Encysted Redworm In Horses

Encysted redworms are the larval stages of a group of small strongyles (cyathostomins) that infect the large intestine of horses. They are among the most common and potentially dangerous parasites affecting equines.

Life Cycle

1. Eggs: Eggs are passed in droppings and hatch into larvae on pasture.
2. Ingestion: Horses eat larvae while grazing.
3. Encystment: Inside the gut, larvae penetrate the intestinal wall and encyst.
4. Dormancy: Larvae may remain encysted for months, especially over winter.
5. Emergence: In spring, thousands can emerge simultaneously, causing larval cyathostominosis, a potentially fatal disease.

Clinical Signs

Encysted redworm syndrome (larval cyathostominosis) often occurs in late winter or early spring, especially in younger horses. Key signs include:

• Sudden weight loss or poor condition
• Diarrhoea (often severe)
• Lethargy and a dull coat
• Colic symptoms
• Peripheral oedema (swelling under the belly or legs)

Diagnosis

• Egg counts (FEC) are often negative - encysted stages don't shed eggs.
• Diagnosis relies on:
  • History (recent deworming, season, pasture management)
  • Clinical signs
  • Blood tests (may show inflammation, low protein, or high globulins)
  • Veterinarian assessment

Treatment

Only certain products target the encysted stage:

• Moxidectin (single-dose treatment; broad spectrum)
• Five-day fenbendazole course (but resistance is common)

Prevention And Management

• Strategic worming: Use faecal egg count-based programmes (FEC) and treat only when necessary.
• Target treatment: Administer moxidectin in late autumn/early winter to reduce encysted burden before spring.
• Pasture hygiene: Regular poo-picking (at least twice weekly).
• Avoid overstocking: Rotate paddocks and rest pastures.
• New arrivals: Quarantine and worm-test before turnout.

Key Times For Action

• Autumn/Winter: Treat for encysted larvae (October-February in most regions).
• Spring/Summer: Conduct regular FECs to monitor adult burdens.