2 year rotational worming programme

The worming programme in the following two illustrations rotates the use of Eraquell (ivermectin) during the summer months one year with pyrantel during the summer months of the following year. Note: Eraquell (ivermectin) used routinely over the summer grazing season effectively treats roundworm and bot infections.

In alternate years, pyrantel should be used as a routine summer wormer. However pyrantel is not effective against immature worms, so must be used more frequently than Eraquell. Also as there have been reports of resistance to pyrantel, it is advisable to have faecal egg counts performed in the rotation year to check that the programme is working.

The revolutionary combination wormer Equimax is the only wormer licensed to kill roundworms, three species of tapeworms and bots in a single dose. By using Equimax in the spring and autumn all of these parasites are treated in a single dose. This reduces the number of annual treatments necessary to give your horse, not only making worming more simple and cost effective, but also helping to reduce the risk of resistance.

New 700kg Equimax syringe: The broad spectrum of Equimax, for treating roundworms, all three species of tapeworm and bots in a single dose is now available with an extra 100kg. If your horse weighs up to 700kg you can now provide effective protection with a single syringe.

Alternative worming programme

5. Pasture Worm Control

Good pasture worm control and horse management practice enhances the effectiveness of your worming programme. The aim is to limit the contamination of grazing with the infestive larval stages of the several species of intestinal parasites (“worms”) which are potentially dangerous to horses.

Brood mares and foals and young stock up to 4 years old require special attention; especially if breeding and rearing are a business and therefore horse numbers are significant; in which case this should be discussed with your vet. In less commercial situations there can, of course be immature stock – up to 4 years old and, increasingly, many geriatrics – over 20 years. These groups should be managed more critically.  Mature horses (and donkeys) will have a more efficient resistance to the ill effects of worms, but no horse is ever always free of gut parasites.

Over stocking can be a problem but this can be reduced by attention to pasture hygiene, laborious unless a motorised “scooper” is available. Faeces should be removed ideally once daily, certainly once weekly in winter months, advisedly every second day in spring and summer – up to first frost – to remove the source of worm infestation. Faeces removal also reduces the development of “sick”, non-appetising grazing. Harrowing helps with this BUT spreads the larvae.

Allow approximately 1.5 acres per horse. Overstocked pasture forces horses to graze around dung piles, increasing the risk of infection. Graze horses with cattle and sheep; they act as natural “vacuum cleaners” as the horse worms and larvae they eat will not develop into egg laying adults, significantly reducing pasture contamination.

If you are employing good pasture management and have faecal egg counts performed it may be possible to worm less frequently. This will slow down the accumulation of resistance.

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