The contents of this page are currently being written, to be available in July/August 2011. For related articles, click on the Guide to Horse Worms, Wormers and Worming.

The main parasites which are treated by wormers are listed below, grouped by category:

Group Common Name Scientific Name
Nematode Hairworms Trichostrongylus axei
  Stomach Hairworms Habronema muscae
  Large Redworms or Large Strongyles

Large Strongyles, including three common species:

  • Strongylus Vulgaris
  • Strongylus Edentatus
  • Strongylus Equinus
  Small Redworms or Small Strongyles Cyathostomes (approximately 40 species infect horses, of which 10 are commonly found)
  Large Roundworms (also know as ascarids) Parascaris equorum
  Lungworms Dictyocaulus arnfieldi
  Pinworms Oxyuris equi
Cestodes Tapeworms

Three species horse tapeworms in USA and UK:

  • Anoplocephala perfoliata
  • Anoplocephala magna
  • Anoplocephaloides mamillana
Arthropods Bots Gasterophilus

There are many different types of worms which can infect horses, with the following being the better known. Note that not all worm types are found in all regions, so you would need to ask a veternarian in your region which of the following are a local concern.

  • Strongyles (large red worms)
  • Cyathastomes (small red worms). The life cycle starts with the horse swallowing the larvae from the pasture, which then enter the bowel lining, where they stay until mature. Once mature, they emerge from the bowel lining and in so doing damage the bowel. Sometimes a large number emerge from the same time (especially at the end of winter), causing substantial damage to the bowel wall, which in some cases is so extensive that it is fatal for the horse. These worms have developed resistance to modern worming medication, making them more difficult to treat. In addition, the larvae can 'hibernate' in the bowel lining, where they are largely resistant to many types of worming medication. 
  • Lung worms (Dictyocaulus Arnfieldi). Unlike most other worms, these are not normally transmitted from horse to horse as they are mainly a donkey parasite. Horses are normally contaminated by grazing on the same field as donkeys or on a field previously grazed by donkeys. Although infected horses are unlikely to spread the disease (as the parasite is normally unable to complete its life cycle in a horse), horses which have high levels of these worms can develop worm cough to the the presence of larvae in their lungs.
  • Pinworm. Although not a significant health risk, they can cause irritation to the horse around the anus. This irritation often results in the horse rubbing its back side, which can result in hair loss in this region or even wounds to the tail or around the anus area.
  • Roundworms (Ascarids). This is mainly found in young horses (foals or yearlings). Due to the larvae migrating through the lung on their way to the intestine, infestation often results in coughing.
  • Tapeworms (Cestodes). Although less dangerous than red worms, they are a potential cause of colic. Unlike other worm types, tapeworm eggs are difficult to detect in a manure sample, although a blood test is available.
  • Bots. Unlike the above, these are actually fly larvae rather than worms. However, the larvae infect the intestine (like intestinal worms) and consequently are often treated the same as worms. The adult fly lays its eggs on the horse's coat, where the eggs hatch into larvae. The larvae produce a chemical which irritates the horse's skin, producing an itching situation. This results in the horse nibbling at the area (to relieve the irritation) and in so doing swallowing the larvae. The larvae develop in the stomach and are then passed in the horse's manure as pupae, which subsequently hatch into the adult fly. The adult fly then lays eggs on a horse, completing the life cycle. Eggs are mainly laid in areas (e.g. forelegs) which are easily reached by the horse with its mouth.