Small Redworm. The hidden parasite that can carry serious risk.

Small Visible Redworms

Small redworm (cyathostomins) are among the most significant internal parasites affecting grazing horses in the UK. Although individually small, they can pose a serious health risk when present in large numbers.

Horses become infected by ingesting larvae while grazing. Once inside the gut, the larvae may burrow into and encyst within the lining of the large intestine. In this dormant state, they can remain hidden for months and are not detected by routine faecal egg counts. Problems arise when large numbers of these encysted larvae emerge simultaneously from the intestinal wall - a process most commonly seen in late winter or early spring, but possible at other times of year.

Mass larval emergence causes inflammation and damage to the intestinal lining, a condition known as larval cyathostominosis. This can lead to significant clinical signs including weight loss, lethargy, dullness, a poor coat, profuse diarrhoea. The diarrhoea may be persistent, watery, and difficult to manage, often requiring intensive veterinary treatment and hospitalisation. In severe cases, protein loss through the damaged gut can lead to fluid accumulation (ventral oedema) and further complications.

Colic is another important risk. Inflammation of the large intestine disrupts normal gut motility, which may result in abdominal pain ranging from mild discomfort to severe, life-threatening colic episodes. Unfortunately, larval cyathostominosis carries a guarded prognosis in advanced cases, making prevention particularly important.

Young horses under six years of age are considered at highest risk, although any grazing horse can be affected, especially where parasite control has been inappropriate.

Strategic parasite management is essential. This includes:

• Careful selection and timing of effective anthelmintics capable of treating encysted larvae

• Good pasture hygiene such as regular poo-picking

• Avoidance of overstocking

If your horse develops unexplained diarrhoea, weight loss, or signs of abdominal discomfort, prompt veterinary assessment is crucial. Early diagnosis and treatment significantly improve the chances of a successful outcome and help protect long-term intestinal health.

Both Photos show visible ascarids (roundworms)

Please call the office on 01834 860871 if you would like to speak to one of our vets about appropriate worming strategies or if you have any other concerns.