There are many species of worm that can live inside a horse, but they can be grouped according to their life cycles, consequences to the horse, and susceptibility to drugs.

Small Strongyles (Redworms)

These small red worms are probably the group that pose the greatest threat to your horse’s health, although strangely the adults cause virtually no problems.  Let’s have a look at their life cycle:

Adult in large intestine –> Eggs Laid –> Eggs in droppings on pasture –> Eggs develop into larvae –> Larvae eaten by horse

Larvae encyst in large intestine wall –> Young adults emerge into large intestine –> Start laying eggs

The adults live in the large intestine, freely within the lumen of the gut and absorb nutrients from the food that passes them by.  In large numbers they will cause poor weight gain/weight loss, and possibly diarrhoea, but due to their uninvasive feeding habits the adults will often not cause a problem.  Looking again at the life cycle, we can see that the strongyle larvae form cysts in the large intestine wall, and these cysts then rupture to release the young adult – it is this cyst formation and rupture that causes the main clinical problems.

The red lesions are cysts formed by small strongyle larvae.

A horse living on contaminated pasture will, throughout the summer, pick up a few worm larvae each day, and these will develop into adults causing minor damage to the intestine as the cysts rupture.  Clinical signs may not be seen as the damage is minor, and the gut can repair itself.  However, towards the end of summer the larvae eaten by the horse will not develop like this – from late summer/autumn, the larvae will form a cyst in the large intestinal wall, but this cyst will not immediately rupture.  These larvae have entered a state known as hypobiosis, and are waiting until the following spring to rupture.

The large intestine in late Summer keeps accumulating more and more of these hypobiotic larvae, none of which will rupture until next Spring.  Finally, the following Spring, all the hypobiotic larvae rupture their cysts in one go, causing massive damage to the large intestinal wall, severe diarrhoea and dehydration, and often death.  The problem is that rather than a gradual eruption of cysts, as over the Summer, all the cysts rupture in one go.  This condition is known aslarval cyathostominosis or larval cyathostomiasis.

The control strategy for larval cyathostominosis is two-fold: Firstly, control the adult population over the Summer to prevent significant amounts of eggs from being deposited onto pasture.  Secondly, prevent the formation of encysted larvae in late Summer/Autumn by the use of a wormer that will kill them.  Using this wormer the following Spring is not only pointless, as the cysts have already formed, but potentially dangerous, as killing all encysted larvae in one go can be even worse than letting them erupt naturally.

Large Strongyles

Large strongyles are a particularly nasty type of worm, that fortunately is rarely seen anymore due to their sensitivity to the now commonly used ivermectin wormers.  They have a more complex life cycle than the small strongyles, involving the formation of cysts within intestinal blood vessels.  This can damage the vessels to the point of cutting off blood supply to large portions of small intestine, with often fatal consequences.

Current control strategies are not usually designed with the large strongyles in mind – this is due to their sensitivity to most types of wormer, meaning that almost all worming programmes designed to control other worms will also control the large strongyles.

Tapeworms

Anoplocephala, the equine tapeworm was, until as recently as 20 years ago considered to be irrelevant, with no effects on the horse at all.  Studies performed at the University of Liverpool have shown that the tapeworm has a considerable role to play in causing certain types of potentially fatal colic, so we are now much more interested in its control.  The tapeworms have an interesting life cycle, involving a type of mite called an oribatid mite.  Eggs layed onto pasture are ingested by these mites, and the parasite develops inside the mite.  The mite must then be eaten by the horse, where it is broken down and the developing tapeworm released.  These mites can be found on almost all pasture, and hence almost all hay/haylage as well!  As horses can therefore become infected due to mites in their hay, then control must be aimed at killing the adults within the horse, rather than preventing your horse from passing eggs onto pasture.  This can be reliably achieved by using a wormer effective against tapeworms on an annual basis, as a one year interval will not allow enough tapeworms to build up to cause a disease problem.

A simple blood test is now available that gauges a horse’s previous exposure to tapeworm, which can be useful in horse’s with an unknown worming history, and also in horses with colic.

Pinworm

The equine pinworm (Oxyuris equi) is a relatively uncommonly seen worm.  The adult lives just inside the rectum of the horse, and lays its eggs in a very sticky material that then sticks to the horse’s backside.  This can be seen as “anal rust.”  This sticky deposit makes the horse extremely itchy, and the eggs are rubbed off onto trees, fencing, and whatever else is used as a scratching post.  The eggs are then ingested by other horses mouthing these areas – this worm is therefore more common in mouthy youngsters, and potentially also horses that wind suck and crib bite.  The only real consequence of infection is severe itching, which may lead to weight loss due to decreased grazing time, and destruction of tail hairs.  This worm is controlled by worming, and thorough cleansing of areas used by the horse to scratch.

Bots

Stomach bots are in fact the larvae of Gasterophilus flies.  The eggs are laid on the horse’s hair (often on the legs), and then ingested when the horse grooms.  The eggs then develop into larvae that attach themselves onto the stomach lining.  These larvae will cause small ulcers where they attach, but these do not appear to cause clinical signs.  Often bots are detected as an incidental finding when performing gastroscopy.  It has also been reported that the bots can re-attach themselves further down the horse’s GI tract, causing mild colic signs.  Most worm control strategies will eradicate bots succesfully, and regular grooming will also help to remove the eggs from your horse’s coat.

Roundworms

The equine roundworm, Parascaris equorum, is a large worm that affects mainly youngsters.  Natural immunity is normally present by 6 months of age, so after this age it rarely causes a problem.  Theoretically, a horse with a compromised immune system could become infected again at an older age.  The common signs of infection are poor weight gain, pot belly and colic.  Worming foals with a high number of these worms can be problematic as many dead worms can cause an obstruction to the small intestine, so prevention is the best strategy.  This can be achieved by grazing mares and foals on clean pasture, with regular poo-picking, and appropriate worming of the mare.  Worming of the foal may also be appropriate in some situations.

Lungworm

Dictyocaulus arnfeldi is the lungworm that can affect horses, although the donkey, not the horse is not its natural host.  Infected horses will not be able to infect other horses, so this is a disease that almost exclusively affects horse that co-graze with donkeys.  Diagnosis can be challenging, as looking for larvae in either faeces or lung secretions is unlikely to be succesful, as these will rarely be produced in a horse.  We must therefore look for historical clues, i.e. donkey co-grazing.  Coughing is the major clinical sign, which can be accompanied by mild weight loss and increased respiratory rate.  This worm is relatively uncommon due to the necessity of the donkey in its life cycle, and its susceptibility to the commonly used wormers.