Worms and your horse

LARGE ROUNDWORM (ascarids, Parascaris equorum)

These are the biggest worms that affect horses and adult worms are easily visible in the faeces.  They look like large pieces of spaghetti!  Young horses are particularly susceptible to infection.  Horses become infected by ingesting eggs from the pasture; these eggs then develop into larvae (young worms) which migrate through the liver and lungs before maturing to egg laying adults in the small intestine.  Their eggs are able to survive in the environment for several years due to a protective outer coating.  Control programmes must therefore include good management of stables and pastures.  The eggs are also very sticky and can often only be removed from stable surfaces by pressure cleaning.  Generally infection is maintained by eggs from previous year’s foals infecting new foals the following year.
General signs that your horse/foal may have an ascarid infection are; coughing often with a greyish nasal discharge, colic, illthriftiness and poor growth rates.  Adults in the intestine can also cause impaction, leading to fatal rupture of the intestines or serious colic requiring surgery.  Ascarid eggs can easily be detected in a faecal egg count

LARGE REDWORMS (large strongyles, Strongylus spp.)

Good worming programs have decreased the infection levels of these worms, however, they remain an important cause of colic in poorly wormed horses.  The larvae of one species burrow into the gut wall and enter arteries eventually migrating to one of the main arteries supplying blood to the intestines where they can stay for several months before returning to the gut wall.  Another type of this worm migrates to the liver instead of the arteries for several months.  The adults attach to the lining of the intestine and can cause damage where they attach resulting in blood loss and anaemia in more serious cases.   General signs of large redworm infection include illthrift, poor appetite, colic and lameness in hindlegs (the worms in the arteries can also affect the blood supply to the hindlegs).

Large Redworms on droppings.  Small redworms are much finer and almost impossible to spot in faeces.

SMALL REDWORMS (small strongyles, Cyathostomum spp.)

These are the most common worms that affect adult horses and are the main target of worm control programs.  They are quite small and thin and can sometimes be visible in the faeces as very fine red worms.  The eggs are not very hardy and are usually eliminated by extreme weather conditions or cleaning.  The eggs develop into larvae on pasture and infection occurs when horses eat the larvae off the pasture.  The larvae go straight to the large intestine where they burrow into the gut wall and continue to develop or can undergo hypobiosis (stop developing).  If they undergo hypobiosis their presence cannot be detected by a faecal egg count.
Severe damage is caused to the gut wall by mass emergence of the worms from hypobiosis. This usually happens in late winter/early spring and is potentially fatal.  Younger horses are most commonly affected (2-3yo) but horses do not acquire immunity so infection is possible throughout life.  Initial signs of infection include anaemia, weight loss and diarrhoea.  In severe cases the diarrhoea is profuse; they lose weight rapidly and can become colicky with swollen legs and a temperature.

THREADWORMS (Strongyloides westeri)

Horses acquire natural immunity by 6mths of age so this worm only really affects young or compromised animals.  It can particularly affect newborn foals as it can migrate into the mare’s udder and pass to the foal in the milk.  Infection in foals can cause diarrhoea, weight loss and anaemia

PINWORMS (Oxyuris equi)

Adult female worms lay their eggs on the skin surrounding the anus which causes the horse intense irritation.  The eggs attach themselves to any surface the horse rubs it hindquarters on where they can then infect others or re-infect the same horse.  This means that control of pinworm infection requires good environmental management and to thoroughly clean all surfaces your horse may have come into contact with.  Cleaning the skin and the underside of the tail with disposable clothes is another measure that can help

WORM CONTROL PROGRAMS

GUIDE TO WORMING FOALS