Deworming in Detail

Internal parasites can cause extensive internal damage to your horse. The effects of these parasites on a horse can range from a dull haircoat and unthriftiness to colic and death. Internal parasites lower the horse’s resistance to infection, deprive the horse of essential nutrients, and in some cases, cause permanent damage to the animals organs.

Common signs of parasitism can include the following:

  • Dull, rough haircoat
  • Lethargy (decreased energy) or depression
  • Decreased stamina
  • Unthriftiness or loss of condition
  • Slowed growth in young horses
  • Pot belly (especially in young horses)
  • Colic
  • Diarrhea

Establishing an effective parasite control program should be ranked second on the priority list, only to be superseded by fresh, clean water and good quality feed and hay.


There are more than 150 species of internal parasites that can infect horses. The ones with the most impact are the big four:

  • Large stronglyes
  • Small strongyles
  • Roundworms
  • Tapeworms

The lifecycle of most internal parasites involves the eggs, larvae (immature worms), and adults (mature worms). Eggs or larvae are deposited onto the ground in the manure of an infected horse. They are then ingested while the horse is grazing, and the eggs hatch and/or larvae mature into adults within the horse’s digestive tract (stomach or intestines). With some species of parasite, the larvae migrate out of the intestine, into other tissues or organs, before returning back to the intestine and maturing into egg-laying adults to complete their life-cycle.


Fecal Egg Counts

One of the most useful tools in a parasite control program is the fecal egg count. This test allows the veterinarian to determine which parasites are present and whether the infection is light, moderate, or heavy. This information is important in developing a deworming program and for monitoring the effectiveness of that program.

Fecal egg counts involve collecting a small fresh manure sample from the horse to be tested. This sample can be collected, placed in plastic bag and put in the refrigerator (not the freezer). Samples should be brought into the clinic within about 24 hours of collection.  Results are expressed as eggs per gram (EPG) of manure. Fecal egg counts of less than 200 EPG suggests a light parasite load, 200-500 EPG, a moderate load, and 500-1000 EPG, a heavy load. Parasitologists believe maintaining parasite burdens at a low level, rather than attempting to eliminate parasites entirely, helps avoid over-treatment, limits the cost of parasite control, and positively impacts a horse’s immune system.

It is important to note that a negative fecal examination does not mean the horse is free of internal parasites. Some types of parasites produce eggs only intermittently. Larvae do not produce eggs at all, and may be present in large numbers in a horse with a fecal egg count of zero.  Seasonal patterns affect egg shedding and hence EPG counts.  Additionally, tapeworm eggs may be missed with routine fecal egg count techniques because they occur in very small numbers and don’t float well in the testing medium.

Your veterinarian will take all of these factors into account when interpreting the fecal egg count. The results are most useful when several horses on a farm are tested on the same day. This information gives the veterinarian and farm manager a good idea of the level of parasitism on the property.



There are several different types of dewormers or anthelmintics, currently available. Most are broad-spectrum, meaning that they are effective against several classes or groups of parasites. No deworming product is 100 percent effective in killing all internal parasites, although it is not necessary for a product to kill every worm in order to improve the horse’s health, minimize the risk of serious disease, improve feed efficiency, and reduce pasture parasite contamination.

Basic Deworming Programs:

  • Strategic
  • Rotational
  • Continuous

There is no single deworming program that suits all horses and all situations. The ideal program for your horse(s) depends on the type, number and ages of the horses on your farm, pasture management and your geographic location.



Strategic deworming or “test and treat” is deworming only at certain times of the year or when fecal egg counts rise. This is the method currently recommended. Having fecal egg counts performed to determine the amount of egg shedding is important. This information will help ensure that the dewormers that are being used are effective and also help determine the frequency of deworming necessary to keep your horse healthy.  It may also help decrease the amount of deworming that your horse requires and therefore reduce the amount of unnecessary chemicals your horse receives, as well as help fight the battle against parasite resistance.


Rotational deworming is the method that recommends administering specific dewormers at set intervals throughout the year based on larval emergence with regard to the seasons. The dewormer chosen is the most effective drug for the prevelance of a particular parasite at that particular time of year. This rotation between chemical classes–not just between brand names of dewormers–is a critical element in the success of a rotational deworming program and maximizes the best attributes of each different chemical compound.  This was the recommended method for many years, but recent studies have shown that the strategic method is more successful in most situations.


Continuous or daily dewormers deliver a minimal quantity of dewormer each day, usually given in a small amount of feed. They effectively prevent new larval infections that are picked up during grazing. Daily dewormers are not designed to resolve existing infections (only to prevent reinfection) and do not kill bots or tapeworms, so they should not be relied upon as the sole method of parasite control. Because this system does not protect against bots or tapeworms, an Equimax should be given in January and July.  The amount of drug given each day with a daily dewormer only prevents infection, it is important to determine if there is an existing infection with a fecal egg count exam and to use an effective dewormer based on the EPG results prior to beginning daily treatment.  This method of treatment is not currently recommended except in very specific situations determined by a veterinarian.


Pasture Maintenance

Good pasture maintenance is an important part of any parasite control program. Pastures become contaminated by the manure of infected horses. Healthy horses can become infected with parasites by ingesting parasite larvae while grazing on contaminated pastures. The goal of pasture maintenance is to reduce the number of parasite larvae in the environment and thus decreasing the horse’s exposure.
Chemical control using dewormers is just one part of a complete parasite control plan. As parasites are primarily transferred through manure, good management is essential:

  • Keep the number of horses per acre to a minimum to prevent overgrazing and reduce pasture contamination with parasite eggs and larvae
  • Pick up and dispose of manure regularly
  • Do not spread manure on fields to be grazed by horses; instead, compost it in a pile away from the pasture
  • Mow and harrow pastures periodically to break up manure piles and expose parasite larvae to the elements The horses should also be removed for 3-4 weeks.  This should be done in hot dry weather. Larvae can survive freezing, but they cannot tolerate extreme heat and drying for very long.
  • Consider rotating pastures by allowing sheep or cattle to graze them, thereby interrupting the life cycles of equine parasites
  • Keep foals and weanlings separate from yearlings and older horses to minimize the foals’ exposure to ascarids and other parasites
  • Use a feeder for hay and grain rather than feeding on the ground
  • Consult your veterinarian to set up a deworming program for your horse(s) and monitor its effectiveness

Monitoring Parasite Levels

Monitoring parasite levels is important in determining the effectiveness of a parasite control program, regardless of the type of method used. All farms should have a fresh fecal sample taken from each enclosure or pasture and submit them to their veterinarian semi-annually (spring and fall). The fecal sample should be collected ten days after the last deworming. Fecal analysis can be performed more frequently on farms beginning a new parasite control program or if parasite problems are suspected.

New Horses

Outside horses entering a farm with an established parasite control program are the major source of re-contamination. New horses on the farm should be dewormed and have a fecal egg count analysis performed upon their arrival regardless of the deworming history. The new arrivals should be isolated from other horses for two weeks and then be placed on the farm’s deworming program.

Common Causes of Program Failure

Even farms with established parasite control programs can experience higher than expected parasite loads. There are several reasons these situations arise. One of the most common is failure to properly administer paste dewormers such as spillage or under dosing. Another complication that occurs is certain parasites may become resistant to some dewormers. This situation may require temporary modifications to the program. A less common cause of program failure is not necessarily a farm issue but an individual horse problem. There are some horses that are very susceptible or sensitive to parasites and these horses may require intensive management.

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