Horses are host to many types of internal parasites and virtually all horses that graze pasture, will at some point get worms! Most horses tolerate low worm burdens well but high worm burdens can lead to clinical disease.
Clinical signs include:
Resistance is when a parasite is no longer affected by an anti-worming drug (anthelmintic). There are only three types of anthelmintics available in the UK although there are many different brand names. Alarming research has revealed that cyathostomes are already showing some resistance to two of these drug types and there are no new anthelmintics currently under development for use in horses in the UK.
Resistance is caused by:
How we can reduce the risk of resistance in our horses:
Factors that increase the risk of high worm burdens:
Harrowing is a common practice but is not useful for worm control in the UK due to the climate. Prolonged periods of hot, dry weather is required before it would become effective in drying out the larvae and eggs, for example in Australia or South Africa.
A specific wormer is used at set intervals throughout the year. For example:
Spring: Target routine worms (Large/small adult red worms, round worms) and tapeworms
Summer: Target routine worms (choose one product and repeat at recommended interval)
Autumn: Target encysted red worms and tapeworms
Winter: Target routine worms and bots
This method is expensive and often unnecessary. Remember research shows that on average, only 20% of horses in a herd at any given time carries a significant worm burden! Wormers are given indiscriminately and too frequently. Overuse of worming drugs increases the risk of resistance developing. Yards that may adopt this protocol would be those with limited or over grazed paddocks or yards with a lot of high risk horses such as stud farms or dealer yards.
This is the use of broad spectrum wormers given at specific times of the year. It aims to disrupt the seasonal life cycle of the worms thereby reducing egg output and pasture contamination. For example:
Horses are dosed 3 times a year, spring, late summer and autumn. This is also a non-specific worming protocol where all of the horses on a yard are wormed. Problems can also arise if abnormal weather patterns occur such as a wet, warm summer or a mild, damp winter leading to early peaks of pasture worm burdens.
Gold Standard Method
Worm Egg Counts (WEC) identify horses with high worm burdens - only these are wormed. N.B. WEC's do not detect encysted (hibernated) red worms or tapeworms. For example:
Spring - Worm for routine worms & tapeworms
Summer - WEC every 8 weeks (worm horses with high WECs as advised by your vet)
Autumn - Worm for everything including encysted red worms. Equest Pramox or 5 day Panacur Guard followed by a tapeworm wormer if any concerns re horse's health.
There are two tests currently available in the UK both of which detect antibodies either in the horse's saliva or blood. Antibodies are produced by the horse's immune system when it comes into contact with tapeworms so the horse may not necessarily be infected but just had a previous exposure to tapeworms.
The commercial salivary test (Equisal) is still very new and has questionable validity - false negatives can occur and veterinary advice is often not sought.
Your veterinary surgeon can take blood for tapeworm testing at your annual vaccination visit, any negative results are reliable and veterinary advice is given on your horse's status and how best to proceed regarding your horse's worming program.
Both tests however have significant limitations - they can only tell you whether your horse has been exposed to tapeworms - they do not tell you if they are currently infected.
To pick up new worm burdens you would need to test every 3-6 months.
Also known as red worms, these are one of the most important worms affecting UK horses. They are very common and can account for up to 90% of a horse’s total worm burden. Redworms hibernate through the colder months in the gut wall and are not easily killed when in this state. Only two worming drugs are effective against this 'encysted' larval stage highlighting the importance of using an effective wormer at the correct time. Mass emergence of these hibernated larvae in the spring can cause severe clinical disease which may lead to death. Worryingly, the redworm is showing increasing levels of resistance to wormers available in the UK.
These large red worms are not very common but are potentially very dangerous. The larval stage migrates through the main artery that supplies the gastro-intestinal tract which can cause colic and possibly death. Thankfully modern wormers are very effective in eliminating this worm.
Often called round worms, these worms tend to affect youngstock and can grow up to 50cm in length! This particular worm is very prolific and produces many many eggs which can survive in the pasture for several years. Modern wormers are effective in eliminating this worm.
Unlike the worms above, tapeworms require a second host to complete their life cycle. Eggs in the paddock are eaten by forage mites (which are everywhere!). These infected mites are then eaten by the horse and 2 months later adult tapeworms can be found in the horse's intestines. Segments rather than eggs are passed in the faeces which is why worm egg counts are not effective in detecting tapeworm infection. Tapeworm infection can cause an intestinal blockage or intussusception which may lead to surgical colic. Only two worming drugs are effective against tapeworm so care must be taken to use the correct wormer.
These worms are common and considered to be non-pathogenic i.e. they don't cause clinical disease. However, egg laying adult worms can cause intense itchiness around the anus causing the horse to rub it's tail. Modern wormers are effective against pinworm but topical preparation may be required if the worms persist.
The main host for this worm is the donkey which can tolerate high burdens. It usually affects young horses grazing with donkeys. Adult worms are found in the small airways resulting in a persistent cough.
Annoying bot flies lay yellow eggs on the horse's legs which are then licked by the horse. Larvae attach to the stomach wall for 10 months then drop off in spring to be passed in the droppings. This worm is also considered non-pathogenic and modern wormers are effective against this worm.