Do I really need to worm my horse?

that is ingested. They too have a damaging migratory pathway from the intestines they migrate through the liver and moult to L3 before progressing to the lungs.  From here they can be coughed up swallowed and moult to L4 and then adulthood in the small intestine before starting the whole cycle again. Liver and lungs may be damaged but impacted colic from a heavy worm burden along with illthrift and a potbelly are common signs.
Dictyocaulus arnfieldi lungworm is another nematode. To my horror it is donkeys that are particularly affected by lungworm and carry it as the life cycle is not actually completed in the horse.  These larvae are ingested and burrow out of the intestinal wall into the bloodstream where they penetrate the lungs. They will cause reactive changes in the respiratory system such as coughing increased mucus production and irritation of the bronchi. Chronic pneumonia secondary infections and pulmonary oedema fluid on the lungs may be other features in heavy burdens.
Anoplocephala tapeworms a cestode not a nematode. It requires an intermediate host to develop to a larva from an egg and it finds a host in the oribatid harvest mite. When the horse eats the mite with the parasite the adult tapeworms can then settle in the caecum the huge fermentation chamber of the horse gut and small intestine. Tapeworms pose a real threat to your horse by associations with spasmodic colic it can cause food impaction and intussusception when the colon telescopes folding in on itself. Inception may be about a dream within a dream so think of intussusception as a colon within a colon.
Other parasites include Oxyurius equi pinworms causing itching and irritation around the anus and Gasterophilus bots actually a fly larva and not known to cause many problems despite settling in the stomach.
What can we do about worms?
I have been on many yards with rigid worming routines as a means of prevention as much as treatment this is called interval dosing. Is it necessary? If I had to fall in a strict yes or no camp I would be in the latter. Wormers known as anthelmintics are becoming less efficacious that is to say anthelminthic resistance is becoming a real problem. The more that worms are exposed to wormers the more the wormer becomes a selection pressure some worms will have innate features which allow them to survive despite these chemicals specifically designed to kill them pesky