Every Horse Boarder's Nightmare: A Young Horse's Death from Severe Worm Damage and the Ten Commandments of Parasite Control

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From the very day that newly-initiated horse owners pick up their crisp new how-to horsecare book or go to that first horse health lecture, the first commandment of horse health management echoes in their ears: Thou shalt worm thy horse religiously.

Is a horse safer in its stall than turned out on untended acreage used for a large number of horses© (Ashley Griffin photo, University of Kentucky, courtesy of ExtensionHorses)

Is a horse safer in its stall than turned out on untended acreage used for a large number of horses© (Ashley Griffin photo, University of Kentucky, courtesy of ExtensionHorses)

But there's a second part to that commandment, and it's one that anyone who keeps a horse in a boarding barn admits to fearing: Thou shalt participate in an overall management program for all horses on a property.

That's a relatively easy one to execute if you're keeping your horse at home, or at a trainer's facility where all horses are under one program. But what if you keep your horse in a "rough board" situation--who can control the pasture management? Who can require the other boarders to keep up a worming schedule? And who can oversee the addition of new horses to the common turnout area, particularly at dangerous times of the year?

Some boarding barns require horse owners to relinquish control of worming to the barn manager. This can range from feed-through worming medication added to a horse's grain ration daily, or regularly scheduled worming of all horses at the same time with the same products, carefully adjusted to each horse's weight so that the proper dose is given.

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Until recently, that first commandment was blindly followed. We wormed our horses whether they needed it or not. We shopped for wormers with a bargain-hunter's vengeance. Now add in the recent publicity about the dangers of overdosing with repeated worming products and the development of resistance to specific worming medications. This has made some horse owners become very lax about worming.

How dangerous can a worming crisis be? Bad enough to kill an otherwise-healthy six-year-old horse, that's how bad. The information for this article was provided by the veterinarian who was called to assist the horse.

Kirsty MacGregor, MRCVS, of Bakewell Veterinary Practice in Derbyshire, England was called out earlier this year to examine a six-year-old horse that had suddenly dropped down while turned out in a field.

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The horse had appeared normal before he was turned out. He was outwardly healthy and had relatively good body condition. He had been seen walking across the field five minutes previously but within minutes he had fallen in mid-stride, indicating a very sudden death. A full post-mortem examination confirmed that the cause of death was ?verminous thromboembolism' -- a fatal blood clot caused by severe worm damage.

MacGregor explained: "The large intestine was loaded with encysted small strongyle larvae and there was evidence that other worms had migrated to the arteries and the liver, causing inflammation and damage. The horse also had lesions in the small intestine, which, although common, are likely to be associated with parasite migration and chronic gastric ulceration in this case."

The horse was kept on a large DIY yard (the British term for what Americans call a "rough board" stable; the horse owners are responsible for the care of their horses). The facility housed around 40 horses and ponies. With so many individual owners, the horse's owner had found it difficult to implement a regular worm control program although the stable owner and her boarders had tried hard to manage the situation.

MacGregor continued: "This unfortunate case serves to highlight the tragic consequences of being unable to coordinate an effective worming program. Subsequently, we have carried out emergency dosing, treating all the horses on the yard with a combination of moxidectin and praziquantel (Equest Pramox - Pfizer Animal Health) to treat encysted small redworm and tapeworm. At the yard owner's request we have also put together a worm control program for all the horses."

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MacGregor's Bakewell Veterinary Practice has seen a number of cases involving encysted small redworm infestation this year. The affected horses presented symptoms such as violent colic and weight loss. In some cases, the infestation proved to be fatal, so the veterinarian has put together what amounts to ten new "commandments" to help horse owners make sure they keep their horses safe from worms:

1. Use diagnostics on a regular basis to build a picture of your horse's worm burden. (usually a fecal sample)

2. Understand your enemies - familiarize yourself with the main types of worms affecting horses.

3.? Select the wormer most appropriate for the parasite you are targeting by looking at the chemical ingredients of each wormer, rather than just choosing the wormer for its name alone.

4.? Use a weight tape or scales to make sure you dose accurately according to weight.

5.? Treat horses as individuals as well as a part of the herd to make sure they are wormed according to their need as well as for their environment.

6.? Worm new horses before they mix with your existing animals.

7.? Don't overstock paddocks and pastures and do rest them wherever possible to let the parasites die off.

8. Grazing sheep or cattle on the same pasture is an excellent way to cut the worm population as they will ?hoover up' (British for "vacuum") horse worm larvae which will then die.

9.? Keep stables hygienic and clean feed buckets well.

10. Collect and dispose of manure from the field promptly; do this at least once a week as this will significantly reduce the number of worm larvae getting on to the pasture.

Photos in this article courtesy of " target="_blank">ExtensionHorses,? The Donkey Sanctuary and the School of Veterinary Medicine and Science, University of Nottingham (UK).