Worming – Small Redworms (Cyathostomes)
These are the most common worms in horses and they are infected from pasture contaminated by eggs passed out in faeces of infected horses. Although these worms are more prevalent in the summer they can be present all year round. The eggs can develop into adults in the gut within 5 weeks, they can either attach to the gut wall or be absorbed into it and reduce the ability of the gut to absorb nutrients leading to weight loss, diarrhoea and general ill thrift. These worms can also encyst in the gut wall as larvae and delay their development to adults. The emergence of these encysted larvae can cause major gut damage leading to severe diarrhoea, weight loss, colic and even death. The adult stages are readily detected by faecal worm egg counts (WEC) and it is sensible to worm your horse based on the results of this. The encysted larvae will not be shown up by WECs so it is important to treat for these once a year (usually in the autumn). You can reduce pasture contamination by avoiding overgrazing/overstocking of pasture and regular poo picking.
Worming – Tapeworms (Anoplocephala spp.)
These worms tend to attach to the gut walls and can interfere with gut motility and cause irritation, they are a major cause of colic (research suggests up to 20% of spasmodic colic can be attributed to tapeworms). It is spread by an intermediate host (the forage mite) and infections occur all year round. Infection can be detected via a blood test or saliva test but will not be shown on a WEC.
Pinworms are becoming increasingly common due to their location at the end of the digestive tract as newer wormers tend to be absorbed before they reach this point. The female lays eggs around the anus and this can lead to perineal irritation and tail rubbing. They are generally not harmful and are more of a nuisance/ irritant to the horse. The adults are readily killed with a double dose of a ‘pyrantel’ based wormer but it is important to also disinfect any area the horse has rubbed on (fences/haynets/rugs etc)as otherwise the horse can become re-infected by ingesting the eggs.
Bots are actually flies that have part of their lifecycle within the horse. Eggs are laid on the hair by a fly, they are consumed by the horse and migrate to the stomach where they attach and can cause some inflammation/ulceration before being passed out in the dung.
Lungworms are very rare in horses unless they are immune-compromised or are grazing alongside donkeys that are relatively commonly infected.
Large Redworms migrate to the blood vessels causing damage to major organs, historically they caused very serious illness but due to regular use of wormers nowadays they are less of a concern.
It is our advice to worm once yearly with a wormer containing moxidectin that kills encysted redworm larvae which do not show up on worm egg counts (such as Equest/Equest Pramox). In between times we recommend worm egg counts are performed and only to worm if these show a high level of adult red worms, this avoids unnecessary worming, reduces the risks of worms becoming resistant to wormers and is likely to reduce your costs! To see if your horse has tapeworms we recommend the new saliva test which you can do yourself and your vet will analyse the results and advise you.