PARASITES AND DEWORMING

Parasitic worms live in the intestines of horses and ponies. Small numbers of worms can be tolerated, causing no effect on well being. Larger worm burdens can cause a range of problems including ill thrift, diarrhoea, colic and death. Pasture management and drug administration assist the horse’s immune system in keeping the intestinal population of worms under control. The immune response to worms is better in some horses and ponies than others and as such some are more susceptible to worm infestation.

What types of worms are there and what problems do they cause?

1. Small redworms (Cyathostomins): These are now by far the commonest equine intestinal parasite. As the horse grazes it swallows the larvae of the small redworms, which can be found on blades of grass in contaminated pastures. The larvae colonise the lining of the large intestine where they develop into adults who lay eggs that are passed with the horse’s droppings onto the pasture and develop into larvae to complete the life cycle. Larvae picked up in the autumn can go into hibernation in cysts in the intestine wall. These ‘encysted larvae’ can survive in this state for a considerable length of time, to develop into adult worms when the conditions become right for them, typically in the spring. They are faiorly resistant to most wormers whilst in this 'hibernating' state. It is the emergence from the bowel wall of large numbers of encysted larvae that can result in severe diarrhoea and weight loss - a disease known as cyathostominosis.

2. Large redworms (Strongyles) (such as Strongylus vulgaris, S. edentatus and S. equinus): Once the scourge of horses, these worms are now very rare. Again, larvae are picked up and swallowed as the horse eats contaminated pasture. The lifecycle of these worms involves the migration of larvae through the blood vessels of the intestine and liver, where they can cause inflammation and obstruction of the blood vessels resulting in damage to the intestine supplied by the damaged vessel. Symptoms of infestation can include colic, ill thrift, and diarrhoea.

3. Parascaris equorum: these worms typically affect younger horses, particularly foals as a good immune response develops with age. Worm eggs are picked up and swallowed whilst grazing. The eggs develop into larvae that migrate through other organs such as the liver and lung before returning to the small intestine to develop into adults, when they can again complete their lifecycle by shedding eggs. Heavy infestation can cause stunted growth, ill thrift, lung damage, diarrhoea and colic due to intestinal obstruction.

4. Strongyloides westeri: this parasite usually affects the small intestine of young foals and is transmitted from the mare to the foal as larvae in the milk. Typically a strong immune response is developed keeping the infection under control as the foal ages, but where large doses of larvae are swallowed and foals are overcrowded or immunocompromised, severe diarrhoea can occur.

5. Tapeworms (Anoplocephala perfoliata): Probably the second most common equine intestinal parasite (after cyathostomes). These typically live at the junction between the small intestine and caecum. A mite, present on the pasture, acts as the intermediate host for the transmission of tapeworms, allowing it to complete its lifecycle. Recently it has become apparent that tapeworms are important causes of spasmodic colic in horses. Also, it can cause some other forms of more serious colic.

6. Bots (Gasterophilus spp.): These are common but generally harmless parasites. They are the larvae of the adult bot fly. The larvae develop in the stomach of horses and are passed in the droppings. They hatch and the adult flies lay eggs on the horses’ coat, usually on the legs. The eggs are then swallowed when the horse licks its legs. Bots can result in mild ulceration of the stomach wall and have been reported to have caused stomach ruptures but are now rarely recognised to cause a problem.

7. Lungworm (Dictyocaulus arnfieldi): the larvae of this worm can be found in the lungs of horses that live with donkeys and can cause parasitic bronchitis and bronchopneumonia. This is a very rare problem.

How are worm infestations diagnosed?

Typical signs and symptoms (including ill-thrift, weight loss, colic, diarrhoea) and poor quality management (overcrowding, inadequate and contaminated grazing, and inadequate use of effective wormers) often suggest that intestinal parasitic problems are most likely. Laboratory investigations are the definitive method of diagnosis. Two types of tests are performed:-

1. Dropping samples are analysed to count the number of worm eggs per gram of faeces. This gives an indication of the types and number of adult worms present in the intestine who are producing eggs. False negative results can occur when the adult worms are not producing enough eggs to be detected and this can sometimes occur when the horse is very unwell. Dropping samples are most useful when collected from all or representative batches of horses on a routine and regular basis to monitor the success of a worm control programme. Generally worm egg counts below 50 eggs per gram (50 epg) are not concerning.

2. Although blood samples are often suggested to help diagnose parasitism, the results of most tests are non-specific and hard to interpret. There is, however, a specific blood test which has been developed to demonstrate tapeworm infestations.

How can I make sure my horses do not suffer from parasitic worms?

Whether you own one horse or a whole studfarm, you should develop a worm control policy with advice from your veterinary surgeon. This will be based upon many factors including your geographical location, the types and ages of horses that you have, your stocking density and the frequency with which horses come and go at your premises. Effective parasite control depends upon both management of grazing to minimise worm egg and larval contamination and the use of wormers to remove parasites from the horses’ intestines. One cannot be adequately effective without the other.

How can I best manage my available grazing?

Horses re-infest themselves by eating parasitic eggs or larvae by grazing contaminated pasture. Therefore:-

1. All horses grazing the pasture should be well wormed to reduce their output of parasitic eggs and larvae.

2. New arrivals should be treated with an effective wormer on arrival and should be stabled separately for 2-3 days before turning out on clean pasture.

3. The pasture should not be overcrowded so horses can avoid eating contaminated grass.

4. Droppings should be frequently and regularly picked up (twice weekly should suffice) and removed from the pasture to minimise contamination. If you have sufficient acreage, mechanical ‘dropping pickers’ (vacuum-type devices powered by small tractors), are particularly effective. This approach is extremely effective at reducing parasitism (perhaps more so than worming!) and should always be encouraged.

5. If possible paddocks should be regularly grazed with other species such as cattle or sheep, in rotation and then rested. These animals are not affected by equine parasites, their parasites do not affect horses, they graze more evenly than horses and their droppings stimulate good grass growth.

How can I best use wormer medicines?

Anthelmintics (wormers) are medicines that are given to the horse by mouth to kill intestinal parasites. When used correctly and efficiently, many are very effective, but cyathostomins (the most common type of worm) have developed widespread resistance to fenbendazole (Panacur). It is inevitable that worms will develop resistance to other wormers too and the best approach to delay this happening is to use 'strategic worming' methods to minimise the amount of wormer used.

For the single horse owner and owners of small stables, an approach is to collect faecal samples and blood samples periodically and then to select anthelmintics depending on the population of worms present, i.e. ‘target worming’. Faecal and blood samples can be used after worming to see if the drug used has been effective. Experience has shown that when regular worm egg counts are performed on populations of horses, many (if not most) frequently have zero worm egg counts and do not require worming. Targetting the worming to only those with positive worm egg counts both reduces the expense of unecessary worming and also delays the onset of resistance to the wormers used.

For commercial studfarms and intensively managed livery and training stables, a more structured approach is required. Foals who are born and raised on commercial studfarms should be first wormed at 4 weeks of age to avoid Strongyloides diarrhoea, and then wormed 4-6 weekly thereafter. The frequency with which worming is required depends upon stocking density, pasture contamination and equine ‘traffic’.

If you own one or a few horses and they have access to large clean paddocks, they will require to be medicated less frequently than if you run busy commercial stables or a studfarm where horses are regularly coming and going from other, often uncertain, managements.

There are 4 main groups of equine anthelmintics that are useful as wormers

• ivermectin and moxidectin (e.g. ivermectin products: Eqvalan, Furexel, Noromectin and Panomec; moxidectin product: Equest). These closely related drugs will kill almost all types of worm (including bots) other than tapeworms. They are incompletely effective against encysted (hibernating) cyathostomins although moxidectin (Equest) seems far better than ivermectin for this purpose. An additional advantage of moxidectin is its 'long acting' effects so redosing should never be required within 3 months of a dose. Moxidectin (Equest) is not suitable for foals.

• fenbendazole (Panacur). In theory this should be a good wormer against most species of parasite other than tapeworms although widespread resitance by cyathostomins limits this drug's usefulness. The drug is still often used as a 5 day course (Panacur equine guard) to target encysted (hibernating) larvae but there is also doubt as to its effectiveness for this purpose too.

• pyrantel (Strongid-P and Pyratape-P). This has reasonable effectiveness against many mature roundworms but is probably mainly used as a treatment for tapeworms. It is very effective against the main species of equine tapeworms when given at double the usual dose.

• praziquantel. This is a specific anti-tapeworm drug which is manufactured in combination with ivernmectin (see above) as the product known as 'Equimax'.

It is recommended that one class of drugs is used as the main wormer routinely for one to two years and then another is used during the next year, to help discourage resistance developing in the worm population. Then, during autumn, to strategically avoid problems with small Strongyle worms (cyathostomins), horses should receive either moxidectin (Equest) or possibly a five day course of fenbendazole (Panacur Equine Guard). A double dose of pyrantel or single dose of praziquantel is probably best given in the autumn to avoid problems with tapeworms. A dose of ivermectin during December will strategically remove stomach Bots.

Can pasture management alone reduce the incidence of worm problems?

Pasture hygiene is of paramount importance in controlling worm burdens. Droppings should be regularly removed from the paddock, ideally daily and certainly at least twice a week. Paddocks should never be overgrazed, reducing the need for horses to graze close to droppings. Paddocks should be rotated and rested. Good management alone may successfully control parasites for a static horse population, but this must be regularly checked by dropping sample analysis and new arrivals should always be wormed before being turned out.

What is the best approach in livery yards?

These yards can be a problem if individual owners are responsible for their own horses’ worm control. Ideally all horses and ponies should follow the same programme either target or interval dosing to prevent those being treated from being re-infected by those not being treated. Similar problems can occur with vaccinations. Best policy, for the health and welfare of the population, is to include routine worming and vaccinations in the costs of livery and for all horses to be dosed at the same time in accordance with stable policy.

In spite of our modern scientific knowledge and the availability of efficient worm medicines, intestinal parasites remain the most important cause of disease in horses and ponies world-wide. The most important reasons are overcrowding on poor quality and highly contaminated grazing, failure to keep grazing areas clean of droppings and the inadequate use of effective wormers. Buying wormers from travelling salesmen on the basis of cheap offers, without specific advice from your own veterinary surgeon, who knows the needs of your horses, is often a recipe for the breakdown of your worm control programme. Formulate a specific programme with your veterinary surgeon.