There are a number of internal parasites that can infect a horse, the most important worms including the following:
Large strongyles
Also known as bloodworms, they are about 10 – 20mm in length and red or grey in colour. They have a long migratory lifecycle of 6 months and are active blood feeders, typically causing anaemia in horses. They are highly pathogenic when they accumulate in the cranial mesenteric arteries. Here they can cause an arterial blockage (thrombosis), damaging the intestine and resulting in possible colic and death.
Small strongyles
Small strongyles or cyathostomins are also commonly known as small redworms. They are generally 5 – 15mm in length and they have a short lifecycle of 6 - 8 weeks, making them highly prevalent and common in horses in Australia. They feed on gut lining and material in the large intestine and can cause mild ulceration, weight loss and diarrhoea. Part of their success is that their larvae (L3) can encyst in the intestinal wall for several months, protecting themselves against many worming treatments. Small strongyle larvae cause severe damage when large numbers emerge from the intestine (called larval cyathostominosis), which can result in signs of diarrhoea, poor condition, colic and possibly death in horses. Only Moxidectin is registered and proven to be effective against encysted small strongyles in one single treatment dose.
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Fig.1 Encysted small strongyles in the intestinal mucosa
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Fig. 2 Adult small strongyles
Ascarids
Ascarids are also known as large roundworms and are of significance in younger horses less than 2 years of age. They are generally white in colour and can be up to 30cms in length. They have a migratory lifecycle of 10 – 16 weeks, travelling through the bloodstream, liver and lungs before residing in the gut as adults. Common clinical signs include respiratory signs, lung damage and more commonly, poor growth and sometimes colic and death. Ascarids have very resilient eggs so it is possible for foals to be infected from one season to the next, as the eggs remain viable in the environment for years.
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Fig.3 Adult ascarids dissected from a 3yo mare
Pinworms
Pinworms can vary from 10mm long (males) up to 100mm long (females) and are white to grey in colour. They are unique in that females migrate to the rectum to lay their eggs outside of the anus of the horse in a gelatinous substance. This can cause itchiness and restlessness, and result in hair loss and possible wounds at the base of the tail as the horse tries to seek some relief by itching themselves. Pinworms predominantly affect younger, stabled horses, mainly as horses can rub their tails (and consequently eggs) into feed bins where they will later ingest them.
Tapeworms
There are three species of tapeworm; however Anoplocephala perfoliata is the most harmful. They are typically 8 – 25cms in length, but can be longer and are found in the small intestine and stomach. Their lifecycle requires the orbatid mite as an intermediate host, which horses ingest whilst grazing on grass, hay or grain. A tapeworm infection can cause signs of weight loss, anaemia, ulceration and colic in high numbers. Praziquantel is the most effective anthelmintic active that targets all three species of tapeworm.
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Fig.4 Tapeworms of about 2cms in length
Stomach bots
The bot fly is not only an annoyance to horses in its environment, but they can also cause harm to horses internally. Typically in the warmer months of August to May, adult female bot flies lay their eggs on the horse’s coat and the eggs are then ingested by the horse. Moisture activates the eggs to moult into larva in the mouth of the horse, causing damage to the tongue and gum tissue, before migrating to the stomach where they develop over a 8 to 12 months. Here they can cause ulceration and lesions and in large numbers possibly colic. In late spring when temperatures start rising, larvae detach and pass out in the faeces to pupate in the soil for a number of weeks before they develop into adults.
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Fig.5 Stomach bot larvae
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