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The basic relationship between parasitic worms and the horse is that the adult worm lives in the horse's bowel, surrounded by food that passes through the digestive tract. Often the worm attaches itself to the bowel wall to stop itself from being carried right through the horse with food and water. it is not in the parasite's interest to kill the horse because, if it does, the worms in that horse will die too. In the wild, where horses roam over a wide area, the rarely develop a worm burden severe enough to be fatal. When they are restricted to grazing a small area continuously, as are domestic horses in a field, the number of worms they taken in can be large and possibly lethal.
Development Of The Worm
A pasture with worm eggs and larvae is like a time bomb, which will explode when the horse grazes there. There are no chemicals that kill worm eggs. Drying out and extremes of temperature will kill them in time, but this can take two or three years. When the eggs hatch, the larvae climb to the top of the grass stalks, where they are ingested when the horse grazes. Topping (cutting the grass) will remove these larvae. Dividing the grazing up and using each area in turn (strip grazing) will allow time for larvae to die in between grazing periods. Removing dropping from the pasture can help to remove worm eggs before they hatch. The larvae develop in the horse, and cause more damage than adult worms. You must therefore worm the horse regularly, not just when you know you have a worm problem.
How Worms Affect The Horse
The following are possible symptoms of worm infestation:
* Weight Loss
You will not be able to tell if your horse has worms by just looking at it. Most worms can cause a loss of condition, but there are other factors that make a horse thin. A large number of adult worms or larvae is usually needed to cause weight loss, anaemia or general poor condition. As worms can be associated with colic, a horse that has repeated attacks of colic or where no other cause is diagnosed, should be wormed.
Worming programmes can vary greatly depending on factors, such as area of country, grazing system, worm history etc. but as a rule the following guidelines should be followed and then adapted as necessary.
There are three "anthelmintics" or worming drugs, which are active against a reasonably wide range of worms: ivermectin, pyrantel and benzimidazoles. It is not possible to control all worms with just one wormer, you must target-worm for the exceptions, and then have one drug for routine use throughout the year. Change the type of wormer every one or two years. You should not change it every time you dose, as this will make it easier for the worms to develop resilience to the wormers. To prevent a significant number of worm eggs from being released on your pasture you should worm every four weeks with a benzimidazole wormer, every six weeks with pyrantel or every eight weeks with ivermectin.
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