It has been identified that the science of worming our horses has changed significantly over the last 5 years. Certain worms that caused severe problems 20 years ago are now rare and other worms previously thought to not be problematic have now emerged to become a serious problem. We now know that about 80% of horses develop a symbiotic relationship with parasites and will be immune to their pathogenic effects. On the other hand about 20% do not develop immunity and parasites are a real threat to these horses health and are a serious source of pasture/paddock contamination and re infection for other horses.

There are 5 basic types of horse parasites (worms).

1) Large Strongyles: These include the blood worms (see image 1) that can cause severe colic related to their larval migration through major arteries that supply blood to the bowel. These parasites are very susceptible to the Eqvalan/Quest family of wormers and have virtually been eliminated. These worms have a six month life cycle. Worming every 6 months will insure that they are not a problem.

2) Small Strongyles: There are over a hundred different types of small strongyles. These worms may appear in the manure as tiny red thread like worms. They have the ability to live in a dormant larval stage in the lining of the bowel (see image 2) for several months to several years. In the spring or as a result of unknown stressors these dormant larvae can emerge in large numbers and cause severe colic, diarrhea and other health problems. These worms have shown to have developed RESISTANCE to some wormers and have emerged as the major worm problem in North America. This type of parasite is of major concern and emphasizes the need for doing fecal egg counts and developing a sound worming program.

3) Ascarids: They are common in all foals and weanlings. Older horses develop, in most instances, a resistance to Ascarid infection. They can be seen in the manure as long (6 – 12 inch) white worms (see image 3). They can cause serious impaction colic, poor thrift, respiratory disease, rough hair coat and diarrhea. All foals should be wormed every 6 weeks starting at 6 weeks of age with either Panacur or Strongid until they are at least 9 months of age. In the fall they should receive a treatment with Eqvalan Gold, Equimax or Quest Plus.

4) Tapeworms: Contrary to popular belief tapeworms (see image 4) do not normally cause a horse to be thin or loose weight. They do however localize at the junction of the small and large intestine and can be a serious cause of colic. The life cycle of the tapeworm involves a pasture mite. It is recommended that your horse be wormed in the fall after coming off of pasture with either Eqvalan Gold, Equimax or Quest Plus.

5) Bots: The Bot fly is responsible for laying the yellow eggs (see image 5) on the horse’s legs in the late summer/fall. Removing these eggs from the horse’s legs is a good idea. Worming after the first heavy frost in the fall with an Eqvalan or Quest product will effectively treat Bots. Untreated bots can cause colic and other digestive disturbances.

It is very important to understand that worming a horse unnecessarily is not only a waste of money it is also the major cause of RESISTANCE. A few worms in a horse is now known to be a good thing and helps stimulate natural immunity. By over worming we allow resistant worms to flourish and become problematic. In some parts of the USA and Canada horses have to be triple dosed with wormer in order to treat worm infections. No new wormers are being developed for horses. Developing RESISTANCE is a serious problem. Many wormers go by the same name. Eqvalan, Biomectin, Zimermectin, Panomec and compounded Ivermectins are all identical wormers. Be sure to know what you are worming with. Certain wormers are best used in the fall, others in the spring, others are important for foals, others are best used to treat clinical cases. Contacting our office for advice based on age, history and fecal eggs per gram analysis will allow us to advise you as to the appropriate wormer for your horse.

We now have improved methods of determining your horse’s parasite load. A simple test can measure the eggs per gram of manure. The number of eggs per gram will classify your horse as a low or high shedder. For horses being tested for the first time we recommend the following:

  1. If a worm load is suspected for any reason (poor doing, weight loss, dull rough hair coat, diarrhea, colic) test your horse immediately.
  2. Otherwise wait for 60 days after the last worming (90 days with Quest). If the test reveals a low shedder worming is not necessary.
  3. A follow up test is recommended in the spring or summer months, as this is a time when some dormant worms mature and begin shedding. If the second test confirms a low shedder then it is fair to say that your horse does not need to be de-wormed other that in the spring and fall.
  4. Annual, in the Spring, fecal egg per gram test is advised.

If your horse is a high shedder then you have a problem and a selective worming program needs to be instituted. A high shedder will develop from high exposure to worm eggs in a contaminated environment, poor built in immunity or RESISTANCE to the wormers that you have been using. If you have a high shedder your horse should be wormed and then retested in 1 month. Depending on the test results a specific and scientifically based worming program can be developed.

Strongyles Larvae
PLEASE NOTE: It is important to test all horses. You cannot tell without testing which horses are high shedders. Identifying high shedders is key. The manure sample collected has to be fresh (warm). The sample can immediately be brought to our lab for testing or it can be kept in the fridge for 10 days before testing. (cooled parasite eggs do no hatch or degenerate).

The most important part of this article is the message about testing and monitoring. Becoming complacent about worming can be as serious as over-worming. New inexpensive tests allow us to accurately monitor your horses’ parasite load. Strategic worming will keep your horse healthy, reduce resistance problems and save you money.