Equine Worms



Large Redworms (Strongylus vulgaris)

Adult Large redworms worms vary in size between 1.5 and 5 cm. Large redworm was once the most important parasite affecting the horse because the migrating larval stage damage the lining of the arteries, particularly those supplying the gut. However in the UK older horses have usually developed immunity due to the efficient use of equine wormers in this country. Large redworm infection is more generally a problem in younger horses. These worms migrate through and damage some of the the body's vital organs.

Large Roundworms Ascarids (Parascaris equorum)
Large roundworm also known as ascarids are very long worms up to 40cms when mature and produce large numbers of tough coated adhesive eggs which can survive for several years and can stick any surrounding environment. The mature worms are white in colour and much thicker than other equine worms. The eggs can stick to the coat and udders of the mare and even to the walls and stable floors. The eggs have very thick shells and therefore can survive on pastures over the winter months and perhaps for many years .These eggs then develop into larvae (young worms) which migrate through the liver and lungs and eventually coughed up then to be ingested and susequently maturing to egg laying adults in small intestine. This complex lifestyle creates great potential for disease can retard growth and development. Respiratory obstruction is common as a result of the presence of larval stages in the lungs.  Intestinal blockage and impaction colic is also common in foals due to the sheer physical size of the adult worms. Their presence in the gut can block the passage of food material as well as leading to nutritional deficiencies. A heavy burden of mature worms in the intestine may well give the classic signs of ill thrift, a pot bellied appearance and or sluggishness. Such a burden has the potential for fatal colic.

Hairworms (Trichostrongylus axei)
Adult hairworms are only about 7cms in length and so are very hard to see with the naked eye.  These worms are usually controlled by products containing Moxidectin or Ivermectin. They are unusual as they are also a parasite of sheep cattle and pigs. As the name suggests they live in the stomach where they feed on blood. The larvae then migrate via the bloodstream to various body tissues and mature on the intestine.

Stomach Hairworms (Habronema muscae)
Stomach hairworms are  1-2.5cms in length and are long slender and white in colour. These worms are usually controlled by the worming programmes containing Moxidectin or Ivermectin but are unusual as they are also a parasite of sheep cattle and pigs. They develop into adults in the stomach where they feed on blood.  These worms are transmitted by flies landing on and ingested dung so fly control measures will help to reduce infections. They are also capable of remaining on and damaging the skin and causing "summer soreness".

Pinworms (Oxyuris equi)
Pinworm burden is becoming an increasing problem for UK horse owners. Pinworm (Oxyuris equi) is not generally considered harmful but more of a nuisance and an irritant. The female can grow up to approximately 10cm in length and are white in colour. They reside in the large intestine and attach them selves to the intestinal wall to ingest the contents for food.
Adults produce eggs approximately 5 months after the initial infection which are found on the pasture, in faeces, contaminated water fences and walls.
Eggs are ingested by the horse and L3 larvae are released in the small intestine, they then migrate to the large intestine to develop into the mucosa to L4 larvae which then emerge and mature into adults. The female adults then migrate from the large intestine to the anus where they lay eggs in clumps on the perineal skin causing irritation around the anus leading to tail rubbing.

Active ingredients that treat for adult and pinworm larvae are Moxidectin,  Ivermectin, Febendazole and Mebendazole with Pyrantel treating adults only. Not all brands are licensed so care needs to be taken by checking the brand’s label.

Extra care should be taken in the stable environment to help reduce the risk of re-contamination from buckets, feed bowls, haynets and rugs etc. Do not share grooming brushes.  A thorough clean with a heavy duty disinfectant in these areas and most importantly the stable after removing all bedding will help and is always a good idea in areas of animal care and management.

Lungworms (Dictyocaulus arnfieldi)
Lungworms are white in colour and are between 6 and 10 cm in length. The lifecycle of lungworms is different to other nematodes as adults settle in the lungs rather than the intestine. Eggs are laid then travel up the trachea, are swallowed and passed out in the faeces. Further development then takes place on the pasture, infective larvae are swallowed by horses or donkeys to further develop into egg laying adults.
It is extremely rare for lungworm larvae to develop to full maturity in horses as the horse is not a good host for that particular parasite.  Horses can be infected with lungworm but as they do not mature in to egg laying larvae in any numbers to sustain a population on the pastures.  Exceptions to this in horses can arise in very young or old horses and seriously depilated horses whose immune systems are impaired.
It has been remarked that a large percentage of donkeys carry lungworms (more recent research is now disputing this with levels been quoted as low as 4%)  often showing no clinical signs of infestation and it is in donkeys where the parasite reaches full maturity.  Donkeys do not always develop the symptomatic cough that is seen in horses but if horses are grazed along side donkeys then particular care must be taken to treat for lungworms accordingly with particular attention to foals as they can sustain permanent lung damage if infected

Intestinal Threadworms (Strongyloides westeri)
Intestinal Threadworms are tiny hair-like parasites being 4-6cm long. They live in the small intestine feeding on intestinal blood. Often larvae will penetrate the mammory tissue and can be transmitted to young pre-immune foals in milk. The life cycle is very short (8-14 days) and foals as young as 4 weeks of age can develop heavy infestations which can damage the intestinal lining causing diarrhea, loss of appetite, anaemia and dullness. Foals usually develop a natural immunity to infection with this worm by around 6 months of age.

Neck Threadworm (Onchocerca species)
The larvae (which are called microfilariae) live in the tissue under the skin and are ingested by midges as they feed, they can also congregate in the eye tissue causing infections. The adult worms live in the tendons and ligaments.They are long and coiled in shape with the males being around 6cm in length and females being around 30cm in length.Neck Threadworms have to depend on an intermediate host, the biting midge - Ceiatopogonidae, to get it to the horse.The neck threadworm microfilariae live just under the horse's skin and wait to ingested up by a visiting midge. Once inside the midge they develop to the infective larvae L3 stage within 24-25 days. When the midge bites another horse the neck threadworms the larvae migrate to the ligaments in the neck and also to the flexor tendons and suspensory ligaments particulary the forelegs.

Symptoms include: Sores along the topline, along the stomach. Sores, irritation and swelling around the eyes. Uveitis - This occurs when there are large quantities of dead microfilariae in the eye which causes the dead to give off large amounts of antigens which cause inflammation in the eye.
A constant water stream out of the eye or eyes often along with a white or yellow mucous in the eye on a regular basis. Hair loss around the head and neck area. Swelling around Ligaments. Swelling around tendons. Lameness. Lumps under the horses skin on the ligaments.
Blindness can occur if the infection around the eye is severe or if treatment is delayed.