Worms are a perennial problem in horses. In severe cases they will cause weight loss, liver damage and colic. An effective worming program is essential in any management system; however there are several different methods of worm control that can tailor to each individual's management.
What type worms infest horses?
There are three species of worm of concern in horses: small redworms, large redworms and tapeworms.
Small red worms (cyathostomes) currently cause the most concern. They may infest in huge numbers resulting in weight loss and colic. As part of their life cycle they "hibernate" in an encysted form within the intestine wall. In the spring these encysted worms can emerge together resulting in massive damage to the intestine wall. This emergence can result in diarrhoea and in extreme cases may be fatal.
Large red worms are a traditional cause of colic in horses. They are now relatively rare in the horse population due to effective worming regimes. These worms migrate through blood vessels in the abdomen and the liver when they are immature larvae causing colic.
Tapeworms are an emerging problem in horses. They are found in a specific site in the intestine next to the caecum. In large numbers they may cause the intestines to telescope into themselves (an intussusception) causing a blockage resulting in a surgical colic.
How do I find out whether my horse has an infestation?
Mature small and large red worms produce eggs that are passed in the droppings. These eggs can be identified and quantified under a microscope giving a rapid and accurate idea of the worm burden of the horse. Immature and encysted worms do not produce eggs and therefore the numbers of these types of worm cannot be assessed.
Tapeworms do not produce large numbers of eggs and microscopic examination of the droppings will not necessarily identify a tapeworm infestation. A blood sample can be taken and this will measure your horse's immune response to any tapeworms that are present in the intestines. The higher the immune response, the greater the number of tapeworms present.
How do I prevent a worm infestation?
The most effective worming regime depends upon how your horse is kept. Those horses that are kept in small groups and that are grazed in paddocks where droppings are picked up require a different regime from those kept in large yards on communal grazing, or those kept solely boxed and never grazed.
What is the best worming regime if my horse is kept in a small group?
This will depend upon whether you can dropping-pick the paddocks daily. If the paddocks can be picked (and it is essential that they are) then a targeted regime is currently recommended. Here wormers are only used when required and the worm burden of the pasture is kept as close to zero as possible. This regime is based around periodic worm egg counts in the droppings. An initial dropping sample is taken and, if deemed necessary, a blood sample may be taken to assess the tapeworm burden. If the dropping sample identifies a worm infestation, an appropriate wormer should be given. If the worm egg count is zero, then no action is required.
Worm egg counts should thereafter be taken at monthly intervals for 3 months. If the last of these counts is zero, worm egg counts are taken every 3 months for 6 months. If these 3-monthly egg counts are zero then a maintenance regime of taking worm egg counts every 6 months can begin. Your horse is only wormed if eggs are identified in the droppings.
What is the best worming regime if my horse is kept in a large group?
Effective droppings picking is very difficult when horses are kept in large groups (greater than 6 or so). Also, horses kept in these situations tend to have new horses with unknown worming history introduced frequently. A targeted regime such as that described above for small groups of horses is not appropriate. It is recommended when your horse is kept in a large group that a periodic worming regime be carried out. With this regime, an effective wormer is given every 6-13 weeks (the length of time depends upon the type of wormer you are using) from March through to November. After the first frost, your horse should be wormed against tapeworms and a wormer given to kill any "hibernating" small red worms. Worming is then re-started the following March.
There are three different "families" of wormers commonly used in horses. To minimize the development of resistance to a particular "family" of wormer each family of wormer used should be rotated on an annual basis, i.e. the same wormer is used through out year one; however, a different wormer family is used in year two and the third family of wormer used in year three.
What is the best worming regime if my horse is kept solely in the box and is never grazed?
This horse should be wormed once against red worms (including the hibernating small red worms) and tapeworms. Once wormed, they do not require to be wormed again, unless they are given access to pasture.