Different types of parasitic worms are present on nearly all pasture occupied by horses. As such it is inevitable that grazing horses will pick up these worms. Fortunately, most adult horses have an effective immune system that will keep their worm burden low and prevent any ill-effects occurring.
Some horses, especially young horses, have an immune system which is underdeveloped or compromised. Also, in some circumstances, horses may be exposed to high levels of worms from their pasture. In this situation it is important to spot and selectively treat the affected individuals before worm burdens become excessive, leading to ill health and on rare occasions even life-threatening.
Which worms affect horses?
There are a number of different types of worms which can cause ill-horse health to horses. These are listed below. It should be noted all worm have a life cycle starting with an egg stage, developing to the larval stage, before becoming adults.
- Small Strongyles – The most common type or horse worm. Found in the hind gut (caecum and large colon). The most serious problems occur when the larval stages burrow into the wall of the intestine and then re-emerge en mass causing severe inflammation. This can lead to weight loss, colic signs, or even death.
- Large strongyles – Adults worms live in the large intestine which can lead to gut damage. Larval stages migrate through the horse’s body affecting the liver or blood vessels which supply the gut.
- Tapeworms – These live specifically at the junction between the small and large intestine where they can cause a blockage resulting in signs of colic.
- Ascarids – These live in the small intestine of young horses (typically up to 2 years of age). They can cause a blockage of the gut presenting as severe colic signs.
- Pinworms – Can cause irritation around the anus as the females crawl from the colon onto the horse’s skin to lay their eggs.
- Threadworm – Found in foals up to 4 months of age. Can cause diarrhoea.
- Bots – Commonly found in the stomach of horse but rarely cause ill health.
- Lungworm – Very rare in the UK
How do I avoid my horse getting ill health from worms?
It is now well recognised that regularly treating adults horses for worms if poor practice. This is because most horse develop an immunity to worms. In fact only around 20% of the adult horse population require worming.
Blanket worming all horses would be unnecessary and expensive. It would also increase worm resistance to the drugs used, and in time cause the wormers to be ineffective in preventing health problems.
Far better is to select the horses which require treatments and then treat those horses appropriately. This can be achieved through use of faecal worm egg counting, thereby assessing the adult population of roundworms worms (strongyles and ascarids) within the intestines.
Unfortunately, this egg counting doesn’t measure two of the most pathological worm types, namely the larval stages of the small strongyles and tapeworms. It is therefore prudent to either use blood or saliva sampling to assess the levels of these two worm types or treat the horses appropriately at the correct time of year.
What are the best wormers to use?
Wormers can be split into three main groups:
- Those which treat adult roundworms
- Those which treat the encysted (enclosed within the gut lining) larval stages of roundworms
- Those which treat tapeworms
Table 1 can help when choosing the most appropriate wormer to use as part of a general worming plan.
Take home message
A good horse health worming protocol doesn’t aim to eliminate worms on a yard but rather keep their levels to a minimum by selecting the horses most likely to be affected by high worm burdens and treating appropriately with the most suitable wormer. This should be supported by other measures of pasture management (e.g. poo picking and stocking density) to help manage worm populations.
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