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Worming

Parasitic intestinal worms are common. All grazing horses are likely to have worms, but most horses have a low number which have no untoward effects and may do no harm. Problems arise if worm burdens increase, which is when the worms can cause disease such as weight loss, colic and diarrhoea. In some cases, heavy worm burdens can be fatal. So it is important to control the numbers of worms in our horses.

The aim is to maintain a low number of worms in the group of horses rather than no worms at all, because a low worm burden stimulates healthy immunity against further infestations.

In adult horses, the strongyles (small and large redworms) and tapeworms are the most important ones that we need to  control.

  • Small redworms can be difficult to eliminate because that can become encysted (i.e. buried and hibernating in the lining of the intestine) which makes them difficult to kill.  This is more common in young horses, particularly in spring. These encysted larvae sometimes all “wake up” and emerge simultaneously from inside the gut lining, which will result in massive damage to the intestine wall and serious life-threatening diarrhoea.
  • Tapeworm can cause colic by blocking the intestine.
  • Pinworms and large redworms cause disease less commonly, but still can require specific treatment.

A good worm control strategy aims:

1.    To minimise the contamination of pasture by worm eggs (redworm eggs are passed in the droppings and contaminate the pasture; larvae develop on the grass and are then eaten and infect horses that are grazing).

2.    To keep the worm burden in an individual horse low enough to prevent disease.
 

There are two main components to a worm control programme:

1.    Pasture management
2.    Drug treatments (“wormers” or “anthelmintics”)

For further information, please see the horse owners booklet on Parasite Control in the 'Health Horses' series by MSD Animal Health.

Helpful hints for pasture management:

  • Horse numbers per acre should be kept low to prevent overgrazing and to reduce contamination by worm eggs. One horse per acre is an appropriate rule of thumb.
  • Pick up and dispose of dung regularly (at least once a week). Do not spread this onto fields grazed by horses as it simply spreads the worms around the fields. Instead please compost it away from grazing.
  • If possible, rotate pastures to give recently grazed pasture time to ‘rest’. A three month rest is sensible, but it is better to rest the pasture longer, if possible.
  • There is no need to treat all horses in a herd prior to movement to a clean pasture so long as regular faecal worm egg counts (FWEC) are being performed.
  • Consider grazing horses with sheep or cattle. Most horse worms will not infect sheep and cattle (and vice versa) and sheep and cattle are very good at “hoovering” horse pastures. 

Heavy use of wormers has caused some equine worms to become resistant to many wormers. To ensure that wormers remain effective, they should be used responsibly and targeted to the horses that need them and will benefit most from them (known as targeted treatment programs).

DO NOT randomly treat horses at frequent fixed intervals year-round. Some wormers can have side effects so they should not be overused. Recent research has shown that targeted treatment usually saves money (an average of nearly £300 per yard per year) when compared with routine treatment of all horses throughout the year.

The ideal strategy varies from yard to yard and should be discussed with BELL EQUINE, please call us on 01622 813700. We may recommend a different approach for different yards with different worming histories. The most important thing is not to over use medication. Only treat when you really need to do so.

The following information is a general guide to worming for horses over 18 months old excluding broodmares.

  • Simplified examples of possible treatment programs can be seen HERE.
  • Information on broodmares and youngstock can be found further down this page.
  • Excellent advice for donkeys is provided by the donkey sanctuary HERE.
  • A list of commonly used wormers and their classes can be found HERE.

Quarentine Policy

A quarantine policy can help prevent worms from spreading between horses. New horses arriving on a yard should be quarantined (suggest 2-3 weeks) before being turned out to pasture with other resident horses.

Horses with an unknown worming history should be treated for encysted redworm and tapeworm using a worming treatment for both types of parasites, e.g. moxidectin and praziquantel such as Equest Pramox®. A faecal worm egg count should then be performed 2 weeks later, if the result is 300 eggs per gram or higher, veterinary advice should be sought.

Horses that have been treated for tapeworm and encysted redworm within the previous year should have a faecal egg count. If the result is 300 eggs per gram or higher, then the horse should be wormed and a faecal egg count reduction test should be performed in order to prevent the introduction of drug-resistant parasites.

Parasitic intestinal worms are common. All grazing horses are likely to have worms, but most horses have a low number, whichhave no untoward effects and may do no harm. Problems arise if worm burdens increase, which is when the worms can cause disease such as weight loss, colic and diarrhorea. In some cases, heavy worm burdens can be fatal.  So it is important to control the numbers of worms in our horses. 

Young horses are more susceptible to picking up worms as well as more vulnerable to diseases caused by worms, so control programs for youngstock need to be more rigorous. The commonest and most important worms that we need to control in youngstock are the ascarids and also the large and small redworms. Strongyloides (thread worms) occasionally cause diarrhoea in young foals, but this is rare. Small redworm eggs eaten by grazing youngstock can cause severe disease the following spring after ‘hibernating’ in the wall of the intestines over the winter, then 'erupting' all at once, which is why encysted redworms can cause problems.


The aims of an effective worm control strategy for horses of any age or type are:
 

1.    To minimise the contamination of pasture by worm eggs (redworm eggs are passed in the droppings and contaminate the pasture; larvae develop on the grass and are then eaten and infect horses that are grazing).

2.    To keep the worm burden in an individual horse low enough to prevent disease.

There are two main components to an equine parasitic worm control programme:

1.    Pasture management
2.    Drug treatments (“wormers” or “anthelmintics”)

1. PASTURE MANAGEMENT

  • Foals should have the 'cleanest' (i.e. least heavily grazed) pasture – ideally pasture that has not been grazed by horses in the last 12 months
  • Horse numbers per acre should be kept low to prevent overgrazing and to reduce contamination by worm eggs. One adult horse per acre is an appropriate rule of thumb.
  • Pick up and dispose of dung more regularly than for adult horses (ideally every other day for youngstock). Do not spread this onto fields grazed by horses, as it simply spreads the worms around the fields. Instead, compost it away from grazed pasture.
  • If possible, rotate pastures to give recently grazed pasture time to ‘rest’ and ideally avoid grazing pasture grazed by foals the previous year
 

2. DRUG TREATMENTS (“Worming”)


As with adult horses, overuse of wormers in foals and broodmares has caused some worms to become resistant to some equine wormers, for example, many ascarid worms are resistant to ivermectin wormers. In mature horses, targeted treatment is used to decide which horses need worming; youngstock are more vulnerable and should be wormed regularly BUT the type of wormer should be chosen carefully to minimise resistance.

The following information is a general guide to worming youngstock (less than 18 months old) and broodmares, but in certain cases we may advise a slightly different approach, so please discuss your specific requirements with one of our vets.


Broodmares:

  • Until the 10th month of pregnancy the mare can follow the same worm control program as other adult horses.
  • A faecal egg count should be performed within the last month of pregnancy and the mare should be treated, only if required. The faecal egg count should be repeated around 3-4 weeks after foaling.

Youngstock: 

PLEASE ENSURE ALL FOALS RECEIVE WORMERS APPROPRIATE FOR THEIR AGE - ALWAYS CHECK THE INSTRUCTIONS BEFORE ADMINISTERING OR CONTACT BELL EQUINE ON 01622 813700 

  • Foals should be wormed regularly until they are 18 months old, regardless of any worm egg counts.
  • The first treatment should be given at 2½ -3 months of age using a benzimidazole (e.g. Panacur®) to treat ascarids.
  • The foal should be wormed according to the type of worms seen e.g. fenbendazole to treat ascarids; pyrantel, ivermectin or moxidectin to treat cyathostomes (moxidectin should not be used on foals less than 4 months of age).
  • After weaning, youngstock should be treated at 3-4 month intervals until they are 18 months old. A treatment for encysted redworm (e.g. moxidectin) should be included each autumn and a product effective against tapeworm (moxidectin and praziquantal - Equest pramox®) should be included at least once yearly, provided the foal is older than 6 ½ months
  • From 18 months of age youngstock can follow a mature horse worming program assuming appropriate pasture management is followed. For yards with a history of worming problems, please speak to one of our vets. 

Every yard is different and specific guidance can be provided by our vets by calling BELL EQUINE on 01622 813700.


Summary – Parasite Control in foals:

PLEASE ENSURE ALL FOALS RECEIVE WORMERS APPROPRIATE FOR THEIR AGE - ALWAYS CHECK THE INSTRUCTIONS BEFORE ADMINISTERING OR CONTACT BELL EQUINE ON 01622 813700 TO DISCUSS BEFORE TREATMENT

  1. Routine treatment with fenbendazole at 2 ½ - 3 months of age.
  2. Perform faecal egg count just prior to weaning and treat depending on what worms are found.
  3. Routine treatment with combined moxidectin and praziquantal in autumn (assuming foal is over 6 ½ months old – if foal is younger please get veterinary advice).
  4. Routine treatment with whichever anthelmintic is routinely used on the premises until animal is 18 months (frequency will depend on which treatment is used).

See HERE for the commonly used worming medications and their trade names.

  • Faecal Worm Egg Counts (FWEC) should be carried out in the spring, summer and autumn (around 3 months apart) in most horses.  Our vets can give you specific advice that is most appropriate for your own horse and your yard.
  • See HERE for details of how to collect and transport samples to our laboratory.
  • Do not treat horses with negative or low FWECs, usually less than 300 eggs per gram as it is unnecessary and increases worm resistance.
  • Only treat horses with worm egg counts greater than 300 eggs per gram using an appropriate wormer, such as pyrantel (e.g. Strongid P®), ivermectin (e.g. Eqvalan®) or moxidectin (e.g. Equest®).
  • Treat all horses in late autumn/early winter with a wormer effective against small redworm encysted larvae (the best product for this use currently is moxidectin - Equest®). This also provides cover against large redworms. A combination of moxidectin and praziquantel (e.g. Equest Pramox®) can be used to treat tapeworms at the same time. Other than this single treatment in autumn/early winter, most horses will not require treatment during the winter. In some cases no treatment will be required if the yard has a long history of excellent parasitic control and healthy horses.

Tapeworm

  • Treat for tapeworms once a year (ideally in the autumn) – using an appropriate wormer e.g. praziquantel (e.g., Equitape®) or a double dose of pyrantel (e.g. Strongid P®). A combination of moxidectin and praziquantel (e.g. Equest Pramox®) could be given in the autumn.
  • A blood or saliva sample can be taken to check the level of antibodies to tapeworms (an indication of the tapeworm burden). This test is more useful if done on several horses in the group rather on an individual horse basis. Horses / herds with high antibody levels should be wormed.
  • Ensure that wormer doses are given as recommended by the manufacturer – always work from an accurate gauge of weight by using scales or a girth tape. If less than one tube is required, re-cap the tube and keep it in a cool, clean place and use next time (unless the packaging instruction state that this should not be done).
  • If possible, avoid using the same class of wormer repeatedly. Remember that ivermectin and moxidectin are in the same class.  However, do not blindly change the class of drug at every dosing, if you are unsure, ring BELL EQUINE on 01622 813700 for advice.
  • Wormer effectiveness can be tested by undertaking a faecal egg count reduction test.  It is sensible to do this every one to two years. Please discuss when this is required with one of our vets.

Wormers can come in the form of paste and granules which are both equally effective. Granules are often cheaper and can be added to the feed although fussy feeders may not consume the whole treatment. Paste is usually quick and easy to administer.

WARNING! Moxidectin and ivermectin can have severe adverse affects on dogs and cats. Keep syringes safely and make sure these animals have no access to feeds containing the paste or the droppings of the horse for 3 days after worming.

Wherever possible, administer the paste directly into the horse’s mouth.

WARNING! Moxidectin can also be toxic to horses if overdosed – this is most likely to happen in foals and Shetland ponies or Miniature horses so please take extra care with dosage. We avoid using it in very thin or poor animals, call BELL EQUINE on 01622 813700 to check.

IT IS ADVISEABLE THAT A VET CHECKS THE WORMING PROGRAMME AT LEAST ONCE A YEAR SINCE TREATMENTS, PROCEDURES AND PARASITES CONTINUALLY CHANGE.

Pinworm (Oxyuris Equi) are small white worms that can cause itching of the skin around the horse's dock.  Pinworm infections were previously uncommon because the worms were easily controlled with most treatments and seemed to only affect young horses. Recently pinworm infections have become more common, now affecting older horses and are more resistant to some wormers.

Usually only one or two horses in a group are affected.  They itch due to the horse's reaction to the worm eggs that are laid on the skin around the dock area.  The itching can be unpleasant and is sometimes confused with sweet itch.  To confirm the diagnosis we use a microscope to look for signs of the pinworm eggs, having initially collected a sample. Sometimes we see the adult worms in faeces; these are white with a very long, thin tail (pin-shaped) and can be up to 15cm long. Unfortunately, pinworm eggs cannot be seen in faecal worm egg counts.

Treatment requires good hygiene and use of wormers.  Daily washing of the affected skin with mild detergent can remove the parasite eggs.  Barrier creams such as vaseline can help to prevent new eggs from sticking.  Cleaning of the stable or areas that the horse itches against can help remove eggs and reduce re-infection.  Some wormers still appear effective but we often treat affected horses more frequently.  Some people suggest that wormers should be given directly into the rectum but this is not helpful and can be harmful; the wormers do not stay there for long enough to work and most of the worms live well out of reach further up the colon.

If you think your horse may have a pinworm infection, please contact us for further advice.

It is vital that we continue to use wormers carefully and correctly to prevent more resistance developing in worms