Nematodes

Large Redworms (Strongylus vulgaris)

Adult Large redworms worms vary in size, ranging from 1.5cm-5cm. Large redworm was once the most dangerous parasite affecting the horse, due to the migrating larval stage of the redworm causing damage to the lining of the arteries, particularly those supplying the gut. However, older horses in the UK have generally developed immunity to large redworms due to the efficient use of equine wormers in the country. These worms migrate through the body and damage some vital organs on the way, commonly being more of a threat to younger horses.

Large Roundworms Ascarids (Parascaris equorum)

Large roundworm (also known as ascarids) are worms white in colour and up to 40cm long when mature, usually being much thicker than other equine worms. They produce large numbers of tough coated adhesive eggs which can survive for several years through harsh conditions and can stick tomany surrounding environments, including horse coats and udders, and even stable walls and floors. These eggs then develop into larvae (young worms) which migrate through the liver and lungs and are eventually coughed up to then be ingested, subsequently maturing into egg laying adults in the small intestine. This complex lifestyle creates potential for disease and can also 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, 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 sluggishness. Such a burden has the potential for fatal colic.

Hairworms (Trichostronglyus axei)

Adult hairworms are around 7cm in length which makes them hard to see with the naked eye, and are usually controlled by products containing Moxidectin or Ivermectin. They are unusual for horses as they are more commonly known as a parasite for 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 1cm-2.5cm in length and are long, slender and white in colour. Similarly to adult hairworms, these worms are usually controlled by the worming programmes containing Moxidectin or Ivermectin and are also primarily a parasite for sheep, cattle and pigs. Again like their adult counterparts, stomach hairworms develop into adults in the stomach where they feed on blood. These worms are transmitted by flies landing on infested dung, so fly control measures will help to reduce infections. They are also capable of remaining on and damaging the skin, 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 themselves 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. The eggs are then ingested by the horse and L3 larvae are released in the small intestine. Next, they migrate to the large intestine to develop into the mucosa to L4 larvae which then eventually 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 is always a good idea.In addition, giving the stable a deep clean after removing all bedding is important 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, before being swallowed and passed out in the faeces. Further development then takes place on the pasture, where 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 the threat level is not as high as other parasites, due to lungworms not maturing into 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, however, more recent research is now disputing this with levels being quoted as low as 4%. Donkeys often show no clinical signs of infestation, and it is within them that the parasite reaches full maturity. Donkeys do not always develop the symptomatic cough that is seen in horses, but if horses are grazed alongside them, 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 that are 4-6cm long that live in the small intestine feeding on intestinal blood. Often, larvae will penetrate the mammary 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. When fully developed, 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 around 30cm. 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 be ingested up by a visiting midge. Once inside the midge they develop into the infective larvae L3 stage within 24-25 days. When the midge bites another horse the neck threadworms migrate to the ligaments in the neck and also to the flexor tendons and suspensory ligaments, particularly the forelegs.

Symptoms include: Sores along the topline and stomach, irritation and swelling around the eyes, and Uveitis. Uveitis occurs when there are large quantities of dead microfilariae in the eye that give off large amounts of antigens, causing inflammation in the eye. This can result in a constant water stream out of the eye or eyes, often along with a white or yellow mucous in the eye which can lead to blindness if not treated efficiently. Other symptoms of neck threadworm infections are hair loss around the head and neck area, swelling around ligaments and tendons, lameness, and lumps under the horses skin on the ligaments.