In the past, it was common to worm your horse regularly throughout the year. However, worms can develop resistance to treatment, making our wormers ineffective. The current recommendation for worming horses is now to perform regular worm egg counts to help us to determine when to give each treatment. This strategy helps to reduce the amount of wormer that we are using, reducing the chance of resistance building up.
Although there are many different parasites that can affect your horse, the worms that we are most concerned about are:
- Strongyles
- Tapeworm
- Cyathostomins
Strongyles
- These worms produce the eggs that vets look for in faeces when performing worm egg counts.
- Worm egg counts should be performed every 8-12 weeks through the grazing season but they may be recommended more often than this if there is a history of a high worm burden.
- Treatment is usually recommended for horses that have a worm egg count of more than 200-250 eggs per gram.
- 80% of the worm eggs are produced from 20% of the horse population. Therefore, only horses that are shedding a high number of worm eggs should be given treatment, rather than blanket worming every horse in the pasture. This is not only better for horses, the parasites, and the environment, but is also more cost effective for you.
- All horses should be weighed prior to worming to ensure that the correct dose is given. This is especially important in Shetlands and other small horses, as overdose has occasionally been associated with signs of toxicity.
- Previously a “dose and move” strategy may have been recommended, this should only be done if there is a high worm burden and when the pasture has not been managed well.
- Horses between 5 and 15 years old (not living with youngstock) are considered to be low risk and therefore should require less worming treatment.
Tapeworm
- Tapeworm can’t be measured in faeces, therefore a sample of saliva or blood needs to be taken, this should be done every 6-12 months.
- Sampling kits are available here.
- Usually the number of tapeworms is higher in Autumn, so if testing is only carried out annually then it should be done at this time of year.
- Treatment should only be given in response to a positive test result and usually would be a product containing praziquantel.
- Following treatment, the horse should be stabled for 3 days and the faeces should be disposed of away from grazing areas. This is because eggs continue to be present in faeces after the adult worm has died.
Cyathostomins (small redworms)
- These worms can cause weight loss, colic and diarrhoea.
- Usually horses are wormed in Autumn to reduce the risk of disease.
- Previously, a Fenbendazolewas given annually, however, this is not recommended any more due to resistance, and moxidectin may be more appropriate.
- If the horse is low risk, has had regular worm egg counts throughout the year and good pasture management has been carried out then worming at this time of year is not usually necessary but a blood sample can be taken to measure the level of risk.
- Traditionally, worming was performed in November, at the time of the first frost. However, the timing of treatment should depend on the type and timing of the previous worming treatment.
Youngstock
- These horses have a lower immunity to worms and therefore are likely to have a higher worm burden.
- Poo picking at least twice a week is essential.
- Rotation of pastures is very important as worms can accumulate rapidly on the pasture.
- Treatment for cyathostomins should be given to all young horses in Autumn.
- If the horse is grazing over winter and the weather is mild, another dose of wormer may be required 3 months after the initial treatment.
- Tapeworm treatment may be required, but saliva or blood testing should be performed first.
Environmental management
Management of the pasture is an critically important way of reducing the amount of worms that your horse will pick up. Without this, more worming treatment will be required and the risk of worms developing resistance is high.
Best practice for managing the pasture includes:
- Reducing stocking density of pastures.
- Poo picking the fields at least twice a week (especially when the conditions are perfect for worms – humid with a temperature above 10℃). Harrowing the fields to spread faeces instead of collecting it is not recommended as this just spreads the worms around the pasture.
- Separating the muck heap from grazing areas to prevent migration of worms onto the pasture.
- Rotating pastures. If there is a high worm burden on a pasture, this area should be rested when the conditions are hot and dry, as the eggs will be killed more quickly.
- Grazing the pasture with sheep and cattle, as they can mop up the parasites.
- Preventing rough areas of pasture where horses regularly pass faeces as this can provide a reservoir of worms.
- Quarantining new horses. A worm egg count should be performed on arrival and if treatment is required, they should be stabled for 3 days to allow for excretion of the parasites, this can then be disposed of away from the grazing area.
How to take a faecal sample
- Order a worm egg count kit and make sure you have everything ready.
- A sample should be collected within 12 hours of the horse passing the faeces.
- A piece should be taken from at least 3 balls of faeces and placed into a plastic bag.
- The total sample should be around the size of a tennis ball.
The specific wormer required for your horse will depend on a number of factors and therefore should be discussed with a vet before use.
Reference:
Rendle, D. et. al., (2019) ‘Equine de-worming: a consensus on current best practice’ UK Vet Equine
