The theory behind changing on a yearly basis is to reduce the risk of resistance building up as frequent changes may actually speed up resistance developing, whereas the theory behind changing every time is just the opposite - frequent changes do not allow resistances to develop.

The drug companies themselves assure us that resistances have not been known to build up.   However research has proven that there can be resistance to certain of the chemicals used in wormer products.  Resistance to single doses of fenbendazole is well established and it is recommended that single doses of products containing fenbendazole should only be used in target worming and not for routine worming.

Resistance to pyrantel is also now confirmed as being prevalent but as yet there is no positive data regarding macrocyclic lactones so the use of invermectin and moxidectin based products as part of the routine working programme is recommended.

Other Considerations

Traditionally owners used to give a regular dose of a worming product every 6 to 8 weeks.  This has resulted in the large redworm being very effectively controlled, but as advances have been made in veterinary science, we are all now aware of the dangers that other worms pose to our horses.  For example, as the small redworm has a shorter life cycle than the large redworm dosing at 6 - 8 weekly intervals does not control them and it is these little devils that then encyst and cause severe problems when they "re-emerge".  We also now know that we also have to give consideration to bots and the devastating effects of tapeworms.

So, now that we know we need to strategically worm for such things as encysted red worms, bots and tapeworms INTERVAL DOSING has become the most popular worming system i.e. regular dosing  just slotting in specific target worming (eg tapeworm protection) at the appropriate times. 

However there are other schools of thought that say we should worm by:-

STRATEGIC DOSING - which involves worming only during the Spring and Summer.

This method does not control tapeworms, bots, etc. but is thought to reduce the risk of resistance as there are much fewer drug doses administered in a year.  However with this method it is very important to have strict pasture hygiene aided some grazing by cattle.

SELECTIVE DOSING - only horses with a known infestation (as confirmed by a faecal egg count) are dosed. 

This method again reduces resistance risks because of the fewer doses given but also does not provide adequate protection against the major nasties and also carries the need for very strict pasture hygiene indeed.

NATURAL CONTROL - adhering to a very strict pasture management regime only.

This method is advocated as the combination of "semi-clean" pasture and the use of drugs is considered to be a sure-fire way of actually speeding up resistance developing. 


Pasture Management

Harrowing fields to scatter dung is NOT an effective worm prevention measure as this just merely scatters the worm larvae over a larger area.  Harrowing is only of benefit in very hot weather - the exposed larvae cannot survive in the heat.  Grazing by sheep or cattle is very benefitial as the parasites that live so well within the horse cannot survive once ingested by other grazing stock - this is known as biological hoover effect.  And on no account should fresh dung be scattered over grassland.

Regular removal of dung from grassland is of great benefit as it reduces the risk of re-infection.  Paddocks should also be rested regularly.

So just what is the best course of action?

At this point (i.e. October 2007) we advise that the current general policy on worming is that too frequent dosing using different wormers i.e changing drug types on each occasion is NOT recommended.

However general advice is still that wormers should be changed on a rotational basis each year BUT more and more "experts" are advocating that the most effective way to minimise/reduce the risk of resistances building up is to actually use wormers that have long dosing intervals and obviously have no known resistance; this is a school of thought which we fully support and is totally in line with our views on the subject.

As wormers have become so highly developed in recent years with ever increasing dosing intervals this is a "positive" on two fronts - less wormer administered = lessened risk of resistance and less chemicals in the horse's body can only be a good move in general as, basically, chemicals are an alien addition to a horse's delicate digestive system and can upset the fine balances of friendly gut bacteria.

Being realistic, most owners are not able to carry out an efficient, effective pasture management regime i.e. the removal of droppings at least every other day because of time constraints.  Also a large proportion of the country's horses are kept at livery and it is just not feasible for yards to cater for differing worming strategies amongst owners; there has to be a uniformity to ensure protection. 

So, as an owner you should work out a worming regime which fits to how your horse is kept - at grass, stabled, out by day all year round, summer grazed only, etc. but obviously if your horse is kept at livery you need to consult with the proprietor. You should also consult with your veterinary surgeon if in any doubt whatsoever about what to give and when or if someone else gives you advice that you are not sure about.

Here, we advocate the use of wormers with the highest intervals between doses so as to limit the "chemical intake", with strategic treatments for tapeworms, bots and encysted redworms.  As we are entrusted with the care and management of other people's horses we naturally carry out a responsible and effective worming programme.

New wormers seem to be appearing on the market with increasing regularity these days so keep an eye on equestrian publications for details. 

General Points to Note

Worm all your horses at the same time and the same type of wormer
• Dose accurately - know your horse's bodyweight>
• Do not adminster doses more frequently than the stated interval
• Do not overstock your pasture
• Do not overlook the older horse - he is more vulnerable than his younger counterparts
• Keep a record of when you wormed and with what
• Worm all new horses to the yard BEFORE turning them out

Worm Counts

It is good management practice to have regular worm counts carried out to ensure against worm build up and of course to check the effectivenss of your worming programme.

If you use benzimidazole based wormers then it is advisable to have a faecal egg count done shortly after worming to ensure adequate dosing.

Blood testing is the only way to check the situation regarding the tapeworm burden and it is recommended that your horse is tested at 18 month intervals.


WORM TYPES

Large Red Worms (Strongyles)(Strongylus vulgaris)

The larvae of these worms do much harm to a horse as the adults as they damage the lining to the arteries, especially those of the gut.  The adults, which are 2cm - 5cm in length, are a cause of colic and also suck blood, so heavy infestation can be a cause of anaemia. Fortunately these are much less common than small redworms and if you adhere to a strict worming regime, older horses do develop an immunity to these worms, so redworm infestation is more of a problem in the younger animal. 

The lifecycle of redworms is about 6 months during which time they migrate throughout the body, one species actually preferring the arteries using them like a motorway network. The movement of these worms through the guts walls causes alot of damage and clots can form.

 

Treat larve with: moxidectin, invermectin or use a 5-day course of fenbendazole based wormer.  The adults will be zapped by all the most commonly used products, but optimal treatment time is late Autumn-December.  Pure Praziquantel-based products are not effective against large red worms.


Small Red Worms (Strongyles); Encysted (Cyathostomes)
 

These make up the major proportion (at least 90%) of the worm infestation of horses and, as they are ingested during the course of grazing, the spring and autumn are the periods of greatest risk when the weather is relatively mild and wet. However research has established that the larvae can be present in pasture during the winter months, even after snow!  The adult worms lay eggs in the intestine which are they passed out in faeces; the grazing horse then ingests the larvae.
 

Redworm larvae develop into adults very quickly during the warmer weather (in just five weeks), but once the temperature drops a little in the autumn the development rate is slowed and an increasing number of the larvae take up their winter quarters by hibernating in the gut wall forming small cysts, making them very resilient and difficult to treat.  Once "encysted" these worm larvae pose a very serious threat because they can trigger fatal diseases; even worse though, once they are prompted back to life (as yet, by an unknown trigger) and decide to emerge, which is usually late winter/early spring, if they do this en mass, the consequences can be fatal because of the serious damage that is done to the intestine walls.  It is important to despatch the encysted worms BEFORE they emerge.

The dangers of encysted redworms warrant a special note here.  Bearing in mind that the mass emergence is late winter/early spring if your horse exhibits any of the following from about December time through to the spring, encysted redworm infestation could well be the cause:

• swellings on the belly or in around the sheath areagradual/rapid weight loss
• recurring bouts of colic that last up to 2/3 days at a time
• diarrhoea - constant or intermittent
• a bloated appearance to the belly
   

Also:

• horses aged under 6 years and over 15 years of age are most at risk
• dung samples will not necessarily contain any worm eggs
• even horses regularly wormed can be affected by encysted redworms
• REMEMBER that a worm count DOES NOT show up encysted larvae
• High risk animals - youngstock and older/retired horses - should be given  a second dose in February.
   
   

When to dose: November/early December when the larvae are dormant (encysted); preferably dose twice a year, especially higher risk animals.

Treat larvae/small worms in the gut with: invermectin or moxidectin
 

Treat when encysted with: moxidectin or a 5-day course of fendendazole.  However the 5-day course is the best option to ensure a targetted and effective treatment. Given over a 5-day period the treatment is gentler on the horse's digestive system and helps reduce the risk of colic attacks when large numbers of the larvae are killed at one time.  Also if the gut wall has been damaged to any degree, it is quite likely that the horse is also suffering from enteritis; a powerful wormer will of course cause considerable discomfort and colic can again be the result.
 

Ascarids (Large Roundworms) (Parascaris equorum)

These are very long worms (up to 50cms!) and as thick as a pencil commonly affecting foals and youngstock, as older horses do develop immunity.  The eggs these worms produce – which they do so in very large numbers – have a tough coating which is adhesive so they literally stick to their surroundings, such as the coat.  They have even been found on the walls and floors of stables.

Roundworm eggs hatch inside the intestine and the larvae migrate through the gut walls and into the veins. Once in the bloodstream they are transported to the lungs from where they are re-swallowed, because the horse coughs them up; Once back into the small intenstine the larvae mature and start to wreak havoc. 

Ascarid infestation causes inflammation of the liver and lungs; they obstruct the workings of the digestive system so a sudden bout of colic can be triggered; blockages and ruptures can prove fatal.
 

Tapeworm (Anoplocephala perfoliata)

Tapeworms are probably the most serious parasite of all and are a major cause of colic.  They lurk where the small and large intestine meet (ileocaecal junction) and cause irritation of the bowel as well as rupturing of the intestine. They also affect the telescopic capacity (intussusception) of the intestines. 

It was once thought that tapeworm infestation was a "grazing season" problem because the forage mite (which is the temporary host "victim" of the worm until it finds a horse) was only to be found out on pasture land; eggs of the tapeworm are eaten by tiny mites which live by their mega-thousands in grass.  These eggs then hatch inside the mites which the grazing horse cannot help but take in.  Once inside the horse the larvae are "realeased" and their development continues.

However it is now known that this little mite is quite happy living in hay and bedding materials.  This is why there is now the recognised requirement to dose twice a year (this applies to stabled horses as well) but the timing of dosing is critical to disrupt the tapeworm lifecycle; for a couple of months or so the worm lives inside the mite, horse eats mite and a month later tapeworm is fully developed and living inside horse!

Horses grazing on land made of up acid soils are more likely to be more heavily infected as the mite loves the herbages that grow more readily in this sort of soil.

When to dose: Grass wintered horses – March and September
                        Winter stabled horses -  June and October

Tapeworm protection should be slotted into your worming programme i.e. regardless of the dosing interval of the product you are using, dose for tapeworm at the appropriate time and then carry on as if you had not administered another drug in between, although it is advisable to wait a couple of weeks after dosing for tapework before adminstering another product.

However, if you are of the mind that you would prefer to give your horse the least amount of chemicals possible, then the good news is that blood tests can now show up tapeworm infestation so your vet can advise whether it is really necessary to worm for tapeworm at all.  If you choose this option, then it is recommended to have a blood test done annually.


PIN WORMS/SEAT WORMS (Oxyuris Equi)

More of an irritation than being actually harmful, these worms live in the colon and migrate to the rectum where they lay their eggs.  The irritation they cause prompts skin and tail rubbing.

Treat with:  All worming products are effective with the exception of those containing praziquantel.
 

BOTS (Gasterophilus)

As you know these are not actually worms, but larvae of the bot fly.  The fly lays eggs on the horse's body particularly the legs and neck (but anywhere can be a suitable spot really as far as the fly is concerned).  These cause a little irritation so the horse then bites them off and into the body they go.  Once hatched, the larvae burrow into the internal body tissue and migrate their way to the stomach; many months later they are passed out in the dung.  Bots can cause inflammation of the stomach wall, ulceration to the lining and, at worst, perforation leading to peritonitis, as well minor infections of the mouth and gums.

When to dose: Traditionally the Autumn - to kill the larvae which by this time will have migrated to the stomach - but dosing is really dependent upon the weather as ideally there should have been a frost so that the adult flies are killed off and the life cycle is broken, so with the milder weather, treatment is usually winter time these days.  If "live" larvae are passed out via the droppings they can happily survive in the underground where they will pupate, so destroy the larvae, destroy the adults and bingo!

Treat with: invermectin or moxidectin.  Also try to remove as many eggs as possible from the coat.
 

As the name implies, these worms affect the workings of the lungs so cause respiratory problems such as coughing. Lungworm is usually a parasite carried byt donkeys although the majority of animals never show any sign of this burden.  Diagnosis is a bit tricky as so few larvae pass lungworm larvae so a negative faeces sample does not necessarily mean all is well; most often a vet will rule out a viral or bacterial infection as the cause of coughing and then turn his attention to lungworm especially if the patient has grazed with or on pasture that has grazed donkeys.

Note that as yet no wormer effectively controls lungworm in donkeys and this is why so many owners are reluctant to mix horses with them on the same pasture.

Treat with: Invermectin or a double dose of fenbendazole
 

STOMACH HAIRWORMS (Trichostrongylus axei)

These tiny worms are not so common in the horse, usually affecting cattle, sheep, and pigs.  However infestation can cause irritation and damage to the lining of the stomach leading to weight loss and/or diarrhoea.
When to dose: late autumn/early winter – after first frosts

Treatment effected within a redworm programme
 

As the name suggests the adult worms take up residence in the stomach.  Hatching eggs (in dung) are picked up by feeding fly maggots, the hatched larvae then get picked up by flies and are transmitted to the horse.

Any larvae deposited on wounds or sores cause intense irritation to the point that the wound just refuses to heal - so-called "summer sores".
 

These are the worms that classically cause diarrhoea in foals which is extremely harmful to them. The worm larvae are passed to foals by the mothers' milk, so it is important to keep brood mares free of these parasites.  A natural immunity to these worms is usually in place by about 6 months of age.  Other symptoms of the their presence in foals is weight loss, dull coat and poor growth/development.


NECK THREADWORMS (Onchocerca spp)

The adults live in tendons and ligaments whilst the larvae live under the skin and are especially fond of the area around the eyes which can cause problems.  When midges bite the skin they are feeding on these larvae.

Please note that in foals diarrhoea should always be considered as serious and that the cause is not necessarily worms; veterinary advice should always be sought as other nasties such as e.coli could be responsible.
 

The main chemical groups and drugs used are:

Tetrahydropyramidines - for use against: large roundworms, large redworms, small redworms, seatworms/pinworms and tapeworms -all species

These drugs paralyse the worm which ultimately kills it.

• Pyrantel [embonates] - e.g. Strongid-P paste, Pyratape P Paste

Benzimidazoles - for use against: roundworms, large redworms and small redworms

These drugs cause damage to the intestines of the worm so that it eventually dies of starvation.

• Fenbendazole - e.g. Panacur paste, Panacur Equine Guard
• Mebendazole  - e.g. Telmin paste, Equivurm, Multiwurva-f 
• Oxibendazole  - e.g. Equitac paste, Lincoln