Response to Nutritionists Article

Horsemen’s Laboratory has received several emails concerning doing FEC’s fecal egg counts on horses.  A recent article has appeared to say that fecal egg counts are of little or no value in evaluating a horse’s worm control program.  However, the article does not appear to have a better way of evaluating the effectiveness of a horse’s worm control program.

My name is Dr. John W. Byrd and I started and own Horsemen’s Laboratory and I do believe that fecal egg counts are the only way of evaluating a horse’s worm control program.  I started doing equine fecal egg counts in 1991 while practicing in southern California. This is when the daily dewormer came out and the drug companies thought that is was going to be the answer to worm control in horses.   The horses that I was caring for were being dewormed every 8 weeks and some were being dewormed as often as every 6 weeks.  At that time when someone asked me how I knew that their horse had worms I always answered because the drug companies told us they do.  After I dewormed horses, I often was asked how do you know the dewormer got rid of the worms my horse had and the answer was the same the drug companies tell us it did.

After making that statement several times, I decided there must be a better answer for these 2 questions.  After thinking about the answers and consulting with 3 equine parasitologists I started checking most of the horses I was deworming before I dewormed them and found that less than 1 out of 20 horses were passing any eggs to indicate they had worms.  In fact, at first, I thought I must be doing the test wrong because it took many samples before I found any eggs. Even when I found eggs the count was very low. That was when I decided to attempt to make it easy for horse owners to evaluate their horses worm control program. This gives the horse owner the peace of mind that they are doing a good job of protecting their horses against worms.

Now let me address the questions that this particular article has caused horse owners to have concerning fecal egg counts.  It is difficult to choose which of these miss conceptions are the most important, so I will address them as stated in the article.


Environmental contamination

By controlling environmental contamination, we can control the rate of parasite infection of the herd and more individuals will be exposed to fewer infective larva. If we can identify the high shedders in a herd, we can separate them from the rest of the herd thereby lower the exposure rate.  Or as the new policy dictates, we may deworm the high shedders also lowering exposure to other individuals.  We can only do this if we know which horses are passing the most eggs.  FEC’s are the only method of knowing which horses are passing the most eggs.


How can we identify horses that may have compromised immunity to parasites in order to protect them from the harmful effects of parasites?

Again, doing timely FEC’s are the only way I know that young horses and other horses’ that have compromised immunity to parasites can be identified.


Damage due to encysted small strongyle larva

Every adult small strongyle that is laying eggs was an encysted larva previously. Before that it was an infective larva in the environment. And before that it was an egg passed by a horse into the environment. So, if we can control the number of eggs passed in the environment, we can control the number of encysted small strongyles.  By knowing where the majority of eggs are coming from, we can control the number of adults presently producing and laying eggs, so we can dramatically reduce the source of eggs by deworming the horses that are high shedders of strongyle eggs.


Many people have made the statement that there is a great seasonal variation in a horse’s egg shedding rate.

However, at Horsemen’s Laboratory we have not found this to be the case. We have analyzed our results over 2 different 3-year periods and found less than a 3% difference when we compared positive and negative samples.  The difference was 3% also when we compared the different egg shedding level (Low, Medium, and High).   In each comparison we compared the months of July, August, and September to December, January, and February, Horsemen’s Laboratory has processed over 100,000 samples.


Tapeworm diagnosis

It is true that making a diagnosis of tape worms is difficult and everyone knows that a standard McMaster fecal test is not very accurate at finding tapeworm eggs in the horse’s stool. Horsemen’s Laboratory has started testing a test that Dr Nielsen describes in his latest book Handbook of Equine Parasite Control. Nielsen states in his book the test is 60% accurate if there is 1 tapeworm present in the horse’s digestive tract.  And the test is 90% accurate if there are 20 or more tapeworms present.  Since no to very little pathology is seen if fewer than 20 tapeworms are present it appears to be a useful test to diagnosis tapeworms.


  • I assume the tissue form of immature larval stages of small strongyles that are spoken of in the article are the encysted stage that are encysted ones in the lining of the large intestine.

True, there is no way to assess the inflammation in the lining of the large intestine caused by encysted strongyle larvae other than doing a biopsy of that lining of the large intestine. Since this carries with it considerable risk Horsemen’s Laboratory feels it is much safer to monitor how many strongyle eggs are being spread in the environment. Because we know that these eggs will shortly become encysted larva in the lining of the large intestine of some horse unless the egg production is stopped. By deworming each horse according to its egg shedding rate (low shedders, 1-2 times/year, medium shedders 2-3 times/year, and high shedders 4 times/year.  If this protocol is followed the large strongyles should also be well controlled.


Yes, bots and pinworms do not show up in fecal egg counts.

However, owners should be able to have a fairly good idea if their horses have bots or pinworms by doing a little reading about these 2 worms and a little detective work. Bot eggs are laid on the hair of the horse on different areas of the horse’s body according to the species of bot fly that lays the eggs.  Pinworm females crawl partially out of the rectum generally at night to lay her eggs and then she dies often to be found by the owner the next morning as a single white worm on the top of a pile of manure.  When she lays her eggs she also passes a sticky fluid that glues the eggs to the exterior rectal ring and causes most horses to rub their tail head breaking off many tail hairs. This can seem to happen overnight.  Anyone having questions about bots or pinworms can call horsemen’s Laboratory at 1 800 5440599 or email us at


  • Strongyloides are rarely (almost never) found in fecal egg count

However, they are unique because they can complete their life cycle either as free living or as a parasite of horses. Foals can have diarrhea due to being infected with strongyloides.  Only females become parasitic and in the third stage larval form penetrate the skin or mucus membranes and if in mares that are pregnant, they are stimulated to go to the mammary gland as the mare gets close to foaling thereby infecting the foal when it nurses.  Eggs generally appear in the foals stool when it is 10-14 days old but have been found at age of 5 days.


  • In 1992 Horsemen’s Laboratory started doing FEC’s through the mail.

I was concerned about a way to preserve the eggs in the samples because I had been taught that the samples had to be cooled. However, after talking with Dr Lyons and Dr. Drudge at the University of Kentucky and they encouraged me to go ahead and try to do FEC’s through the mail. After 1 year we found that most of the samples we received had many eggs that looked just like they had been passed from the horse.  However, some of the eggs were embyronated and a few had hatched, and larvae were swimming in the solution. This continued for several years. One of the strange things that seemed to occur was that often we received several samples in one envelope and some of the samples had larva and embyronated eggs and non-embyronated while all the eggs in other samples appeared as they had just been passed from the horse.  Many years ago I met Dr. Nielsen and we were discussing this phenomena of the embyronated eggs.  He suggested it was likely due to the fact the eggs need oxygen to embryonate and hatch. He felt it was due to some samples not being firmly packed and contained air that contained oxygen of course. This oxygen was enough to allow the eggs to embryonate and hatch.  Since then we have stressed that owners firmly pack our containers full so no or very little oxygen is present. Now we seldom find any embyronated eggs or larvae in the sample even if it takes 2 weeks for the samples to get to Horsemen’s Laboratory. Horsemen’s Laboratory receives 85-90% of the samples within 3-5 days of the date the sample was taken.  So the real key to sending fecal samples through the mail successfully is limiting the eggs exposure to oxygen by packing the samples firmly in Horsemen’s Laboratory’s sample container.

The conclusion is that doing periodic fecal egg counts on your horses is much better than guessing if your horse has worms.  Fecal egg counts will also indicate which horses in a pasture are distributing the most eggs (infective larvae) per square inch.  And remember that horses that are low shedders of parasite eggs are not likely to be responsible for their own re-infection or the infection of other horses in the pasture.