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Are deworming pastes enough?

Amanda Edwards - Sunday, June 05, 2016

Most horse owners worm their horses regularly.  Some worm every time the farrier comes, some at the start of every season.  But how many of us really know what we are doing and is it the right thing to do?

According to mounting research this probably isn’t the best way of protecting our horses into the future.  What the researchers are finding is that some worms are becoming resistant to the drugs in the worming paste.  This means they may not work anymore.  As there are not very many different types and no new drugs being developed this could become a big problem.

First things first what are worms and why do we need to worry about them?

Worms are parasites that live off the horse.  There are several different types but essentially they follow the same pattern.  The eggs are in the grass and on the ground, horse eats the eggs which then hatch inside as larvae (baby worms).  These then live in different parts of the horse and feeding either directly off the horse’s insides or off the food the horse eats.  Once they grow up into adults and then lay eggs which leave the horse via the manure.  Then the cycle starts again.

They are dangerous to the horse because the worms are either taking away the nutrients the horse is eating or causing damage to the gut lining by embedding themselves into it.  If there are a lot of worms this can cause the horse to lose condition, have a low blood count,  get stomach ulcers, develop colic and even die as a result.

These are the more common worms affecting horses.

Common name


Fancy name

Horse problem


Large Redworms


Large strongyles

Fever, tiredness, gut problems, anaemia through damage to blood vessels and internal bleeding.  These worms hook into the blood vessels in the bowel and suck the blood from the horse.  Very important for foals to be wormed against these.


Small Redworms



These little guys form cysts and attach themselves in the wall of the bowel.  They can stay there for up to 3 years.  They are hard to detect too. The number of worms encysted in the wall of the intestine does not equal the number of eggs in the manure.  Treat for these whether they are identified in faecal egg count testing or not. *Faecal egg count will NOT tell you if your horse has encysted cystathomes and they are killers.

Spring, Winter


(©Tracy Chapman)


Bot eggs are laid on the horse.  Horse scratches and licks the eggs.  Eggs hatch, worms attach themselves to the stomach lining.  Large numbers can cause the horse to lose condition and have gut ulcers. There are often lots of them .  Gastric ulcers, colic, poor condition are often signs.





These worms are nasty.  They are long and can grow up to 60 cm!  They get into the liver, bile ducts and lungs.  The horse may cough excessively without other symptoms of a cold.  The worm eats the food the horse digests and so the horse doesn’t get the nutrients it needs.




These tiny worms sit on the outside of the anus causing itchiness.  The horse scratches its tail a lot and can rub it out.  Other than that they don’t cause serious problems.




Gut ulcers, colic, poor condition,



Pasacaris equorum

These guys are why it’s critical to worm foals in their first months of life.  Ascarids are only found in foals and they grow up to 10-15cm inside the horse.  They get into the liver and lungs as well as the bowel.  If they haven’t been treated early when the horse does get wormed and the worms die they get stuck in the bowel and cause constipation, obstruction and colic.

In the past Large Redworms were a major problem in Australia and were the reason that 6-8 week worming was recommended.  Now, however, these have been mostly eradicated tend to only be an issue for foals.

Now, small redworms (cyathostomes), bots and tapeworms are more of a problem.  Small redworms travel as larvae to the large bowel, form cysts that attach to the wall.   If there are lots of them and they all hatch at once they release toxins which cause the horse to scour, develop colic or even die.  This is triggered by worming and killing the strongyles in the gut which sends a signal to the encysted strongyles to ‘hatch’.  The massive release of strongyles and their waste products are toxic to the horse.  This is called Larval Cyathostomosis and can look a lot like colic.  The horse develops scouring, weight loss and becomes very ill and may die.  The worst thing about this is that the horse can look very well before this happens and it’s often misdiagnosed and incorrectly treated.

This is why it is important to get worming right.

Bot worms found inside a horse who died of colic

Managing worms is not as simple as buying a worm paste and giving it often.  Latest advice about deworming suggests the following:

  1. Manage the Poo!  Pick up poo in small paddocks or yards every 2-3 days, in stables at least daily.  In larger paddocks avoid overgrazing and rotate horses to other paddocks when the grass gets shorter than 5cm.  Harrow the paddock and leave to rest for 6-8 weeks.  If you don’t rest the paddock you are just spreading the worms around and feeding them to your horse which makes harrowing a problem not a solution!
  2. Some horses will need worming more often than others.  Doing fecal egg count tests can tell you which horses need worming and whether the worming has been effective.  (note this is not the case for redworms as they don’t produce eggs)  Check out stockwatch lab for more info.
  3. Get some dung beatles – they love to eat poo and this helps break the worm life cycle.  Buy them from John Feehan at the Dung Beetle Expert
  4. Use the right wormer at the right time, that is,  the point at which it will kill the most worms.  This is largely dependent on the weather.  Worms can survive the cold, thrive in temperate weather but cannot cope with temperatures above 30 degrees Celsius.  Use this to your advantage and deworm around this.
  5. Making sure that new horses are wormed on arrival and kept separate from the others for 4 days.  Oh, and pick up their poo too!
  6. Make sure that you give the dose according to the weight of the horse……….if you don’t know this use a weight tape and add 10%.  Better to overdose a little than underdose, except in foals, ponies and minis.  And don’t let him spit it out either!  Weight tapes are a guide and I’ve found that they underestimate weight. 
  7. Suspected small strongyle encystation should be treated with 5 days of Panacur followed up with a wormer in Spring and then again in Autumn that has a moxidectin in it.
  8. Don’t Rotate dewormers - Rotation of dewormers is no longer recommended and is now considered to be causing resistance to dewormers across the world.

Wormer family

Includes the following active drugs

Active against

Common Brand names that include these drugs

Macrolytic lactones (mectins)




Round worms and bot larvae

Equimax, equiminth, equaquell, equimax LV, equiminth invermectin liquid, equest

Benzimidazoles (dazoles)




Redworms at all stages and if they are embedded in the gut

Strategy T, oximinth, oximinth plus, Fencar 100, panacur 100




Equimax, equimax LV

Tetrahydropyrimadines (antels)



Tapeworms and roundworms.  (morantel minimal effect on tapeworms)

Strategy T, ammo

Organophosphates (chlor…)



Bot worms and larvae

Oxyminth plus

Telmin – Plus

I have read lots of papers and information on deworming but my favourite and best resource on deworming is written by fellow Australian Dr. Ann Nyland.  I’m going to give this little ebook ‘What you don’t’ know about worms will surprise you’  a big plug here.  The information is evidence based, practical and easy to understand.  She gives a clear outline of what to worm and when in much greater detail than is covered here as well as covering other worms and worm related issues.  For around $5.00 it’s a great buy and something every horse owner should read. On top of that the proceeds go towards helping Ann look after rescue horses.   Get it here https://www.smashwords.com/books/view/600944 .  PS:  I don’t have any connection to Ann – I just think it’s a great reference!

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