The Horse and the Worm


Over the last decade drug resistant parasites have been reported, despite the
use of an increasing number of wormer drugs. This is worrying as there are no new drugs in the pipeline and eventually worms will become resistant to all the currently available drugs.

Up until now the conventional wisdom in horse husbandry has been INTERVAL DOSING.

“Interval dosing”, which many horse owners / yard managers use (and it is promoted by drug companies), involves regular wormer administration based on the egg reappearance periods after treatment with a wormer. It is important to realize if using this method that the egg reappearance time differs for each wormer drug.

This is an easy protocol to follow; however, this regimen will result in the routine worming of many horses that do not need it. Studies have shown that in any given population, 80% of worms are carried by only 20% of horses. Not only is this a waste of money and you are giving your horse
unnecessary drugs, but more importantly this ‘over-worming’ is believed to have made a substantial contribution to drug resistance.


Well obviously if you see worms in your horses poop, something must be done. If you are not very knowledgeable, you should take a poop specimen to your local lab to identify which worms your horse has. Then make sure you use a wormer that covers that particular species.

Here are some common symptoms that your horse may exhibit if it has worms:

Signs of Parasitism:

– Dull, rough haircoat
– Lethargy (decreased energy) or depression
– Decreased stamina
– Unthriftiness or loss of condition
– Slowed growth in young horses
– Potbelly (esp. young horses)
– Colic
– Diarrhea
– Excessive tail rubbing

If you suspect worms, the American Association of Equine Practitioners (AAEP) is emphasizing the importance of selective use of the remaining effective dewormers to reduce resistance. Testing is simple; you provide a fresh fecal sample (<24 hours old) from each horse, and it is evaluated for parasite eggs. The fewer the eggs, the fewer the parasites and the less often your
horse must be dewormed.

Here are some practical tips to lessen the chance of your horse picking worms up:

1. Remove manure regularly (at least twice a week), and spread on crop land or un-grazed land or
compost in a covered pit away from pasture.
2. Practice frequent mowing and dragging of pastures. This exposes eggs and larvae to sunlight
and environmental factors to decrease their chance of survival. (larvae can survive freezing, but
can’t tolerate extreme heat and drying)
3. Rotate pastures every few months to reduce parasite build-up and increase chances for natural
death of parasite eggs and larvae.
4. Avoid overcrowding and prevent overgrazing of pastures; this contributes to parasite build-up,
close grazing and increase rates of re-infection.
5. Avoid feeding horses from the floor or ground. Provide feeders for hay and grain.
6. Graze weanling and yearlings separate from older horses.
7. Isolate new horses until dewormed and fecal negative.
8. Provide a clean water supply free of manure contamination.
9. Coordinate your deworming; horses that share pasture, stable, etc should be dewormed at the
same time.
10. If possible, graze horses alternately with cattle or sheep, this interrupts the life cycle of equine
11. Remove bot eggs regularly from haircoat (a flea comb works well in some cases).
12. If you suspect worms, test for them first so that you can choose the proper product.

For more detailed information we have included a link to a well-written article.

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