Parasites & Worming :: Nixon Equine Vets

Horses can be infected by 3 groups of internal parasites: roundworms, tapeworms and bots that can cause weight loss, diarrhoea, poor performance and general ill thrift. More seriously they are a major cause of colic; the single largest cause of equine mortality.

Tapeworm & roundworm need different horse wormer active ingredients

Tapeworm parasitism incrreases the risk of colic which can be lethal. The larvae are carried in grass mites and hibernated from them when digested. It is usually necessary to dose for worms at the end of summer grazing and in some situations e.g. when hay is made from grassland where horses have grazed at some time, at the end of winter feeding.

Tapeworm are not susceptible to routine wormer but require active ingredients such as Pyrantel or Praziquantel.

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.

Only one drug which will “get at” roundworm / redworm with just one dose; it is moxidectin.

Expert advice on the complexities of effective worm control in horses

"Worm control in horses is complex. We advise that horse owners work with their vet to devise the best solution for their horse." So says the Diagnosteq website run by Liverpool University Equine Veterinary Faculty to give vets access to expert interpretation of test results compiled by experts in parasitic disease and parasite control in horses. Whilst aimed primarily at vets, much of the Diagnosteq website information can be used by knowledgeable horse owners as it includes advice on pasture management and hygiene, choosing and using horse wormers.

Overuse of wormers contributes to resistence in the worm population

Over-use of wormers can contribute to resistance in the worm population; in addition to being bad for your pocket! If the worm control advice provided in the Information & Advice section of the Nixon Equine Vets website (and on the Diagnosteq website) is adhered to, especially when armed with the knowledge contained in a faecal egg count report from Nixon Equine Vets, it is possible to reduce the number of worm treatments your horse requires each year, thereby minimising the chance of worm resistence occurring within your own horse's environment.

click here to read more about effective parasite control and horse worming

Worming horses on large stable yards

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.

Horse wormer products and active ingredients

The number of different UK licensed horse wormer products, each with a different “active ingredient”, can make choosing the most appropriate horse wormer to use at certain times of the year 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.

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. Furthermore, 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.

Not surprisingly many horse owners ask us for advice on worming their horse. We offer free client telephone consultations Monday to Saturday between 8 am and 9 am when an experienced equine vet will be available to discuss your particular horse requirements, including worming advice.

So which horse wormer group should you choose and when?

The table below provides details of the activity of the different wormer groups.

Horse wormer dosage

It is important to 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, worms are exposed to a sub-lethal dose of the drug. This may leave a worm population 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.

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, that 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.

Horse worming programmes

For a one 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.

One of the key techniques for minimising resistance is to to use a rotational strategic worm control programme in which the group or type of wormer used is rotated 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.

Rotational horse worming programmes

The worming programme in the two illustrations below rotate 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.

An example of a rotational worming programme

An alternative example of a rotational worming programme

Pasture management and worm control hygiene

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.