Types of roundworms in horses

• These the the most common roundworms infecting grazing horses and ponies
• Cyathostomins have a very quick life cycle ~5-6 weeks
• This worm species can develop in horses in two different ways:
  1. Classic roundworm infection, larvae develop to adult stage immediately and begin to lay eggs (identifiable by faecal egg count testing)
  2. Arrested development. This is where larvae burrow into the gut wall and become dormant (encysted – pictured on the right). Larvae can stay in this state for months/years and encysted larvae can build up to large numbers. Damage to horses occurs when these larvae re-emerge to complete their development to adult. Re-emergence can be triggered by changing seasons (particularly winter to spring), a which point large numbers can emerge at once, causing significant inflamation in the gut (colitis). Encysed larvae cannot be detected by faecal egg counting as only adult worms lay eggs.

• These worms are less common now due to the frequent use of anthelmintics (womers)
• Large redworms can grow up to 5cm long
• Larvae migrate through blood vessels and in large numbers can cause blockages in the vessels
• Adults are found in the large intestine/colon where they feed on blood and tissue
• Clinical signs include anaemia, poor condition and temporary lameness

Hairworm (Trichostrongylus)

• The eggs of Trichostrongylus look the same as Strongylus eggs (pictured on the right)
• This roundworm species can also infect sheep and cattle so is largely an issue for horses which are co-grazed with livestock

• Primarily infect foals
• “Risk areas” for infection are paddocks and stalls which housed foals the previous year
• The larvae migrate and in large numbers can cause respiratory symptoms (e.g. coughing)

• Horse tapeworms can grow very long and are found within the intestines
• Clinical disease is rare but cases have been reported of intestinal obstructions, poor condition, enteritis and colic

• Infection more common in foals
• Clinical signs of acute infection include diarrhoea, weakness and poor condition

• Found in the large intestine and rectum of infected horses
• A key clinical sign of infection is itching around the anus which in turn causes horses to rub against objects, resulting in hair loss and scaly skin on the rump and tail head
• Horses may appear restless