Management of parasites is an essential component to maintaining overall horse health and often goes hand in hand with providing optimum nutrition. Although there is no such thing as a parasite-free horse, it is important to control internal parasites to help minimize associated problems including damage to the gastrointestinal tract which can cause diarrhoea, anaemia, and colic. Associated symptoms also include poor coat, weight loss, a pot-bellied appearance and possible adverse effects on the growth of young horses. In some cases, owners may think some or all of these problems are associated with nutrition and will try to overcome them by changing or increasing the horse’s feed. However, if a worm burden is present, deworming the horse as needed will often improve the condition without requiring dietary changes.

Some common intestinal worms that affect horses are the large and small strongyles (redworms), roundworms, and pinworms. The larvae of bot flies are also a parasite, however, they rarely cause digestive issues. The common signs of worm infestation are tail rubbing, pale gums, ill-thrift, colic, and poor coats. Identifying the signs of parasites is essential as if left untreated, severe infestation can cause chronic diarrhoea or sudden death.

Large strongyles are gray, blood-sucking worms, approximately 20 mm long, that are often referred to as redworms or bloodworms. The most important of the group is Strongylus vulgaris. The eggs from adult worms living in the horse’s large intestine are passed out with manure onto pasture where, if the conditions are right, they hatch and develop into infective larvae. When eaten by a horse, the larvae follow a complex migration, passing through the intestinal wall to the inner lining of small arteries, then moving up these to the large arteries supplying the digestive tract. The larval migrations last five to seven months and the larvae eventually return to the intestine where they become adults. The migration of Strongylus vulgaris larvae can cause serious weakening of the walls of arteries and the formation of blood clots. When these clots break away and block smaller arteries supplying sections of the intestines, colic commonly results. On rare occasions, a weakened section of the artery wall may rupture, resulting in sudden collapse and death. Although horses usually pick up worm larvae from pasture, any grassy areas around yards and stables that have been contaminated with manure will also be heavily infested.

Small strongyles are small redworms usually found in the large intestine. The larvae migrate into the gut wall, causing the formation of small nodules. The immature stage hibernates in the gut wall and then emerges to cause substantial damage to the gut. Heavy infestations of small strongyles can cause ill-thrift and diarrhoea. These worms have developed resistance to many of the common deworming products, and they are now a great problem for horses.

Roundworms are a common problem in foals and yearlings. Roundworms are creamy-white, almost the thickness of a pencil and up to 50 cm long. You may notice the worms in droppings after deworming young horses. The adult roundworm produces eggs that have considerable resistance to adverse conditions, and they can survive for several years on pastures and in yards and stables before hatching into larvae. This means eggs from one year’s foals can infect next year’s foals. Once ingested, the larvae migrate through the liver and lungs before becoming adults in the small intestine. These very active worms irritate the intestinal lining and can cause intermittent diarrhoea, ill-thrift, and a potbelly. Pneumonia in foals is a common side effect to the lung damage caused by the migrating larvae. Heavy burdens of adult roundworms can cause intestinal obstruction, colic, and occasionally death.

Pinworms are up to 10 cm long and grayish-white. The eggs are eaten with feed and hatch into larvae in the small intestine. The adult female lives in the large intestine and lays eggs near the horse’s anus, causing intense irritation. The commonest indication of pinworm infestation is persistent tail rubbing. Pinworms are most often a problem of stabled horses. Although pinworms are not a particularly serious parasite, horses that continually rub their tails can develop abrasions and unsightly bare areas.

Tapeworms may occasionally cause colic, general unthriftiness, and diarrhoea. Problems usually arise in late summer and autumn following infections in late spring and early summer. Although rare, acute symptoms of colic or sudden death can occur from blockage and then rupture of the bowel. Not all deworming products eliminate tapeworms, so check the labels of the products you are using.

Threadworms may cause diarrhoea in young foals that are infected through the mare’s milk. Treatment of the mare on the day of foaling and rotation of foaling paddocks will assist control.

Lungworm infestation is a problem when horses are grazed with donkeys. Affected horses may show overall weakness or coughing, but clinical problems are seen more commonly in donkeys.

A key factor in controlling parasites is developing an effective plan for the management of manure. On average, a medium-sized horse can produce about fourteen kilograms of manure and ten litres of urine every day. Multiply this by several horses and owners suddenly have a lot of material to deal with. While the main objective of manure management should be for optimum control of infections and parasites, there are added benefits to improving the cleanliness and appearance of an equine property,

Collecting manure is by far the most effective way of breaking the parasite cycle and preventing worm infections in horses. Parasite eggs are passed out in manure and if this is collected before these eggs have a chance to hatch into larvae, it will reduce the risk of the infection being passed on. While it is impossible to completely eradicate worms from every property, manure collection will minimise the number of larvae picked up by horses and can therefore reduce the number of times horses will require drugs to prevent and manage worm burdens.

Manure collection may be achievable on properties with few horses but it is not always a convenient solution for large equine farms and in this case, alternative management practices are required.

Harrowing pastures through towing chains to break up and distribute manure can be an effective technique for controlling parasites however it is essential that this is carried out when environmental factors are correct. Hot and dry climates are ideal, as the heat and wind will help to kill eggs and larvae. Any other weather is likely to result in spreading the parasites and creating further pasture contamination.

Cross-grazing by introducing other animals to a paddock after horses have grazed on it can help to ‘clean up’ the paddock and minimise worm burden. There is only one parasite, Trichostrongylus, that infects a wide range of animal species and can live in the stomach of horses, cattle, sheep and goats. Notably, though, infections with this parasite are rare in adult ruminants.

Paddock rotation timing is also an important part of manure management and optimum timings will depend on paddock size, stocking rates and cross-grazing. Ideally, paddocks should be grazed by horses once a year for two to three months followed by a three-week rest to allow eggs to hatch before introducing another species. Paddocks should then be grazed for at least a month by sheep, cows or goats before being rested again.

If these rotational timings are not achievable due to grazing area being limited or high stocking rates, drug treatments to control parasites will be necessary at least twice a year. Advice on drug treatments and worming programmes should be sought from veterinarians or nutritionists, as well as other local horse owners.

A relevant problem in parasite management is the increasing resistance of parasites to the available drugs in deworming medication. To minimise this, it is recommended to avoid deworming regularly without obtaining faecal egg counts. These are available from your veterinarian and are great tools to determine when drug treatments are needed.

For further assistance, consult a qualified Equine Nutritionist. https://nrm.co.nz/

Article supplied by Luisa Wood, Equine Nutritionist.