Six Problems That Can Be Due To Worms

By John Byrd, DVM
Problem #2  

Wormy Horse Syndrome:  Weight Loss, Diarrhea, Stunting, Poor Hair Coat, Depression, Poor Appetite, and Potbelly appearance
I have elected to discuss all these problems together because they all appear to occur together.  The problems are generally seen in young horses less than 2 years of age.  They are most often seen in poorly managed breeding operations where young horses have been raised in the same pasture or paddocks for several years.  There are 3 major factors that seem to be responsible for this.  

  1. A young horse’s immune system has not developed the ability to fight off the worm infections or other infections that often complicate worm infections by adding stress to the young horse’s life.
  2. The exposure to the infective stages of the worms is highest at this time in the life of a young horse. 
  3. For weanlings, autumn is when they are weaned, adding more stress to their life and causing the immune system to be less effective at protecting them.

Weight loss, stunting, poor hair coat, and depression can also be due to the heavy load of worms using up a great deal of the energy that these young horses need to grow.  In the case of roundworms, just the large mass of roundworms in the small intestine can interfere with the absorption of nutrients that takes place in the small intestine.  Due to the lack of needed nutrients the horse will become depressed and its appetite will also be depressed.  Often these young horses are infected with several different classes of worms.  It is not uncommon to find large numbers of roundworm eggs and strongyle eggs in a young horse’s fecal sample.  Recently, we had a couple yearlings that also had tapeworm eggs in their stool.  These worms are all competing with the young horse for the nutrients it is taking in.  The large strongyles are particularly bad because the adults attach to the intestinal wall and actually suck the horse’s blood.  Soon after I graduated from vet school I recall deworming a large herd of horses, it had snowed a few inches overnight and the next morning the owner called me in a panic because he thought I had poisoned his horses.  From each of the fresh manure piles there were little red streaks on the fresh snow.  I rushed back to the farm and what we found was that each streak had a dead large strongyle at the end of it filled with pure blood.  Many weanlings have suffered and died of anemia due to severe large strongyle infections.  Fortunately, shortly after that Ivermectin came on the market and has tremendously reduced the population of large strongyles, but has not completely eradicated them as we had hoped.
Diarrhea can occur when there are extremely large numbers of encysted strongyle larvae in the wall of the large intestine in a horse.  The cysts can cause an interference with the absorption of fluid from the large intestine thereby causing the fluid to be passed in the stool as diarrhea. 
Diarrhea also can occur when a large number of larvae all leave their cysts in a relatively short period of time.  On rare occasions, this encystation causes enough swelling and inflammation to cause poor absorption of fluid from the large intestine.  This syndrome is termed Larval Cyathostminosis and can occur at any age, but is more prevalent in horses between 1 and 4 years of age according to Dr. Nielsen and Dr. Reinemeyer in their book the HANDBOOK of EQUINE PARASITE CONTROL.