How to Effectively Worm your Horse

How to Effectively Worm your Horse

Promoting good horse health


Good health is essential for the wellbeing and growth and development of any animal. Horses are no exception and the prevention of internal (endo) and external (ecto) parasites is paramount for every horse owner or keeper. Parasites can affect any age of horse with the stomach, small intestine, caecum and colon providing the habitat for them to breed. The main internal parasites affecting horses are:


Bots are the larval stage of the bot fly Gasterophilus sp. These larvae are found adhering to the stomach lining. Bots cause a mild inflammation of the stomach.


The removal of the larval stages in the late summer or winter will break the life cycle of this parasite. The Group 3 class of anthelmintic will effectively remove this infection.


This parasite lives in the stomach and is unique in that it can affect both horses and ruminants (sheep and cattle).


The hairworm is susceptible to the broad spectrum anthelmintics.


Heavy infections with this parasite are common in foals and yearlings. The clinical signs associated with this infection include coughing, a nasal discharge, unthriftiness and colic.


Regular worming at 4-6 week intervals.


Infection with this nematode is common in foals aged 2 weeks to 4 months of age but seldom persists in animals over 6 months of age. Foals are infected by larvae present in the milk of the mare.


Most wormers will control the intestinal stages of this parasite.


The adult tapeworm lives in the small intestine and caecum. The damage done to the intestine is not great as they feed of the contents of the food within the gut rather than off the horse itself. However they do attach themselves to the gut wall and may cause ulcers to develop. These ulcers may predispose the horse to the
development of colic, abscesses and other more serious gut conditions.


As it is not possible to control the intermediate host, control of tapeworms is usually carried out after mite activity has reduced in the Autumn (October). Depending upon whether the horse is stabled or left at grass over the winter another treatment could
be carried out in April or July. Pyrantel and Praziquantel are currently the only drugs available to treat tapeworm infection in horses in the U.K.

Large Strongyles

The signs produced are loss of condition and anaemia. The migratory stages of this infection may damage the arteries leading to the intestines, causing blood clots to be swept into the small blood vessels of the gut. This results in damage to the intestinal
wall and colic.


Twice yearly anthelmintic treatments theoretically should control the infection. Two factors need to be taken into consideration when choosing an anthelmintic. One is that the wormer is effective against the larval stages in the arteries and the other is that the parasite is susceptible to the anthelmintic being used i.e. there is no
anthelmintic resistance to the class of anthelmintic.

Small Strongyles/Small Redworms

There are many species of worms which come under the classification of Cyathostomims (formerly Cyathostomes). The characteristic, apart from size, which differentiates them from the large Strongyles, is they migrate into the wall of the intestine
where they form small cysts. At the end of migration and the period of inhibition a mass emergence of larvae occurs and this can have serious implications for the animal. Diarrhoea, mucosal ulcers and bleeding, and colic are common signs.


Prevention/reduction of the accumulation of inhibited larvae is one method which is used and this involves minimising reinfection either by regular use of an effective anthelmintic and by reducing pasture contamination by collecting the faeces of animals using the pasture.


Infection by this nematode is common but it is not a serious condition. The signs of the disease are an irritation of the skin surrounding the anus (perineum).


The local irritation caused by the eggs can be alleviated by wiping away the eggs, however the infection is easily removed by a regular worming routine.


The lifecycle of this parasite is similar to that of the cattle lungworm. D arnfieldi is primarily a parasite of donkeys and horses become infected by sharing a pasture with them. In spite of carrying the infection, both donkeys and horses do not often
show signs of a respiratory infection, however the damage done by the adult parasites in the lungs may contribute to the development of chronic respiratory disease in later life.


The regular use of avermectin or benzimidazole anthelmintics for gut roundworm infections will simultaneously reduce lungworm numbers.

Endoparasite control

Internal parasites are controlled in horses by regular dosing every 6-13 weeks depending on the length of action of the product (anthelmintic/wormer) from spring through to autumn. During the Winter, dosing every 3 months only is necessary. If larval cyaththostomiasis is a problem then treatment against the larval stages with an effective anthelmintic is indicated at the beginning of the winter period.

The classification of the anthelmintics is printed on labels, packaging and data sheets and is as follows:

1-BZ Benzimidazoles and probenzimidazoles

2-LM Imidazothiazoles and tetrahydropyrimidines

3-AV Avermectins and milbemycins

A fourth group of anthelmintics is now available but only for sheep and distributed only through veterinary surgeons.

It is essential to weigh the animal or that the weight is estimated by means of a tape measure prioir to dosing and purchasing animal medicines.

Husbandry factors can also manage parasites. Clean grazing systems can be used so that the host is not available for completion of the life cycle, thus all strains of worms are affected. Routines involving frequent removal of faeces or mixing of grazing animal species or rotational grazing can help reduce or eliminate challenge.

The change of the anthelmintic group used on an annual basis is likely to delay the onset of resistance by preventing large scale development strains resistant to any one group. Simply changing anthelmintic within a chemical group is not satisfactory.

The interval between treatments depends upon several factors which include:

  • The age of the animal. Younger animals may need more frequent treatment.
  • The challenge, stocking density and the frequency of dung removal
  • The length of activity of the anthelmintic
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