|The redworm is the common name given to an important group of horse worms, also referred to as small strongyles or cyathostomes. Redworms are considerably smaller than, but closely related to, the bloodworm but they do not have the same extensive damaging effects. Redworms affect horses of all ages.
Redworms are small red-coloured worms, which inhabit the large intestine of horses. They are not easily visible to the naked eye. Heavy burdens of redworms can cause severe ill-health, diarrhea and colic: such burdens are not uncommon on properties where stringent worm control programs are not carried out. Redworms generally occur in much larger numbers than bloodworms, and their overall effects although different, may be as severe.
The life cycle of the redworm is shown above.
Adult female worms living in the horse's large intestine pass eggs into the horse's droppings. Having been deposited on the pasture, the eggs hatch and develop into infective larvae in much the same way as the bloodworm.  The mare is the major source of pasture infection for the foal.
Horses of all ages become infected by picking up infective larvae with herbage. Once in the large intestine, the larvae emerge from their protective sheath and burrow into the lining of the intestine. Unlike the bloodworm larvae, however, they go no further. Large numbers of redworm larvae form nodules under the lining of the intestine where they remain dormant or go through a period of development. After a period of 6 weeks to many months (or years), they re-emerge as young adult worms into the intestine and attach themselves to the gut wall. Redworms do not feed on the horse's blood after they have emerged into the intestine, but do so when they are developing in their nodules and may cause bleeding into the gut when they first emerge.
Some of the worms in nodules in the lining of the intestine may lie dormant for considerable periods of time. It is not fully understood how they become reactivated, but it is known that dormant redworms may emerge rapidly if a large number of adult worms are removed from the intestine of a horse following worming. This is an important factor, which needs to be taken into consideration in worming programs.
The time from initial infection to the appearance of eggs in the horse's droppings is variable but eggs have been seen in the droppings of foals as young as six weeks. Although the life cycle is generally longer than this, dormant worms in the horse' s intestines may resume development and repopulate the intestine with adult worms within a few weeks of worming.
The removal of adult worms from the intestine is relatively simple and most common wormers are effective. .
No wormer is highly effective against the immature forms of the larvae, which are protected by virtue of their inactivity in their nodules of the lining of the intestine. As mentioned above, it is highly likely that one treatment of a horse will be followed by the activation and emergence of dormant larvae within days to reestablish new infection. It may be necessary to treat a horse several times at six weekly intervals before an infliction is finally cleared. This is why it is necessary to regularly treat even those horses, which are always boxed or kept in dry yards away from sources of contamination.
The Principles which apply to the property control of the bloodworm also apply to the redworm. Although some types of redworm may begin laying eggs in less than six weeks after emergence into the intestine, a six weekly worming interval is for practical purposes adequate.
A four weekly treatment of mares and foals and a six weekly treatment of all other horses with a oral wormer, coupled with a sensible pasture management program will greatly reduce the level of infection by this worm on a property, at the same time as controlling roundworm and bloodworm.