All grazing horses will have worms, but most horses have a low number which is healthy and does no harm. Small numbers of worms are actually healthy because they promote a healthy immune system. Problems arise if worm burdens become large, which is when the worms can cause disease such as weight loss, colic and diarrhoea, or contaminate the pasture with large numbers of of infective larval stages of worms which pass out in the faeces (droppings). In some cases, heavy worm burdens can be fatal. So it is very important to control the numbers of worms in our horses but in a way to ensure our treatments remain effective.

The aim is to maintain a low number of worms in the group of horses and on the pasture.

In adult horses, the cyathostomes and large strongyles (small and large redworms) and tapeworms are the most important ones that we need to  control. You can learn about the life cycle of the small and large redworm HERE.

• Small redworms can be difficult to eliminate because that can become encysted (i.e. buried and hibernating in the lining of the intestine) which makes them difficult to kill.  This is more common in young horses, particularly in autumn and winter. These encysted larvae sometimes all “wake up” and emerge simultaneously from inside the gut lining, which will result in massive damage to the intestine wall and serious life-threatening diarrhoea (acute larval cyathostominosis)

• Tapeworm can cause colic by damaging and blocking the intestine.

• Large redworms are now uncommon in the UK, but they can cause serious, life-threatening colic.

• Pinworms cause disease less commonly, but can still cause irritation to teh skin around the anus and require specific treatment.

A good worm control strategy aims:

1.    To minimise the contamination of pasture by worm eggs:

- Redworm eggs are passed in the droppings and contaminate the pasture; larvae develop on the grass and are then eaten and infect horses that are grazing)

- Tapeworm eggs are also passed out in horses' droppings but are then eaten by soil mites (oribatid mites), where larvae start developing. Horses become infected when the eat the mites on grass

2.    To keep the worm burden in an individual horse low enough to prevent disease, but not eliminate worms completely.
 

There are two main components to a worm control programme:

1.    Pasture management - this is the MOST important way of controlling worms
2.    Drug treatments ('dewormers' or 'anthelmintics')

For further information, please see the link to CANTER (Controlling ANTIparasitic Resistance in Equines Responsibly).