September 2009

 A Rational Approach to Parasite Control

By Charlene B. Cook DVM

Perhaps I should say how to deworm the intelligent way. Right now you are probably asking why should I care?  I already deworm every 60 days and my horses look really good. The fact is that you should care a whole lot about resistance that is developing to anthelmintics (dewormers).
A few years ago I started running fecals on every colic case that I treated, the results were alarming; a very high percentage of horses with colic had high levels of parasites even though their owners were regularly deworming them. That lead me to look closer at horses that were boarding at our farm. Once again, I was alarmed with the results. Many of the horses arriving for breeding and treatment had very high levels of parasites in spite of having had some type of regular parasite control. So I took this a step further and started running fecals on all of my patients that were getting an annual physical exam. We saw that horses who were not receiving their daily dewormer by either owner error, pasture feeding or the horsesí own sloppy eating habits had developed resistance. In one case a local farm was shipping mares out for foaling and breeding and the foals were coming back loaded with resistant roundworms that were leading to pneumonia.
So you might ask how do horses get worms in the first place? The most important and damaging parasite in horses are small Strongyles. Adult Strongyles live in the large intestine and the females lay eggs which pass into the environment in the horsesí manure. The eggs hatch under favorable environmental conditions and small wormlike larvae emerge. The larvae will molt twice and the third stage (L3) larvae climb up the blades of grass where they are ingested by a grazing horse. The larvae can migrate away from the manure pile for a distance of about 18 inches spreading the level of contamination. Once ingested by a horse the larvae then burrow into the intestinal lining forming a protective capsule of scar tissue, much like the cocoon of a moth. The larvae can stay in the capsules for a few weeks or more than 2 years waiting for the right environmental conditions to hatch out and repeat the cycle. Large numbers of larvae can hatch at once causing diarrhea that can be fatal. Even small numbers of larvae can cause colic, weight loss, poor growth, hypoproteinemia, loss of condition and rough hair coats. Even in the best barns parasite loads cause a drop in performance because the blood sucking parasites rob the horse of nutrients. In addition the owner suffers financial loss due to higher feed bills, more supplements etc while trying to get the horse up to peak condition.
Because the life cycle involves grazing, stalled horses are at lower risk however, because the encysted stage can last 2 years or longer we still find horses with significant worm burdens that have been in stalls 24 hours a day for over 2 years.
These parasites have evolved for maximum survival. The larvae can survive when temperatures range from 45 to 85 degrees Fahrenheit. The fecal pile is moist and dark providing the protection for developing larvae. During the hot, dry summer months the larvae will die quickly due to desiccation (drying). During the spring and fall months it is a parasite wonderland in Georgia. The parasites are capable of conserving energy and they can survive even northern winters with ease. In Georgia, fall is the start of the parasite cycle and is the most important time to test and deworm horses.
Horses have some protective mechanisms of their own. If you look over a pasture you will notice that some areas have manure piles (roughs) and other areas have none (lawns). Lawns are for grazing, roughs are toilets. Ever noticed that horses donít like to graze around manure? It is a natural protective mechanism. Horses will not graze within 18 inches of a manure pile unless they are forced to do so. If you can see a manure pile from a distance your horses donít have enough to eat. You may think that you are helping by dragging your pastures to spread the manure but, unless we are currently having a drought you are only spreading the parasites. In Europe land is at a premium and it is very popular to remove the manure piles either by hand or with a pasture vacuum. Removing the manure before the parasites can go thorough the molting cycles will greatly reduce the level of pasture infectivity.

        Overgrazed Pasture                                                     Non-Overgrazed Pasture

So how does parasite resistance develop? Many times it is by error. If a horse weights 800 pounds and you only give him a 750 pound dose the horse will not receive adequate medication and some of the parasites will be left in the intestine. These parasites that have been exposed to inadequate levels of product can develop a genetic code for resistance that they can also pass along to future generation of worms. Some horses make the job of deworming easy while others toss their heads, spit out paste and otherwise do their best not to take their medication. Think about all the times that you dewormed your horse and when you were done there was paste on the horsesí lips, the side of their face and your shoes. Do you really think that he got the full dose? Or how about the horse that spit the entire tube out, you only had one tube for each horse so you didn't go back to the store for another? Likewise many farms donít have a deworming program; horse A is dewormed today, horse B is dewormed 2 weeks later and horse C is never dewormed. The horses are constantly exposing each other and resistance develops quickly in these cases. Now let one of those horses move to a new farm and he takes his resistant worms with him depositing the eggs on the new pastures.
The fact is there are no new products being developed at this time. The drugs that we have available today are all that we have to treat our horses. Once you have resistance to a drug using more of the drug is of no benefit.  This is why deworming and parasite control should matter to you.  
We test the fecal samples using a specific test called a McMasterís fecal examination. To do this we weigh out a 1 gram sample of fresh manure. 1 gram of manure is about the size of a quarter, a single ďRoad AppleĒ is plenty enough for us to test. We then take the 1 gram sample and float it in a concentrated solution of sodium nitrate. The eggs produced by the worms are buoyant and will float to the top of the tube in the solution just as you would float in a swimming pool. Fluid from the top of the tube is carefully pipetted into the McMasterís chamber and then the slide is then viewed under a microscope. The number of eggs that are present in the chamber grids are counted and a mathematical formula is used to calculate the fecal egg count. A scoring system was developed by parasitology experts: A low shedder has 150 or fewer eggs, a moderate shedder has 150-500 is a moderate shedder and those with 500 or more are high shedders.
So, why is this important to you? Well, we know that approximately 20% of the horses on any farm have excellent immunity to parasites and these horses will be in the low shedder group, most of these horses only need to be dewormed twice a year. Another 20% of the population will have poor immunity and fall in the high shedder classification. These high shedding horses will need to be dewormed at least every 90 days or 4-6 times per year. The remaining horses will fall into the moderate shedder classification and will require deworming about 3-4 times per year. By identifying which horses are in which category most owners can reduce their deworming bill in half.
The important message in all of this is that we need to identify the horses that are at most risk. The high parasite loads will cause progressive damage to the intestinal wall and predispose these horses to colic. In additional the horses that are shedding high numbers of parasite eggs are contaminating your pastures only to be shared with other horses on the farm.
Many of the dewormers on the market have been around for decades. Resistance has developed as a result of repeated exposure and misuse of these products. The only way to know if a product is working is the test horses after deworming. Letís say for example that a horse has a parasite load of 500 eggs per gram. If you deworm this horse with product XXX and retest the horse 10-14 days later the fecal egg count should be 50 or less. If the fecal egg count remains high the horse has parasites that are resistant to that drug. This means that product XXX can no longer be used on that farm.
The fastest way to develop resistance is to under dose horses. The second fastest route is to use the same product over and over to the same group of horses. The product that should be used depends on multiple factors: the horsesí age, the horsesí environment (stall vs. pasture), the time of year and the resistance patterns on the farm. Your neighbors deworming program may not be the right program for your horses. We can customize deworming programs for the best possible control with minimal environmental impact.
Rational parasite control is best achieved through a combination of targeted anthelmintics use, good husbandry and non-chemical methods of control.

So in summary letís hit some important points.
  1. Fall is the most important time to test horses to determine which category they belong in.
  2. Spring and fall are the most important times to deworm horses.
  3. You must test positive horses to know if resistance to a product is present on your farm.
  4. Bring a fresh fecal sample in a zip-lock bag or air-tight container for testing. Keep samples cool until testing.
  5. The best time to test is 4 months after Moxidectin, 3 months after Ivermectin and 9-10 weeks after Pyrantel or Benzimidazoles.
  6. Test all new arrivals coming to the farm BEFORE you allow them to go out on pastures.
  7. When giving anthelmintics be sure to give an appropriate dose.
  8. Only deworm horses that require deworming based on fecal egg counts.

Non Chemical Methods of Parasite Control
  1. Rotate pastures if possible, it is best to rest pastures for a minimum of 4 weeks during the summer heat.
  2. Cross graze pastures with cattle, sheep or goats which act as biological vacuums.
  3. Compost manure. Properly composted manure will kill Strongyle larvae and many ascarid eggs.
  4. Keep pastures roughs mowed to 3-8 inches.
  5. Feed hay and grain in raised containers and not directly on the ground.
  6. Remove manure from pastures and paddocks every 24-72 hours before Strongyle eggs can get a chance to hatch and develop into infective larvae.
  7. Drag or harrow pastures only during hot, dry periods and keep horses off the pasture for at least 4 weeks.
  8. Avoid overstocking pastures, 1 horse per acre minimum.
  9. Clean water sources regularly to prevent fecal contamination.
  10. Plant pastures with winter grazing to reduce exposure.

For more information visit
Come by our office or ask us to bring you a Parasite Control Kit on your next visit.
Got questions? Please feel free to call or write me at

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