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Encysted Small Redworm

Worm control is important! 

Many horse owners know why worm control is so important, but the most recent National Equine Health Survey found that almost 1 in 5 horse owners had never heard of one of the most dangerous parasite threats- the encysted small redworm.

This parasite is picked up by horses when they are grazing. It then burrows into the gut to become enveloped in a cyst within the gut wall (encysted). This can potentially cause very serious problems with diarrhoea and potentially fatal colic.

How to spot small redworm

During the spring, summer and early autumn months, you can check for small redworm by carrying out regular faecal worm egg counts (FWECs). This test uses a small sample of your horse’s faeces to identify eggs produced by female adult small redworm, as well as other types of worms. A FWEC is a useful routine check that can be done by your vet or by a commercial service, but it will not pick up redworm in their encysted stages.

Winter FWECs: a warning

FWECs carried out in the winter months can be misleading. Your horse may have ingested small redworm larvae but instead of developing into adult redworm, they can ‘hibernate’ inside cysts within your horse’s gut wall. These encysted small redworms don’t lay eggs, so a FWEC gives you no indication that they are there. When the weather starts to warm up, the encysted redworm start developing and they emerge as adults in the gut lumen. This mass emergence of the worms can cause very severe damage to the horse’s gut resulting in diarrhoea, colic and death in 50% of cases in which signs have developed.

Controlling small redworm

There was another worrying find in last year’s Nation Equine Health Survey. It showed that 23% of horse owners did not intend treating their horse for encysted small redworm. The reasons they gave included low egg counts and a healthy appearance.

Unfortunately, appearances can be deceiving, when it comes to both winter egg counts and how your horse looks. Every horse over approximately 6 months of age should be treated with a wormer effective against encysted small redworm in November/December, to prevent the dangerous and undetected build-up of these small redworm larvae. This should be done even if your horse’s FWEC comes back negative.

When grazing season arrives, you should carry out FWECs every two to three months. Your prescriber will be able to tell you whether the test results warrant a worming treatment. Regular testing means you can avoid unnecessary worming, which helps to prevent resistance to wormers and cuts costs. Foals will usually need worming more than adult horses but it is best to get expert advice.

Looking after your horse’s environment will also help to reduce the spread of redworm. To do this you can pick up poo from the field at least twice a week, rotate grazing areas and avoid overcrowding.

Targeted treatment

There are several different types of ingredients in horse wormers, and they are not all able to treat the encysted small redworm. It is important to have the help of an expert such as, your vet, a suitably qualified person or a veterinary pharmacist, will be able to prescribe the most effective wormer for your horse, whether you are tackling small redworm or any other type of worm.


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