
With the increasing threat of worms becoming resistant against the wormers we currently have and the development of new classes of wormers for use in horses being slow at best, it is important to have a strategic plan for worm control. We can no longer rely on routine, regular worming, and rotation of wormers as was common practice in the past. There have already been quite a few reports of resistance even against the ‘old reliable’ moxidectin (best known from Equest/Equest Pramox). There is also growing evidence that once resistance has developed, there is no reversing it. One study found that 40 years after stopping the use of a particular wormer, worms on the same field were still resistant to that wormer!
Currently, the most important worms to control are the small red worm (Cyathostomes) and tapeworm (Anoplocephala). Although pinworm (Oxyuris Equi) can cause frustrating problems like itchiness around the backend in some horses, it is not a cause for weight loss or life-threatening disease like the other two. For that reason, only horses clearly affected by pinworm will undergo targeted treatment and we will not discuss them here.
Small red worm
During the summer months, adult small red worms in the horse’s intestine lay eggs which shed in droppings and develop on pasture to larvae. After being taken in via the mouth by grazing horses, these larvae can either develop into adult worms straight away or go into ‘hibernation’ in the gut wall. There they are protected in a capsule that makes them harder to treat, and some wormers do not have any effect against them. Larvae do not produce eggs and therefore are not picked up on faecal worm egg counts!
Large numbers of adult small red worms in the intestine can cause lethargy, weight loss and diarrhoea. However, the most serious problem these worms can cause is when larvae that are present in the gut wall (encysted larvae) emerge en masse, usually during late winter or early spring. This condition called ‘larval cyasthomiasis’, causes damage to the intestine resulting in severe diarrhoea, weight loss and potentially colic. Even with aggressive treatment about 50% of horses with this condition sadly die.
Tapeworm
Tapeworms are taken in via ingestion of mites carrying the immature tapeworm. They mostly attach to a very specific part of the horse’s intestine, the area where the small intestine goes into the large intestine (caecum). Due to motility changes and blockage, they can cause colic which may need surgical treatment. Contrary to myths, weightloss is not a sign of tapeworm infection!
The good news is that less than 50% of adult horses in the UK are infected with adult tapeworms.
So, what should be done to make the most of our wormers?
1 - Good field management
Minimise the number of horses per acre and try to keep a stable group of horses in a field.
Poo pick at least twice weekly.
Rest and rotate fields, red worm larvae can survive as long as 9 months in a field but are killed more quickly in hot, dry weather (a few weeks).
Larvae can travel on pasture for a short distance: do not let horses graze close to a muck heap and prevent rough areas forming where they defecate regularly.
Having cows and sheep co-graze helps to reduce the worm burden.
2 - New Arrivals – the ideal situation
New animals should be kept off pasture.
On arrival perform a worm egg count followed by administration of Equest Pramox.
Repeat the worm egg count 10-14 days later. If the reduction of worm eggs compared to the first is less than 95%, a different wormer should be used before the horse is turned out.
3 - Only worm if necessary
Perform faecal worm egg counts in February, May and August and worm if there is a significant worm burden. Avoid Equest/Equest Pramox, these are reserved for autumn worming! Ivermectin wormers are likely most applicable.
Test for tapeworm (saliva or blood) once or twice yearly, depending on the horse’s risk (November and/or May) and treat only if required.
Test for small red worm (blood test only) between September and December and treat if required. In this case we recommend using Equest/Equest Pramox.
Testing for tapeworm and small red worm larvae is not useful within 4 months of worming your horse.
4 - If blood tests are not an option
A less than ideal but still safe option is to combine worm egg counts throughout the year with Equest/Equest Pramox around November, this applies even if worm egg counts have been negative throughout the year. This is to deal with the potential presence of small red worm larvae.
Worming Program
February
Worm egg count (WEC) all horses
Low WEC result horses= no need to worm
High WEC result horses= Worm with Equest at recommended dose
May
Worm egg count (WEC) all horses
Low WEC result horses= no need to worm
High WEC result horses= Worm with Equest at recommended dose
August
Worm egg count (WEC) all horses
Low WEC result horses= no need to worm
High WEC result horses= Worm with Equest at recommended dose
November
No need to WEC
Instead, worm all horses with correct dosage of Equest Pramox
Day to day worming measures
Give all new horses on the yard an Equest Pramox at recommended dose, before turning out onto grass.
Regular poo picking (not spreading) of grazing land.
No need to keep off the grazing when worming if regularly poo picking.