Worming

A LOGICAL APPROACH TO CONTROLLING WORMS IN HORSES

WHY SHOULD I WORM MY HORSE?

Most horse owners are well aware of the importance of good worm control in maintaining their horses’ health and reducing the incidence of colic. However, horses are often overdosed with wormer which is costly to the owner, prevents the development of some immunity by the horse, and hastens the development of worm resistance to currently available anthelmintic products.

Worms of particular importance in the adult horse are small redworms (small strongyles/Cyathastomes) and tapeworms (Anaplocephala sp.). Other worms, which are less significant as they are readily controlled by pasture management and worming, are: stomach bots (Gastrophilus intestinalis); large redworms (large strongyles); pinworms (Oxyuris equi); stomach hairworms (Trichostrongylus axei) and lungworms (Dictyocaulus sp.). White worms (large roundworms/Parascaris sp.) and threadworms (Strongyloides westeri) are mostly a problem for young stock, i.e. horses less than three years old, as older horses will have developed a good resistance to these.

HOW CAN I REDUCE THE WORM BURDEN ON MY PASTURES?

Pasture management has proven to be highly effective in the control of worms so appropriate hygiene strategies should be agreed for all premises.

Droppings should be removed once a week in winter and twice a week in the summer. Turnout paddocks should be cleared daily. Removing droppings is particularly important during mild autumns and wet summers as these conditions favour the survival and spread of worms on the pasture.
Ideally horses should be stocked one per acre, although this is often impractical.
Mixed or alternate grazing of pasture with cattle and sheep is a good idea as they act as ‘biological vacuum cleaners’ and ensure that the pasture is evenly grazed. With the exception of the stomach hairworm, cattle and sheep share none of the important horse worms and this worm is readily controlled in programmes built around redworm control.
Pastureland should be harrowed only in very dry conditions to expose larvae to desiccation.
Where possible, grazing should be rested from horses for at least five months of the year as this is how long the small redworm larvae can survive on the pasture.
Young stock should be managed separately where possible, as they have lower worm immunity and within an older group they may be forced to graze worm heavy areas of pasture.
Other important adjuncts to a good control programme are manually removing bot eggs from the hair coat and cleaning the dock and head areas with separate sponges to control pinworms. Horses should not be fed directly from the ground - hay racks or nets and feed bowls should be used.

WHAT WORMERS ARE AVAILABLE AND HOW CAN THEY BE USED MOST EFFECTIVELY?

The main groups of wormers available include macrocyclic lactones [ivermectin (Eqvalan/Furexel) and moxidectin (Equest)], benzimidazoles [fenbendazole (Panacur Equine Guard)] and tetrahydropyramidines [pyrantel (Strongid P/Pyratape)]. A good worm control programme involves using an effective wormer at regular intervals throughout the year with additional specific treatment for tapeworms and small redworms.

In the past, horse owners have been recommended to rotate between products for routine worming on an annual basis. However, there are reports of resistance to pyrantel in the UK and elsewhere but to date there are no reports of resistance to macrocyclic lactones (ivermectin and moxidectin). Therefore we recommend using either ivermectin or moxidectin for routine worming. There is no advantage in rotating between these two as they are from the same family and have similar modes of action.

Traditionally, owners have been advised to treat for tapeworms with a double dose of pyrantel once or twice a year. Recently, a blood test developed at Liverpool University allows horses to be tested for tapeworms. This way it can be decided if it is necessary to double dose at all. If the test reveals low or medium values, testing or double dosing may be delayed for 18 to 24 months.

Small redworms must be specifically targeted. The larvae of these worms burrow into the wall of the gut (encysting) and may become inhibited over the winter (similar to hibernation). Five days of fenbendazole in the autumn (November) is helpful in removing larvae that might otherwise overwinter within the horse’s intestine and cause diarrhoea and other problems. This is the most effective wormer for this problem. It should be noted that resistance to single doses of fenbendazole is widespread, so single doses should not be used for routine worming during the rest of the year.

Many owners treat specifically for bots with an ivermectin product in November. However, this is unnecessary if the horse has been treated with an ivermectin or moxidectin product during the year. Lungworms tend only to be a problem when horses are grazed with donkeys as they are the natural host for this parasite. If this type of mixed grazing is unavoidable, it is imperative that donkeys are routinely wormed too.

When using a wormer, it is important to dose accurately according to the horse’s body weight. Using special measuring tapes can help this. Include all horses on a premises in the same worm control programme, and worm all horses grazing together on the same day.

COMMON MISTAKES IN A WORM CONTROL PROGRAMME

Don’t rely solely on drugs – good pasture management is the most important element of worm control

The aim of worming is to reduce the number of worms and eggs on the pasture - both inside and outside of your horse – it is imperative that all horses on a pasture receive the same drug on the same day. Horses’ weights vary tremendously. Ideally you should use a weight tape to help dose your horse correctly.
The manufacturer’s recommended dosing intervals for products containing moxidectin and ivermectin are 13 and 8 weeks respectively. You should follow these recommendations, or perform faecal egg counts as described below to allow longer intervals between dosing.

HOW WILL I KNOW IF MY WORM CONTROL PROGRAMME IS WORKING?

Most owners are used to worming their horses at the recommended dosing interval e.g. 13 weeks for Equest. However, an improvement on this strategy is to collect a faecal sample for each horse and have it analysed for worm eggs at the time when the next dose of wormer is due. If the egg count (eggs per gram/epg) is low, i.e. between 100 and 200, the use of the wormer can be delayed. This may save you money but more importantly avoids the overuse of wormer which speeds the development of a resistant worm population and prevents the potential development of some immunity by the horse itself. Resistance develops in this way as only the sensitive worms are killed by each dose of wormer allowing resistant worms to become the greater part of the population. Dung samples may be collected by the owner and handed in to the practice for egg counts to be performed. In large populations, such as livery yards, samples may be taken from a representative number of horses in each age group and the whole population wormed on the basis of the results.

SPECIFIC PROGRAMMES

ADULT HORSES

Once good pasture management is in place for adult horses we recommend that every horse receives the following programme at the same time

(1) During the grazing season administer either Equest every 13 weeks (3 months) or Eqvalan/Furexel every 8 weeks (2 months)

(2) Five day fenbendazole (Panacur Equine Guard) in autumn to treat and prevent encysted/inhibited cyathastome larvae.

(3) Monitor faecal egg counts every three months initially to ensure your programme is adequate. If the result is less than 200 eggs per gram the interval between doses of wormer may be extended.

(4) Each horse should be blood tested for tapeworms every 12 – 18 months and treated according to the results.

SPECIAL SITUATIONS

FOALS

Foals should be wormed from 6 weeks of age with ivermectin every eight weeks. They can then be included in the adult programmes as yearlings.

MARES IN FOAL

No extra anthelmintic treatment is needed for the mare as long as she is included in a sensible control programme. However, to reduce her output of worm eggs she should receive an additional dose of ivermectin approximately 4 weeks before foaling.

NEW HORSES

These horses should be blood tested for tapeworms and treated with pyrantel according to the results. On arrival, they should also be treated with moxidectin and kept off the pasture for two days at least. This should be followed shortly after by a five-day fenbendazole course to treat any inhibited and encysted small redworms present.