How do wormers work?

In playing my part in being green and sustainable, I have ditched routine worming throughout the year in favour of getting faecal egg counts done. It's really interesting as, despite being on identical deworming schedules, some of the horses definitely seem more prone to picking up worms and parasites than others (younger animals have lower immunity and pick them up easily). I am completely won over by this method, as it is not only far cheaper (I normally pay €5 per sample!), but I know they're only getting dewormed when needed and I am also reducing the chance of them developing drug-resistant worms. Another bonus is that because I am using fewer wormers, I am also using and disposing of less plastic! Win-win all round!

For many years, worming routines aimed to eliminate all parasites from horses, but we now know this is not achievable but also counterproductive. There are now three distinct goals when worming:

  • Eradicate enough parasites so as to prevent clinical disease (when a horse shows signs and symptoms of having a worm burden).  
  • Control environmental contamination via eggs being shed in the horse's faeces
  • Reduce anthelmintic resistant parasites by only using wormers when and if they are needed. 

Worm resistance is becoming a huge problem, in fact, it's becoming more prevalent worldwide. These mutated and resistant parasites reproduce and pass on their drug-resistant genes thus the parasitic population within an animal grows rapidly but is unaffected by anthelmintics. Anthelmintics are the group of drugs that stun or kill parasites such as worms, more commonly known as wormers!

The main reasons for wormer resistance are:

  • Underdosing the animal - the worms do not come into contact with enough of the chemicals in the wormer, so rather than die they become damaged but recover. As they recover their body learns how to protect itself from future contact with the wormer. 
  • Overuse of wormer - similar to the above, anything that survives coming into contact with the wormer will genetically change and be able to survive future 
  • Conventional worming routines - wormers should be used to target specific parasites at specific and strategic times of the year. It still amazes me that there are people who think there are wormers that will kill ALL worms and use the same wormers periodically throughout the year

To overcome and kill parasites that have become resistant, a different class of wormer must be used. There are currently four different types that are commonly used and they all work differently. Additionally, they also target and effect parasites differently, so despite what some companies say, no wormer will kill every parasite!

Fenbendazole
This is absorbed by the parasite and blocks its ability to produce energy. It then starves and dies. Interestingly, fenbendazole is attracted to a parasite's cells rather than the host's cells (ie. the horse) which means the animal's healthy cells are not damaged in the process.

Ivermectin and Moxidectin 
These chemicals don't actually kill parasites, but rather they paralyse them. With their immune system blocked, parasites are then killed by the horse's white blood cells.

Praziquantel 
Praziquantel's mode of action is not completely known at present, however, it is understood that it causes severe spasms and paralyses the parasite.

Pyrantel 
This chemical also causes sudden paralyses the parasite, via muscle contraction. It then loses it's grip on the intestinal wall and is passed out of the horse via the faeces.


There is so much that could be discussed regarding worming horses and there are many conflicting views and opinions. While I highly recommend getting faecal egg counts done, and knowing what wormer, if any, your horse needs, they are not totally foolproof. They don't tell you the population of encysted redworm that your horse may be carrying, as the worms aren't mature enough to lay eggs (thus none will be seen in their poo). So it's worth worming your horse in winter for these regardless. Tapeworm also fails to be picked up, therefore unless you do a (fairly costly) saliva test, you need to routinely worm for them twice a year.

However, by utilising the latest information and working with science, we can not only reduce the prevalence of drug-resistant parasites but also save ourselves a bit of money! Money that could be spent on other things, such as a new saddle pad or show jacket ;)  

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