Worming CPD September 2016


Optimising parasite control

By Chris Taylor BVSc MRCVS, Technical director, Virbac Limited

The basic aim of 3D Worming – which stands for direction, dosage and delivery - is optimising parasite control via:

• The appropriate use of wormers 
• Good pasture management 
• Monitoring the effectiveness of worming
• Avoiding high parasite burdens
• Preventing or delaying anthelmintic resistance
Remember…The only way to ensure the correct approach is via the core SQP responsibility, which is asking the right questions – each time, every time.


What is the best direction to take when worming a horse? 

The basic questions are: 

• Is the horse one of the 20% with a high worm burden or not? In any multiple horse establishment there will be enormous variations in the worm burden of individuals. In general, it is likely that 20% of such horses will have higher burdens then the rest.

• When a wormer is needed, what is the best way to dose and which product and at what time of year?

The two main objectives are:

• to prevent disease
• to reduce pasture contamination
So the main aim is targeted treatment.

First of all you need to establish whether the horse is likely to have a significant worm burden so you should advise a faecal worm egg count (FWEC) before treatment and again 14 days post treatment. A FWEC does not provide any indication of how many worms are infecting a horse but how much the horse is contaminating the pasture. Horses with high counts generally need more regular treatments with wormers; those with low counts (FWEC results of < 200 eggs per gram) may only require key strategic treatments. 

But do remember that FWECs can mislead:

• They do not detect the encysted stages of small redworms.
• They do not detect tapeworms.
• They provide little or no information on levels of parasite infection over the winter months.
• FWECs are not directly correlated with the number of worms.
• There can be inaccuracies if the faeces are not collected and stored properly – fresh faecal samples are mandatory as eggs can hatch before the count is performed yielding a false negative result.

Which product and when?

Tapeworm treatments may be given in Spring and autumn using either praziquantel or pyrantel. Bots should be treated in November/December using ivermectin.

Encysted cyathostomes (small redworm) need to be treated between November and late January/early February using moxidectin or five day fenbendazole.

Worming should be strategic at other times (through the grazing season) using ivermectin/fenbendazole.

Expert recommendation is to reserve moxidectin only for once yearly treatment of cyathostomes. A quote from a well-known parasitologist, Dr Gerald Coles, (In Vet Times No 51 December 2009): “Since moxidectin is so valuable because of its activity against inhibited larvae, it is best confined to use for an autumn or early winter treatment.”

Targeted strategic worming in practice 

The following flow chart summarises the direction needed.


Giving the correct dose is absolutely vital as under-dosing leads to:

• Poor efficacy – health risks and owner dissatisfaction.

• Exposure of worms to sub-lethal dose which encourages resistance.

The accurate weight of the horse must be known prior to prescription so you should always recommend a weighbridge if available or a weigh tape and use of the standard weight calculation formula below.

You should always bear in mind that horse owners are notoriously bad at weight ‘guesstimation’.

On average, horse owners underestimate weight by 20% and frequently more. It is also strange to note that over estimation of weight is extremely rare! 

You need to be aware of ‘convenience’ too – a significant number of horses weigh over 600kg, but the tendency is to use just one syringe for heavy horses.


The correct dose must be delivered properly. One of the biggest everyday problems with syringe wormers is ‘spit out’.

In a study on 480 horses at the French Ministry of Defence, 22% of them spat out a significant amount of the wormer administered. And those horses spat out between 29% and 68% of the contents of the syringe wormers they were given. Indeed, ‘spit out’ can be a problem, even for experienced equestrian handlers.

So, what’s in a syringe?

Bear in mind that the plastic of the barrel used in equine syringe wormers is very thick; it’s designed to withstand chewing! This means that the tube containing the worm paste or gel is very narrow. Basically, syringe wormers contain between 5 and 10ml, or only one to two teaspoonsful.

Spit out, therefore, is not only a potential poor efficacy problem – it can also be a significant safety problem as dogs will be poisoned should they ingest wormer residue from the floor. It is an inescapable part of your prescribing duties to warn owners about such situations as well as the safe disposal of used and part-used syringes.

For horses that are known to be ‘difficult’ with syringe wormers, there is a viable alternative. Ivermectin and ivermectin/praziquantel wormers are available in palatable (apple grounds, not flavour) tablet form. Presented in a simple dose form - one tablet per 100kgs and eight tablets per tube, they can be presented in feed and are usually well accepted provided the horse does not see the owner putting the tablets in the feed!