All horses have worms. Small numbers are healthy, but large worm burdens can cause serious disease, as well as contaminating the pasture that they are grazing, thereby increasing the risk to other horses grazing the same pasture. It is essential, therefore, that worms are kept to manageable numbers both in our horses and on the pastures that they graze.
For the last fifty years we have had drugs (called anthelmintics) that we can use to kill horse worms, but recently many of the worms have become increasingly resistant to these drugs. In addition, we now know that these drugs can have serious adverse effects on invertebrates and the environment. This has necessitated a change in the way that we manage horses to control worm burdens, and the way that we use the deworming drugs
Worming advice for adult horses (over 18 months)
All grazing horses will have worms, but most horses have a low number which is healthy and does no harm. Small numbers of worms are actually healthy because they promote a healthy immune system. Problems arise if worm burdens become large, which is when the worms can cause disease such as weight loss, colic and diarrhoea, or contaminate the pasture with large numbers of infective larval stages of worms which pass out in the faeces (droppings). In some cases, heavy worm burdens can be fatal. So it is very 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 and on the pasture.
In adult horses, the cyathostomes and large strongyles (small and large redworms) and tapeworms are the most important ones that we need to control. You can learn about the life cycles of the small and large redworms here.
- 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 autumn and winter. These encysted larvae sometimes all “wake up” and emerge simultaneously from inside the gut lining, which will result in damage to the intestine wall and serious life-threatening diarrhoea (acute larval cyathostominosis).
- Tapeworm can cause colic by damaging and blocking the intestine.
- Large redworms are now uncommon in the UK, but they can cause serious, life-threatening colic.
- Pinworms cause disease less commonly, but still can cause irritation to the skin around the anus and require specific treatment.
A good worm control strategy aims:
- To minimise the contamination of pastures 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).
- To keep the worm burden in an individual horse low enough to prevent disease, but not eliminate worms completely.
There are two main components to a worm control programme:
- Pasture management – this is the most important way of controlling worms
- Drug treatments (known as “dewormers”, “wormers” or “anthelmintics”)
For further information, please visit the links below:
Key Fact: Minimise use of anthelmintics, otherwise known as wormers or de-wormers, to prevent resistance to these products. Excessive and unnecessary use of anthelmintics also causes serious damage to the environment because they are toxic to many insects and aquatic animals.
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 for one or one and a half acres is an appropriate rule of thumb.
- Pick up and dispose of droppings regularly (at least twice a week during the grazing season, March to October). Do not spread this onto fields grazed by horses as it simply spreads the worms around the fields. Instead, please compost it somewhere away from the grazing area.
- Position the muck heap as far as possible away from water courses / streams etc (at least 10 metres)
- 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 – ideally from the end of one grazing period to halfway through the next grazing period.
- 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. Beware liver fluke in wet areas.
Drug treatments (‘Wormers’, ‘anthelmintics’)
Heavy use of wormers has caused many equine worms to become resistant to the 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 adult horses at frequent fixed intervals year-round (known as “interval treatment”). 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.
It has become common practice to treat all horses in the autumn / winter with a dewormer that targets the inhibited larvae of small redworms (eg moxidectin or Equest). This is no longer recommended – instead a risk assessment based on the age of the horse, its management and results of regular FWECs obtained during the summer should be used to determine whether or not the horse requires a dewormer for redworms.
The ideal strategy varies from yard to yard and should be discussed with Bell Equine, please call us on 01622 813700 and speak to one of our vets or one of our SQPs (suitably qualified people). 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.
Dangers of exposing the environment to wormer drugs:
- Worming drugs are chemicals that can have negative environmental impacts. The drugs pass out of the body in the faeces (droppings). The toxicity of these drugs will have harmful effects to dung beetles, insects and other microorganisms as well as some aquatic life, meaning they pose a real threat to our rural ecosystems.
- To prevent contamination, bring the horse onto a surfaced area to administer the wormer, that can be easily cleaned if necessary – rather than worm them in the field.
- Poo pick regularly post worming, so any contaminated droppings are not left on the field- these droppings should be disposed of separately if your muck heap is used for slurry spreading or else composted for at least six months before being spread back onto the fields.
How do you know if the dewormer is working?
Since many horse worms, including redworm, ascarids and tapeworms, are becoming resistant to the different dewormers that we have available (and with no new dewormers becoming available for the foreseeable future), it is sensible to check for resistance on your property at least once a year. This can be done using a Faecal Egg Count Reduction Test (FECRT). Speak to one of our vets or SQPs about how you can do this.
We advise checking for tapeworm in all grazing horses at least once a year. On particular premises where there is a known tapeworm problem, and in young horses (less than 2-3 years), checking more frequently may be required.
Since it is generally impossible to detect tapeworm eggs in a normal faecal egg count test, the tapeworm saliva test, generally performed once a year in the autumn, is recommended. Horses that have a high result of this test should receive treatment with a dewormer that is effective against tapeworm (eg praziquantal).
- 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 have a faecal worm egg count, and possibly a small redworm blood test and tapeworm saliva test, performed. – speak to one of our vets or a SQP. If needed, they can then 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®. Another faecal worm egg count should then be performed 2 weeks later – if the result is 50 eggs per gram or higher, veterinary advice should be sought because it suggests that the horse may be carrying resistant worms.
- Horses that are known to have been treated for tapeworm and encysted redworm within the previous year should still have a faecal egg count performed. If the result is 300 eggs per gram or higher, then the horse should be wormed and a faecal egg count reduction test (FECRT) should be performed (i.e. repeat the worm egg count two weeks later) in order to prevent the introduction of drug-resistant parasites.
Advice for worming broodmares and youngstock
Foals and young horses (up to 18-24 months of age) 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 (roundworms) 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 the larvae ‘hibernate’ in the wall of the intestines over the winter, then ‘erupt’ all at once.
The aims of an effective worm control strategy for horses of any age or type are:
- 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).
- 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:
- Pasture management
- Drug treatments (“dewormers”,“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 for one or one and a half acres 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 and away from water courses.
- 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 dewormers in foals and broodmares has caused some worms to become resistant to some equine dewormers, 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 much 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 or SQPs.
- 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.
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 always be given at 2 -3 months of age using a benzimidazole (e.g. Panacur®) to treat ascarids (regardless of FEC results).
- Subsequently (after 4 months of age), the foal should be wormed according to the type of worms seen in routine faecal egg counts performed every eight weeks, 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. Recently weaned foals and yearlings are more likely to be high egg shedders so routine monitoring (ideally every 3 months) of faecal egg shedding is Faecal worm egg counts can be used to identify the predominant type of worms (ie ascarids or small redworms) that the foal has – and this can help to determine the most appropriate class of wormer to use. In most cases, ascarid egg excretion peaks at 4-5 months of age and then wanes.
- In the UK many foals will be weaned in the autumn/early winter when a cyathostomin larvicidal treatment (i.e., moxidectin) should be considered. There can be a second wave of ascarid infection in some yearlings and FEC should inform on the presence of this species.
- A product effective against tapeworm (moxidectin and praziquantal – Equest pramox®) should be included at least once yearly, providedthe 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 or SQPs.
Every yard is different and specific guidance can be provided by our vets by calling Bell Equine on 01622 813700.
Bell Equine’s top tips for optimum worm control
- Faecal Worm Egg Counts (FWEC) should be carried out in the spring, summer and autumn (around 3 months apart) in all grazing horses to check for redworm. Our vets or SQPs 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 mature horses with negative or low FWECs, usually less than 300 eggs per gram as it is unnecessary and increases worm resistance.
- Only treat mature 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®).
- Once a year perform a faecal egg count reduction test using either pyrantel (Strongid P) or a macrocyclic lactone (ivermectin [eg Eqvalan] or moxidectin [eg Equest]) to check for small redworm resistance on your premises.
- Treat all young (up to 5 years of age) and high risk horses (base on FEC results, pasture management, etc) 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.
- Low risk horses (horses 5-20 years of age, with regularly low FEC results over the summer, well managed pastures) will not require a routine dewormer treatment in the autumn. If there is any doubt about this, consider running the small redworm blood test on low risk horses to determine whether or not they require a dewormer treatment.
- Other than this possible single treatment in autumn/early winter, most mature horses will not require treatment for redworm during the winter. In some cases no treatment will be required at all if the yard has a long history of excellent parasitic control and healthy mature horses.
- Consider treatment for tapeworms once a year (ideally in the autumn) – using an appropriate wormer e.g. praziquantel 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 if needed.
- A saliva (or blood) sample can be taken to check the level of antibodies to tapeworms (an indication of the tapeworm burden). Horses / herds with high antibody levels should be wormed with a product effective against tapeworms, but horses with low antibody levels should not be treated.
- Ensure that the dewormer 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 every time. 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 [FECRT]. It is sensible to do this every year. Please discuss when this is required with one of our vets or SQPs.
- 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 effects 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 at least 3 days after worming.
- Wherever possible, administer the paste directly into the horse’s mouth. Click here for advice about how to correctly administer a wormer.
- 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 one of our vets or SQPs 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 reliably 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. Hygiene is most important, as on many occasions wormer medication is ineffective.
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 will not work and can be harmful.
- 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.
Faecal Worm Egg Count (FWEC)
The heavy use of wormers over the past 20 years has unfortunately led to a significant amount of worm resistance within the equine population. This means that some dewormers are no longer effective at eliminating worm burdens in some horses. These problems can be overcome relatively easily by monitoring the faecal worm egg count (FWEC) of all horses on a yard at regular intervals. This ensures only those individuals with a significant worm burden are treated using appropriate wormers at appropriate times, thereby reducing the problem of resistance.
The FWEC is a relatively simple procedure that only requires a small amount of input from the horse owner. To do this we need you to provide a small amount of your horse’s droppings, approximately 10g or a heaped teaspoon size (selected from more than one nugget – ideally at least three). The sample should be as fresh as possible, ideally being delivered to the clinic on the day of collection. Alternatively, samples can be sent by post using one of our free-post sample kits. Request your free kit/s here. Samples can also be collected into a clearly labelled sealable container, such as a plastic sandwich bag and brought in within 24 hours along with a completed request form.
Click FWEC Guidance Notes for a simple guide to collecting a faecal worm egg count sample.
To download a FWEC request form (if you are not using one of our free kits) which is required for sample submission: click FWEC Postal Form for the submission form for 4 samples or less and click FWEC Postal Form – Yard for the submmission form for 5 samples or more.
For those samples coming from a larger yards, it helps to call our team in advance on 01622 813 700.
Following microscopic analysis of each sample individually, we aim to contact you with your results within 48 hours. This gives your vet or SQP the opportunity to advise you on any further treatment, considering our knowledge of your horse. This also gives you the opportunity to ask any questions, a service of which is all included in the lab fee. By running worm egg count tests 14 days after worming, we can see how effective the wormer has been at eradicating the worms. This is know as the ‘faecal worm egg count reduction test’.
Unfortunately tapeworms are not detected by routine worm egg counts, so a tapeworm saliva (or blood) test should be performed at least once a year (usually in the autumn). Often with the results showing no treatment is required.
Young horses (up to five years of age) are most susceptible to picking up large numbers of small redworm larvae, many thousands of which can accumulate as encysted larvae in the walls of the large intestine. These larvae can then cause a serious disease called larval cyathostominos.is Unfortunately there is no available diagnostic test to determine if horses are carrying large burdens of ‘encysted’ (immature) small strongyles. Therefore, it is advisable that all young horses less than 5 years old or others at particular risk are treated for these larval worms in the autumn / winter (usually with moxidectin which is believed to be effective against the larval stages of the parasite). Older horses that are considered to be at low risk of accumulating large numbers of larvae do not necessarily need treating; the small redworm blood test may be useful in these cases to confirm that no treatment is required. The test estimates the total burden of small redworm in the horse’s body (adults and larvae) and therefore is of little value in horses that have high FEC results since the result of the blood test will also be high. Our vets can advise you further if you have any questions.
Example worming plans for adult horses
Please note, this is just an example of a possible worming programme that we may recommend for adult horses. The best program will vary from yard to yard depending on the age and number of horses, the type of grazing, pasture management and previous worm related disease.
Example 1) Simple worming for adult horses, with routine worming for tapeworm in the Autumn.
|Spring (March, April, May)||Worm egg count every 2-3 months. Only worm high risk horses with more than 300 eggs per gram.|
|Summer (June, July, August)||Worm egg count every 2-3 months. Only worm high risk horses with more than 300 eggs per gram. For any horses which will require treatment because of a high WEC, perform a FECRT.|
|Autumn (September, October, November)||Treat all high risk horses with Moxidectin and Praziquantal (e.g. Equest Pramox) (if high saliva test result) once. Low risk horses with low FEC and low tapeworm saliva test results do not require treating.|
|Winter (December, January February)||No worming or testing required (unless on heavily grazed pasture all year, a very mild winter or any signs of disease).|