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.

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.

2. Blood samples are used to evaluate the severity of the worm larval activity in individual horses. This is done by measuring total and differential white cell counts and specific protein levels. Recently, a blood test has been developed to demonstrate tapeworm infestations.

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.

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 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.

5. 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.

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 some populations of worms have developed resistance to the effects of some of the drugs, over the last decade, and they need to be used strategically, according to age of horse and time of year. Strategic worming also minimises the chances of stimulating the development of resistance.

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.

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 now two main classes of equine anthelmintics that are useful as routine wormers against a broad spectrum of parasites. These are the ivermectins (e.g. Vectin and Equest) and the pyrantel drugs (e.g. Equitape and Pyratape-P). There are several formulations made by different manufacturers and your veterinary surgeon will make recommendations. Some formulations are claimed to need less frequent dosing but this should always be checked by periodic dropping sample tests as local management factors may result in varying efficacy. It is recommended that one class of drugs is used routinely for one year and then the other is used during the next year, to help discourage resistance developing in the worm population. Then, during spring and autumn, to strategically avoid problems with small Strongyle worms (cyathostomes), horses should receive a specific five day course of fenbendazole (Panacur Equine Guard) and then a double dose of pyrantel to avoid problems with tapeworms. A dose of ivermectins during December will strategically remove stomach Bots.

For horses kept under intensive management conditions, strategic monthly worming may be necessary, in addition to good management of grazing.

Pasture hygiene is of paramount importance in controlling worm burdens. Droppings should be regularly removed from the paddock, ideally daily and certainly at least once or 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.

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.

This may occur if management or worming failures occur or if an individual horse’s immunity becomes compromised by, for instance, another illness. Typical signs and symptoms may develop (e.g. ill-thrift, weight loss, colic, diarrhoea). Horses with colic associated with damage to intestinal blood vessels or twists or telescoping of sections of bowel may require emergency abdominal surgery to try to save their lives. Horses with severe diarrhoea and toxaemia may require hospitalisation and intensive care. In terms of specific anthelmintic treatment for ‘out of control’ small Strongyle (cyathostome) infestations, clinical experience suggests that 5 daily double doses of fenbendazole is most reliably helpful and for tapeworms, double doses of pyrantel drugs are required.

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 us.