Worming can be a confusing subject! The correct worming strategy for your horse is one tailored for your specific circumstances. Every property and pasture is different and our vets will be happy to discuss the solution that best fits your needs. For most adult horses, a simple strategy of regular egg counts through the grazing season with treatments targeted to high counts will be advised.
Firstly, there are some simple considerations when designing a worm control programme:
- Faecal egg counts (FECs) are a cheap and effective way of estimating a horses’ likelihood to contaminate pasture with worm eggs
- Poo pick! Regardless of the programme you follow, droppings should be removed from grazing pasture regularly, at least twice weekly
- New arrivals should be kept stabled initially and wormed with an effective broad spectrum wormer before being allowed to graze – ask our vets for the current recommended product
- Cross-grazing with sheep and/or cattle is a useful way of minimising pasture contamination. Avoid fields becoming poached or over-stocked and where possible ‘rest’ them regularly
- Avoid spreading horse manure on pasture grazed by horses
Did you know…?
- For every ten horses gazing, it is likely that only one or two of them will require de-worming at any one time
- Overuse of the anthelmintic drugs used to kill parasites encourages resistance to develop, reducing their effectiveness
- Resistance to many classes of wormers is already widespread and there is little likelihood of new classes being developed soon
- A minimal worm burden may be beneficial to your horses’ health.
When worm control plans fail, the consequences can be severe. Weight loss, diarrhoea and in some cases death can be the result. The aim of your plan should be to reduce pasture contamination to a background level, minimise the possibility of transmission between horses and slow the development of drug-resistant worms.
What is resistance?
When parasites can no longer be controlled with anthelmintic drugs, this is termed ‘resistance’. Parasites become resistant by mutating over generations, so in general parasites with shorter life cycles are the most likely to develop resistance – a small number survive exposure to drugs and it is these survivors that progress to adulthood, passing on their resistance to subsequent generations. A great but annoying example of evolution! The risk of resistance is increased by using anthelmintics too frequently and by incorrectly estimating body weight and using sub-lethal doses of drugs.
Key parasitic species
Many worm species use the horse as a host at some point in their life cycles.
- For adult horses, small red worms (Cyathastomes) are the most common and can account for up to 90% of a horses’ worm burden. They are adept at developing resistance to the drugs we use to control them, and have the ability to encyst and hibernate within the gut wall, surviving through winter. The mass emergence of these worms can cause damage to the gut and trigger colic
- Large red worms (Strongyles), which have historically been the most dangerous to horses, are well controlled by modern drugs
- Tapeworm are located in the horses’ gut and target the junction between the small and large intestine, where in large numbers they can cause mechanical blockage (adults could be 8cm long and 1.5cm wide).
Ascarids do not normally concern adult horses as immunity is quickly acquired, but are a concern in youngstock. Pinworm lay their eggs around the horses’ rectum and can cause intense irritation and itching, while lungworm may be an issue particularly in horses sharing grazing with donkeys.
Finally, bot flies, while not worms per se, are a common irritant to grazing horses, laying their small, sticky eggs on the horses’ coat for them to swallow while grooming.
Faecal egg counts
Kits are available from the office or via our vets (let us know how many you need one when you book your appointment). A ping-pong ball sized sample of faeces is sent to our lab where it is processed and the number of worm eggs present counted. The test should be carried out at least twice yearly, but it must be noted that it only provides information about the number of adult egg-producing worms present, and does not determine the presence of tapeworm or encysted larvae. A blood test or cheek swab is required if a tapeworm problem is suspected.
How should I worm my horses?
Do you already have resistance? If you are on pasture that has a lot of horses or have been using routine treatments without egg counts for many years, then you would be strongly advised to check for resistance. To this, perform a ‘fecal egg count reduction test’ by submitting a faecal sample for a worm count both before and 14 days after the administration of an anthelmintic drug. This should always be performed with veterinary input and is most useful if all co-grazing horses (or a minimum of 6) on a yard are tested together.
Special considerations for youngsters
How to worm