Parasitic Worms

Parasitic Worms

Bots

Arthropods/Gasterophilus: Bots are not a worm but an egg or larvae of a bot fly. The fly lays their eggs on the legs of the horse (summer and early fall), and is ingested during grooming or itching. The eggs develop in the stomach and are passed through dung. While bots are not considered dangerous, they do cause inflammation and ulcerated stomachs. Worming with Ivermectin or Moxidectin products in December or January ensures the bots are destroyed in cold winter temperatures. It is important to remove the bot eggs from your horse. They are tiny yellow eggs around the top end of the horses legs, usually front legs. Some bot eggs can be found around the face and neck. The bot fly is about the size of a honey bee, and moves about very quickly.

Large Redworms

Strongylus vulgaris: These are the most concerning worms for the horse owner. Large redworm larvae migrate from the bowel into the abdomen before returning to the bowel as adults. These worms specifically attack the arteries that supply the bowel and can obstruct the blood flow, causing colic. Large redworm infection is more of a problem in younger horses. Adult large redworms vary in size: 1.5 to 5 cm.

Large Roundworms/Ascarids

Parascaris equorum: these worms tend to affect younger horses. The larvae migrate through the lung on their way to the intestine, causing the horse to dry cough. They are very long - up to 40 cm. the eggs are tough and sticky in order to adhere to their environment (horse, stall and paddock). Their thick shells enable them to survive colder winters.

Hairworms

Trichostrongylus axel: hairworms live in cows, sheep and pigs as well as horses. They live in the stomach and feed on blood. They migrate through the bloodstream to parts of the body tissues and mature in the intestine. They can move through the mammary tissue and be ingested through the mare's milk to her foal. They are very tiny, 7cm, and difficult to see with the human eye.

Stomach Hairworms

Habronema muscae: these parasites develop into adults in the stomach and feed on blood. They are transmitted by flies, so keeping pastures and paddocks free from manure will help control them. They are also found in cows, sheep and pigs. They are 1 to 2.5 cm long and white.

Pinworms

Oxyuris equi: this is the nasty little worm that causes the itching around the anus. They are not considered harmful and have a short lifespan. They inhabit the large and small colon, laying their sticky eggs around the opening of the anus, and being pushed out with each bowel movement, only to enter the horse again during feeding. They can be as long as 10cm and are white in color.

Large Mouthed Stomach Worms

These parasites have an indirect life cycle and require an intermediate host, often fulfilled by house and stable flies. The worm larvae are ingested by fly maggots, which turn into adult flies around 3 weeks, who then deposit infectious larvae in skin lesions on the horse. They are white and about 2.5 cm. Horses suffer gastritis and tumor-like growths in the stomach that can rupture and cause internal bleeding. External wounds from these parasites are referred to as 'summer sores', becoming chronic, non-healing wounds. Larvae deposited around the eyes can cause conjunctivitis. Symptoms include diarrhea and sudden weight loss, and summer sores.

Lungworms

Dictyocaulus arnfieldi: Donkeys are the host for this parasite, so be warned if your horse shares his spaces with a donkey. While horses can pick up the worm, the worm's lifecycle is often incomplete, and not detectable in fecal counts. Horses with a large number of lungworms will have a dry cough from the worm in their lungs. Adult lungworms are white and from 5-10cm. They are known to live out full life cycles in very young or very old horses, causing permanent lung damage.

Small Redworm

Cyathostomins/small strongyle: this is the most common horse parasite. Their numbers are declining partly due to the short life cycle, but they have the ability to develop resistance to wormers. These worms hibernate in the lining of the bowel and gut and many will remain within the lining for years. They reduce the efficacy of the gut wall to absorb nutrients, leading to unthrifty horses unable to utilize their diet. As the larvae re-emerge in vast numbers near the end of winter, they can cause severe damage to the bowel and gut walls. In some cases it is fatal. Spring is the time when they reactivate in the large intestine, leading to springtime diarrhoea (rare) a potential life threatening condition. The small redworm can also inhabit the lining without any form of inflammation.

They are around 2.5cm, come in red and white and are thin. Horses pick them up grazing. Symptoms are dry cough, colic, weight loss, diarrhea. Horses at high risk are those grazing contaminated pasture, not had regular worming or are under the age of 6. This worm is one of the strong reasons to have regular fecal tests. Pancur Equine Guard and Equest are the only anthelmintics to claim efficient elimination.

Tapeworm (flatworms)

Cestodes/Anoplocephala perfoliata/anoplocephala manga/anoplocephaloides mamilana: Tapeworms are a potentially damaging parasite and a major cause for colic. Blood tests are better at finding them than manure tests. They attach to the small and large intestine in the ileocaecal junction, where they cause bowel irritation, intussusception, rupture, thichening at the site of attachment, and twisting of the intestine. They are passed through the horse and become food for oribatid mites. Acidic soils are the favorite breeding ground and tapeworms are found in various quantities in most horses all year round. Tapeworms can also live in hays and bedding. They range in size (8cm - 20cm in horses), flat (1.5cm wide), white, and triangular in shape and live in massive numbers. Tapeworms cause severe colic (sometimes fatal).

Threadworms

Strongyloides westeri: horses can be infected by ingesting larvae (eating grass) or through the skin, where the larvae migrate to the lungs and wind pipe, where they are coughed up and swallowed. Larvae grow into adults in the small intestine. Threadworm larvae in the lungs can cause bleeding and respiratory problems. Untreated foals can suffer diarrhea, weakness, weight loss and poor growth.