Page 67 - The Veterinary Care of the Horse

Life Cycle The adult tapeworms live in the ileo-caecal area (where the small and large

intestine join) of the intestine of the horse. Here they cause inflammation of the gut wall and
VetBooks.ir are thought to release chemicals which decrease the normal movement of the gut. The mature
tapeworms release segments full of eggs which are passed in the droppings onto the pasture.

Here they are eaten by an intermediate host, the oribatid (forage) mite and the eggs hatch and
the larvae begin to develop inside the mite. The horse will ingest these mites while grazing

and the larvae develop into adults over the next 6–10 weeks. This life-cycle takes
approximately 5–6 months.

Clinical signs Tapeworms may cause no symptoms at all. However, large numbers in the
ileo-caecal area can lead to symptoms of ill thrift, weight loss, diarrhoea and spasmodic colic,

especially in young animals. Tapeworm-infected horses are more likely to experience
spasmodic colic than uninfected horses. Very heavy parasite burdens can cause impaction of

the ileum, which may result in intussusception or ileal or caecal rupture.
Treatment Tapeworms are effectively controlled by dosing with a suitable anthelmintic once

or twice a year (spring and autumn) when indicated by positive blood or saliva tests. Where
they cause intestinal obstruction, surgery or euthanasia is indicated.

ROUNDWORMS (ASCARIDS) – Parascaris equorum

Adult size and colour These large white worms can grow up to 50 cm (1 ft 8 in) in length.

Life Cycle The adult worms are commonly found in the small intestine of foals, yearlings and
young horses up to 2 years of age. They produce large numbers of eggs which pass out onto
the pasture. Larvae develop inside the eggs which are sticky and can stick to the udder and

teats of mares. When ingested by a foal, the larvae hatch and burrow through the small
intestine wall. They enter the bloodstream and are carried to the liver and then the lungs

where they develop further. The larvae in the lungs are then coughed up and swallowed. On
reaching the small intestine they develop into adult worms. This life cycle takes

approximately 3 months. The eggs can survive on the pasture for several years.
Clinical signs Poor coat, reduced growth rate, coughing, nasal discharge, diarrhoea, colic,

bowel obstruction and rupture, peritonitis.

Treatment Migrating Parascaris larvae are not detected by a faecal egg count (FEC) test, so
treatment of foals with a suitable anthelmintic is recommended at 2–3 months of age and
again at 5–6 months of age. Resistance has been reported to ivermectin, moxidectin and

pyrantel embonate so the choice of anthelmintic should be discussed with your vet. Once the
foal is 7–8 months old, a routine FEC should be performed to determine the type of worm

eggs being excreted.
Surgery is sometimes necessary to relieve an impaction caused by large numbers of these