top of page

Large Strongyles - Large Redworms


  • Strongylus vulgaris is the main species of large strongyle in the UK

  • This parasite used to be the most common in our horses in the 1960’s to the 1980’s, however, it is now present in low numbers

  • There is no documented anthelmintic resistance in this parasite


Life Cycle


The horse ingests the 3rd stage larvae (L3) from the pasture, these migrate through to the large intestine. The larvae penetrate the intestinal mucosa and moult to L4. They then travel around the associated blood vessels over the next few months. During this time they cause damage to the blood vessel walls that leads to weakening. They also cause blood clotting, and the combination of clotting and blood vessel damage can lead to fatal colic by disrupting blood flow to the gut. After migrating around the blood vessels they penetrate the gut mucosa back to the large intestine moult to L5 and become adult. The eggs are excreted in the dung, hatch and moult through L1 and L2 to L3 for the cycle to start again.





















What do they look like?


  • Red worms that are typically 14-24mm in length

  • Feed on blood so often dark red but can be white





















What damage can they cause?


  • Damage to the blood vessels, causing weakening to the walls

  • Blood clotting

  • Fatal colic caused by disruption of the blood supply to the gut through clotting or rupture

  • Gastrointestinal problems

  • Damage to other organs such as the liver and the pancreas


Clinical Signs


  • Anaemia

  • Weakness

  • Poor appetite

  • Weight loss

  • Abdominal discomfort

  • Diarrheoa


Severe infections:

  • Colic

  • Gangrenous enteritis (death or decay of tissue)

  • Rupture of the intestines

  • Death


Diagnosis and Detection


  • Can be detected through faecal egg counts

  • It is normal to have a mixed infection of large and small strongyles; faecal egg counts are unable to differentiate between the species

  • Mixed infections will be measured as low (0-200epg), medium (200-500epg) or high (more than 500epg)

















  • Large strongyles are sensitive to all classes of wormers

  • Treatment is carried out according to the results of faecal egg counts – low readings need not be treated as the horse’s natural immune system should be able to cope with the infection. Medium and high readings should be treated accordingly.

  • Faecal egg counts cannot differentiate between large and small stronglye eggs. Therefore, if a faecal egg count result is positive your horse should be treated with a wormer that can treat both large and small strongyle infections.  


Preventative Measures


  • Avoid overstocking pasture with horses

  • Good pasture management


Current Issues in 2015


  • Very sensitive to wormers and has been virtually eliminated from the UK

  • Only seen occasionally in horses that have not received wormers





bottom of page