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Horse worming – The how & why

How do horses become infested with worms?

Worms in the gut produce eggs that are passed in the manure onto the pasture. Horses then graze the pasture and ingest the eggs. Once inside the body, the eggs hatch and resume their lifecycle as larvae. This often involves a period of migrating through the tissues, including the guts, lungs and liver, before returning to the gut as an adult, egg-producing worm. This is a basic explanation. Different parasites have slightly different variations on this cycle, but this information is not essential for this article.

What are the types of worms that affect horses?

  • Small strongyles/redworms (cyathostomes)
  1. These are common and cause direct damage to the gut wall. They are the most important class of worms currently as they are responsible for most worm-related illness in horses. In winter the immature stages encyst in the gut wall. When they emerge in spring they can cause colic and diarrhoea due to damage to the gut wall.
  2. Susceptible to:
  • Moxidectin
  • 5 day course of fenbendazole at 10mg/kg

 

  • Large strongyles/redworms (e.g. Strongylus vulgaris)
  1. These can invade the blood vessels in the abdomen and can cause anaemia, colic or sudden death due to rupture of a blood vessel. They used to be a major problem in horses but rarely cause problems nowadays as they are susceptible to the major classes of wormer drugs used.
  2. Susceptible to:
  • Benzimidazoles
  • Macrocyclic lactones

 

  • Tapeworms (Anoplocephala perfoliata)
  1. Can cause colic and death.
  2. Susceptible to:
  • Pyrantel at double the normal dose
  • Praziquantel

 

  • Roundworms (Parascaris equorum)
  1. Affect foals only as immunity develops in older horses resulting in spontaneous expulsion of the worm. They live in the small intestine, and large numbers of them can cause obstruction of the intestine. This usually occurs 1-5 days after the foal is wormed, causing colic. Affected foals are usually 2-6 months of age and unthrifty (poorly grown for their age, possibly underweight, poor hair coat). The worm is susceptible to the common wormers used. If you suspect that your foal may be infested with roundworms and by worming it with a regular wormer you may cause colic, it is best to worm the foal with low-dose fenbendazole (Panacur), a type of benzimidazole, which will kill off some of the worms but not all of them. One week later, worm with the normal dose of fenbendazole to remove the rest of the worms. By worming in two stages, the risk of causing an obstruction is reduced.
  2. Susceptible to:
  • Benzimidazoles
  • Macrocyclic lactones

 

  • Threadworm (Strongyloides westeri)
  1. Adult horses often do not have a patent infection (they do not carry the worm within their gut), but larvae are present in the tissues, where they do not cause a problem. Parturition (giving birth) can stimulate re-activation of these larvae, which migrate to the mare’s udder and infect the milk. The foal drinks the infected milk and becomes infected. This worm is rarely a problem but has been associated with diarrhoea in foals.
  2. Susceptible to:
  • Macrocylic lactones

 

  • Pinworms (Oxyuris equi)
  1. These worms are not a major problem, but they reside in the anus and can cause irritation around the anus, resulting in the horse rubbing its bottom to try to scratch the itch. This results in the horse rubbing out its tail, which can be a problem in show horses. These worms are very susceptible to most classes of wormer drugs.
  2. Susceptible to:
  • Benzimidazoles
  • Macrocyclic lactones

 

  • Lungworm (Dictyocaulus arnfieldi)
  1. Lungworm is an uncommon problem in horses. Again, this type of worm is very susceptible to most types of wormer drugs. Donkeys often carry the worm without showing any clinical signs, and they are usually the source of infection for horses. In horses it causes a cough, and in extreme cases can cause pneumonia.
  2. Susceptible to:
  • Benzimidazoles
  • Macrocyclic lactones

 

  • Bots
  1. Very common. Flies lay their eggs on the horse’s coat. The horse licks its coat and ingests the eggs, which hatch in the stomach and attach to the lining of the stomach. Can cause ulcers in rare cases. They are not a worm, but for the purpose of this article they will be included as one.
  2. Susceptible to:
  • Macrocyclic lactones

Thanks Veresdale Equine Veterinary Services for the info!

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