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Endoparasite Control In Horses

By David Rendle BVSc MVM CertEM(IntMed) DipECEIM MRCVS

All horses are susceptible to endoparasites (or internal parasites) and will be affected at some time in their lives. Level of infection depends upon the type of parasite, the degree of exposure to the parasite and the immunity of the horse. Immunity tends to increase with age such that older horses generally have lower levels of infection. As horses become very old their immunity may decrease and they may start to carry higher parasite burdens, however they are unlikely to suffer from parasite-related disease. Horses with Cushing's have been shown to carry greater numbers of parasites. Small numbers of parasites are not harmful to horses and need not be treated; indeed excessive treatment can generate resistance and be detrimental to populations of horses.

 

Small Strongyles (or Cyathostomins, or Small Redworms)

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Cyathostome larvae in faeces from a horse with cyathostomin infection

These are the most common endoparasites in horses. When faecal worm egg counts are performed the overwhelming majority of the eggs will be cyathostomin (formerly called cyathostome) eggs and over 95% of a horse's parasite burden will be cyathostomins. There are hundreds of different but closely related species of cyathostomin. Adults live within the large intestine (the colon or the caecum) attached to the wall of the intestine. The larval stages migrate into the wall of the intestine and can become encysted when they go into a period of dormancy that is similar to hibernation. These larval stages are thought to stay dormant until numbers of adult worms in the intestine become low at which time  they emerge  to replace  the  adults. Mass emergence can cause disease and may occur after treatment removes the adult worms. Disease is caused by the extensive damage to the lining of the large intestine by parasites breaking out of their cysts and migrating to the surface of the intestine. The most common clinical signs are diarrhoea, dramatic weight loss and low levels of protein in the blood leading to fluid swelling (oedema) under the abdomen. This typically occurs in young horses in the late winter or spring after ingestion of large numbers of worm eggs the previous summer. Adult worms are easy to kill although resistance to the benzimidazole class of womers is common. Encysted larval stages are difficult to kill and only two types of wormer have a manufacturer's claim that they are effective; moxidectin (Equest) and fenbendazole (Panacur Equine Guard). If the  adult  worms  are  resistant  to  fenbendazole  then larval stages will also be resistant leaving moxidectin as the only type of wormer which is effective. Treatment of larval stages can cause further damage to the intestine as the worms die releasing toxins and also trigger development of other larval stages.

 

Large Strongyles (Large Redworms)

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Strongylus vulgaris and the thickened cranial mesenteric artery from which it was removed

The three most common types of Large Redworms are Strongylus vulgaris, Strongylus edentatus and Strongylus equinus. All 3 types are very sensitive to wormers and have been virtually eliminated from horses in the UK. Strongylus vulgaris is seen occasionally in horses that have not received wormers. When it does occur it can be devastating as sudden death often results. Adults live in the intestine and larval stages of the  parasite  migrate  to  the  major  arteries  in  the abdomen and live in the artery walls potentially disrupting the blood supply to the intestines or causing aneurisms, blood vessel rupture and sudden death. Fortunately these worms are sensitive to all classes of wormers.

Tapeworms (Anoplocephala  perfoliata and Anoplocephala magna)

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Anoplocephala perfoliata segments

Most tapeworms found in horses in the UK are Anoplocephala perfoliata species, however Anoplocephala magna is thought to be becoming more common. The adults live in the caecum or ileum and tend to congregate around the ileocaecal junction; the transition between the small and large intestines. A number of types of colic including colic that requires surgery have been associated with tapeworm infection. Tapeworm eggs are difficult to identify in faeces and diagnosis is therefore dependent upon blood tests identifying antibodies to tapeworms. A limitation of the blood test is its inability to distinguish between horses that have infection and horses that have been infected within the previous 3 months. Juvenile stages of the tapeworm live within forage mites (orbatid mites) in the stable. The mites are inadvertantly eaten by the horse allowing  the  tapeworm  eggs  to  develop  into  adults within the intestine. Fortunately tapeworms can be killed easily if appropriate classes of wormer are used; both pyrantel (at twice the dose required to kill roundworms) and praziquantel are effective. There have been some concerns over resistance to pyrantel.

 

Bots (Gasterophilus species)

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Bots in the stomach of a horse

Bots lay eggs on horses' skin typically over the lower limbs. When horses lick themselves the eggs are ingested and the larvae develop in the stomach where they remain until they detach and pass through the intestine. Once passed in faeces they pupate on the ground and hatch into flies. It is currently considered that bots do not cause any harm to the horse in which they live. Bots are extremely sensitive to wormers and the presence of bots is therefore an indication that wormers have not been administered recently and other parasites may be present.

 

Ascarids (Parascaris equorum)

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Ascarid impaction in the small intestine of a foal

Ascarids are only a problem in foals. Eggs tend to pass from one generation of foals to the next but can survive on pasture for many years. The eggs are ingested and the larvae burrow through the gut wall, migrate through the liver and the lung to the trachea and are coughed- up and swallowed. Once back in the intestine the adults can become very large (10-15cm), and can be numerous, potentially resulting in complete obstruction of the intestine. Obstruction may occur after worming when large numbers of dead worms produce an impaction. This may result in acute colic and surgery may be required to remove the impacted worms. Other less dramatic clinical signs include poor growth, diarrhoea and coughing.

 

Strongyloides westeri

These worms also cause disease in foals exclusively. Foals ingest the eggs in mares milk after adult worms in the mare have migrated to the mammary gland. Once eggs are ingested by the foal, adults develop very quickly in the small intestine and infection tends to peak at 4 weeks of age. Heavy infection may result in diarrhoea. Immunity develops rapidly and by 4 months of age foals naturally rid themselves of infection and remain immune to disease caused by this parasite.

 

Pinworms (Oxyuris equi)

Pinworms are very uncommon in the UK and are not associated with any signs of intestinal disease. The adults live in the small and large colons and migrate to the anus where they lay their eggs on the peri-anal skin. The eggs are subsequently ingested and moult to become  adults  within  the  large  intestine. The only clinical signs are related to irritation around the anus which may cause horses to rub and demonstrate signs of a "rat-tail" similar to what is observed in horses with sweet itch. These worms are easily killed by all classes of wormer and as a result are very rarely seen. Diagnosis is not based on faecal egg counts but on identification of eggs on the skin around the anus. This may be facilitated by collecting eggs on adhesive tape and looking at the tape under a microscope.

 

Lungworm (Dictyocaulus arnfieldii)

Lungworm is a very uncommon cause of coughing in the  horse  and  even  if  horses  become  infected  they rarely show clinical signs. Donkeys, asses and mules are the natural hosts for the parasite and can develop respiratory disease. Horses are unlikely to pick up the parasite unless they are kept with donkeys, asses or mules and the worms are rarely able to reach adulthood and produce eggs in horses. Animals that do develop disease typically develop a chronic cough. Diagnosis is usually by examination of the respiratory tract with an endoscope and collection of washes from the lower airway. Very large numbers of eosinophils (a type of white blood cell) are characteristic of lungworm infection and occasionally the washes contain worm eggs, larvae or even adult worms. Eggs are coughed-up and swallowed and pass out in the faeces. The eggs may be detected by Baermann flotation of faeces but this is unreliable especially in horses in which eggs are rarely produced. Fortunately these worms are very sensitive to most types of wormers.

 

Hydatid cysts (Echinococcus granulosus)

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Hydatid cyst (cut in half) in the lung of a horse

Horses can be intermediate hosts for the canine tapeworm  Echinococcus granulosus.  Infection  is  rare but can occur following ingestion of dog faeces.  The eggs hatch in the intestine and larvae migrate through the tissues, usually to the lung or liver, where they form large cysts that contain numerous developing parasites. Unless they occur in very large numbers hydatid cysts are not thought to result in disease in horses. Treatment can be attempted with albendazole but this drug is not licensed for horses. The cysts can be drained but this is often unsuccessful and can have severe consequences if the cysts rupture. For the parasites lifecycle to be completed dogs have to eat the infected meat or organs and ingest the contents of the cysts.

 

Liver fluke (Fasciola hepatica)

Little is known about liver fluke in horses but very rare cases have been reported. Diagnosis is generally presumptive based on changes in biopsies. Triclabendazole can be used in treatment but is not licensed for use in horses.

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