Six Problems That Can Be Due To Worms

By John Byrd, DVM
Problem #6:  Death
Please be sure to read the last paragraph of this article.
Deaths in horses have occurred in the past due to worms and may continue to occur, but at a much reduced rate.  The introduction of Ivermectin as a deworming medication has been a large factor in the decreased death rate from worms.  It has nearly eliminated Strongyle vulgaris and other large Strongyles that were the major cause of death in horses from worms.  As indicated previously, Strongyle larvae can cause severe damage to the arterial supply to the digestive tract.  Blood clots, emboli, and aneurisms were the result of large Strongyle infections.  The finding of these problems is seen much less today during colic surgery and necropsies.  Small Strongyles now can cause death due to large excystation of small strongyle larvae with development of the syndrome termed Larval Cyathostminosis.
Most deaths in horses thought to be due to worms are due to several factors and worms are only one contributing factor.  Horses that are suffering from inadequate nutrition, viral or bacterial infections, and other stress are much more likely to die from worm infections.
The number of horses that die from worms is a very small (probably minuscule) number and owners should not read this article and become worried that their horses are going to die or even that worms are going to effect their performance.  I know for a fact that I could put 5 horses in a ring and ask expert horsemen to rate them according to how many worm eggs they are passing and it would be pure luck for them to get the rating correct.  I say this not to be critical, but to show that horses can be fat, slick (have a beautiful hair coat) and energetic and still we find 2000-3000 eggs per gram of strongyle eggs in their manure.  One of the first horses I tested when I moved back to Illinois had just won a major Quarter horse futurity a few days before I personal took the sample and it was positive for 5000-strongyle eggs/gm of manure.  Dr. M. K. Nielsen one of the authors of Handbook of EQUINE PARASITE CONTROL did a study on fecal egg counts in 213 racing Standardbred horses in Denmark with the following results.  There was a statistical difference in the fecal egg count in the horses finishing 1-3 in a race as compared to those horses finishing unplaced.  The actual results were unexpected.  Horses finishing 1-3 actually had a higher fecal egg count than those finishing unplaced.
To keep from worrying about worms in your horses do periodic (Horsemen’s Laboratory recommends every 3 month at least until it is established what egg shedding category they are in) fecal egg counts to evaluate the effectiveness of your horses worm control program and go enjoy your horse.