Dr. Tania Sundra

24 October 2018


Imagine if you owned 3 horses : a shetland pony, a thoroughbred and a clydesdale. Would you use the same saddle and bridle for all three ? Probably not, because simply, one piece of equipment is not going to fit all three horses. 

The same can be said when making decisions about your horse’s health. 

 “there is no one-size-fits-all” solution. 

We believe in offering customised veterinary care, tailored to the specific needs of our patients. And when it comes to deworming your horse, this same rule still applies! 

But before we get into more specifics, you need to understand the issue of worm resistance and why this is so important to your horse’s health. 

What is resistance ? 

Resistance is the ability of the worms in your horse to survive deworming treatments that were previously effective in killing them. From a worm’s point of view, they have evolved over the years because their survival depends on it. 

From an owner’s perspective – resistance is a man-made problem. It has come about due to our over-use of dewormers in recent years. In Australia, recent reports have shown that Roundworms (Ascarids) are resistant to ivermectin, moxidectin and oxfenbendazole. More concerningly, in Brazil, there have been reports of stronglye worms being resistant to all three classes of dewormers as well as combination drenches! 

A new study has also shown that once resistance develops in your horse, you cannot reverse it… not even if you withhold worming treatments for almost 10 years! 

Keep this in mind, when I tell you that there are currently NO new classes of dewormers being researched in the equine field. We are on the verge of a potentially devastating health crisis for horses. 

What does this mean for your horse ? 

If we continue down the path of deworming horses every 6-12 weeks with no justification other than “this is how we’ve always done it” or “I have competition horses” we will rapidly find ourselves in the same situation as the horse owners in Brazil – where there are no effective dewormers to protect your horse from resistant worms. 

What can you do about it ? 

As we previously covered, you shouldn’t be using a “cookie cutter” approach to anything related to your horse’s health – and deworming is NO EXCEPTION!  This way of thinking is not only old and outdated, but is also an ineffective way to manage parasites within your herd. 

What works for the horse in the paddock-next door is not necessarily going to be the best thing for your horse. 

The most effective parasite control program for your horse is : STRATEGIC DEWORMING. 

What are the goals of Strategic Deworming ? 
  1. To minimise the risk of worm-related disease
  2. To control parasite egg shedding 
  3. To maintain the effectiveness of drugs and avoid further development of resistance  

You might say, that’s all very well and good, but it sounds expensive and somewhat confusing. In actual fact, it is quite straightforward, and you may even save money by avoiding the need to purchase unnecessary worming treatments for your horse. 

Remember, even the cheapest dewormer is expensive if it doesn’t work! 

The following is a guide, laid out step-by-step to help you start a deworming program for your horse. 

There are three key components to developing a STRATEGIC DEWORMING program : 

  1. Perform fecal egg counts 2-3 times per year to determine the parasite egg shedding status of your horse
  2. Deworm the right horses at the right time of year with the right compound based on the results of your fecal egg count and advice from your veterinarian 
  3. Decrease worm burdens by paddock management 


The cornerstone to modern worm control in horses is the fecal egg count test (FEC). We have literally performed hundreds of FEC’s in our dedicated lab over the past 12mths for educated owners wanting to invest in their horse’s health. Depending on the results of your horse’s FEC we will lay out a customised plan for your horse in a full report. 

It is important to note that a negative fecal examination does not mean the horse is free of internal parasites. Some types of parasites produce eggs only intermittently. Larvae do not produce eggs at all, and may be present in a in a horse with a low fecal egg count. In addition, tapeworms, bots and pinworms are not seen on a routine faecal egg count. The results are most useful when several horses on a farm are tested on the same day. Whilst fecal egg counts may not be an ‘exact science’ it is still the most useful tool we currently have to determine the egg shedding status of your horse.

Your input in this step is easy : simply submit a sample of your horse’s manure into our lab for testing. All the instructions can be found at the bottom of this post. The cost is $15 per sample. 


Adult horses have very different deworming needs compared with young horses and some older horses. It is important to base how often to deworm your horse and with what compound on the results of the fecal egg count and age of your horse. Special attention must be paid to deworming foals and young horses as well as geriatric animals. Young horses, for instance, tend to be more susceptible to the effects of ascarids and must be dewormed appropriately. However, ascarids rarely cause a problem in mature horses due to the development of immunity.

In addition, a properly timed single, annual treatment which targets tapeworms is recommended for most horses. The timing on this treatment is very dependent on the season. For example, tapeworms are best targeted at the end of autumn/early winter, which means you should use a dewormer containing PRAZIQUANTEL – the compound which is effective against tapeworms. If you are one of our clients, we will send you email reminders on when to deworm your horses based on the season here in WA.

For wormers to be effective at treating internal parasites, it is essential that your horse is dosed with the appropriate amount of dewormer according to their bodyweight. Under-dosing your horse will accelerate the development of resistant worms.

This is probably also a good time to address the use of dewormers in oil drenches. The following information is based on my personal views and experience. I feel the effectiveness of the dewormer is reduced when included as part of a paraffin oil drench and we no longer recommend clients include a dewormer in their drenches. This is because I simply question how much of the dewormer is actually absorbed when administered in combination with 4+L of mineral oil.  Furthermore, if only part of the dewormer is effective, it would be similar to deworming your horse with half its recommended dose thereby increasing the chance for worm resistance to develop in your horse.

We recommend testing the manure prior to deworming to determine if your horse actually needs a deworming treatment in the first place!


Eggs shed by your horse in manure, hatch into larvae when conditions are favourable (ie. when there is warmth and moisture). Cold weather slows the development of larvae and very high temperatures kill both eggs and larvae. 

By far the most effective way of reducing the worm eggs on your paddocks is via removal of the manure. Studies have show that removing manure every 1-3 days is optimal to reduce the number of larvae. However, this is obviously not practical for some farms. Other strategies which may be more effective on your farm include : 

  • Proper composting of manure 
  • Resting paddocks / Rotational grazing 
  • Cross-grazing with other species (eg sheep/cattle) 

Current worming practices for horses are based largely on concepts and knowledge that is more than 40 years old. A lot has changed over that time. Worms have evolved… isn’t it time our deworming practices evolve too ? 



About the author : Dr Tania Sundra is an equine veterinarian and director of Avon Ridge Equine Veterinary Services. In addition to her role as an ambulatory practitioner, Dr Sundra is also serves as a member of the The Australian Equine Parasitology Advisory Panel.