Equine Worming

Much is written about worming and everyone has their opinion.

Here is ours! The following tries to give you some basic rules and advice which you can use to design your own worming program. This page deals solely with the use of wormers and does not cover worm prevention strategies such as picking up droppings etc. These strategies are equally, if not more important than correct worming strategies.

What types of worms are there?

Here is a very quick summary of the main worms, detailing where they live, whether or not they cause problems and how effective wormers are against them:

Large Strongyles.

They live in the large blood vessels around the gut.
They are not really a problem mainly because they have not developed much resistance to any wormers so any reasonable worming program will control them.

Small Strongyles / small redworms / cyathostomes.

They live in the large intestine. They can kill horses and they are good at acquiring resistance to wormers. There is less resistance to the avermectin type wormers (see below) but some multi-resistant strains do exist.

Pinworms.

They live in the rectum. They are normally more of an irritation (for owner and horse!)then a danger. Resistance to wormers can be a problem but it may be that getting a high enough wormer dose to that location is the hardest part of treating them.

Parascoris equorum.

These only really affects foals and young horses and live in the small intestines. They can cause problems. They are large (like big bamboo shoots) and can cause a physical obstruction
There is resistance to ivermectin wormers but they are sensitive to Panacur.

Bots.

These live in the stomach. They are not really a problem. They are killed effectively by ivermectin wormers.

Tapeworms.

They live in the small intestine. They definitely can cause problems. They are implicated in colics. Only dedicated tapeworm drugs will kill tapeworms - normal wormers will not. The best is praziquantel – look for this ingredient in the wormer.

“Worm the pasture not the horse”

The aim of worming is to maintain a pasture relatively free from worm egg contamination. The aim is not to remove every last worm from your horse. Having a totally worm free horse is practically not possible and overworming your horse may not be healthy and will contribute towards drug resistance in the parasites. Studies show that 100% of horses have small strongyles in their intestines, 5% have large strongyles and 70% of horses have tapeworms. When you are worming, try to think in terms of the egg output from your horse; you are trying to prevent your horse contaminating the pasture.

Tapeworm Twice a Year

Most standard wormers do not treat tapeworms. Standard worm egg counts often do not look for tapeworm eggs and even if they do, they are a poor way of detecting tapeworm infestation (the tapeworms shed eggs at intervals not continuously like the other worms)

The main combination wormers that kill tapeworms as well as normal worms are Equimax, Equest Pramox and Eqvalan Duo.

Double dose Strongid-P will treat normal worms and tapeworms.

Equitape treats just tapeworms.

Most of the other wormers do nothing against tapeworms.

Controlling tapeworms is simple: treat your horse twice a year, preferably with a wormer that contains praziquantel

Treat Horses As Individuals But Treat The Herd

Some individual horses excrete far more eggs than others. Ideally these horses need identifying so they can be treated more aggressively. Those horses that are not excreting eggs do not generally need treating. Worm counts should be carried out on individual horses to identify which horses need worming. This strategy will also give you advance warning if your wormers are not working. Rather than waiting for a horse to get sick, you will see frustratingly high worm counts despite regular worming. All horses should be treated with wormers (or worm counted and left untreated if appropriate) at the same time. Worm counting is simple and costs less than a wormer. In addition, horses are individuals and are different weights. Please use a weigh tape to estimate your horse’s weight before dosing. Under-dosing is a major factor in the development of drug resistance.

Wormer Resistance

Increasingly wormers are not working. Some worm types are becoming resistant to the wormers we use. A recent study testing the efficacy of the 3 groups of worming products on small strongyles gave the following results:

Panacur does not work on 14 out of 17 yards

Strongid P does not work on 4 out of 22 yards

Ivermectin type wormers (not including Equest) do not work in 2 out of 22 yards

You should bear this in mind when selecting the wormer you use. It is relatively easy to check that your wormer is working – you simply take a worm count before and after using the wormer to make sure the worm count is knocked down to zero. You need to discuss this test with your vet.

Reducing the amount of wormer you use.

No one wants to fill their horse full of chemicals and drugs. Ideally, the minimum number of wormers should be used each year. The easiest way to do this is to do a few worm counts each year. If the counts are low then that wormer can be skipped for that horse. The exact cut-off level should be discussed with your vet. Take care using this strategy in young horses – they will be sensitive to worm infestation. Extra worm counts may be advisable.

New Horse on the Yard?

A new horse on the yard may have drug resistant worms in its intestines. If nothing is done, eggs from these worms will be passed onto the pasture and the rest of the herd will be infected. Subsequent worming may then kill only the sensitive worms and gradually the proportion of resistant worms will increase.

New horses should be wormed with Equest or Equest Pramox on arrival and stabled for 3 days before being allowed out on pasture. Equest is used because there is less resistance to it than any other drug on the market. If the horse has been wormed recently, you should check with your vet to make sure it is safe to give Equest. Panacur Equine Guard is not a great choice for new arrivals; there is a lot of resistance to this drug and if the worms are resistant then a 5 day course is not going to be much better than a 1 day dose.

Types of wormers

The following is a summary of the main wormers on the market:

Group 1:

'Benzimidazoles' including Panacur and Panacur Equine Guard

Group 2:

'Pyrantel type' including Strongid P

Group 3:

'Avermectins'

The avermectin group contains 2 main drugs. Equest contains the drug ‘moxidectin’ which is a modern avermectin. All the other wormers contain ‘ivermectin’. These include: Animec Oral Paste, Eqvalan Oral Paste, Eraquell, Eraquell Tabs, Noromectin, Maximec Oral Paste, Vectin Chewable Tablets, Vectin Oral Paste, Equimax, Eqvalan Duo

Tapeworm Only: Equitape

Should I Rotate Wormer Class?

That is a tricky one. The theory is that using a wormer from each group in a 3 year rotation will prevent the development of drug resistance. Unfortunately, the evidence from farm animals is that this may not make as much difference as hoped. It is probably more important to get a really good program up and running that includes worm counts and is based on the Group 3 wormers since they have the least drug resistance. Get some worm count data in for a couple of years and maybe then think about using a Group 1 or Group 2 wormer. When you switch to a Group that has resistance problems like Panacur, you can easily design a worm count system to make sure the wormer is working.