Internal parasites are able to cause extensive damage, often without you realising that your animals are infected. At the very least, worms may rob the horse of valuable nutrients, cause gastrointestinal irritation and loss of condition. At their worst they may lead to colic, intestinal rupture and death.

In terms of management priorities, establishing an effective parasite control programme is second only to supplying your horse with high quality feed and clean, plentiful water.

Identify the enemy
There are more than 150 internal parasites that afflict horses, including several major species. Among the most common and harmful are: Small strongyles (cyathostomes), large strongyles (large bloodworms), ascarids, tapeworms, pinworms and bots.

Any or all of these parasites may be present in the horse at one time, at different stages of their lifecycles. This will influence the deworming programme needed to combat them. Keep in mind that some worms may lay more that 20 000 eggs a day, causing a rapid increase in pasture contamination.

Signs of parasitism
Different parasites may harm the horse in different ways. They may damage tissue and vital organs, including the intestines, lungs, liver and stomach as they migrate through the horse's system to complete their lifecycle. They may cause ulceration and obstructions within the horse's digestive tract, or intense irritation as they lay their eggs.

Contrary to popular belief, many horses that harbour internal parasites may appear to be externally healthy, while on the inside the worms are causing significant damage. In contrast, in some horses (especially weanlings and yearlings), parasites can take a visible toll.

Signs of worm infestation include dull hair coat, weightloss, colic, diarrhoea, tail rubbing, summer sores, coughing and nasal discharge.

Faecal examination
Faecal examination for worm eggs and larvae is a useful tool in setting up and monitoring an effective parasite control programme. This procedure involves taking two to three fresh faecal balls to your veterinarian for laboratory analysis. If the faecal balls cannot be delivered immediately, they should be stored in a sealed plastic container in a fridge.

The worms themselves are rarely visible in the manure, but by counting the numbers and types of parasite eggs or larvae present in the faecal sample, your veterinarian may recommend the most suitable deworming product.

Faecal eggs per gram (EPG) counts provide an indication of the degree of parasite infestation within an individual animal or within a group of horses prior to deworming, and can also be applied within two weeks after deworming, to indicate whether the dewormer has worked. Note that a negative EPG does not mean that an individual horse has no worms, as the immature larval stages which do not produce eggs, could still be present.

Controlling worms in your horse is a two-pronged attack aimed at preventing the worm from completing its lifecycle through pasture management and deworming.

Pasture management
Remove horse droppings from your pasture regularly, ideally twice a week. Divide paddocks into smaller areas so that they can be alternately grazed and rested to reduce pressure on the pasture. This will make it easier for you to remove droppings.

Try not to overstock pastures. Ideally fields should contain no more than one or two horses per hectare. Interval resting of pastures may be applied, but is not always effective. Worm eggs and larvae can survive for long periods on pastures, so simply resting the pasture does not guarantee that it is worm-free.

Graze pastures in rotation with cattle or sheep. Worms which affect horses are host-specific and cannot survive in cattle or sheep. Any larvae eaten by cattle or sheep will be destroyed – this is known as a biological vacuum-cleaning effect.

Harrowing is not recommended, as it may only act to spread the infective worm stages. It is generally only successful in hot countries where exposed parasites are killed by the heat and dry conditions.

Deworming
Consult your local veterinarian for an appropriate deworming programme for your horse’s particular needs. Ideally select a deworming programme which is effective against both adult small strongyles, as well as their encysted larval stages. Aim to use a longer dosing interval, as this will help to reduce the likelihood of resistance developing.

Deworm all new arrivals, using a dewormer known to control benzimidazole-resistant worms and larvae. Keep new arrivals in a stable or isolation paddock for 48 hours after deworming, to limit pasture contamination by eggs that are passed. Weigh your horse to ensure accurate dosing, using a weigh tape or scale.

Deworm pregnant mares before and after foaling down. Utilise faecal egg counts to identify horses requiring deworming. Always keep a record of when you dewormed and which product you used.